ARE THE ULTRA-FAINT DWARF GALAXIES JUST CUSPS?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zolotov, Adi; Hogg, David W.; Willman, Beth, E-mail: az481@nyu.edu

2011-01-20

We develop a technique to investigate the possibility that some of the recently discovered ultra-faint dwarf satellites of the Milky Way might be cusp caustics rather than gravitationally self-bound systems. Such cusps can form when a stream of stars folds, creating a region where the projected two-dimensional surface density is enhanced. In this work, we construct a Poisson maximum likelihood test to compare the cusp and exponential models of any substructure on an equal footing. We apply the test to the Hercules dwarf (d {approx} 113 kpc, M{sub V} {approx} -6.2, e {approx} 0.67). The flattened exponential model is stronglymore » favored over the cusp model in the case of Hercules, ruling out at high confidence that Hercules is a cusp catastrophe. This test can be applied to any of the Milky Way dwarfs, and more generally to the entire stellar halo population, to search for the cusp catastrophes that might be expected in an accreted stellar halo.« less

The Least Luminous Galaxies in the Universe

NASA Astrophysics Data System (ADS)

Willman, Beth

2011-05-01

In the past six years, more than two dozen dwarf galaxies have been discovered around the Milky Way and M31. Many of these discoveries are 100 times less luminous than any galaxy previously known, and a million times less luminous than the Milky Way itself. These discoveries have made astronomers question the very meaning of the word "galaxy", and hint that such ultra-faint dwarf galaxies may be the most numerous type of galaxy in the universe. This talk will highlight i. how we can see galaxies that are effectively invisible in images of the sky, ii. the brewing controversy over the definition of the term "galaxy", and iii. what ultra-faint galaxies can reveal about the distribution of dark matter in our Universe.

Variable Stars in the Field of the Hydra II Ultra-faint Dwarf Galaxy

NASA Astrophysics Data System (ADS)

Vivas, A. Katherina; Olsen, Knut; Blum, Robert; Nidever, David L.; Walker, Alistair R.; Martin, Nicolas F.; Besla, Gurtina; Gallart, Carme; van der Marel, Roeland P.; Majewski, Steven R.; Kaleida, Catherine C.; Muñoz, Ricardo R.; Saha, Abhijit; Conn, Blair C.; Jin, Shoko

2016-05-01

We report the discovery of one RR Lyrae star in the ultra-faint satellite galaxy Hydra II based on time series photometry in the g, r and I bands obtained with the Dark Energy Camera at Cerro Tololo Inter-American Observatory, Chile. The association of the RR Lyrae star discovered here with Hydra II is clear because is located at 42\\prime\\prime from the center of the dwarf, well within its half-light radius of 102\\prime\\prime . The RR Lyrae star has a mean magnitude of I=21.30+/- 0.04 which is too faint to be a field halo star. This magnitude translates to a heliocentric distance of 151 ± 8 kpc for Hydra II; this value is ˜ 13% larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of {76}-10+12 pc and a brighter absolute magnitude of {M}V=-5.1+/- 0.3, which keeps this object within the realm of the dwarf galaxies. A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.

STELLAR ARCHEOLOGY IN THE GALACTIC HALO WITH ULTRA-FAINT DWARFS. VII. HERCULES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Musella, Ilaria; Ripepi, Vincenzo; Marconi, Marcella, E-mail: ilaria@na.astro.it, E-mail: ripepi@na.astro.it, E-mail: marcella@na.astro.it

2012-09-10

We present the first time-series study of the ultra-faint dwarf galaxy Hercules. Using a variety of telescope/instrument facilities we secured about 50 V and 80 B epochs. These data allowed us to detect and characterize 10 pulsating variable stars in Hercules. Our final sample includes six fundamental-mode (ab-type) and three first-overtone (c-type) RR Lyrae stars, and one Anomalous Cepheid. The average period of the ab-type RR Lyrae stars, (P{sub ab}) = 0.68 days ({sigma} = 0.03 days), places Hercules in the Oosterhoff II group, as found for almost the totality of the ultra-faint dwarf galaxies investigated so far for variability.more » The RR Lyrae stars were used to obtain independent estimates of the metallicity, reddening, and distance to Hercules, for which we find [Fe/H] = -2.30 {+-} 0.15 dex, E(B - V) = 0.09 {+-} 0.02 mag, and (m - M){sub 0} = 20.6 {+-} 0.1 mag, in good agreement with the literature values. We have obtained a V, B - V color-magnitude diagram (CMD) of Hercules that reaches V {approx} 25 mag and extends beyond the galaxy's half-light radius over a total area of 40' Multiplication-Sign 36'. The CMD and the RR Lyrae stars indicate the presence of a population as old and metal-poor as (at least) the Galactic globular cluster M68.« less

THE PRIMEVAL POPULATIONS OF THE ULTRA-FAINT DWARF GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, Thomas M.; Tumlinson, Jason; Kalirai, Jason S.

We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter-dominated, and least chemically evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminarymore » analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient globular cluster M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within {approx}1 Gyr of each other, as might be expected if their star formation was truncated by a global event, such as reionization.« less

Eight ultra-faint galaxy candidates discovered in year two of the Dark Energy Survey

DOE PAGES

Drlica-Wagner, A.

2015-11-04

Here, we report the discovery of eight new ultra-faint dwarf galaxy candidates in the second year of optical imaging data from the Dark Energy Survey (DES). Six of these candidates are detected at high confidence, while two lower-confidence candidates are identified in regions of non-uniform survey coverage. The new stellar systems are found by three independent automated search techniques and are identified as overdensities of stars, consistent with the isochrone and luminosity function of an old and metal-poor simple stellar population. The new systems are faint (M V > –4.7more » $$\\mathrm{mag}$$) and span a range of physical sizes (17 $$\\mathrm{pc}$$ < r 1/2 < 181 $$\\mathrm{pc}$$) and heliocentric distances (25 kpc < D ⊙ < 214 kpc). All of the new systems have central surface brightnesses consistent with known ultra-faint dwarf galaxies (μ ≳ 27.5 $$\\mathrm{mag}$$ $$\\mathrm{arcsec}$$ –2). Roughly half of the DES candidates are more distant, less luminous, and/or have lower surface brightnesses than previously known Milky Way satellite galaxies. Most of the candidates are found in the southern part of the DES footprint close to the Magellanic Clouds. We find that the DES data alone exclude (p < 10 –3) a spatially isotropic distribution of Milky Way satellites and that the observed distribution can be well, though not uniquely, described by an association between several of the DES satellites and the Magellanic system. Our model predicts that the full sky may hold ~100 ultra-faint galaxies with physical properties comparable to the DES satellites and that 20%–30% of these would be spatially associated with the Magellanic Clouds.« less

Eight ultra-faint galaxy candidates discovered in year two of the Dark Energy Survey

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drlica-Wagner, A.

Here, we report the discovery of eight new ultra-faint dwarf galaxy candidates in the second year of optical imaging data from the Dark Energy Survey (DES). Six of these candidates are detected at high confidence, while two lower-confidence candidates are identified in regions of non-uniform survey coverage. The new stellar systems are found by three independent automated search techniques and are identified as overdensities of stars, consistent with the isochrone and luminosity function of an old and metal-poor simple stellar population. The new systems are faint (M V > –4.7more » $$\\mathrm{mag}$$) and span a range of physical sizes (17 $$\\mathrm{pc}$$ < r 1/2 < 181 $$\\mathrm{pc}$$) and heliocentric distances (25 kpc < D ⊙ < 214 kpc). All of the new systems have central surface brightnesses consistent with known ultra-faint dwarf galaxies (μ ≳ 27.5 $$\\mathrm{mag}$$ $$\\mathrm{arcsec}$$ –2). Roughly half of the DES candidates are more distant, less luminous, and/or have lower surface brightnesses than previously known Milky Way satellite galaxies. Most of the candidates are found in the southern part of the DES footprint close to the Magellanic Clouds. We find that the DES data alone exclude (p < 10 –3) a spatially isotropic distribution of Milky Way satellites and that the observed distribution can be well, though not uniquely, described by an association between several of the DES satellites and the Magellanic system. Our model predicts that the full sky may hold ~100 ultra-faint galaxies with physical properties comparable to the DES satellites and that 20%–30% of these would be spatially associated with the Magellanic Clouds.« less

Dwarf galaxies: a lab to investigate the neutron capture elements production

NASA Astrophysics Data System (ADS)

Cescutti, Gabriele

2018-06-01

In this contribution, I focus on the neutron capture elements observed in the spectra of old halo and ultra faint galaxies stars. Adopting a stochastic chemical evolution model and the Galactic halo as a benchmark, I present new constraints on the rate and time scales of r-process events, based on the discovery of the r-process rich stars in the ultra faint galaxy Reticulum 2. I also show that an s-process activated by rotation in massive stars can play an important role in the production of heavy elements.

A common origin for globular clusters and ultra-faint dwarfs in simulations of the first galaxies

DOE PAGES

Ricotti, Massimo; Parry, Owen H.; Gnedin, Nickolay Y.

2016-11-09

In this study, the first in a series on galaxy formation before reionization, we focus on understanding what determines the size and morphology of stellar objects in the first low-mass galaxies, using parsec-scale cosmological simulations performed with an adaptive mesh hydrodynamics code. Although the dense gas in which stars are formed tends to have a disk structure, stars are found in spheroids with little rotation. Halos with masses betweenmore » $${10}^{6}\\,{M}_{\\odot }$$ and $$5\\times {10}^{8}\\,{M}_{\\odot }$$ form stars stochastically, with stellar masses in the range $${10}^{4}\\,{M}_{\\odot }$$ to $$2\\times {10}^{6}\\,{M}_{\\odot }$$. We observe, nearly independent of stellar mass, a large range of half-light radii for the stars, from a few parsecs to a few hundred parsecs and surface brightnesses and mass-to-light ratios ranging from those typical of globular clusters to ultra-faint dwarfs. In our simulations, stars form in dense stellar clusters with high gas-to-star conversion efficiencies and rather uniform metallicities. A fraction of these clusters remain bound after the gas is removed by feedback, but others are destroyed, and their stars, which typically have velocity dispersions of 20–40 km s –1, expand until they become bound by the dark matter halo. We thus speculate that the stars in ultra-faint dwarf galaxies may show kinematic and chemical signatures consistent with their origin in a few distinct stellar clusters. On the other hand, some globular clusters may form at the center of primordial dwarf galaxies and may contain dark matter, perhaps detectable in the outer parts.« less

A common origin for globular clusters and ultra-faint dwarfs in simulations of the first galaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ricotti, Massimo; Parry, Owen H.; Gnedin, Nickolay Y.

In this study, the first in a series on galaxy formation before reionization, we focus on understanding what determines the size and morphology of stellar objects in the first low-mass galaxies, using parsec-scale cosmological simulations performed with an adaptive mesh hydrodynamics code. Although the dense gas in which stars are formed tends to have a disk structure, stars are found in spheroids with little rotation. Halos with masses betweenmore » $${10}^{6}\\,{M}_{\\odot }$$ and $$5\\times {10}^{8}\\,{M}_{\\odot }$$ form stars stochastically, with stellar masses in the range $${10}^{4}\\,{M}_{\\odot }$$ to $$2\\times {10}^{6}\\,{M}_{\\odot }$$. We observe, nearly independent of stellar mass, a large range of half-light radii for the stars, from a few parsecs to a few hundred parsecs and surface brightnesses and mass-to-light ratios ranging from those typical of globular clusters to ultra-faint dwarfs. In our simulations, stars form in dense stellar clusters with high gas-to-star conversion efficiencies and rather uniform metallicities. A fraction of these clusters remain bound after the gas is removed by feedback, but others are destroyed, and their stars, which typically have velocity dispersions of 20–40 km s –1, expand until they become bound by the dark matter halo. We thus speculate that the stars in ultra-faint dwarf galaxies may show kinematic and chemical signatures consistent with their origin in a few distinct stellar clusters. On the other hand, some globular clusters may form at the center of primordial dwarf galaxies and may contain dark matter, perhaps detectable in the outer parts.« less

Distinguishing CDM dwarfs from SIDM dwarfs in baryonic simulations

NASA Astrophysics Data System (ADS)

Strickland, Emily; Fitts, Alex B.; Boylan-Kolchin, Michael

2017-06-01

Dwarf galaxies in the nearby Universe are the most dark-matter-dominated systems known. They are therefore natural probes of the nature of dark matter, which remains unknown. Our collaboration has performed several high-resolution cosmological zoom-in simulations of isolated dwarf galaxies. We simulate each galaxy in standard cold dark matter (ΛCDM) as well as self-interacting dark matter (SIDM, with a cross section of σ/m ~ 1 cm2/g), both with and without baryons, in order to identify distinguishing characteristics between the two. The simulations are run using GIZMO, a meshless-finite-mass hydrodynamical code, and are part of the Feedback in Realistic Environments (FIRE) project. By analyzing both the global properties and inner structure of the dwarfs in varying dark matter prescriptions, we provide a side-by-side comparison of isolated, dark-matter-dominated galaxies at the mass scale where differences in the two models of dark matter are thought to be the most obvious. We find that the edge of classical dwarfs and ultra-faint dwarfs (at stellar masses of ~105 solar masses) provides the clearest window for distinguishing between the two theories. At these low masses, our SIDM galaxies have a cored inner density profile, while their CDM counterparts have “cuspy” centers. The SIDM versions of each galaxy also have measurably lower stellar velocity dispersions than their CDM counterparts. Future observations of ultra faint dwarfs with JWST and 30-m telescopes will be able to discern whether such alternate theories of dark matter are viable.

Galaxias enanas: las voces de la mayoría

NASA Astrophysics Data System (ADS)

Cellone, S. A.

More than twenty years after photographic surveys of nearby clusters of galaxies revealed that low-luminosity, or ``dwarf'', galaxies (M_B ≳ -18 mag) are the numerically dominant population, research on these objects has been boosted by new instrumental and theoretical developments. Among several breakthroughs that have re-shaped our knowledge abut dwarf galaxies, we should point out: the detection of underlying spiral structure, disks/bars in dwarf ``elliptical'' galaxies; the possible evolutionary relation between (some?) dwarf ellipticals and spiral galaxies; the discoveries of ultra-compact and ultra-faint dwarfs; the universality of the color-luminosity relation extending along ˜ 10 mag. A brief review on these subjects is presented, with emphasis on early-type dwarfs and their possible evolutionary relations with other galaxy types. I will particularly address the controversy about which are the objects that extend the E sequence down to the lowest luminosities (if such objects really exist). FULL TEXT IN SPANISH

Looking for Galaxies in All the Right Places: A Search for Stellar Populations in ALFALFA’s Ultra-compact High Velocity Clouds

NASA Astrophysics Data System (ADS)

Janesh, William; Rhode, Katherine L.; Salzer, John J.; Janowiecki, Steven; Adams, Elizabeth; Haynes, Martha P.; Giovanelli, Riccardo; Cannon, John M.

2018-01-01

Nearby gas-rich dwarf galaxies are excellent laboratories for investigating the baryonic feedback processes that govern star formation and galaxy evolution in galaxies at the extreme end of the mass function. Detecting and studying such objects may help resolve the well-known tension between cosmological model predictions for low-mass dark matter halos and observations. The ALFALFA neutral hydrogen (Hi) survey has detected a sample of isolated ultra-compact high-velocity Hi clouds (UCHVCs) with kinematic properties that make them likely members of the Local Volume, but that have no optical counterparts in existing optical surveys. This UCHVC sample possesses Hi properties (at 1 Mpc, Hi masses of ~105-106 M⊙, Hi diameters of ~2-3 kpc, and dynamical masses of ~107-108 M⊙) similar to other known ultra-faint dwarf galaxies like Leo T. Following the discovery of Leo P, an extremely metal-poor, gas-rich star-forming dwarf galaxy associated with an ALFALFA UCHVC, we have initiated a campaign to obtain deep optical imaging of 56 UCHVCs using the wide field-of-view, high-resolution ODI camera on the WIYN 3.5-m telescope. Here we present a brief overview of our campaign to search for resolved stellar populations associated with the UCHVCs in our optical images, and initial results from our survey.After creating a stellar catalog from the pipeline-reduced and stacked ODI g- and i-band images, we apply a color-magnitude filter tuned for old, metal-poor stellar populations to select red giant branch stars at distances between 250 kpc and 2 Mpc. The spatial distribution of the stars selected by the filter is then smoothed, and overdensities in the fields are identified. Of the 22 targets analyzed to date, seven have associated stellar populations detected at a high confidence (92% to 99.9% significance). The detected objects have a range of distances (from 350 kpc to 1.6 Mpc) and have optical properties similar to those of ultra-faint dwarf galaxies. These objects have extreme Hi-to-stellar mass ratios, and given their isolation, may represent a progenitor population to the ultra-faint dwarfs. They also help constrain the conditions needed for star formation in the lowest-mass galaxies.

WHERE ARE THE FOSSILS OF THE FIRST GALAXIES? II. TRUE FOSSILS, GHOST HALOS, AND THE MISSING BRIGHT SATELLITES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bovill, Mia S.; Ricotti, Massimo, E-mail: msbovill@astro.umd.edu

We use a new set of cold dark matter simulations of the local universe to investigate the distribution of fossils of primordial dwarf galaxies within and around the Milky Way. Throughout, we build upon previous results showing agreement between the observed stellar properties of a subset of the ultra-faint dwarfs and our simulated fossils. Here, we show that fossils of the first galaxies have galactocentric distributions and cumulative luminosity functions consistent with observations. In our model, we predict {approx}300 luminous satellites orbiting the Milky Way, 50%-70% of which are well-preserved fossils. Within the Milky Way virial radius, the majority ofmore » these fossils have luminosities L{sub V} < 10{sup 6} L{sub sun}. Despite our multidimensional agreement with observations at low masses and luminosities, the primordial model produces an overabundance of bright dwarf satellites (L{sub V} > 10{sup 4} L{sub sun}) with respect to observations where observations are nearly complete. The 'bright satellite problem' is most evident in the outer parts of the Milky Way. We estimate that, although relatively bright, the primordial stellar populations are very diffuse, producing a population with surface brightnesses below surveys' detection limits, and are easily stripped by tidal forces. Although we cannot yet present unmistakable evidence for the existence of the fossils of first galaxies in the Local Group, the results of our studies suggest observational strategies that may demonstrate their existence: (1) the detection of 'ghost halos' of primordial stars around isolated dwarfs would prove that stars formed in minihalos (M < 10{sup 8} M{sub sun}) before reionization and strongly suggest that at least a fraction of the ultra-faint dwarfs are fossils of the first galaxies; and (2) the existence of a yet unknown population of {approx}150 Milky Way ultra-faints with half-light radii r{sub hl} {approx} 100-1000 pc and luminosities L{sub V} < 10{sup 4} L{sub sun}, detectable by future deep surveys. These undetected dwarfs would have the mass-to-light ratios, stellar velocity dispersions, and metallicities predicted in this work.« less

HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: BOÖTES II

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ji, Alexander P.; Frebel, Anna; Simon, Joshua D.

2016-01-20

We present high-resolution Magellan/MIKE spectra of the four brightest confirmed red giant stars in the ultra-faint dwarf galaxy Boötes II (Boo II). These stars all inhabit the metal-poor tail of the Boo II metallicity distribution function. The chemical abundance pattern of all detectable elements in these stars is consistent with that of the Galactic halo. However, all four stars have undetectable amounts of neutron-capture elements Sr and Ba, with upper limits comparable to the lowest ever detected in the halo or in other dwarf galaxies. One star exhibits significant radial velocity variations over time, suggesting it to be in a binary system. Itsmore » variable velocity has likely increased past determinations of the Boo II velocity dispersion. Our four stars span a limited metallicity range, but their enhanced α-abundances and low neutron-capture abundances are consistent with the interpretation that Boo II has been enriched by very few generations of stars. The chemical abundance pattern in Boo II confirms the emerging trend that the faintest dwarf galaxies have neutron-capture abundances distinct from the halo, suggesting the dominant source of neutron-capture elements in halo stars may be different than in ultra-faint dwarfs.« less

The Chemical Evolution of the Bootes I Ultra-faint Dwarf Galaxy

NASA Astrophysics Data System (ADS)

Frebel, Anna; Norris, John E.; Gilmore, Gerard; Wyse, Rosemary F. G.

2016-08-01

We present chemical abundance measurements of two metal-poor red giant stars in the ultra-faint dwarf galaxy Boötes I, based on Magellan/MIKE high-resolution spectra. For Boo-980, with {{[Fe/H]}}=-3.1, we present the first elemental abundance measurements, while Boo-127, with {{[Fe/H]}}=-2.0, shows abundances in good agreement with previous measurements. Light and iron-peak element abundance ratios in the two Boötes I stars, as well as those of most other Boötes I members, collected from the literature, closely resemble those of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba are systematically lower than the main halo trend and also show a significant abundance spread. Overall, this is similar to what has been found for other ultra-faint dwarf galaxies. We apply corrections to the carbon abundances (commensurate with stellar evolutionary status) of the entire sample and find 21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13% without using the carbon correction. We reassess the metallicity distribution functions for the CEMP stars and non-CEMP stars, and confirm earlier claims that CEMP stars might belong to a different, earlier population. Applying a set of abundance criteria to test to what extent Boötes I could be a surviving first galaxy suggests that it is one of the earliest assembled systems that perhaps received gas from accretion from other clouds in the system, or from swallowing a first galaxy or building block type object. This resulted in the two stellar populations observable today. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: LEO IV

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simon, Joshua D.; McWilliam, Andrew; Thompson, Ian B.

2010-06-10

We present high-resolution Magellan/MIKE spectroscopy of the brightest star in the ultra-faint dwarf galaxy Leo IV. We measure an iron abundance of [Fe/H] = -3.2, adding to the rapidly growing sample of extremely metal-poor (EMP) stars being identified in Milky Way satellite galaxies. The star is enhanced in the {alpha} elements Mg, Ca, and Ti by {approx}0.3 dex, very similar to the typical Milky Way halo abundance pattern. All of the light and iron-peak elements follow the trends established by EMP halo stars, but the neutron-capture elements Ba and Sr are significantly underabundant. These results are quite similar to thosemore » found for stars in the ultra-faint dwarfs Ursa Major II, Coma Berenices, Booetes I, and Hercules, suggesting that the chemical evolution of the lowest-luminosity galaxies may be universal. The abundance pattern we observe is consistent with predictions for nucleosynthesis from a Population III supernova explosion. The extremely low metallicity of this star also supports the idea that a significant fraction ({approx}>10%) of the stars in the faintest dwarfs have metallicities below [Fe/H] = -3.0.« less

Ghostly Halos in Dwarf Galaxies: a probe of star formation in the Early Universe

NASA Astrophysics Data System (ADS)

Kang, Hoyoung; Ricotti, Massimo

2016-01-01

We carry out numerical simulations to characterize the size, stellar mass, and stellar mass surface density of extended stellar halos in dwarf galaxies as a function of dark matter halo mass. We expect that for galaxies smaller than a critical value, these ghostly halos will not exist because the smaller galactic subunits that build it up, do not form any stars. The detection of ghostly halos around isolated dwarf galaxies is a sensitive test of the efficiency of star formation in the first galaxies and of whether ultra-faint dwarf satellites of the Milky Way are fossils of the first galaxies.

Faint blue galaxies revisited

NASA Astrophysics Data System (ADS)

Ferguson, Henry C.

If dwarf-elliptical galaxies formed their stars very rapdily (on timescales of less than 1 Gyr), they may in principle be detectable out to high redshift. Prior to the discovery of cosmic acceleration, it appeared that rapid and late formation dwarf elliptical galaxies might be required to explain the number counts of faint galaxies. A plausible hypothesis emerged: that photoionization by the UV background prevents gas cooling in low-mass halos until z ≲ 1.5. The discovery of cosmic acceleration eased the tension between predicted galaxy number counts and galaxy-evolution models. Nevertheless, there is some evidence for relatively late star formation in nearby dE's, and the photoionization delay mechanism still appears to have some merit. It is thus of interest to look back in time to see if we can find starbursting dwarf galaxies at moderate redshift. We review the connection between faint-blue galaxies and bursting-dwarf galaxies and discuss some attempts to identify progenitors to dE galaxies in the Hubble Ultra Deep Field (HUDF) observations. We find roughly 85 galaxies in the HUDF with redshifts 0.6 that appear to have formed most of their stars at z. Of these, 70% have half-light radii less than 1.5 kpc. These are thus "smoking gun" candidates for dwarf galaxies that are either collapsing for the first time at moderate redshifts or have otherwise been unable to form stars for more than 1/3 of a Hubble time.

The quenching of the ultra-faint dwarf galaxies in the reionization era

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, Thomas M.; Tumlinson, Jason; Kalirai, Jason S.

2014-12-01

We present new constraints on the star formation histories of six ultra-faint dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I. Our analysis employs a combination of high-precision photometry obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, medium-resolution spectroscopy obtained with the DEep Imaging Multi-Object Spectrograph on the W. M. Keck Observatory, and updated Victoria-Regina isochrones tailored to the abundance patterns appropriate for these galaxies. The data for five of these Milky Way satellites are best fit by a star formation history where at least 75% of the starsmore » formed by z ∼ 10 (13.3 Gyr ago). All of the galaxies are consistent with 80% of the stars forming by z ∼ 6 (12.8 Gyr ago) and 100% of the stars forming by z ∼ 3 (11.6 Gyr ago). The similarly ancient populations of these galaxies support the hypothesis that star formation in the smallest dark-matter sub-halos was suppressed by a global outside influence, such as the reionization of the universe.« less

Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II

NASA Astrophysics Data System (ADS)

Contenta, Filippo; Balbinot, Eduardo; Petts, James A.; Read, Justin I.; Gieles, Mark; Collins, Michelle L. M.; Peñarrubia, Jorge; Delorme, Maxime; Gualandris, Alessia

2018-05-01

We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ˜45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results, therefore, favour a DM core. This implies that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation or we are seeing an evidence for physics beyond cold DM.

On the Nature of Ultra-faint Dwarf Galaxy Candidates. I. DES1, Eridanus III, and Tucana V

NASA Astrophysics Data System (ADS)

Conn, Blair C.; Jerjen, Helmut; Kim, Dongwon; Schirmer, Mischa

2018-01-01

We use deep Gemini/GMOS-S g, r photometry to study the three ultra-faint dwarf galaxy candidates DES1, Eridanus III (Eri III), and Tucana V (Tuc V). Their total luminosities, M V (DES1) = ‑1.42 ± 0.50 and M V (Eri III) = ‑2.07 ± 0.50, and mean metallicities, [{Fe}/{{H}}]=-{2.38}-0.19+0.21 and [{Fe}/{{H}}]=-{2.40}-0.12+0.19, are consistent with them being ultra-faint dwarf galaxies, as they fall just outside the 1σ confidence band of the luminosity–metallicity relation for Milky Way satellite galaxies. However, their positions in the size–luminosity relation suggest that they are star clusters. Interestingly, DES1 and Eri III are at relatively large Galactocentric distances, with DES1 located at {D}{GC}=74+/- 4 {kpc} and Eri III at {D}{GC}=91+/- 4 {kpc}. In projection, both objects are in the tail of gaseous filaments trailing the Magellanic Clouds and have similar 3D separations from the Small Magellanic Cloud (SMC): {{Δ }}{D}{SMC,{DES}1}=31.7 kpc and {{Δ }}{D}{SMC,{Eri}{III}}=41.0 kpc, respectively. It is plausible that these stellar systems are metal-poor SMC satellites. Tuc V represents an interesting phenomenon in its own right. Our deep photometry at the nominal position of Tuc V reveals a low-level excess of stars at various locations across the GMOS field without a well-defined center. An SMC Northern Overdensity–like isochrone would be an adequate match to the Tuc V color–magnitude diagram, and the proximity to the SMC (12.°1 {{Δ }}{D}{SMC,{Tuc}{{V}}}=13 kpc) suggests that Tuc V is either a chance grouping of stars related to the SMC halo or a star cluster in an advanced stage of dissolution.

Foreground effect on the J-factor estimation of ultra-faint dwarf spheroidal galaxies

NASA Astrophysics Data System (ADS)

Ichikawa, Koji; Horigome, Shun-ichi; Ishigaki, Miho N.; Matsumoto, Shigeki; Ibe, Masahiro; Sugai, Hajime; Hayashi, Kohei

2018-05-01

Dwarf spheroidal galaxies (dSphs) are promising targets for the gamma-ray dark matter (DM) search. In particular, DM annihilation signal is expected to be strong in some of the recently discovered nearby ultra-faint dSphs, which potentially give stringent constraints on the O(1) TeV WIMP DM. However, various non-negligible systematic uncertainties complicate the estimation of the astrophysical factors relevant for the DM search in these objects. Among them, the effects of foreground stars particularly attract attention because the contamination is unavoidable even for the future kinematical survey. In this article, we assess the effects of the foreground contamination on the astrophysical J-factor estimation by generating mock samples of stars in the four ultra-faint dSphs and using a model of future spectrographs. We investigate various data cuts to optimize the quality of the data and apply a likelihood analysis which takes member and foreground stellar distributions into account. We show that the foreground star contaminations in the signal region (the region of interest) and their statistical uncertainty can be estimated by interpolating the foreground star distribution in the control region where the foreground stars dominate the member stars. Such regions can be secured at future spectroscopic observations utilizing a multiple object spectrograph with a large field of view; e.g. the Prime Focus Spectrograph mounted on Subaru Telescope. The above estimation has several advantages: The data-driven estimation of the contamination makes the analysis of the astrophysical factor stable against the complicated foreground distribution. Besides, foreground contamination effect is considered in the likelihood analysis.

CHEMICAL DIVERSITY IN THE ULTRA-FAINT DWARF GALAXY TUCANA II

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ji, Alexander P.; Frebel, Anna; Ezzeddine, Rana

We present the first detailed chemical abundance study of the ultra-faint dwarf galaxy Tucana II, based on high-resolution Magellan/MIKE spectra of four red giant stars. The metallicities of these stars range from [Fe/H] = −3.2 to −2.6, and all stars are low in neutron-capture abundances ([Sr/Fe] and [Ba/Fe] < −1). However, a number of anomalous chemical signatures are present. One star is relatively metal-rich ([Fe/H] = −2.6) and shows [Na, α , Sc/Fe] < 0, suggesting an extended star formation history with contributions from AGB stars and SNe Ia. Two stars with [Fe/H] < −3 are mildly carbon-enhanced ([C/Fe] ∼more » 0.7) and may be consistent with enrichment by faint supernovae, if such supernovae can produce neutron-capture elements. A fourth star with [Fe/H] = −3 is carbon-normal, and exhibits distinct light element abundance ratios from the carbon-enhanced stars. This carbon-normal star implies that at least two distinct nucleosynthesis sources, both possibly associated with Population III stars, contributed to the early chemical enrichment of this galaxy. Despite its very low luminosity, Tucana II shows a diversity of chemical signatures that preclude it from being a simple “one-shot” first galaxy yet still provide a window into star and galaxy formation in the early universe.« less

Feedback by Massive Black Holes in Gas-rich Dwarf Galaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Silk, Joseph; AIM-Paris-Saclay, CEA/DSM/IRFU, CNRS, Univ Paris 7, F-91191, Gif-sur-Yvette; Department of Physics and Astronomy, The Johns Hopkins University, Homewood Campus, Baltimore, MD 21218

Could there be intermediate-mass black holes in essentially all old dwarf galaxies? I argue that current observations of active galactic nuclei in dwarfs allow such a radical hypothesis that provides early feedback during the epoch of galaxy formation and potentially provides a unifying explanation for many, if not all, of the dwarf galaxy anomalies, such as the abundance, core-cusp, “too-big-to-fail,” ultra-faint, and baryon-fraction issues. I describe the supporting arguments, which are largely circumstantial, and discuss a number of tests. There is no strong motivation for modifying the nature of cold dark matter in order to explain any of the dwarfmore » galaxy “problems.”.« less

THE DETECTION OF ULTRA-FAINT LOW SURFACE BRIGHTNESS DWARF GALAXIES IN THE VIRGO CLUSTER: A PROBE OF DARK MATTER AND BARYONIC PHYSICS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Giallongo, E.; Menci, N.; Grazian, A.

2015-11-01

We have discovered 11 ultra-faint (r ≲ 22.1) low surface brightness (LSB, central surface brightness 23 ≲ μ{sub r} ≲ 26) dwarf galaxy candidates in one deep Virgo field of just 576 arcmin{sup 2} obtained by the Large Binocular Camera at the Large Binocular Telescope. Their association with the Virgo cluster is supported by their distinct position in the central surface brightness—total magnitude plane with respect to the background galaxies of similar total magnitude. They have typical absolute magnitudes and scale sizes, if at the distance of Virgo, in the range −13 ≲ M{sub r} ≲ −9 and 250 ≲more » r{sub s} ≲ 850 pc, respectively. Their colors are consistent with a gradually declining star formation history with a specific star formation rate of the order of 10{sup −11} yr{sup −1}, i.e., 10 times lower than that of main sequence star-forming galaxies. They are older than the cluster formation age and appear to be regular in morphology. They represent the faintest extremes of the population of low luminosity LSB dwarfs that has recently been detected in wider surveys of the Virgo cluster. Thanks to the depth of our observations, we are able to extend the Virgo luminosity function down to M{sub r} ∼ −9.3 (corresponding to total masses M ∼ 10{sup 7} M{sub ⊙}), finding an average faint-end slope α ≃ −1.4. This relatively steep slope puts interesting constraints on the nature of the dark matter and, in particular, on warm dark matter (WDM) often invoked to solve the overprediction of the dwarf number density by the standard cold dark matter scenario. We derive a lower limit on the WDM particle mass >1.5 keV.« less

Searches for new Milky Way satellites from the first two years of data of the Subaru/Hyper Suprime-Cam survey: Discovery of Cetus III

NASA Astrophysics Data System (ADS)

Homma, Daisuke; Chiba, Masashi; Okamoto, Sakurako; Komiyama, Yutaka; Tanaka, Masayuki; Tanaka, Mikito; Ishigaki, Miho N.; Hayashi, Kohei; Arimoto, Nobuo; Garmilla, José A.; Lupton, Robert H.; Strauss, Michael A.; Miyazaki, Satoshi; Wang, Shiang-Yu; Murayama, Hitoshi

2018-01-01

We present the results from a search for new Milky Way (MW) satellites from the first two years of data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) ˜300 deg2 and report the discovery of a highly compelling ultra-faint dwarf galaxy candidate in Cetus. This is the second ultra-faint dwarf we have discovered after Virgo I reported in our previous paper. This satellite, Cetus III, has been identified as a statistically significant (10.7 σ) spatial overdensity of star-like objects, which are selected from a relevant isochrone filter designed for a metal-poor and old stellar population. This stellar system is located at a heliocentric distance of 251^{+24}_{-11}kpc with a most likely absolute magnitude of MV = -2.4 ± 0.6 mag estimated from a Monte Carlo analysis. Cetus III is extended with a half-light radius of r_h = 90^{+42}_{-17}pc, suggesting that this is a faint dwarf satellite in the MW located beyond the detection limit of the Sloan Digital Sky Survey. Further spectroscopic studies are needed to assess the nature of this stellar system. We also revisit and update the parameters for Virgo I, finding M_V = -0.33^{+0.75}_{-0.87}mag and r_h = 47^{+19}_{-13}pc. Using simulations of Λ-dominated cold dark matter models, we predict that we should find one or two new MW satellites from ˜300 deg2 HSC-SSP data, in rough agreement with the discovery rate so far. The further survey and completion of HSC-SSP over ˜1400 deg2 will provide robust insights into the missing satellites problem.

CEMP Stars in the Halo and Their Origin in Ultra-Faint Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Beers, Timothy C.

2018-06-01

The very metal-poor (VMP; [Fe/H] < –2.0) and extremely metal-poor (EMP; [Fe/H] < –3.0) stars provide a direct view of Galactic chemical and dynamical evolution; detailed spectroscopic studies of these objects are the best way to identify and distinguish between various scenarios for the enrichment of early star-forming gas clouds soon after the Big Bang. It has been recognized that a large fraction of VMP (15-20%) and EMP stars (30-40%) possess significant over-abundances of carbon relative to iron, [C/Fe] > +0.7. This fraction rises to at least 80% for stars with [Fe/H] < –4.0. Recent studies show that the majority of CEMP stars with [Fe/H] < –3.0 belong to the CEMP-no sub-class, characterized by the lack of strong enhancements in the neutron-capture elements (e.g., [Ba/Fe] < 0.0). The CEMP-no abundance signature is commonly observed among stars ultra-faint dwarf spheroidal galaxies such as SEGUE-1. In addition, kinematic studies of CEMP-no stars strongly suggest an association with the outer-halo population of the Galaxy, which was likely formed from the accretion of low-mass mini-halos. These observations, and other lines of evidence, indicate that the CEMP-no stars of the Milky Way were born in low-mass dwarf galaxies, and later subsumed into the halo.

Understanding r-process nucleosynthesis with dwarf galaxies

NASA Astrophysics Data System (ADS)

Ji, Alexander P.

2018-06-01

The Milky Way's faintest dwarf galaxy satellites each sample short, independent bursts of star formation from the first 1-2 Gyr of the universe. Their simple formation history makes them ideal systems to understand how rare events like neutron star mergers contribute to early enrichment of r-process elements. I will focus on the ultra-faint galaxy Reticulum II, which experienced a single prolific r-process event that left ~80% of its stars extremely enriched in r-process elements. I will present abundances of ~40 elements derived from the highest signal-to-noise high-resolution spectrum ever taken for an ultra-faint dwarf galaxy star. Precise measurements of elements from all three r-process peaks reaffirm the universal nature of the r-process abundance pattern from Ba to Ir. The first r-process peak is significantly lower than solar but matches other r-process enhanced stars. This constrains the neutron-richness of r-process ejecta in neutron star mergers. The radioactive element thorium is detected with a somewhat low abundance. Naive application of currently predicted initial production ratios could imply an age >20 Gyr, but more likely indicates that the initial production ratios require revision. The abundance of lighter elements up to Zn are consistent with extremely metal-poor Milky Way halo stars. These elements may eventually provide a way to test for other hypothesized r-process sites, but only after a more detailed understanding of the chemical evolution in this galaxy. Reticulum II provides a clean view of early r-process enrichment that can be used to understand the increasing number of r-process measurements in other dwarf galaxies.

STAR FORMATION IN ULTRA-FAINT DWARFS: CONTINUOUS OR SINGLE-AGE BURSTS?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webster, David; Bland-Hawthorn, Joss; Sutherland, Ralph, E-mail: d.webster@physics.usyd.edu.au

2015-01-30

We model the chemical evolution of six ultra-faint dwarfs (UFDs): Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I based on their recently determined star formation histories. We show that two single-age bursts cannot explain the observed [α/Fe] versus [Fe/H] distribution in these galaxies and that some self-enrichment is required within the first burst. An alternative scenario is modeled, in which star formation is continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from the center of the system. This model allows for self-enrichment and can reproduce themore » chemical abundances of the UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier work, the observed properties of UFDs can be explained by formation at a low mass (M{sub vir}∼10{sup 7} M{sub ⊙}), rather than being stripped remnants of much larger systems.« less

The contribution of dissolving star clusters to the population of ultra faint objects in the outer halo of the Milky Way

NASA Astrophysics Data System (ADS)

Contenta, Filippo; Gieles, Mark; Balbinot, Eduardo; Collins, Michelle L. M.

2017-04-01

In the last decade, several ultra faint objects (UFOs, MV ≳ -3.5) have been discovered in the outer halo of the Milky Way. For some of these objects, it is not clear whether they are star clusters or (ultra faint) dwarf galaxies. In this work, we quantify the contribution of star clusters to the population of UFOs. We extrapolated the mass and Galactocentric radius distribution of the globular clusters using a population model, finding that the Milky Way contains about 3.3^{+7.3}_{-1.6} star clusters with MV ≳ -3.5 and Galactocentric radius ≥20 kpc. To understand whether dissolving clusters can appear as UFOs, we run a suite of direct N-body models, varying the orbit, the Galactic potential, the binary fraction and the black hole (BH) natal kick velocities. In the analyses, we consider observational biases such as luminosity limit, field stars and line-of-sight projection. We find that star clusters contribute to both the compact and the extended population of UFOs: clusters without BHs appear compact with radii ˜5 pc, while clusters that retain their BHs after formation have radii ≳ 20 pc. The properties of the extended clusters are remarkably similar to those of dwarf galaxies: high-inferred mass-to-light ratios due to binaries, binary properties mildly affected by dynamical evolution, no observable mass segregation and flattened stellar mass function. We conclude that the slope of the stellar mass function as a function of Galactocentric radius and the presence/absence of cold streams can discriminate between dark matter-free and dark matter-dominated UFOs.

A New Milky Way Satellite Discovered in the Subaru/Hyper Suprime-Cam Survey

NASA Astrophysics Data System (ADS)

Homma, Daisuke; Chiba, Masashi; Okamoto, Sakurako; Komiyama, Yutaka; Tanaka, Masayuki; Tanaka, Mikito; Ishigaki, Miho N.; Akiyama, Masayuki; Arimoto, Nobuo; Garmilla, José A.; Lupton, Robert H.; Strauss, Michael A.; Furusawa, Hisanori; Miyazaki, Satoshi; Murayama, Hitoshi; Nishizawa, Atsushi J.; Takada, Masahiro; Usuda, Tomonori; Wang, Shiang-Yu

2016-11-01

We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way (MW) based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program. This new satellite, Virgo I, which is located in the constellation of Virgo, has been identified as a statistically significant (5.5σ) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in the color-magnitude diagram. The significance of this overdensity increases to 10.8σ when the relevant isochrone filter is adopted for the search. Based on the distribution of the stars around the likely main-sequence turnoff at r ˜ 24 mag, the distance to Virgo I is estimated as 87 kpc, and its most likely absolute magnitude calculated from a Monte Carlo analysis is M V = -0.8 ± 0.9 mag. This stellar system has an extended spatial distribution with a half-light radius of {38}-11+12 pc, which clearly distinguishes it from a globular cluster with comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey. This demonstrates the power of this survey program to identify very faint dwarf satellites. This discovery of Virgo I is based only on about 100 square degrees of data, thus a large number of faint dwarf satellites are likely to exist in the outer halo of the MW.

The Hunt for Missing Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2015-11-01

Theories of galaxy formation and evolution predict that there should be significantly more dwarf galaxies than have been observed. Are our theories wrong? Or are dwarf galaxies just difficult to detect? Recent results from a survey of a galaxy cluster 62 million light-years away suggest there may be lots of undiscovered dwarf galaxies hiding throughout the universe!Hiding in FaintnessThe missing dwarf problem has had hints of a resolution with the recent discovery of Ultra-Diffuse Galaxies (UDGs) in the Coma and Virgo galaxy clusters. UDGs have low masses and large radii, resulting in a very low surface brightness that makes them extremely difficult to detect. If many dwarfs are UDGs, this could well explain why weve been missing them!But the Coma and Virgo galaxy clusters are similar in that theyre both very massive. Are there UDGs in other galaxy clusters as well? To answer this question, an international team of scientists is running the Next Generation Fornax Survey (NGFS), a survey searching for faint dwarf galaxies in the central 30 square degrees of the Fornax galaxy cluster.The NGFS uses near-UV and optical observations from the Dark Energy Camera mounted on the 4m Blanco Telescope in Chile. The survey is still underway, but in a recent publication led by Roberto P. Muoz (Institute of Astrophysics at the Pontifical Catholic University of Chile), the team has released an overview of the first results from only the central 3 square degrees of the NGFS field.Surprising DetectionGalaxy radii vs. their absolute i-band magnitudes, for the dwarfs found in NGFS as well as other stellar systems in the nearby universe. The NGFS dwarfs are similar to the ultra-diffuse dwarfs found in the Virgo and Coma clusters, but are several orders of magnitude fainter. [Muoz et al. 2015]In just this small central field, the team has found an astounding 284 low-surface-brightness dwarf galaxy candidates 158 of them previously undetected. At the bright end of this sample are dwarf galaxies that resemble the UDGs found in Virgo and Coma clusters, verifying that such objects exist in environments beyond only massive clusters.And at the faint end of the sample, the authors find additional extremely low-surface-brightness dwarfs that are several orders of magnitude fainter even than classical UDGs.The authors describe the properties of these galaxies and compare them to systems like classical UDGs and dwarf spheroidal galaxies in our own Local Cluster. The next step is to determine which of the differences between the sample of NGFS dwarfs and previously known systems are explained by the environmental factors of their host cluster, and which are simply due to sample biases.With much more data from the NGFS still to come, it seems likely that we will soon be able to examine an even larger sample of no-longer-missing dwarfs!CitationRoberto P. Muoz et al 2015 ApJ 813 L15. doi:10.1088/2041-8205/813/1/L15

What the Local Group can tell us about the first stars and first galaxies

NASA Astrophysics Data System (ADS)

Jablonka, Pascale

2018-06-01

I will present a series of new observational results on the most pristine stellar population in some of the Local Group dwarf spheroidal galaxies, with particular focus on the level of homogeneity of their abundance ratios. I will draw comparisons with the Milky Way halo and discuss the very specific question of the neutron capture elements. Our understanding of the formation of dwarf galaxies strongly rely on our ability to model them. I will present the status of our recent high resolution chemo-dynamical simulations, describe our latest results on galaxy build-up processes, and discuss the challenges that modeling has to face, in particular in the mass range of the ultra-faint dwarfs.

WHERE ARE THE FOSSILS OF THE FIRST GALAXIES? I. LOCAL VOLUME MAPS AND PROPERTIES OF THE UNDETECTED DWARFS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bovill, Mia S.; Ricotti, Massimo, E-mail: msbovill@astro.umd.edu, E-mail: ricotti@astro.umd.edu

We present a new method for generating initial conditions for {Lambda}CDM N-body simulations which provides the dynamical range necessary to follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5-10 Mpc, scales. The initial distribution of particles represents the position, velocity, and mass distribution of the dark and luminous halos extracted from pre-reionization simulations. We confirm previous results that ultra-faint dwarfs have properties compatible with being well-preserved fossils of the first galaxies. However, because the brightest pre-reionization dwarfs form preferentially in biased regions, they most likely merge into non-fossil halos with circular velocities >20-30more » km s{sup -1}. Hence, we find that the maximum luminosity of true fossils in the Milky Way is L{sub V} < 10{sup 6} L{sub sun}, casting doubts on the interpretation that some classical dSphs are true fossils. In addition, we argue that most ultra-faints at small galactocentric distance, R < 50 kpc, had their stellar properties modified by tides, while a large population of fossils is still undetected due to their extremely low surface brightness log ({Sigma}{sub V}) < -1.4. We estimate that the region outside R{sub 50} ({approx}400 kpc) up to 1 Mpc from the Milky Way contains about a hundred true fossils of the first galaxies with V-band luminosity 10{sup 3}-10{sup 5} L{sub sun} and half-light radii, r{sub hl} {approx} 100-1000 pc.« less

The Evolution of Dwarf Galaxy Satellites with Different Dark Matter Density Profiles in the ErisMod Simulations. I. The Early Infalls

NASA Astrophysics Data System (ADS)

Tomozeiu, Mihai; Mayer, Lucio; Quinn, Thomas

2016-02-01

We present the first simulations of tidal stirring of dwarf galaxies in the Local Group carried out in a fully cosmological context. We use the ErisDARK cosmological simulation of a Milky Way (MW)-sized galaxy to identify some of the most massive subhalos (Mvir > 108 M⊙) that fall into the main host before z = 2. Subhalos are replaced before infall with extremely high-resolution models of dwarf galaxies comprising a faint stellar disk embedded in a dark matter halo. The set of models contains cuspy halos as well as halos with “cored” profiles (with the cusp coefficient γ = 0.6) consistent with recent results of hydrodynamical simulations of dwarf galaxy formation. The simulations are then run to z = 0 with as many as 54 million particles and resolutions as small as ∼4 pc using the new parallel N-body code ChaNGa. The stellar components of all satellites are significantly affected by tidal stirring, losing stellar mass, and undergoing a morphological transformation toward a pressure supported spheroidal system. However, while some remnants with cuspy halos maintain significant rotational flattening and disk-like features, all the shallow halo models achieve vrot/σ⋆ < 0.5 and round shapes typical of dSph satellites of the MW and M31. Mass loss is also enhanced in the latter, and remnants can reach luminosities and velocity dispersions as low as those of ultra-faint dwarfs.

BEASTS OF THE SOUTHERN WILD: DISCOVERY OF NINE ULTRA FAINT SATELLITES IN THE VICINITY OF THE MAGELLANIC CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koposov, Sergey E.; Belokurov, Vasily; Torrealba, Gabriel

2015-06-01

We have used the publicly released Dark Energy Survey (DES) data to hunt for new satellites of the Milky Way (MW) in the southern hemisphere. Our search yielded a large number of promising candidates. In this paper, we announce the discovery of nine new unambiguous ultra-faint objects, whose authenticity can be established with the DES data alone. Based on the morphological properties, three of the new satellites are dwarf galaxies, one of which is located at the very outskirts of the MW, at a distance of 380 kpc. The remaining six objects have sizes and luminosities comparable to the Seguemore » 1 satellite and cannot be classified straightforwardly without follow-up spectroscopic observations. The satellites we have discovered cluster around the LMC and the SMC. We show that such spatial distribution is unlikely under the assumption of isotropy, and, therefore, conclude that at least some of the new satellites must have been associated with the Magellanic Clouds in the past.« less

Ultra faint dwarf galaxies: an arena for testing dark matter versus modified gravity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Weikang; Ishak, Mustapha, E-mail: wxl123830@utdallas.edu, E-mail: mishak@utdallas.edu

2016-10-01

The scenario consistent with a wealth of observations for the missing mass problem is that of weakly interacting dark matter particles. However, arguments or proposals for a Newtonian or relativistic modified gravity scenario continue to be made. A distinguishing characteristic between the two scenarios is that dark matter particles can produce a gravitational effect, in principle, without the need of baryons while this is not the case for the modified gravity scenario where such an effect must be correlated with the amount of baryonic matter. We consider here ultra-faint dwarf (UFD) galaxies as a promising arena to test the twomore » scenarios based on the above assertion. We compare the correlation of the luminosity with the velocity dispersion between samples of UFD and non-UFD galaxies, finding a significant loss of correlation for UFD galaxies. For example, we find for 28 non-UFD galaxies a strong correlation coefficient of −0.688 which drops to −0.077 for the 23 UFD galaxies. Incoming and future data will determine whether the observed stochasticity for UFD galaxies is physical or due to systematics in the data. Such a loss of correlation (if it is to persist) is possible and consistent with the dark matter scenario for UFD galaxies but would constitute a new challenge for the modified gravity scenario.« less

Stellar kinematics and metallicities in the ultra-faint dwarf galaxy Reticulum II

DOE PAGES

Simon, J. D.

2015-07-23

With this study, we present Magellan/M2FS, Very Large Telescope/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity ofmore » $$62.8\\pm 0.5\\;\\mathrm{km}\\;{\\rm{s}}^{-1}$$ and a velocity dispersion of $$3.3\\pm 0.7\\;\\mathrm{km}\\;{\\rm{s}}^{-1}$$. The mass-to-light ratio of Ret II within its half-light radius is $$470\\pm 210\\ {M}_{\\odot }/{L}_{\\odot }$$, demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 $$\\mathrm{km}\\ {{\\rm{s}}}^{-1}$$, respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span 1.3 dex in metallicity, with a dispersion of 0.28 ± 0.09 dex, and we identify several extremely metal-poor stars with $${\\rm{[Fe/H]}}\\lt -3$$. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of $${\\rm{[Fe/H]}}=-2.65\\pm 0.07$$, Ret II matches Segue 1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of the dark matter density squared is $${\\mathrm{log}}_{10}(J)=18.8\\pm 0.6\\;\\;\\mathrm{GeV}{\\;}^{2}\\;{\\mathrm{cm}}^{-5}\\;$$ within 0fdg2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.« less

DISCOVERY OF AN ULTRA-FAINT DWARF GALAXY IN THE INTRACLUSTER FIELD OF THE VIRGO CENTER: A FOSSIL OF THE FIRST GALAXIES?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jang, In Sung; Lee, Myung Gyoon, E-mail: isjang@astro.snu.ac.kr, E-mail: mglee@astro.snu.ac.kr

2014-11-01

Ultra-faint dwarf galaxies (UFDs) are newcomers among galaxies, and are the faintest galaxies in the observed universe. To date, they have only been found around the Milky Way Galaxy and M31 in the Local Group. We present the discovery of a UFD in the intracluster field in the core of the Virgo cluster (Virgo UFD1), which is far from any massive galaxies. The color-magnitude diagram of the resolved stars in this galaxy shows a narrow red giant branch, similar to those of metal-poor globular clusters in the Milky Way. We estimate its distance by comparing the red giant branch withmore » isochrones, and we obtain a value 16.4 ± 0.4 Mpc. This shows that it is indeed a member of the Virgo cluster. From the color of the red giants we estimate its mean metallicity to be very low, [Fe/H] =–2.4 ± 0.4. Its absolute V-band magnitude and effective radius are derived to be M{sub V} = –6.5 ± 0.2 and r {sub eff} = 81 ± 7 pc, much fainter and smaller than the classical dwarf spheroidal galaxies. Its central surface brightness is estimated to be as low as μ {sub V,} {sub 0} = 26.37 ± 0.05 mag arcsec{sup –2}. Its properties are similar to those of the Local Group analogs. No evidence of tidal features are found in this galaxy. Considering its narrow red giant branch with no asymptotic giant branch stars, low metallicity, and location, it may be a fossil remnant of the first galaxies.« less

Discovery of Distant RR Lyrae Stars in the Milky Way Using DECam

NASA Astrophysics Data System (ADS)

Medina, Gustavo E.; Muñoz, Ricardo R.; Vivas, A. Katherina; Carlin, Jeffrey L.; Förster, Francisco; Martínez, Jorge; Galbany, Lluís; González-Gaitán, Santiago; Hamuy, Mario; de Jaeger, Thomas; Maureira, Juan Carlos; San Martín, Jaime

2018-03-01

We report the discovery of distant RR Lyrae stars, including the most distant known in the Milky Way, using data taken in the g-band with the Dark Energy Camera as part of the High cadence Transient Survey (HiTS; 2014 campaign). We detect a total of 173 RR Lyrae stars over a ∼120 deg2 area, including both known RR Lyrae and new detections. The heliocentric distances d H of the full sample range from 9 to >200 kpc, with 18 of them beyond 90 kpc. We identify three sub-groups of RR Lyrae as members of known systems: the Sextans dwarf spheroidal galaxy, for which we report 46 new discoveries, and the ultra-faint dwarf galaxies Leo IV and Leo V. Following an MCMC methodology, we fit spherical and ellipsoidal profiles of the form ρ(R) ∼ R n to the radial density distribution of RR Lyrae in the Galactic halo. The best fit corresponds to the spherical case, for which we obtain a simple power-law index of n=-{4.17}-0.20+0.18, consistent with recent studies made with samples covering shorter distances. The pulsational properties of the outermost RR Lyrae in the sample (d H > 90 kpc) differ from the ones in the halo population at closer distances. The distribution of the stars in a period-amplitude diagram suggest they belong to Oosterhoff-intermediate or Oosterhoff II groups, similar to what is found in the ultra-faint dwarf satellites around the Milky Way. The new distant stars discovered represent an important addition to the few existing tracers of the Milky Way potential in the outer halo.

Reconstructing the Accretion History of the Galactic Halo Using Stellar Chemical Abundance Ratio Distributions

NASA Astrophysics Data System (ADS)

Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva; Jessop, Will

2016-08-01

In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from eleven ``MW-like'' halos to generate satellite template sets of 2D CARDs of accreted dwarf satellites which are comprised of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ~ 103-4 mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those eleven halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the satellite template set (STS) used and the sample size. For certain STS used we typically can identify the relative mass contributions of all accreted satellites to within a factor of 2. We also find that this method is particularly sensitive to older accretion events involving low-luminous dwarfs e.g. ultra-faint dwarfs - precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early Universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ~ 6-9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us (given the development of new CARD-generating dwarf models) to recover the luminosity function of infalling dwarf galaxies - and the detailed accretion history of the halo - across cosmic time.

Reconstructing the Accretion History of the Galactic Halo Using Stellar Chemical Abundance Ratio Distributions

NASA Astrophysics Data System (ADS)

Lee, Duane Morris; Johnston, Kathryn V.; Sen, Bodhisattva; Jessop, Will

2015-08-01

In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from eleven ``MW-like'' halos to generate satellite template sets of 2D CARDs of accreted dwarf satellites which are comprised of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ~103-4 mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those eleven halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the satellite template set (STS) used and the sample size. For certain STS used we typically can identify the relative mass contributions of all accreted satellites to within a factor of 2. We also find that this method is particularly sensitive to older accretion events involving low-luminous dwarfs e.g. ultra-faint dwarfs --- precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early Universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ~6-9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us (given the development of new CARD-generating dwarf models) to recover the luminosity function of infalling dwarf galaxies --- and the detailed accretion history of the halo --- across cosmic time.

Distinguishing cold dark matter dwarfs from self-interacting dark matter dwarfs in baryonic simulations

NASA Astrophysics Data System (ADS)

Strickland, Emily; Fitts, Alex; Boylan-Kolchin, Michael

2018-01-01

Our collaboration has simulated several high-resolution (mbaryon = 500Mo, mdm = 2500Mo) cosmological zoom-in simulations of isolated dwarf galaxies. We simulate each galaxy in standard cold dark matter (ΛCDM) as well as a self-interacting dark matter (SIDM) (with a cross section of σ/m ~ 1 cm2/g), both with and without baryons, to identify distinguishing characteristics between the two. The simulations are run using GIZMO, a meshless-finite-mass (MFM) hydrodynamical code, and are part of the Feedback in Realistic Environments (FIRE) project. By analyzing both the global properties and inner structure of the dwarfs in varying dark matter prescriptions, we provide a side-by-side comparison of isolated, dark matter dominated galaxies at the mass scale where differences in the two models of dark matter are thought to be the most obvious. We find that the edge of classical dwarfs and ultra-faint dwarfs (UFDs) (at ~105 Mo) provides the clearest window for distinguishing between the two theories. Here our SIDM galaxies continue to display a cored inner profile unlike their CDM counterparts. The SIDM versions of each galaxy also have measurably lower stellar velocity dispersions than their CDM counterparts.

A Hero’s Dark Horse: Discovery of an Ultra-faint Milky Way Satellite in Pegasus

NASA Astrophysics Data System (ADS)

Kim, Dongwon; Jerjen, Helmut; Mackey, Dougal; Da Costa, Gary S.; Milone, Antonino P.

2015-05-01

We report the discovery of an ultra-faint Milky Way satellite galaxy in the constellation of Pegasus. The concentration of stars was detected by applying our overdensity detection algorithm to the SDSS-DR 10 and confirmed with deeper photometry from the Dark Energy Camera at the 4 m Blanco telescope. Fitting model isochrones indicates that this object, Pegasus III, features an old and metal-poor stellar population ([Fe/H] ˜ -2.1) at a heliocentric distance of 205 ± 20 kpc. The new stellar system has an estimated half-light radius of {{r}h}=78-24+30 pc and a total luminosity of {{M}V}˜ -4.1+/- 0.5 that places it into the domain of dwarf galaxies on the size-luminosity plane. Pegasus III is spatially close to the MW satellite Pisces II. It is possible that the two might be physically associated, similar to the Leo IV and Leo V pair. Pegasus III is also well aligned with the Vast Polar Structure, which suggests a possible physical association.

A chemical confirmation of the faint Boötes II dwarf spheroidal galaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koch, Andreas; Rich, R. Michael, E-mail: akoch@lsw.uni-heidelberg.de

2014-10-10

We present a chemical abundance study of the brightest confirmed member star of the ultra-faint dwarf galaxy Boötes II from Keck/HIRES high-resolution spectroscopy at moderate signal-to-noise ratios. At [Fe/H] = –2.93 ± 0.03(stat.) ± 0.17(sys.), this star chemically resembles metal-poor halo field stars and the signatures of other faint dwarf spheroidal galaxies at the same metallicities in that it shows enhanced [α/Fe] ratios, Solar Fe-peak element abundances, and low upper limits on the neutron-capture element Ba. Moreover, this star shows no chemical peculiarities in any of the eight elements we were able to measure. This implies that the chemical outliersmore » found in other systems remain outliers pertaining to the unusual enrichment histories of the respective environments, while Boo II appears to have experienced an enrichment history typical of its very low mass. We also re-calibrated previous measurements of the galaxy's metallicity from the calcium triplet (CaT) and find a much lower value than reported before. The resulting broad metallicity spread, in excess of one dex, the very metal-poor mean, and the chemical abundance patterns of the present star imply that Boötes II is a low-mass, old, metal-poor dwarf galaxy and not an overdensity associated with the Sagittarius Stream as has been previously suggested based on its sky position and kinematics. The low, mean CaT metallicity of –2.7 dex falls right on the luminosity-metallicity relation delineated over four orders of magnitude from the more luminous to the faintest galaxies. Thus Boötes II's chemical enrichment appears representative of the galaxy's original mass, while tidal stripping and other mass loss mechanisms were probably not significant as for other low-mass satellites.« less

Isolated and companion young brown dwarfs in the taurus and chamaeleon molecular clouds

PubMed

Tamura; Itoh; Oasa; Nakajima

1998-11-06

Infrared imaging observations have detected a dozen faint young stellar objects (YSOs) in the Taurus and Chamaeleon molecular clouds whose near-infrared colors are similar to those of classical T Tauri stars (TTS). They are around four magnitudes fainter than low-luminosity YSOs in Taurus detected in earlier surveys and as much as eight magnitudes fainter than typical TTS. The extreme faintness of the objects and their lower luminosity relative to previously identified brown dwarfs in the Pleiades indicate that these faint YSOs are very young brown dwarfs on the order of 1 million years old.

Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites

NASA Astrophysics Data System (ADS)

Gennaro, Mario; Tchernyshyov, Kirill; Brown, Thomas M.; Geha, Marla; Avila, Roberto J.; Guhathakurta, Puragra; Kalirai, Jason S.; Kirby, Evan N.; Renzini, Alvio; Simon, Joshua D.; Tumlinson, Jason; Vargas, Luis C.

2018-03-01

Using deep observations obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST), we demonstrate that the sub-solar stellar initial mass function (IMF) of six ultra-faint dwarf Milky Way satellites (UFDs) is more bottom light than the IMF of the Milky Way disk. Our data have a lower-mass limit of ∼0.45 M ⊙, while the upper limit is ∼0.8 M ⊙, set by the turnoff mass of these old, metal-poor systems. If formulated as a single power law, we obtain a shallower IMF slope than the Salpeter value of ‑2.3, ranging from ‑1.01 for Leo IV to ‑1.87 for Boötes I. The significance of these deviations depends on the galaxy and is typically 95% or more. When modeled as a log-normal, the IMF fit results in a higher peak mass than in the Milky Way disk, but a Milky Way disk value for the characteristic system mass (∼0.22 M ⊙) is excluded at only 68% significance, and only for some UFDs in the sample. We find that the IMF slope correlates well with the galaxy mean metallicity, and to a lesser degree, with the velocity dispersion and the total mass. The strength of the observed correlations is limited by shot noise in the number of observed stars, but future space-based missions like the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope ( WFIRST) will enhance both the number of dwarf Milky Way satellites that can be studied in such detail and the observation depth for individual galaxies. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-12549.

Hunting Faint Dwarf Galaxies in the Field Using Integrated Light Surveys

NASA Astrophysics Data System (ADS)

Danieli, Shany; van Dokkum, Pieter; Conroy, Charlie

2018-03-01

We discuss the approach of searching the lowest mass dwarf galaxies, ≲ {10}6 {M}ȯ , in the general field, using integrated light surveys. By exploring the limiting surface brightness-spatial resolution (μ eff,lim‑θ) parameter space, we suggest that faint field dwarfs in the Local Volume, between 3 and 10 Mpc, are expected to be detected very effectively and in large numbers using integrated light photometric surveys, complementary to the classical star counts method. We use a sample of dwarf galaxies in the Local Group to construct relations between their photometric and structural parameters, M *–μ eff,V and M *–R eff. We use these relations, along with assumed functional forms for the halo mass function and the stellar mass–halo mass (SMHM) relation, to calculate the lowest detectable stellar masses in the Local Volume and the expected number of galaxies as a function of the limiting surface brightness and spatial resolution. The number of detected galaxies depends mostly on the limiting surface brightness for distances >3 Mpc, while spatial resolution starts to play a role for galaxies at distances >8 Mpc. Surveys with μ eff,lim ∼ 30 mag arcsec‑2 should be able to detect galaxies with stellar masses down to ∼104 M ⊙ in the Local Volume. Depending on the form of the SMHM relation, the expected number of dwarf galaxies with distances between 3 and 10 Mpc is 0.04–0.35 per square degree, assuming a limiting surface brightness of ∼29–30 mag arcsec‑2 and a spatial resolution <4″. We plan to search for a population of low-mass dwarf galaxies in the field by performing a blank wide field photometric survey with the Dragonfly Telephoto Array, an imaging system optimized for the detection of extended ultra low surface brightness structures.

Serendipitous discovery of a faint dwarf galaxy near a Local Volume dwarf

NASA Astrophysics Data System (ADS)

Makarova, L. N.; Makarov, D. I.; Antipova, A. V.; Karachentsev, I. D.; Tully, R. B.

2018-03-01

A faint dwarf irregular galaxy has been discovered in the HST/ACS field of LV J1157+5638. The galaxy is resolved into individual stars, including the brightest magnitude of the red giant branch. The dwarf is very likely a physical satellite of LV J1157+5638. The distance modulus of LV J1157+5638 using the tip of the red giant branch (TRGB) distance indicator is 29.82 ± 0.09 mag (D = 9.22 ± 0.38 Mpc). The TRGB distance modulus of LV J1157+5638 sat is 29.76 ± 0.11 mag (D = 8.95 ± 0.42 Mpc). The distances to the two galaxies are consistent within the uncertainties. The projected separation between them is only 3.9 kpc. LV J1157+5638 has a total absolute V magnitude of -13.26 ± 0.10 and linear Holmberg diameter of 1.36 kpc, whereas its faint satellite LV J1157+5638 sat has MV = -9.38 ± 0.13 mag and Holmberg diameter of 0.37 kpc. Such a faint dwarf was discovered for the first time beyond the nearest 4 Mpc from us. The presence of main-sequence stars in both galaxies unambiguously indicates the classification of the objects as dwarf irregulars with recent or ongoing star formation events in both galaxies.

Reconstructing the Accretion History of the Galactic Stellar Halo from Chemical Abundance Ratio Distributions

NASA Astrophysics Data System (ADS)

Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva; Jessop, Will

2015-03-01

Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ˜103-4 mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ˜6-9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time.

A wide-field survey for high-redshift quasars

NASA Astrophysics Data System (ADS)

Kakazu, Yuko K. M.

2008-02-01

The present thesis reports the results from the Hawaii Quasar and T dwarf survey (HQT survey), which is a wide-field optical imaging survey conducted with Subaru/Suprime-Cam. The HQT survey was designed to search for low- luminosity quasars ( M 1450 < -22.5) at high-redshift ( z > 5.7) as well as T dwarfs, both of which are selected by their very red optical I -- z ' colors. We developed a new color selection technique using a narrowband NB 816 filter in order to break a well-known color degeneracy between quasars and foreground M and L dwarfs. The follow-up Keck/DEIMOS spectroscopy and near-IR imaging with various instruments on Mauna Kea have demonstrated the effectiveness of our technique, and have successfully revealed six faint T dwarfs ( J < 20). These dwarfs are among the most distant spectroscopically known (60 - 170 pc) and they provide an indirect support for the high binary fraction at L/ T transition. The non-detection of z > 5.7 quasars in our survey is consistent with the present picture of the cosmic reionization in which quasars are negligible contributor to the cosmic reionization. With our survey area coverage (9.3 deg 2 ) and depths ( Z AB < 23.3), we were able to set strong constraints on the faint-end slope of the quasar luminosity function. Majority of our candidate quasars turned out to be strong emission line galaxies at z < 1, whose large equivalent widths and low metal contents suggest they are very young systems which have just undergone starbursts within a few Myrs. In order to systematically search for these Ultra-Strong Emission Line galaxies (USELs), we used narrowband selected samples from Hu's ultra-deep multiwavelength data. The followup Keck/DEIMOS spectra have revealed their high star formation density (5-10% of UV measurements at z = 0-1), which is a significant contribution at a epoch when cosmic star formation is in its peak. Many of the USELs show [OIII]l4363 auroral lines and about a dozen satisfy the criteria for eXtremely Metal Poor Galaxies (XMPGs). Our XMPGs are the most distant known today. Our high yield rate of XMPGs suggests that narrowband method is powerful in finding such populations. Strikingly, a few of our XMPGs have metallities close to the most metal-poor galaxy. Our discovery indicates that galaxies are still forming in relatively chemically pristine sites at z ~ 1

DEEP ULTRAVIOLET LUMINOSITY FUNCTIONS AT THE INFALL REGION OF THE COMA CLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hammer, D. M.; Hornschemeier, A. E.; Jenkins, L.

2012-02-01

We have used deep GALEX observations at the infall region of the Coma cluster to measure the faintest ultraviolet (UV) luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are measured to M{sub UV} = -10.5 in the GALEX FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach the dwarf early-type galaxy population in Coma for the first time. The Schechter faint-end slopes ({alpha} Almost-Equal-To -1.39 in both GALEX bands) are shallower than reported in previous Coma UV LF studies owing to a flatter LF at faint magnitudes. A Gaussian-plus-Schechtermore » model provides a slightly better parameterization of the UV LFs resulting in a faint-end slope of {alpha} Almost-Equal-To -1.15 in both GALEX bands. The two-component model gives faint-end slopes shallower than {alpha} = -1 (a turnover) for the LFs constructed separately for passive and star-forming galaxies. The UV LFs for star-forming galaxies show a turnover at M{sub UV} Almost-Equal-To -14 owing to a deficit of dwarf star-forming galaxies in Coma with stellar masses below M{sub *} = 10{sup 8} M{sub Sun }. A similar turnover is identified in recent UV LFs measured for the Virgo cluster suggesting this may be a common feature of local galaxy clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did not identify an excess of passive galaxies as would be expected if the missing dwarf star-forming galaxies were quenched inside the cluster. In fact, the LFs for both dwarf passive and star-forming galaxies show the same turnover at faint magnitudes. We discuss the possible origin of the missing dwarf star-forming galaxies in Coma and their expected properties based on comparisons to local field galaxies.« less

Deep UV Luminosity Functions at the Infall Region of the Coma Cluster

NASA Technical Reports Server (NTRS)

Hammer, D. M.; Hornschemeier, A. E.; Salim, S.; Smith, R.; Jenkins, L.; Mobasher, B.; Miller, N.; Ferguson, H.

2011-01-01

We have used deep GALEX observations at the infall region of the Coma cluster to measure the faintest UV luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are measured to M(sub uv) = -10.5 in the GALEX FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach the dwarf early-type galaxy population in Coma for the first time. The Schechter faint-end slopes (alpha approximately equal to -1.39 in both GALEX bands) are shallower than reported in previous Coma UV LF studies owing to a flatter LF at faint magnitudes. A Gaussian-plus-Schechter model provides a slightly better parametrization of the UV LFs resulting in a faint-end slope of alpha approximately equal to -1.15 in both GALEX bands. The two-component model gives faint-end slopes shallower than alpha = -1 (a turnover) for the LFs constructed separately for passive and star forming galaxies. The UV LFs for star forming galaxies show a turnover at M(sub UV) approximately equal to -14 owing to a deficit of dwarf star forming galaxies in Coma with stellar masses below M(sub *) = 10(sup 8) solar mass. A similar turnover is identified in recent UV LFs measured for the Virgo cluster suggesting this may be a common feature of local galaxy clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did not identify an excess of passive galaxies as would be expected if the missing dwarf star forming galaxies were quenched inside the cluster. In fact, the LFs for both dwarf passive and star forming galaxies show the same turnover at faint magnitudes. We discuss the possible origin of the missing dwarf star forming galaxies in Coma and their expected properties based on comparisons to local field galaxies.

The Stellar Populations of Ultra-Compact Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Karick, Arna; Gregg, M. D.

2006-12-01

We have discovered an intracluster population of ultra-luminous compact stellar systems in the Fornax cluster. Originally coined "ultra-compact dwarf galaxies" (UCDs), these objects were thought to be remnant nuclei of tidally stripped dE,Ns. Subsequent searches in Fornax (2dF+VLT) have revealed many fainter UCDs; making them the most numerous galaxy type in the cluster and fueling controversy over their origin. UCDs may be the bright tail of the globular cluster (GCs) population associated with NGC1399. Alternatively they may be real intracluster GCs, resulting from hierarchical cluster formation and merging in intracluster space. Determining the stellar populations of these enigmatic objects is challenging. UCDs are unresolved from the ground but our HST/STIS+ACS imaging reveals faint halos around the brightest UCDs. Here we present deep u'g'r'i'z' images of the cluster core using the CTIO 4m Mosaic. Combined with GALEX/UV imaging and using SSP isochrones, UCDs appear to be old, red and unlike cluster dEs. In contrast, our recent IMACS and Keck/LRIS+ESI spectroscopy shows that UCDs are unlike GCs and have intermediate stellar populations with significant variations in their Mg and Hβ line strength indices. This work is supported by National Science Foundation Grant No. 0407445 and was done at the Institute of Geophysics and Planetary Physics, under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

RECONSTRUCTING THE ACCRETION HISTORY OF THE GALACTIC STELLAR HALO FROM CHEMICAL ABUNDANCE RATIO DISTRIBUTIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva

Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfsmore » from various mass regimes and epochs of accretion. Next, we randomly drew samples of ∼10{sup 3–4} mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ∼6–9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time.« less

Pushing the boundaries: probing the halo of the Milky Way beyond 100 kpc with RR Lyrae

NASA Astrophysics Data System (ADS)

Carlin, Jeffrey L.; Medina, Gustavo; Munoz, Ricardo R.; Vivas, Anna Katherina; Willman, Beth

2018-01-01

Stars in the outermost halo of the Milky Way are vital tracers of the mass of our Galaxy. Furthermore, beyond ~100 kpc from the Galactic center, most (or perhaps all) of the stars are likely to be in faint dwarf galaxies or tidal debris from recently accreted dwarfs, making the outer reaches of the Galaxy important for understanding the Milky Way’s accretion history. However, confirmed stars are scarce at these distances because they are difficult to securely identify among the more numerous foreground stars. Pulsating variables such as RR Lyrae are ideal probes of the distant halo because they are readily identified in time-series data, are intrinsically bright and thus can be seen at large distances, and follow well-known period-luminosity relations that enable precise distance measurements. We present results from our program to find RR Lyrae using deep DECam time series data (from the HiTS supernova survey as well as our own observing program) covering ~300 square degrees. Our sample of distant RR Lyrae more than doubles the number of known Milky Way stars beyond distances of ~150 kpc. Among these, we find two distinct groups of two and three stars that are members of the Leo IV and Leo V ultra-faint dwarf galaxies, located at distances of ~145 kpc and ~175 kpc, respectively. We derive the stellar density as a function of Galactocentric radius, extending to more than 250 kpc from the Galactic center. This sample of RR Lyrae provides a set of important probes of the mass of the Milky Way and the accretion origin of the outer Galactic halo.

Antlia B: A Faint Dwarf Galaxy Member of the NGC 3109 Association

NASA Astrophysics Data System (ADS)

Sand, D. J.; Spekkens, K.; Crnojević, D.; Hargis, J. R.; Willman, B.; Strader, J.; Grillmair, C. J.

2015-10-01

We report the discovery of Antlia B, a faint dwarf galaxy at a projected distance of ˜72 kpc from NGC 3109 ({M}V ˜ -15 {mag}), the primary galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The tip of the red giant branch distance to Antlia B is D = 1.29 ± 0.10 Mpc, which is consistent with the distance to NGC 3109. A qualitative analysis indicates the new dwarf's stellar population has both an old, metal-poor red giant branch (≳ 10 {{Gyr}}, [Fe/H] ˜ -2), and a younger blue population with an age of ˜200-400 Myr, analogous to the original Antlia dwarf, another likely satellite of NGC 3109. Antlia B has H i gas at a velocity of {v}{helio,{{H}} {{I}}} = 376 km s-1, confirming the association with NGC 3109 (vhelio = 403 km s-1). The H i gas mass (MH i = 2.8 ± 0.2 × 105 {M}⊙ ), stellar luminosity (MV = -9.7 ± 0.6 mag) and half light radius (rh = 273 ± 29 pc) are all consistent with the properties of dwarf irregular and dwarf spheroidal galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy. The discovery of Antlia B is the initial result from a Dark Energy Camera survey for halo substructure and faint dwarf companions to NGC 3109 with the goal of comparing observed substructure with expectations from the Λ+Cold Dark Matter model in the sub-Milky Way regime.

No Assembly Required: Mergers are Mostly Irrelevant for the Growth of Low-mass Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Fitts, Alex; Boylan-Kolchin, Michael; Bullock, James S.; Weisz, Daniel R.; El-Badry, Kareem; Wheeler, Coral; Faucher-Giguère, Claude-André; Quataert, Eliot; Hopkins, Philip F.; Kereš, Dušan; Wetzel, Andrew; Hayward, Christopher C.

2018-06-01

We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 high-resolution cosmological zoom-in simulations, all with masses of Mhalo ≈ 1010 M⊙ (and M⋆ ˜ 105 - 107 M⊙) at z = 0, from the Feedback in Realistic Environments (FIRE) project. The stellar populations of these dwarf galaxies at z = 0 are formed essentially entirely "in situ": over 90% of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have >70% of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to z ˜ 3, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo 5 galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at z = 0. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for next-generation facilities. The small contribution of accreted stars make it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star color-magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual star formation histories of merged dwarfs.

The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

DOE PAGES

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

2016-04-01

In this paper, we study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M vir ~ 10 12.1 M ⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M star ~ 10 8–10 10M ⊙. Halos with more quiescent accretion histories tendmore » to have lower mass progenitors (10 8–10 9 M ⊙), and lower overall accreted stellar masses. Ultra-faint mass (M star < 10 5 M ⊙) dwarfs contribute a negligible amount (<<1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (~2%–5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 10 5 < M star/M ⊙ < 10 8 provide a substantial amount of the very metal-poor stellar material (~40%–80%), and even relatively metal-rich dwarfs with M star > 10 8 M ⊙ can contribute a considerable fraction (~20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. In conclusion, we suggest that the MW could be a "transient fossil"; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.« less

THE STELLAR INITIAL MASS FUNCTION OF ULTRA-FAINT DWARF GALAXIES: EVIDENCE FOR IMF VARIATIONS WITH GALACTIC ENVIRONMENT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geha, Marla; Brown, Thomas M.; Tumlinson, Jason

2013-07-01

We present constraints on the stellar initial mass function (IMF) in two ultra-faint dwarf (UFD) galaxies, Hercules and Leo IV, based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. The Hercules and Leo IV galaxies are extremely low luminosity (M{sub V} = -6.2, -5.5), metal-poor (([Fe/H]) = -2.4, -2.5) systems that have old stellar populations (>11 Gyr). Because they have long relaxation times, we can directly measure the low-mass stellar IMF by counting stars below the main-sequence turnoff without correcting for dynamical evolution. Over the stellar mass range probed by our data, 0.52-0.77 M{sub Sun }, the IMFmore » is best fit by a power-law slope of {alpha}= 1.2{sub -0.5}{sup +0.4} for Hercules and {alpha} = 1.3 {+-} 0.8 for Leo IV. For Hercules, the IMF slope is more shallow than a Salpeter ({alpha} = 2.35) IMF at the 5.8{sigma} level, and a Kroupa ({alpha} = 2.3 above 0.5 M{sub Sun }) IMF slope at 5.4{sigma} level. We simultaneously fit for the binary fraction, f{sub binary}, finding f{sub binary}= 0.47{sup +0.16}{sub -0.14} for Hercules, and 0.47{sup +0.37}{sub -0.17} for Leo IV. The UFD binary fractions are consistent with that inferred for Milky Way stars in the same mass range, despite very different metallicities. In contrast, the IMF slopes in the UFDs are shallower than other galactic environments. In the mass range 0.5-0.8 M{sub Sun }, we see a trend across the handful of galaxies with directly measured IMFs such that the power-law slopes become shallower (more bottom-light) with decreasing galactic velocity dispersion and metallicity. This trend is qualitatively consistent with results in elliptical galaxies inferred via indirect methods and is direct evidence for IMF variations with galactic environment.« less

The Carnegie-Chicago Hubble Program: Discovery of the Most Distant Ultra-faint Dwarf Galaxy in the Local Universe

NASA Astrophysics Data System (ADS)

Lee, Myung Gyoon; Jang, In Sung; Beaton, Rachael; Seibert, Mark; Bono, Giuseppe; Madore, Barry

2017-02-01

Ultra-faint dwarf galaxies (UFDs) are the faintest known galaxies, and due to their incredibly low surface brightness, it is difficult to find them beyond the Local Group. We report a serendipitous discovery of a UFD, Fornax UFD1, in the outskirts of NGC 1316, a giant galaxy in the Fornax cluster. The new galaxy is located at a projected radius of 55 kpc in the south-east of NGC 1316. This UFD is found as a small group of resolved stars in the Hubble Space Telescope images of a halo field of NGC 1316, obtained as part of the Carnegie-Chicago Hubble Program. Resolved stars in this galaxy are consistent with being mostly metal-poor red giant branch (RGB) stars. Applying the tip of the RGB method to the mean magnitude of the two brightest RGB stars, we estimate the distance to this galaxy, 19.0 ± 1.3 Mpc. Fornax UFD1 is probably a member of the Fornax cluster. The color-magnitude diagram of these stars is matched by a 12 Gyr isochrone with low metallicity ([Fe/H] ≈ -2.4). Total magnitude and effective radius of Fornax UFD1 are MV ≈ -7.6 ± 0.2 mag and reff = 146 ± 9 pc, which are similar to those of Virgo UFD1 that was discovered recently in the intracluster field of Virgo by Jang & Lee. Fornax UFD1 is the most distant known UFD that is confirmed by resolved stars. This indicates that UFDs are ubiquitous and that more UFDs remain to be discovered in the Fornax cluster. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #10505 and #13691.

The Origins of the Ultra Compact Dwarfs in the halos of the central cluster galaxies in Fornax and Virgo

NASA Astrophysics Data System (ADS)

Voggel, Karina Theresia

2015-08-01

Ultra-Compact Dwarf Galaxies (UCDs) have filled the size gap (10-100pc) in the scaling relations of early-type stellar systems. Before their discovery, no objects were known in the parameter space between globular clusters (GCs) and dwarf galaxies. The nature of UCDs is widely debated. Two formation channels have been suggested: either UCDs are surviving nuclei of tidally stripped dwarf galaxies, or they constitute the high mass end of the GC population. In this work we establish new strategies to constrain the formation channel of UCDs, looking for the observational signatures of stripped nuclei.Before falling into a galaxy cluster dwarf galaxies initially host their own GC system. Through tidal interaction the GCs outside of the shrinking tidal radius are lost and disperse in the general GC population of the cluster, whereas GCs inside the tidal radius remain bound to the dwarf galaxy. Therefore, we expect to find some GCs close to the stripped nuclei that have not been removed yet, but dragged towards the nucleus via dynamical friction.We tested this prediction in the halo of NGC 1399, the central Fornax cluster galaxy, where we find a local overabundance of GCs on scales of 0.5 to 1 kpc around UCDs. A similar analysis of GC overdensities around UCDs in the halo of M87, the central Virgo cluster galaxy, is ongoing. Such a clustering signal of GCs around UCDs could be a hint that these UCDs formed as nuclei, and what we see is the remnant GC population of the ancestor galaxy.We also have studied the detailed structural composition of ~100 UCDs in the halo of NGC 1399 by analyzing their surface brightness profiles. We present new evidence for faint asymmetric structures and tidal tails around several UCDs, possible tracers for the assembly history of the central cluster galaxy. With new numbers on the abundance of tidal features and close GC companions within large UCD samples, the contribution of each formation channel to the GC/UCD populations in galaxy halos can be constrained.

Resolving the faint end of the satellite luminosity function for the nearest elliptical Centaurus A

NASA Astrophysics Data System (ADS)

Crnojevic, Denija

2014-10-01

We request HST/ACS imaging to follow up 15 new faint candidate dwarfs around the nearest elliptical Centaurus A (3.8 Mpc). The dwarfs were found via a systematic ground-based (Magellan/Megacam) survey out to ~150 kpc, designed to directly confront the "missing satellites" problem in a wholly new environment. Current Cold Dark Matter models for structure formation fail to reproduce the shallow slope of the satellite luminosity function in spiral-dominated groups for which dwarfs fainter than M_V<-14 have been surveyed (the Local Group and the nearby, interacting M81 group). Clusters of galaxies show a better agreement with cosmological predictions, suggesting an environmental dependence of the (poorly-understood) physical processes acting on the evolution of low mass galaxies (e.g., reionization). However, the luminosity function completeness for these rich environments quickly drops due to the faintness of the satellites and to the difficult cluster membership determination. We target a yet unexplored "intermediate" environment, a nearby group dominated by an elliptical galaxy, ideal due to its proximity: accurate (10%) distance determinations for its members can be derived from resolved stellar populations. The proposed observations of the candidate dwarfs will confirm their nature, group membership, and constrain their luminosities, metallicities, and star formation histories. We will obtain the first complete census of dwarf satellites of an elliptical down to an unprecedented M_V<-9. Our results will crucially constrain cosmological predictions for the faint end of the satellite luminosity function to achieve a more complete picture of the galaxy formation process.

Mapping the Tidal Destruction of the Hercules Dwarf: A Wide-field DECam Imaging Search for RR Lyrae Stars

NASA Astrophysics Data System (ADS)

Garling, Christopher; Willman, Beth; Sand, David J.; Hargis, Jonathan; Crnojević, Denija; Bechtol, Keith; Carlin, Jeffrey L.; Strader, Jay; Zou, Hu; Zhou, Xu; Nie, Jundan; Zhang, Tianmeng; Zhou, Zhimin; Peng, Xiyan

2018-01-01

We investigate the hypothesized tidal disruption of the Hercules ultra-faint dwarf galaxy (UFD). Previous tidal disruption studies of the Hercules UFD have been hindered by the high degree of foreground contamination in the direction of the dwarf. We bypass this issue by using RR Lyrae stars, which are standard candles with a very low field-volume density at the distance of Hercules. We use wide-field imaging from the Dark Energy Camera on CTIO to identify candidate RR Lyrae stars, supplemented with observations taken in coordination with the Beijing–Arizona Sky Survey on the Bok Telescope. Combining color, magnitude, and light-curve information, we identify three new RR Lyrae stars associated with Hercules. All three of these new RR Lyrae stars lie outside its published tidal radius. When considered with the nine RR Lyrae stars already known within the tidal radius, these results suggest that a substantial fraction of Hercules’ stellar content has been stripped. With this degree of tidal disruption, Hercules is an interesting case between a visibly disrupted dwarf (such as the Sagittarius dwarf spheroidal galaxy) and one in dynamic equilibrium. The degree of disruption also shows that we must be more careful with the ways we determine object membership when estimating dwarf masses in the future. One of the three discovered RR Lyrae stars sits along the minor axis of Hercules, but over two tidal radii away. This type of debris is consistent with recent models that suggest Hercules’ orbit is aligned with its minor axis.

Constraining the Nature of Dark Matter with the Star-formation History of the Faintest Local Group Dwarf Galaxy Satellites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chau, Alice; Mayer, Lucio; Governato, Fabio

Λ warm dark matter (ΛWDM), realized by collisionless particles of 1–3 keV, has been proposed as an alternative scenario to Λ-Cold-Dark Matter (ΛCDM) for the dwarf galaxy scale discrepancies. We present an approach to test the viability of such WDM models using star-formation histories (SFHs) of the dwarf spheroidal galaxies (dSphs) in the Local Group. We compare their high-time-resolution SFHs with the collapse redshift of their dark halos in CDM and WDM. Collapse redshift is inferred after determining the subhalo infall mass. This is based on the dwarf current mass inferred from stellar kinematics, combined with cosmological simulation results onmore » subhalo evolution. WDM subhalos close to the filtering mass scale, forming significantly later than CDM, are the most difficult to reconcile with early truncation of star formation ( z ≥ 3). The ultra-faint dwarfs (UFDs) provide the most stringent constraints. Using six UFDs and eight classical dSphs, we show that a 1 keV particle is strongly disfavored, consistently with other reported methods. Excluding other models is only hinted for a few UFDs. Other UFDs for which the lack of robust constraints on halo mass prevents us from carrying out our analysis rigorously, show a very early onset of star formation that will strengthen the constraints delivered by our method in the future. We discuss the various caveats, notably the low number of dwarfs with accurately determined SFHs and the uncertainties when determining the subhalo infall mass, most notably the baryonic physics. Our preliminary analysis may serve as a pathfinder for future investigations that will combine accurate SFHs for local dwarfs with direct analysis of WDM simulations with baryons.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

In this paper, we study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M vir ~ 10 12.1 M ⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M star ~ 10 8–10 10M ⊙. Halos with more quiescent accretion histories tendmore » to have lower mass progenitors (10 8–10 9 M ⊙), and lower overall accreted stellar masses. Ultra-faint mass (M star < 10 5 M ⊙) dwarfs contribute a negligible amount (<<1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (~2%–5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 10 5 < M star/M ⊙ < 10 8 provide a substantial amount of the very metal-poor stellar material (~40%–80%), and even relatively metal-rich dwarfs with M star > 10 8 M ⊙ can contribute a considerable fraction (~20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. In conclusion, we suggest that the MW could be a "transient fossil"; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.« less

The MESSIER surveyor: unveiling the ultra-low surface brightness universe

NASA Astrophysics Data System (ADS)

Valls-Gabaud, David; MESSIER Collaboration

2017-03-01

The MESSIER surveyor is a small mission designed at exploring the very low surface brightness universe. The satellite will drift-scan the entire sky in 6 filters covering the 200-1000 nm range, reaching unprecedented surface brightness levels of 34 and 37 mag arcsec-2 in the optical and UV, respectively. These levels are required to achieve the two main science goals of the mission: to critically test the ΛCDM paradigm of structure formation through (1) the detection and characterisation of ultra-faint dwarf galaxies, which are predicted to be extremely abundant around normal galaxies, but which remain elusive; and (2) tracing the cosmic web, which feeds dark matter and baryons into galactic haloes, and which may contain the reservoir of missing baryons at low redshifts. A large number of science cases, ranging from stellar mass loss episodes to intracluster light through fluctuations in the cosmological UV-optical background radiation are free by-products of the full-sky maps produced.

Ultra-cool dwarfs viewed equator-on: surveying the best host stars for biosignature detection in transiting exoplanets

NASA Astrophysics Data System (ADS)

Miles-Paez, Paulo; Metchev, Stanimir; Burgasser, Adam; Apai, Daniel; Palle, Enric; Zapatero Osorio, Maria Rosa; Artigau, Etienne; Mace, Greg; Tannock, Megan; Triaud, Amaury

2018-05-01

There are about 150 known planets around M dwarfs, but only one system around an ultra-cool (>M7) dwarf: Trappist-1. Ultra-cool dwarfs are arguably the most promising hosts for atmospheric and biosignature detection in transiting planets because of the enhanced feature contrast in transit and eclipse spectroscopy. We propose a Spitzer survey to continuously monitor 15 of the brightest ultra-cool dwarfs over 3 days. To maximize the probability of detecting transiting planets, we have selected only targets seen close to equator-on. Spin-orbit alignment expectations dictate that the planetary systems around these ultra-cool dwarfs should also be oriented nearly edge-on. Any planet detections from this survey will immediately become top priority targets for JWST transit spectroscopy. No other telescope, present or within the foreseeable future, will be able to conduct a similarly sensitive and dedicated survey for characterizeable Earth analogs.

Indications of M-Dwarf Deficits in the Halo and Thick Disk of the Galaxy

NASA Technical Reports Server (NTRS)

Konishi, Mihoko; Shibai, Hiroshi; Sumi, Takahiro; Fukagawa, Misato; Matsuo, Taro; Samland, Matthias S.; Yamamoto, Kodai; Sudo, Jun; Itoh, Yoichi; Arimoto, Nubuo;

Indications of M-Dwarf Deficits in the Halo and Thick Disk of the Galaxy

NASA Technical Reports Server (NTRS)

Konishi, Mihoko; Shibai, Hiroshi; Sumi, Takahiro; Fukagawa, Misato; Matsuo, Taro; Samland, Matthias S.; Yamamoto, Kodai; Sudo, Jun; Itoh, Yoichi; Arimoto, Nobuo;

Ultra-compact High Velocity Clouds as Minihalos and Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Faerman, Yakov; Sternberg, Amiel; McKee, Christopher F.

2013-11-01

We present dark matter minihalo models for the Ultra-Compact, High-Velocity H I Clouds (UCHVCs) recently discovered in the 21 cm ALFALFA survey. We assume gravitational confinement of 104 K H I gas by flat-cored dark-matter subhalos within the Local Group. We show that for flat cores, typical (median) tidally stripped cosmological subhalos at redshift z = 0 have dark-matter masses of ~107 M ⊙ within the central 300 pc (independent of total halo mass), consistent with the "Strigari mass scale" observed in low-luminosity dwarf galaxies. Flat-cored subhalos also resolve the mass discrepancy between simulated and observed satellites around the Milky Way. For the UCHVCs, we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed H I mass and predict the associated (projected) H I half-mass radii, assuming the clouds are embedded in distant (d >~ 300 kpc) and unstripped subhalos. For a typical UCHVC (0.9 Jy km s-1), we predict physical H I half-mass radii of 0.18 to 0.35 kpc (or angular sizes of 0.'6 to 2.'1) for distances ranging from 300 kpc to 2 Mpc. As a consistency check, we model the gas-rich dwarf galaxy Leo T, for which there is a well-resolved H I column density profile and a known distance (420 kpc). For Leo T, we find that a subhalo with M 300 = 8 (± 0.2) × 106 M ⊙ best fits the observed H I profile. We derive an upper limit of P HIM <~ 150 cm-3 K for the pressure of any enveloping hot intergalactic medium gas at the distance of Leo T. Our analysis suggests that some of the UCHVCs may in fact constitute a population of 21 cm-selected but optically faint dwarf galaxies in the Local Group.

Understanding the Milky Way Halo through Large Surveys

NASA Astrophysics Data System (ADS)

Koposov, Sergey

This thesis presents an extensive study of stellar substructure in the outskirts of the Milky Way(MW), combining data mining of SDSS with theoretical modeling. Such substructure, either bound star clusters and satellite galaxies, or tidally disrupted objects forming stellar streams are powerful diagnostics of the Milky Way's dynamics and formation history. I have developed an algorithmic technique of searching for stellar overdensities in the MW halo, based on SDSS catalogs. This led to the discovery of unusual ultra-faint ~ (1000Lsun) globular clusters with very compact sizes and relaxation times << t_Hubble. The detailed analysis of a known stellar stream (GD-1), allowed me to make the first 6-D phase space map for such an object along 60 degrees on the sky. By modeling the stream's orbit I could place strong constraints on the Galactic potential, e.g. Vcirc(R0)= 224+/-13 km/s. The application of the algorithmic search for stellar overdensities to the SDSS dataset and to mock datasets allowed me to quantify SDSS's severe radial incompleteness in its search for ultra-faint dwarf galaxies and to determine the luminosity function of MW satellites down to luminosities of M_V ~ -3. I used the semi-analytical model in order to compare the CDM model predictions for the MW satellite population with the observations; this comparison has shown that the recently increased census of MW satellites, better understanding of the radial incompleteness and the suppression of star formation after the reionization can fully solve the "Missing satellite problem".

WEAK GALACTIC HALO-DWARF SPHEROIDAL CONNECTION FROM RR LYRAE STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fiorentino, Giuliana; Bono, Giuseppe; Monelli, Matteo

2015-01-01

We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in 6 dwarf spheroidals (dSphs) and 11 ultra faint dwarf galaxies (∼1300) show a Gaussian period distribution well peaked around a mean period of (Pab) = 0.610 ± 0.001 days (σ = 0.03). The Halo RRL (∼15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dSphs apart from Sagittariusmore » are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P ≲ 0.48 days and A{sub V} ≥ 0.75 mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered 18 globulars covering a broad range in metallicity (–2.3 ≲ [Fe/H] ≲ –1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal-rich than [Fe/H] ∼ –1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of ∼50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.« less

Using r-process enhanced galaxies to estimate the neutron star merger rate at high redshift

NASA Astrophysics Data System (ADS)

Roederer, Ian

2018-01-01

The rapid neutron-capture process, or r-process, is one of the fundamental ways that stars produce heavy elements. I describe a new approach that uses the existence of r-process enhanced galaxies, like the recently discovered ultra-faint dwarf galaxy Reticulum II, to derive a rate for neutron star mergers at high redshift. This method relies on three assertions. First, several lines of reasoning point to neutron star mergers as a rare yet prolific producer of r-process elements, and one merger event is capable of enriching most of the stars in a low-mass dwarf galaxy. Second, the Local Group is cosmologically representative of the halo mass function at the mass scales of low-luminosity dwarf galaxies, and the volume that their progenitors spanned at high redshifts can be estimated from simulations. Third, many of these dwarf galaxies are extremely old, and the metals found in their stars today date from the earliest times at high redshift. These galaxies occupy a quantifiable volume of the Universe, from which the frequency of r-process enhanced galaxies can be estimated. This frequency may be interpreted as lower limit to the neutron star merger rate at a redshift (z ~ 5-10) that is much higher than is accessible to gravitational wave observatories. I will present a proof of concept demonstration using medium-resolution multi-object spectroscopy from the Michigan/Magellan Fiber System (M2FS) to recover the known r-process galaxy Reticulum II, and I will discuss future plans to apply this method to other Local Group dwarf galaxies.

The very faint X-ray binary IGR J17062-6143: a truncated disc, no pulsations, and a possible outflow

NASA Astrophysics Data System (ADS)

van den Eijnden, J.; Degenaar, N.; Pinto, C.; Patruno, A.; Wette, K.; Messenger, C.; Hernández Santisteban, J. V.; Wijnands, R.; Miller, J. M.; Altamirano, D.; Paerels, F.; Chakrabarty, D.; Fabian, A. C.

2018-04-01

We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in 2006. Analysing NuSTAR, XMM-Newton, and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disc at 77^{+22}_{-18} gravitational radii (˜164 km) assuming a high inclination, although a low inclination and a disc extending to the neutron star cannot be excluded. The high-resolution spectroscopy reveals evidence for oxygen-rich circumbinary material, possibly resulting from a blueshifted, collisionally ionized outflow. Finally, we do not detect any pulsations. We discuss these results in the broader context of possible explanations for the persistent faint nature of weakly accreting neutron stars. The results are consistent with both an ultra-compact binary orbit and a magnetically truncated accretion flow, although both cannot be unambiguously inferred. We also discuss the nature of the donor star and conclude that it is likely a CO or O-Ne-Mg white dwarf, consistent with recent multiwavelength modelling.

Unveiling the extreme nature of the hyper faint galaxy Virgo I

NASA Astrophysics Data System (ADS)

Crnojevic, Denija

2017-08-01

We request HST/ACS imaging to obtain a deep color-magnitude diagram of the newly discovered candidate Milky Way satellite Virgo I. With an estimated absolute magnitude of only M_V -0.8 and a Galactocentric radius of 90 kpc, Virgo I is one of the faintest and most distant dwarfs ever observed, and could be identified as a prototype ''hyper'' faint galaxy. The detailed characterization of the smallest inhabited dark matter subhalos is crucial to guide hierarchical galaxy formation models, and in particular to constrain reionization, the nature of the dark matter particle, etc. With the advent of deep wide-field, ground-based surveys, the potential of uncovering these lowest-mass galaxies is quickly turning into reality, as demonstrated by the discovery in the past two years of tens of new Local Group members in the ultra-faint regime (M_V>-8). Virgo I represents a new record in galaxy physical properties, and urges us to be prepared for the likely emergence of an entirely new class of such objects in the era of future wide-field surveys (e.g., LSST). Only high resolution HST observations can enable us to confirm the nature of Virgo I, providing significantly more accurate estimates for its distance and structural properties, when compared to the discovery Subaru/HyperSuprimeCam imaging. Our proposed dataset will constitute a fundamental step in the upcoming hunt for galaxies with similarly extreme properties.

Faint blue counts from formation of dwarf galaxies at z approximately equals 1

NASA Technical Reports Server (NTRS)

Babul, Arif; Rees, Martin J.

1993-01-01

The nature of faint blue objects (FBO's) has been a source of much speculation since their detection in deep CCD images of the sky. Their high surface density argues against them being progenitors of present-day bright galaxies and since they are only weakly clustered on small scales, they cannot be entities that merged together to form present-day galaxies. Babul & Rees (1992) have suggested that the observed faint blue counts may be due to dwarf elliptical galaxies undergoing their initial starburst at z is approximately equal to 1. In generic hierarchical clustering scenarios, however, dwarf galaxy halos (M is approximately 10(exp 9) solar mass) are expected to form at an earlier epoch; for example, typical 10(exp 9) solar mass halos will virialize at z is approximately equal to 2.3 if the power-spectrum for the density fluctuations is that of the standard b = 2 cold dark matter (CDM) model. Under 'ordinary conditions' the gas would rapidly cool, collect in the cores, and undergo star-formation. Conditions at high redshifts are far from 'ordinary'. The intense UV background will prevent the gas in the dwarf halos from cooling, the halos being released from their suspended state only when the UV flux has diminished sufficiently.

Serendipitous Discovery of RR Lyrae Stars in the Leo V Ultra-faint Galaxy

NASA Astrophysics Data System (ADS)

Medina, Gustavo E.; Muñoz, Ricardo R.; Vivas, A. Katherina; Förster, Francisco; Carlin, Jeffrey L.; Martinez, Jorge; Galbany, Lluis; González-Gaitán, Santiago; Hamuy, Mario; de Jaeger, Thomas; Maureira, Juan Carlos; San Martín, Jaime

2017-08-01

During the analysis of RR Lyrae stars (RRLs) discovered in the High Cadence Transient Survey (HiTS) taken with the Dark Energy Camera at the 4 m telescope at Cerro Tololo Inter-American Observatory, we found a group of three very distant, fundamental mode pulsator RR Lyrae (type ab). The location of these stars agrees with them belonging to the Leo V ultra-faint satellite galaxy, for which no variable stars have been reported to date. The heliocentric distance derived for Leo V based on these stars is 173 ± 5 kpc. The pulsational properties (amplitudes and periods) of these stars locate them within the locus of the Oosterhoff II group, similar to most other ultra-faint galaxies with known RRLs. This serendipitous discovery shows that distant RRLs may be used to search for unknown faint stellar systems in the outskirts of the Milky Way.

An Ultra-faint Galaxy Candidate Discovered in Early Data from the Magellanic Satellites Survey

NASA Astrophysics Data System (ADS)

Drlica-Wagner, A.; Bechtol, K.; Allam, S.; Tucker, D. L.; Gruendl, R. A.; Johnson, M. D.; Walker, A. R.; James, D. J.; Nidever, D. L.; Olsen, K. A. G.; Wechsler, R. H.; Cioni, M. R. L.; Conn, B. C.; Kuehn, K.; Li, T. S.; Mao, Y.-Y.; Martin, N. F.; Neilsen, E.; Noel, N. E. D.; Pieres, A.; Simon, J. D.; Stringfellow, G. S.; van der Marel, R. P.; Yanny, B.

2016-12-01

We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (μ ={28.5}-1+1 {mag} {arcsec}{}-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of {45}-4+5 {kpc}. The physical size ({r}1/2={46}-11+15 {pc} ) and low luminosity ({M}V=-{3.2}-0.5+0.4 {mag} ) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located {11.3}-0.9+3.1 {kpc} from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H., E-mail: adeason@stanford.edu

We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M{sub vir} ∼ 10{sup 12.1} M{sub ⊙}) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M{sub star} ∼ 10{sup 8}–10{sup 10}M{sub ⊙}. Halos with more quiescent accretion histories tend to have lower mass progenitors (10{sup 8}–10{sup 9} M{sub ⊙}), andmore » lower overall accreted stellar masses. Ultra-faint mass (M{sub star} < 10{sup 5} M{sub ⊙}) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < −2. Dwarfs with masses 10{sup 5} < M{sub star}/M{sub ⊙} < 10{sup 8} provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M{sub star} > 10{sup 8} M{sub ⊙} can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil”; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.« less

An ultra-faint galaxy candidate discovered in early data from the Magellanic Satellites Survey

DOE PAGES

Drlica-Wagner, A.; Bechtol, Keith; Allam, S.; ...

2016-11-30

Here, we report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644–5953 (Pictor II or Pic II) is a low surface brightness (more » $$\\mu ={28.5}_{-1}^{+1}\\,\\mathrm{mag}\\,\\,\\mathrm{arcsec}{}^{-2}$$ within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of $${45}_{-4}^{+5}\\,\\mathrm{kpc}$$. The physical size ($${r}_{1/2}={46}_{-11}^{+15}\\,\\mathrm{pc}\\,$$) and low luminosity ($${M}_{V}=-{3.2}_{-0.5}^{+0.4}\\,\\mathrm{mag}\\,$$) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644–5953 (Pic II) is located $${11.3}_{-0.9}^{+3.1}\\,\\mathrm{kpc}\\,$$ from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644–5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.« less

An ultra-faint galaxy candidate discovered in early data from the Magellanic Satellites Survey

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drlica-Wagner, A.; Bechtol, Keith; Allam, S.

Here, we report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644–5953 (Pictor II or Pic II) is a low surface brightness (more » $$\\mu ={28.5}_{-1}^{+1}\\,\\mathrm{mag}\\,\\,\\mathrm{arcsec}{}^{-2}$$ within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of $${45}_{-4}^{+5}\\,\\mathrm{kpc}$$. The physical size ($${r}_{1/2}={46}_{-11}^{+15}\\,\\mathrm{pc}\\,$$) and low luminosity ($${M}_{V}=-{3.2}_{-0.5}^{+0.4}\\,\\mathrm{mag}\\,$$) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644–5953 (Pic II) is located $${11.3}_{-0.9}^{+3.1}\\,\\mathrm{kpc}\\,$$ from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644–5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.« less

Dark Matter Search Using XMM-Newton Observations of Willman 1

NASA Technical Reports Server (NTRS)

Lowenstein, Michael; Kusenko, Alexander

2012-01-01

We report the results of a search for an emission line from radiatively decaying dark matter in the ultra-faint dwarf spheroidal galaxy Willman 1 based on analysis of spectra extracted from XMM-Newton X-ray Observatory data. The observation follows up our analysis of Chandra data of Willman 1that resulted in line flux upper limits over the Chandra bandpass and evidence of a 2.5 keY feature at a significance below the 99% confidence threshold used to define the limits. The higher effective area of the XMM-Newton detectors, combined with application of recently developing methods for extended-source analysis, allow us to derive improved constraints on the combination of mass and mixing angle of the sterile neutrino dark matter candidate. We do not confirm the Chandra evidence for a 2.5 keV emission line.

Dwarfs in Coma Cluster

NASA Technical Reports Server (NTRS)

2007-01-01

[figure removed for brevity, see original site] Click on image for larger poster version

This false-color mosaic of the central region of the Coma cluster combines infrared and visible-light images to reveal thousands of faint objects (green). Follow-up observations showed that many of these objects, which appear here as faint green smudges, are dwarf galaxies belonging to the cluster. Two large elliptical galaxies, NGC 4889 and NGC 4874, dominate the cluster's center. The mosaic combines visible-light data from the Sloan Digital Sky Survey (color coded blue) with long- and short-wavelength infrared views (red and green, respectively) from NASA's Spitzer Space Telescope.

Deep UBVRI photometric calibration of high-latitude fields: SA 57 (1307+30) and Hercules (1720+50)

NASA Technical Reports Server (NTRS)

Majewski, S. R.; Kron, R. G.; Koo, D. C.; Bershady, M. A.

1994-01-01

We present CCD photometric calibration sequences in the magnitude range V = 17-22 for two fields at high Galactic latitude: SA 57 (at the North Galactic Pole) and Hercules (l = 77, b = 35). Photometry to a precision of about 0.02 mag at V = 20 and, in general, better than 0.10 mag at V = 22 was obtained in the Johnson UBV as well as the Kron-Cousins R and I bands. These data are suitable for setting magnitude zero-points in catalogues of faint stars, galaxies, and QSOs, and we apply them to our own photographic catalogs in these two fields. We also note a significant deviation in the (V-R, R-I) color-color diagram for the locus of faint (V is greater than 20) M dwarfs compared to the locus provided by much brighter M dwarfs. This deviation may indicate differences in spectral properties between Population I and older populations of late dwarfs; however we do not discount the possibility that this locus for the faint stars, which appears as a saturation in V-R color with increasing R-I color, is the result of systematic photometric error.

CHEMICAL SIGNATURES OF THE FIRST GALAXIES: CRITERIA FOR ONE-SHOT ENRICHMENT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frebel, Anna; Bromm, Volker, E-mail: afrebel@mit.edu, E-mail: vbromm@astro.as.utexas.edu

We utilize metal-poor stars in the local, ultra-faint dwarf galaxies (UFDs; L {sub tot} {<=} 10{sup 5} L {sub Sun }) to empirically constrain the formation process of the first galaxies. Since UFDs have much simpler star formation histories than the halo of the Milky Way, their stellar populations should preserve the fossil record of the first supernova (SN) explosions in their long-lived, low-mass stars. Guided by recent hydrodynamical simulations of first galaxy formation, we develop a set of stellar abundance signatures that characterize the nucleosynthetic history of such an early system if it was observed in the present-day universe.more » Specifically, we argue that the first galaxies are the product of chemical 'one-shot' events, where only one (long-lived) stellar generation forms after the first, Population III, SN explosions. Our abundance criteria thus constrain the strength of negative feedback effects inside the first galaxies. We compare the stellar content of UFDs with these one-shot criteria. Several systems (Ursa Major II, and also Coma Berenices, Bootes I, Leo IV, Segue 1) largely fulfill the requirements, indicating that their high-redshift predecessors did experience strong feedback effects that shut off star formation. We term the study of the entire stellar population of a dwarf galaxy for the purpose of inferring details about the nature and origin of the first galaxies 'dwarf galaxy archaeology'. This will provide clues to the connection of the first galaxies, the surviving, metal-poor dwarf galaxies, and the building blocks of the Milky Way.« less

A MEGACAM SURVEY OF OUTER HALO SATELLITES. II. BLUE STRAGGLERS IN THE LOWEST STELLAR DENSITY SYSTEMS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Santana, Felipe A.; Munoz, Ricardo R.; Geha, Marla

2013-09-10

We present a homogeneous study of blue straggler stars across 10 outer halo globular clusters, 3 classical dwarf spheroidal galaxies, and 9 ultra-faint galaxies based on deep and wide-field photometric data taken with MegaCam on the Canada-France-Hawaii Telescope. We find blue straggler stars to be ubiquitous among these Milky Way satellites. Based on these data, we can test the importance of primordial binaries or multiple systems on blue straggler star formation in low-density environments. For the outer halo globular clusters, we find an anti-correlation between the specific frequency of blue stragglers and absolute magnitude, similar to that previously observed formore » inner halo clusters. When plotted against density and encounter rate, the frequency of blue stragglers is well fit by a single trend with a smooth transition between dwarf galaxies and globular clusters; this result points to a common origin for these satellites' blue stragglers. The fraction of blue stragglers stays constant and high in the low encounter rate regime spanned by our dwarf galaxies, and decreases with density and encounter rate in the range spanned by our globular clusters. We find that young stars can mimic blue stragglers in dwarf galaxies only if their ages are 2.5 {+-} 0.5 Gyr and they represent {approx}1%-7% of the total number of stars, which we deem highly unlikely. These results point to mass-transfer or mergers of primordial binaries or multiple systems as the dominant blue straggler formation mechanism in low-density systems.« less

The Resolved Stellar Populations Early Release Science Program

NASA Astrophysics Data System (ADS)

Gilbert, Karoline; Weisz, Daniel; Resolved Stellar Populations ERS Program Team

2018-06-01

The Resolved Stellar Populations Early Release Science Program (PI D. Weisz) will observe Local Group targets covering a range of stellar density and star formation histories, including a globular cluster, and ultra-faint dwarf galaxy, and a star-forming dwarf galaxy. Using observations of these diverse targets we will explore a broad science program: we will measure star formation histories, the sub-solar stellar initial mass function, and proper motions, perform studies of evolved stars, and map extinction in the target fields. Our observations will be of high archival value for other science such as calibrating stellar evolution models, studying variable stars, and searching for metal-poor stars. We will determine optimal observational setups and develop data reduction techniques that will be common to JWST studies of resolved stellar populations. We will also design, test, and release point spread function (PSF) fitting software specific to NIRCam and NIRISS, required for the crowded stellar regime. Prior to the Cycle 2 Call for Proposals, we will release PSF fitting software, matched HST and JWST catalogs, and clear documentation and step-by-step tutorials (such as Jupyter notebooks) for reducing crowded stellar field data and producing resolved stellar photometry catalogs, as well as for specific resolved stellar photometry science applications.

WFIRST: Resolving the Milky Way Galaxy

NASA Astrophysics Data System (ADS)

Kalirai, Jason; Conroy, Charlie; Dressler, Alan; Geha, Marla; Levesque, Emily; Lu, Jessica; Tumlinson, Jason

2018-01-01

WFIRST will yield a transformative impact in measuring and characterizing resolved stellar populations in the Milky Way. The proximity and level of detail that such populations need to be studied at directly map to all three pillars of WFIRST capabilities - sensitivity from a 2.4 meter space based telescope, resolution from 0.1" pixels, and large 0.3 degree field of view from multiple detectors. In this poster, we describe the activities of the WFIRST Science Investigation Team (SIT), "Resolving the Milky Way with WFIRST". Notional programs guiding our analysis include targeting sightlines to establish the first well-resolved large scale maps of the Galactic bulge aand central region, pockets of star formation in the disk, benchmark star clusters, and halo substructure and ultra faint dwarf satellites. As an output of this study, our team is building optimized strategies and tools to maximize stellar population science with WFIRST. This will include: new grids of IR-optimized stellar evolution and synthetic spectroscopic models; pipelines and algorithms for optimal data reduction at the WFIRST sensitivity and pixel scale; wide field simulations of Milky Way environments including new astrometric studies; and strategies and automated algorithms to find substructure and dwarf galaxies in the Milky Way through the WFIRST High Latitude Survey.

Retainment of r-process material in dwarf galaxies

NASA Astrophysics Data System (ADS)

Beniamini, Paz; Dvorkin, Irina; Silk, Joe

2018-04-01

The synthesis of r-process elements is known to involve extremely energetic explosions. At the same time, recent observations find significant r-process enrichment even in extremely small ultra-faint dwarf (UFD) galaxies. This raises the question of retainment of those elements within their hosts. We estimate the retainment fraction and find that it is large ˜0.9, unless the r-process event is very energetic (≳ 1052erg) and / or the host has lost a large fraction of its gas prior to the event. We estimate the r-process mass per event and rate as implied by abundances in UFDs, taking into account imperfect retainment and different models of UFD evolution. The results are consistent with previous estimates (Beniamini et al. 2016b) and with the constraints from the recently detected macronova accompanying a neutron star merger (GW170817). We also estimate the distribution of abundances predicted by these models. We find that ˜0.07 of UFDs should have r-process enrichment. The results are consistent with both the mean values and the fluctuations of [Eu/Fe] in galactic metal poor stars, supporting the possibility that UFDs are the main 'building blocks' of the galactic halo population.

DISCOVERY OF A NEW FAINT DWARF GALAXY ASSOCIATED WITH NGC 253

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sand, D. J.; Crnojević, D.; Strader, J.

2014-09-20

We report the discovery of a new faint dwarf galaxy, which we dub Scl-MM-Dw1, at a projected distance of ∼65 kpc from the spiral galaxy NGC 253. The discovery results from the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), a program with the Magellan/Megacam imager to study faint substructure in resolved stellar light around massive galaxies outside of the Local Group. We measure a tip of the red giant branch distance to Scl-MM-Dw1 of D = 3.9 ± 0.5 Mpc, consistent with that of NGC 253, making their association likely. The new dwarf's stellar population is complex, with an old, metal-poor red giant branch (≳10 Gyr, [Fe/H] ∼ –2), andmore » an asymptotic giant branch with an age of ∼500 Myr. Scl-MM-Dw1 has a half-light radius of r{sub h} = 340 ± 50 pc and an absolute magnitude of M{sub V}  = –10.3 ± 0.6 mag, comparable to the Milky Way's satellites at the same luminosity. Once complete, our imaging survey of NGC 253 and other nearby massive galaxies will provide a census of faint substructure in halos beyond the Local Group, both to put our own environment into context and to confront models of hierarchical structure formation.« less

TOWARD A NETWORK OF FAINT DA WHITE DWARFS AS HIGH-PRECISION SPECTROPHOTOMETRIC STANDARDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Narayan, G.; Matheson, T.; Saha, A.

We present the initial results from a program aimed at establishing a network of hot DA white dwarfs to serve as spectrophotometric standards for present and future wide-field surveys. These stars span the equatorial zone and are faint enough to be conveniently observed throughout the year with large-aperture telescopes. The spectra of these white dwarfs are analyzed in order to generate a non-local-thermodynamic-equilibrium model atmosphere normalized to Hubble Space Telescope colors, including adjustments for wavelength-dependent interstellar extinction. Once established, this standard star network will serve ground-based observatories in both hemispheres as well as space-based instrumentation from the UV to themore » near IR. We demonstrate the effectiveness of this concept and show how two different approaches to the problem using somewhat different assumptions produce equivalent results. We discuss the lessons learned and the resulting corrective actions applied to our program.« less

Faint Dwarf Galaxies in Hickson Compact Group 90

NASA Astrophysics Data System (ADS)

Ordenes-Briceño, Y.; Taylor, M. A.; Puzia, T. H.; Muñoz, R. P.

2017-07-01

We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U- and I-band images obtained with VLT/VIMOS. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies, which share properties with dwarf galaxies found throughout the Local Volume and in nearby galaxy clusters such as Fornax. Among them, we find a pair of candidates with ˜2 kpc projected separation and a nucleated dwarf candidate, with nucleus size of reff≅46-63 pc.

Four faint T dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Southern Stripe

NASA Astrophysics Data System (ADS)

Chiu, Kuenley; Liu, Michael C.; Jiang, Linhua; Allers, Katelyn N.; Stark, Daniel P.; Bunker, Andrew; Fan, Xiaohui; Glazebrook, Karl; Dupuy, Trent J.

2008-03-01

We present the optical and near-infrared photometry and spectroscopy of four faint T dwarfs newly discovered from the UKIDSS first data release. The sample, drawn from an imaged area of ~136 deg2 to a depth of Y = 19.9 (5σ, Vega), is located in the Sloan Digital Sky Survey (SDSS) Southern Equatorial Stripe, a region of significant future deep imaging potential. We detail the selection and followup of these objects, three of which are spectroscopically confirmed brown dwarfs ranging from type T2.5 to T7.5, and one is photometrically identified as early T. Their magnitudes range from Y = 19.01 to 19.88 with derived distances from 34 to 98 pc, making these among the coldest and faintest brown dwarfs known. The T7.5 dwarf appears to be single based on 0.05-arcsec images from Keck laser guide star adaptive optics. The sample brings the total number of T dwarfs found or confirmed by UKIDSS data in this region to nine, and we discuss the projected numbers of dwarfs in the future survey data. We estimate that ~240 early and late T dwarfs are discoverable in the UKIDSS Large Area Survey (LAS) data, falling significantly short of published model projections and suggesting that initial mass functions and/or birth rates may be at the low end of possible models. Thus, deeper optical data have good potential to exploit the UKIDSS survey depth more fully, but may still find the potential Y dwarf sample to be extremely rare.

Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane’s {S}_{K}^{2} Kinematic Scaling, and the Spin-Ellipticity Diagram

NASA Astrophysics Data System (ADS)

Graham, Alister W.; Janz, Joachim; Penny, Samantha J.; Chilingarian, Igor V.; Ciambur, Bogdan C.; Forbes, Duncan A.; Davies, Roger L.

2017-05-01

Selected from a sample of nine, isolated, dwarf early-type galaxies (ETGs) with the same range of kinematic properties as dwarf ETGs in clusters, we use LEDA 2108986 (CG 611) to address the nature versus nurture debate regarding the formation of dwarf ETGs. The presence of faint disk structures and rotation within some cluster dwarf ETGs has often been heralded as evidence that they were once late-type spiral or dwarf irregular galaxies prior to experiencing a cluster-induced transformation into an ETG. However, CG 611 also contains significant stellar rotation (≈20 km s-1) over its inner half-light radius ({R}{{e},{maj}}=0.71 kpc), and its stellar structure and kinematics resemble those of cluster ETGs. In addition to hosting a faint young nuclear spiral within a possible intermediate-scale stellar disk, CG 611 has accreted an intermediate-scale, counter-rotating gas disk. It is therefore apparent that dwarf ETGs can be built by accretion events, as opposed to disk-stripping scenarios. We go on to discuss how both dwarf and ordinary ETGs with intermediate-scale disks, whether under (de)construction or not, are not fully represented by the kinematic scaling {S}0.5=\\sqrt{0.5 {V}{rot}2+{σ }2}, and we also introduce a modified spin-ellipticity diagram λ (R)-ɛ (R) with the potential to track galaxies with such disks.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roederer, Ian U., E-mail: iur@umich.edu

The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies in the Milky Way halo are generally underabundant by an order of magnitude or more when compared with stars in the halo field. Among the heavy elements produced by n -capture reactions, only Sr and Ba can be detected in red giant stars in most UFD galaxies. This limited chemical information is unable to identify the nucleosynthesis process(es) responsible for producing the heavy elements in UFD galaxies. Similar [Sr/Ba] and [Ba/Fe] ratios are found in three bright halo field stars, BD−18°5550, CS 22185–007, and CS 22891–200. Previous studiesmore » of high-quality spectra of these stars report detections of additional n -capture elements, including Eu. The [Eu/Ba] ratios in these stars span +0.41 to +0.86. These ratios and others among elements in the rare Earth domain indicate an r -process origin. These stars have some of the lowest levels of r -process enhancement known, with [Eu/H] spanning −3.95 to −3.32, and they may be considered nearby proxies for faint stars in UFD galaxies. Direct confirmation, however, must await future observations of additional heavy elements in stars in the UFD galaxies themselves.« less

Sub-percent Photometry: Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories

NASA Astrophysics Data System (ADS)

Narayan, Gautham; Axelrod, Tim; Calamida, Annalisa; Saha, Abhijit; Matheson, Thomas; Olszewski, Edward; Holberg, Jay; Holberg, Jay; Bohlin, Ralph; Stubbs, Christopher W.; Rest, Armin; Deustua, Susana; Sabbi, Elena; MacKenty, John W.; Points, Sean D.; Hubeny, Ivan

2018-01-01

We have established a network of faint (16.5 < V < 19) hot DA white dwarfs as spectrophotometric standards for present and future wide-field observatories. Our standards are accessible from both hemispheres and suitable for ground and space-based covering the UV to the near IR. The network is tied directly to the most precise astrophysical reference presently available - the CALSPEC standards - through a multi-cycle program imaging using the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We have developed two independent analyses to forward model all the observed photometry and ground-based spectroscopy and infer a spectral energy distribution for each source using a non-local-thermodynamic-equilibrium (NLTE) DA white dwarf atmosphere extincted by interstellar dust. The models are in excellent agreement with each other, and agree with the observations to better than 0.01 mag in all passbands, and better than 0.005 mag in the optical. The high-precision of these faint sources, tied directly to the most accurate flux standards presently available, make our network of standards ideally suited for any experiments that have very stringent requirements on absolute flux calibration, such as studies of dark energy using the Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey Telescope (WFIRST).

Exploding Satellites—The Tidal Debris of the Ultra-faint Dwarf Galaxy Hercules

NASA Astrophysics Data System (ADS)

Küpper, Andreas H. W.; Johnston, Kathryn V.; Mieske, Steffen; Collins, Michelle L. M.; Tollerud, Erik J.

2017-01-01

The ultra-faint satellite galaxy Hercules has a strongly elongated and irregular morphology with detections of tidal features up to 1.3 deg (3 kpc) from its center. This suggests that Hercules may be dissolving under the Milky Way’s gravitational influence, and hence could be a tidal stream in formation rather than a bound, dark-matter-dominated satellite. Using Bayesian inference in combination with N-body simulations, we show that Hercules has to be on a very eccentric orbit (ɛ ≈ 0.95) within the Milky Way in this scenario. On such an orbit, Hercules “explodes” as a consequence of the last tidal shock at pericenter 0.5 Gyr ago. It is currently decelerating toward the apocenter of its orbit with a velocity of V = 157 km s-1—of which 99% is directed radially outwards. Due to differential orbital plane precession caused by the non-spherical nature of the Galactic potential, its debris fans out nearly perpendicular to its orbit. This explains why Hercules has an elongated shape without showing a distance gradient along its main body: it is in fact a stream that is significantly broader than it is long. In other words, it is moving perpendicular to its apparent major axis. In this scenario, there is a spike in the radial velocity profile created by the dominant debris component that formed through the last pericenter passage. This is similar to kinematic substructure that is observed in the real Hercules. Modeling a satellite on such a highly eccentric orbit is strongly dependent on the form of the Galactic potential. We therefore propose that detailed kinematic investigation of Hercules and other exploding satellite candidates can yield strong constraints on the potential of the Milky Way.

An Unusual Transient in the Extremely Metal-Poor Galaxy SDSS J094332.35+332657.6 (Leoncino Dwarf)

NASA Astrophysics Data System (ADS)

Filho, Mercedes E.; Sánchez Almeida, J.

2018-05-01

We have serendipitously discovered that Leoncino Dwarf, an ultra-faint, low-metallicity record-holder dwarf galaxy, may have hosted a transient source, and possibly exhibited a change in morphology, a shift in the center of brightness, and peak variability of the main (host) source in images taken approximately 40 yr apart; it is highly likely that these phenomena are related. Scenarios involving a Solar System object, a stellar cluster, dust enshrouding, and accretion variability have been considered, and discarded, as the origin of the transient. Although a combination of time-varying strong and weak lensing effects, induced by an intermediate mass black hole (104 - 5 × 105 M⊙) moving within the Milky Way halo (0.1 - 4 kpc), can conceivably explain all of the observed variable galaxy properties, it is statistically highly unlikely according to current theoretical predictions, and, therefore, also discarded. A cataclysmic event such as a supernova/hypernova could have occurred, as long as the event was observed towards the later/late-stage descent of the light curve, but this scenario fails to explain the absence of a post-explosion source and/or host HII region in recent optical images. An episode related to the giant eruption of a luminous blue variable star, a stellar merger or a nova, observed at, or near, peak magnitude may explain the transient source and possibly the change in morphology/center of brightness, but can not justify the main source peak variability, unless stellar variability is evoked.

Automated detectionof very low surface brightness galaxiesin the Virgo cluster

NASA Astrophysics Data System (ADS)

Prole, D. J.; Davies, J. I.; Keenan, O. C.; Davies, L. J. M.

2018-07-01

We report the automatic detection of a new sample of very low surface brightness (LSB) galaxies, likely members of the Virgo cluster. We introduce our new software, DeepScan, that has been designed specifically to detect extended LSB features automatically using the DBSCAN algorithm. We demonstrate the technique by applying it over a 5 deg2 portion of the Next Generation Virgo Survey (NGVS) data to reveal 53 LSB galaxies that are candidate cluster members based on their sizes and colours. 30 of these sources are new detections despite the region being searched specifically for LSB galaxies previously. Our final sample contains galaxies with 26.0 ≤ ⟨μe⟩ ≤ 28.5 and 19 ≤ mg ≤ 21, making them some of the faintest known in Virgo. The majority of them have colours consistent with the red sequence, and have a mean stellar mass of 106.3 ± 0.5 M⊙ assuming cluster membership. After using ProFit to fit Sérsic profiles to our detections, none of the new sources have effective radii larger than 1.5 Kpc and do not meet the criteria for ultra-diffuse galaxy (UDG) classification, so we classify them as ultra-faint dwarfs.

Automated detection of very Low Surface Brightness galaxies in the Virgo Cluster

NASA Astrophysics Data System (ADS)

Prole, D. J.; Davies, J. I.; Keenan, O. C.; Davies, L. J. M.

2018-04-01

We report the automatic detection of a new sample of very low surface brightness (LSB) galaxies, likely members of the Virgo cluster. We introduce our new software, DeepScan, that has been designed specifically to detect extended LSB features automatically using the DBSCAN algorithm. We demonstrate the technique by applying it over a 5 degree2 portion of the Next-Generation Virgo Survey (NGVS) data to reveal 53 low surface brightness galaxies that are candidate cluster members based on their sizes and colours. 30 of these sources are new detections despite the region being searched specifically for LSB galaxies previously. Our final sample contains galaxies with 26.0 ≤ ⟨μe⟩ ≤ 28.5 and 19 ≤ mg ≤ 21, making them some of the faintest known in Virgo. The majority of them have colours consistent with the red sequence, and have a mean stellar mass of 106.3 ± 0.5M⊙ assuming cluster membership. After using ProFit to fit Sérsic profiles to our detections, none of the new sources have effective radii larger than 1.5 Kpc and do not meet the criteria for ultra-diffuse galaxy (UDG) classification, so we classify them as ultra-faint dwarfs.

WISE Brown Dwarf Binaries: The Discovery of a T5+T5 and a T8.5+T9 System

NASA Astrophysics Data System (ADS)

Gelino, Christopher R.; Kirkpatrick, J. Davy; Cushing, Michael C.; Eisenhardt, Peter R.; Griffith, Roger L.; Mainzer, Amanda K.; Marsh, Kenneth A.; Skrutskie, Michael F.; Wright, Edward L.

2011-08-01

The multiplicity properties of brown dwarfs are critical empirical constraints for formation theories, while multiples themselves provide unique opportunities to test evolutionary and atmospheric models and examine empirical trends. Studies using high-resolution imaging cannot only uncover faint companions, but they can also be used to determine dynamical masses through long-term monitoring of binary systems. We have begun a search for the coolest brown dwarfs using preliminary processing of data from the Wide-field Infrared Survey Explorer and have confirmed many of the candidates as late-type T dwarfs. In order to search for companions to these objects, we are conducting observations using the Laser Guide Star Adaptive Optics system on Keck II. Here we present the first results of that search, including a T5 binary with nearly equal mass components and a faint companion to a T8.5 dwarf with an estimated spectral type of T9. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Detection of an Optical Counterpart to the ALFALFA Ultra-compact High-velocity Cloud AGC 249525

NASA Astrophysics Data System (ADS)

Janesh, William; Rhode, Katherine L.; Salzer, John J.; Janowiecki, Steven; Adams, Elizabeth A. K.; Haynes, Martha P.; Giovanelli, Riccardo; Cannon, John M.

2017-03-01

We report on the detection at >98% confidence of an optical counterpart to AGC 249525, an ultra-compact high-velocity cloud (UCHVC) discovered by the Arecibo Legacy Fast ALFA survey blind neutral hydrogen survey. UCHVCs are compact, isolated H I clouds with properties consistent with their being nearby low-mass galaxies, but without identified counterparts in extant optical surveys. Analysis of the resolved stellar sources in deep g- and I-band imaging from the WIYN pODI camera reveals a clustering of possible red giant branch stars associated with AGC 249525 at a distance of 1.64 ± 0.45 Mpc. Matching our optical detection with the H I synthesis map of AGC 249525 from Adams et al. shows that the stellar overdensity is exactly coincident with the highest-density H I contour from that study. Combining our optical photometry and the H I properties of this object yields an absolute magnitude of -7.1≤slant {M}V≤slant -4.5, a stellar mass between 2.2+/- 0.6× {10}4 {M}⊙ and 3.6+/- 1.0× {10}5 {M}⊙ , and an H I to stellar mass ratio between 9 and 144. This object has stellar properties within the observed range of gas-poor ultra-faint dwarfs in the Local Group, but is gas-dominated.

Evidence of enrichment by individual SN from elemental abundance ratios in the very metal-poor dSph galaxy Boötes I

NASA Astrophysics Data System (ADS)

Feltzing, S.; Eriksson, K.; Kleyna, J.; Wilkinson, M. I.

2009-12-01

Aims. We establish the mean metallicity from high-resolution spectroscopy for the recently found dwarf spheroidal galaxy Boötes I and test whether it is a common feature for ultra-faint dwarf spheroidal galaxies to show signs of inhomogeneous chemical evolution (e.g. as found in the Hercules dwarf spheroidal galaxy). Methods: We analyse high-resolution, moderate signal-to-noise spectra for seven red giant stars in the Boötes I dSph galaxy using standard abundance analysis techniques. In particular, we assume local thermodynamic equilibrium and employ spherical model atmospheres and codes that take the sphericity of the star into account when calculating the elemental abundances. Results: We confirm previous determinations of the mean metallicity of the Boötes I dwarf spheroidal galaxy to be -2.3 dex. Whilst five stars are clustered around this metallicity, one is significantly more metal-poor, at -2.9 dex, and one is more metal-rich at, -1.9 dex. Additionally, we find that one of the stars, Boo-127, shows an atypically high [Mg/Ca] ratio, indicative of stochastic enrichment processes within the dSph galaxy. Similar results have previously only been found in the Hercules and Draco dSph galaxies and appear, so far, to be unique to this type of galaxy. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

The Coma Cluster Luminosity Function from Ultraviolet to Near-Infrared

NASA Astrophysics Data System (ADS)

Andreon, S.; Cuillandre, J.-C.; Pello, R.

The Coma cluster luminosity function (LF) from ultraviolet (2000 AA ) to the near-infrared (H band) is summarized. In the UV the LF is very steep, much steeper than in the optical. The steep Coma UV LF implies that faint and bright galaxies give similar contributions to the total UV flux and to the total metal production rate. The ComaUV LF is dominated in number and luminosity by blue galaxies, which are often faint in the optical. Therefore the Coma UV LF is dominated by star forming galaxies, not by massive and large galaxies. The optical Coma LF is relatively steep (alpha=-1.4) over the 11 magnitudes sampled, but its slope and shape depend on considered filter and magnitude. We found a clear steeping of the FL going from B to R bands, indicative of the presence of a large number of red dwarfs, as faint as three bright globular clusters. Furthermore, using Hubble Space Telescope images, we discover that blends of globular clusters, not resolved in individual components due to seeing, look like dwarf galaxies when observed from the ground and are numerous and bright. The existence of these fake extended sources increases the steepness of the LF at faint magnitudes, if not deal on. This concern affects previous deep probing of the luminosity function, but not the present work. The near-infrared LF wa s computed on a near-infrared selected sample of galaxies which photometry is complete down to the typical dwarf (M* +5) luminosity. The Coma LF can be described by a Schechter function with intermediate slope (alpha sim-1.3), plus a dip at MH~-22 mag. The shape of the Coma LF in H band is quite similar to th e one found in the B band. The similarity of the LF in the optical and H bands implies that in the central region of Coma there is no new population of galaxies which is too faint to be observed in the optical band (because dust enshrouded, for instance), down to the magnitudes of dwarfs. The exponential cut of the LF at the bright end is in good agreement with the one derived from shallower near-infrared samples o f galaxies, both in clusters and in the field. The faint end of the LF, reaching MH~-19 mag (roughly MB~ -15), is steep, but less than previously suggested from shallower near-infrared observations of an adjacent region in the Coma cluster.

A Spectroscopic Survey of Lensed Dwarf Galaxies at 1

NASA Astrophysics Data System (ADS)

Alavi, Anahita; Siana, Brian; gburek, Timothy; Richard, Johan; Teplitz, Harry; Rafelski, Marc; Stark, Daniel P.; Anahita Alavi

2018-01-01

High-redshift dwarf galaxies (M<109 M⊙) are one of the primary targets of the James Web Space Telescope. Recent studies have suggested that these galaxies are different than their bright counterparts, as they follow a divergent evolutionary history of star formation. In our previous study, utilizing the magnification from massive clusters of galaxies (Hubble Frontier Fields), we found a large sample of dwarf star-forming galaxies at the peak epoch of star formation (1

Stellar Abundances for Galactic Archaeology Database. IV. Compilation of stars in dwarf galaxies

NASA Astrophysics Data System (ADS)

Suda, Takuma; Hidaka, Jun; Aoki, Wako; Katsuta, Yutaka; Yamada, Shimako; Fujimoto, Masayuki Y.; Ohtani, Yukari; Masuyama, Miyu; Noda, Kazuhiro; Wada, Kentaro

2017-10-01

We have constructed a database of stars in Local Group galaxies using the extended version of the SAGA (Stellar Abundances for Galactic Archaeology) database that contains stars in 24 dwarf spheroidal galaxies and ultra-faint dwarfs. The new version of the database includes more than 4500 stars in the Milky Way, by removing the previous metallicity criterion of [Fe/H] ≤ -2.5, and more than 6000 stars in the Local Group galaxies. We examined the validity of using a combined data set for elemental abundances. We also checked the consistency between the derived distances to individual stars and those to galaxies as given in the literature. Using the updated database, the characteristics of stars in dwarf galaxies are discussed. Our statistical analyses of α-element abundances show that the change of the slope of the [α/Fe] relative to [Fe/H] (so-called "knee") occurs at [Fe/H] = -1.0 ± 0.1 for the Milky Way. The knee positions for selected galaxies are derived by applying the same method. The star formation history of individual galaxies is explored using the slope of the cumulative metallicity distribution function. Radial gradients along the four directions are inspected in six galaxies where we find no direction-dependence of metallicity gradients along the major and minor axes. The compilation of all the available data shows a lack of CEMP-s population in dwarf galaxies, while there may be some CEMP-no stars at [Fe/H] ≲ -3 even in the very small sample. The inspection of the relationship between Eu and Ba abundances confirms an anomalously Ba-rich population in Fornax, which indicates a pre-enrichment of interstellar gas with r-process elements. We do not find any evidence of anti-correlations in O-Na and Mg-Al abundances, which characterizes the abundance trends in the Galactic globular clusters.

Deep Subaru Hyper Suprime-Cam Observations of Milky Way Satellites Columba I and Triangulum II

NASA Astrophysics Data System (ADS)

Carlin, Jeffrey L.; Sand, David J.; Muñoz, Ricardo R.; Spekkens, Kristine; Willman, Beth; Crnojević, Denija; Forbes, Duncan A.; Hargis, Jonathan; Kirby, Evan; Peter, Annika H. G.; Romanowsky, Aaron J.; Strader, Jay

2017-12-01

We present deep, wide-field Subaru Hyper Suprime-Cam photometry of two recently discovered satellites of the Milky Way (MW): Columba I (Col I) and Triangulum II (Tri II). The color-magnitude diagrams of both objects point to exclusively old and metal-poor stellar populations. We re-derive structural parameters and luminosities of these satellites, and find {M}{{V},{Col}{{I}}}=-4.2+/- 0.2 for Col I and {M}{{V},{Tri}{II}}=-1.2+/- 0.4 for Tri II, with corresponding half-light radii of {r}{{h},{Col}{{I}}}=117+/- 17 pc and {r}{{h},{Tri}{II}}=21+/- 4 pc. The properties of both systems are consistent with observed scaling relations for MW dwarf galaxies. Based on archival data, we derive upper limits on the neutral gas content of these dwarfs, and find that they lack H I, as do the majority of observed satellites within the MW virial radius. Neither satellite shows evidence of tidal stripping in the form of extensions or distortions in matched-filter stellar density maps or surface-density profiles. However, the smaller Tri II system is relatively metal-rich for its luminosity (compared to other MW satellites), possibly because it has been tidally stripped. Through a suite of orbit simulations, we show that Tri II is approaching pericenter of its eccentric orbit, a stage at which tidal debris is unlikely to be seen. In addition, we find that Tri II may be on its first infall into the MW, which helps explain its unique properties among MW dwarfs. Further evidence that Tri II is likely an ultra-faint dwarf comes from its stellar mass function, which is similar to those of other MW dwarfs. Based in part on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

Young, metal-enriched cores in early-type dwarf galaxies in the Virgo cluster based on colour gradients

NASA Astrophysics Data System (ADS)

Urich, Linda; Lisker, Thorsten; Janz, Joachim; van de Ven, Glenn; Leaman, Ryan; Boselli, Alessandro; Paudel, Sanjaya; Sybilska, Agnieszka; Peletier, Reynier F.; den Brok, Mark; Hensler, Gerhard; Toloba, Elisa; Falcón-Barroso, Jesús; Niemi, Sami-Matias

2017-10-01

Early-type dwarf galaxies are not simply featureless, old objects, but were found to be much more diverse, hosting substructures and a variety of stellar population properties. To explore the stellar content of faint early-type galaxies, and to investigate in particular those with recent central star formation, we study colours and colour gradients within one effective radius in optical (g - r) and near-infrared (I - H) bands for 120 Virgo cluster early-type galaxies with - 19 mag

Chemical Abundances of New Member Stars in the Tucana II Dwarf Galaxy

NASA Astrophysics Data System (ADS)

Chiti, Anirudh; Frebel, Anna; Ji, Alexander P.; Jerjen, Helmut; Kim, Dongwon; Norris, John E.

2018-04-01

We present chemical abundance measurements for seven stars with metallicities ranging from Fe/H] = ‑3.3 to [Fe/H] = ‑2.4 in the Tucana II ultra-faint dwarf galaxy (UFD), based on high-resolution spectra obtained with the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. For three stars, we present detailed chemical abundances for the first time. Of those, two stars are newly discovered members of Tucana II and were selected as probable members from deep narrowband photometry of the Tucana II UFD taken with the SkyMapper telescope. This result demonstrates the potential for photometrically identifying members of dwarf galaxy systems based on chemical composition. One new star was selected from the membership catalog of Walker et al. The other four stars in our sample have been reanalyzed, following additional observations. Overall, six stars have chemical abundances that are characteristic of the UFD stellar population. The seventh star shows chemical abundances that are discrepant from the other Tucana II members and an atypical, higher strontium abundance than what is expected for typical UFD stars. While unlikely, its strontium abundance raises the possibility that it may be a foreground metal-poor halo star with the same systemic velocity as Tucana II. If we were to exclude this star, Tucana II would satisfy the criteria to be a surviving first galaxy. Otherwise, this star implies that Tucana II has likely experienced somewhat extended chemical evolution. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

A supermassive black hole in an ultra-compact dwarf galaxy.

PubMed

Seth, Anil C; van den Bosch, Remco; Mieske, Steffen; Baumgardt, Holger; den Brok, Mark; Strader, Jay; Neumayer, Nadine; Chilingarian, Igor; Hilker, Michael; McDermid, Richard; Spitler, Lee; Brodie, Jean; Frank, Matthias J; Walsh, Jonelle L

2014-09-18

Ultra-compact dwarf galaxies are among the densest stellar systems in the Universe. These systems have masses of up to 2 × 10(8) solar masses, but half-light radii of just 3-50 parsecs. Dynamical mass estimates show that many such dwarfs are more massive than expected from their luminosity. It remains unclear whether these high dynamical mass estimates arise because of the presence of supermassive black holes or result from a non-standard stellar initial mass function that causes the average stellar mass to be higher than expected. Here we report adaptive optics kinematic data of the ultra-compact dwarf galaxy M60-UCD1 that show a central velocity dispersion peak exceeding 100 kilometres per second and modest rotation. Dynamical modelling of these data reveals the presence of a supermassive black hole with a mass of 2.1 × 10(7) solar masses. This is 15 per cent of the object's total mass. The high black hole mass and mass fraction suggest that M60-UCD1 is the stripped nucleus of a galaxy. Our analysis also shows that M60-UCD1's stellar mass is consistent with its luminosity, implying a large population of previously unrecognized supermassive black holes in other ultra-compact dwarf galaxies.

Formation of Compact Ellipticals in the merging star cluster scenario

NASA Astrophysics Data System (ADS)

Urrutia Zapata, Fernanda Cecilia; Theory and star formation group

2018-01-01

In the last years, extended old stellar clusters have been observed. They are like globular clusters (GCs) but with larger sizes(a limit of Re=10 pc is currently seen as reasonable). These extended objects (EOs) cover a huge range of mass. Objects at the low mass end with masses comparable to normal globular clusters are called extended clusters or faint fuzzies Larsen & Brodie (2000) and objects at the high-mass end are called ultra compact dwarf galaxies (UCDs). Ultra compact dwarf galaxies are compact object with luminositys above the brigtest known GCs. UCDs are more compact than typical dwarf galaxies but with comparable luminosities. Usually, a lower mass limit of 2 × 10^6 Solar masses is applied.Fellhauer & Kroupa (2002a,b) demostrated that object like ECs, FFs and UCDs can be the remnants of the merger of star clusters complexes, this scenario is called the Merging Star Cluster Scenario. Amore concise study was performed by Bruens et al. (2009, 2011).Our work tries to explain the formation of compact elliptical(cE). These objects are a comparatively rare class of spheroidal galaxies, possessing very small Re and high central surface brightnesses (Faber 1973). cEs have the same parameters as extended objects but they are slightly larger than 100 pc and the luminosities are in the range of -11 to -12 Mag.The standard formation sceanrio of these systems proposes a galaxy origin. CEs are the result of tidal stripping and truncation of nucleated larger systems. Or they could be a natural extension of the class of elliptical galaxies to lower luminosities and smaller sizes.We want to propose a completely new formation scenario for cEs. In our project we try to model cEs in a similar way that UCDs using the merging star cluster scenario extended to much higher masses and sizes. We think that in the early Universe we might have produced sufficiently strong star bursts to form cluster complexes which merge into cEs. So far it is observationally unknown if cEs are dark matter dominated objects. If our scenario is true, then they would be dark matter free very extended and massive "star clusters".

Get Ready for Gaia: Cool White Dwarfs in Common Proper Motion with Tycho Stars

NASA Astrophysics Data System (ADS)

Hambly, N.; Rowell, N.; Lam, M.

2017-03-01

We discuss the Gaia Data Release 1 (September 2016) and preliminary work on maximising the benefit for cool white dwarf (WD) science in advance of the full parallax catalogue which will appear around one year later in DR2. The Tycho catalogue is used in conjunction with the all-sky ground based astrometric/ photometric SuperCOSMOS Sky Survey in order to identify candidate faint common proper motion objects to the Tycho stars. Gaia DR1 is supplemented by the Tycho-Gaia Astrometric Solution catalogue containing some 2 million parallaxes with Hipparcos-like precision for Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples are much rarer (if present at all) and CPM offers one method to infer precision distances for a statistically useful sample of these very faint WDs.

Mapping out the origins of compact stellar systems

NASA Astrophysics Data System (ADS)

Romanowsky, Aaron J.; Brodie, Jean P.; SAGES Collaboration

2017-03-01

We present a suite of extragalactic explorations of the origins and nature of globular clusters (GCs) and ultra-compact dwarfs (UCDs), and the connections between them. An example of GC metallicity bimodality is shown to reflect underlying, distinct metal-poor and metal-rich stellar halo populations. Metallicity-matching methods are used to trace the birth sites and epochs of GCs in giant E/S0s, pointing to clumpy disk galaxies at z ~ 3 for the metal-rich GCs, and to a combination of accreted and in-situ formation modes at z ~ 5-6 for the metal-poor GCs. An increasingly diverse zoo of compact stellar systems is being discovered, including objects that bridge the gaps between UCDs and faint fuzzies, and between UCDs and compact ellipticals. Many of these have properties pointing to origins as the stripped nuclei of larger galaxies, and a smoking-gun example is presented of an ω Cen-like star cluster embedded in a tidal stream.

The T dwarf population in the UKIDSS LAS .

NASA Astrophysics Data System (ADS)

Cardoso, C. V.; Burningham, B.; Smith, L.; Smart, R.; Pinfield, D.; Magazzù, A.; Ghinassi, F.; Lattanzi, M.

We present the most recent results from the UKIDSS Large Area Survey (LAS) census and follow up of new T brown dwarfs in the local field. The new brown dwarf candidates are identified using optical and infrared survey photometry (UKIDSS and SDSS) and followed up with narrow band methane photometry (TNG) and spectroscopy (Gemini and Subaru) to confirm their brown dwarf nature. Employing this procedure we have discovered several dozens of new T brown dwarfs in the field. Using methane differential photometry as a proxy for spectral type for T brown dwarfs has proved to be a very efficient technique. This method can be useful in the future to reliably identify brown dwarfs in deep surveys that produce large samples of faint targets where spectroscopy is not feasible for all candidates. With this statistical robust sample of the mid and late T brown dwarf field population we were also able to address the discrepancies between the observed field space density and the expected values given the most accepted forms of the IMF of young clusters.

The U.S. survey for faint blue objects

NASA Technical Reports Server (NTRS)

Mitchell, K. J.; Howell, S. B.; Usher, P. D.

1987-01-01

A spectrophotometric study of the blue and UV-excess starlike objects in the U.S. survey (Usher, 1981) has been conducted. Observations were obtained with a resolution of about 20 A over the 3500-7000-A wavelength range. Considered within the sample are 42 DA white dwarfs, 4 DB/DO white dwarfs, 13 subdwarf B stars, 12 subdwarf O stars, and 13 horizontal branch stars. The sample is analyzed using numerical convolution photometry.

A coronagraphic search for brown dwarfs around nearby stars

NASA Technical Reports Server (NTRS)

Nakajima, T.; Durrance, S. T.; Golimowski, D. A.; Kulkarni, S. R.

1994-01-01

Brown dwarf companions have been searched for around stars within 10 pc of the Sun using the Johns-Hopkins University Adaptive Optics Coronagraph (AOC), a stellar coronagraph with an image stabilizer. The AOC covers the field around the target star with a minimum search radius of 1 sec .5 and a field of view of 1 arcmin sq. We have reached an unprecedented dynamic range of Delta m = 13 in our search for faint companions at I band. Comparison of our survey with other brown dwarf searches shows that the AOC technique is unique in its dynamic range while at the same time just as sensitive to brown dwarfs as the recent brown dwarf surveys. The present survey covered 24 target stars selected from the Gliese catalog. A total of 94 stars were detected in 16 fields. The low-latitude fields are completely dominated by background star contamination. Kolmogorov-Smirnov tests were carried out for a sample restricted to high latitudes and a sample with small angular separations. The high-latitude sample (b greater than or equal to 44 deg) appears to show spatial concentration toward target stars. The small separation sample (Delta Theta less than 20 sec) shows weaker dependence on Galactic coordinates than field stars. These statistical tests suggest that both the high-latitude sample and the small separation sample can include a substantial fraction of true companions. However, the nature of these putative companions is mysterious. They are too faint to be white dwarfs and too blue for brown dwarfs. Ignoring the signif icance of the statistical tests, we can reconcile most of the detections with distant main-sequence stars or white dwarfs except for a candidate next to GL 475. Given the small size of our sample, we conclude that considerably more targets need to be surveyed before a firm conclusion on the possibility of a new class of companions can be made.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Herrmann, Kimberly A.; Hunter, Deidre A.; Elmegreen, Bruce G., E-mail: kah259@psu.edu, E-mail: dah@lowell.edu, E-mail: bge@us.ibm.com

In this second paper of a series, we explore the B  −  V , U  −  B , and FUV−NUV radial color trends from a multi-wavelength sample of 141 dwarf disk galaxies. Like spirals, dwarf galaxies have three types of radial surface brightness profiles: (I) single exponential throughout the observed extent (the minority), (II) down-bending (the majority), and (III) up-bending. We find that the colors of (1) Type I dwarfs generally become redder with increasing radius, unlike spirals which have a blueing trend that flattens beyond ∼1.5 disk scale lengths, (2) Type II dwarfs come in six different “flavors,” one of whichmore » mimics the “U” shape of spirals, and (3) Type III dwarfs have a stretched “S” shape where the central colors are flattish, become steeply redder toward the surface brightness break, then remain roughly constant beyond, which is similar to spiral Type III color profiles, but without the central outward bluing. Faint (−9 >  M{sub B}  > −14) Type II dwarfs tend to have continuously red or “U” shaped colors and steeper color slopes than bright (−14 >  M{sub B}  > −19) Type II dwarfs, which additionally have colors that become bluer or remain constant with increasing radius. Sm dwarfs and BCDs tend to have at least some blue and red radial color trend, respectively. Additionally, we determine stellar surface mass density (Σ) profiles and use them to show that the break in Σ generally remains in Type II dwarfs (unlike Type II spirals) but generally disappears in Type III dwarfs (unlike Type III spirals). Moreover, the break in Σ is strong, intermediate, and weak in faint dwarfs, bright dwarfs, and spirals, respectively, indicating that Σ may straighten with increasing galaxy mass. Finally, the average stellar surface mass density at the surface brightness break is roughly 1−2  M {sub ⊙} pc{sup −2} for Type II dwarfs but higher at 5.9  M {sub ⊙} pc{sup −2} or 27  M {sub ⊙} pc{sup −2} for Type III BCDs and dIms, respectively.« less

Modeling Neutron stars as r-process sources in Ultra Faint Dwarf galaxies

NASA Astrophysics Data System (ADS)

Safarzadeh, Mohammadtaher; Scannapieco, Evan

2018-06-01

To explain the high observed abundances of r-process elements in local ultrafaint dwarf (UFD) galaxies, we perform cosmological zoom simulations that include r-process production from neutron star mergers (NSMs). We model star formation stochastically and simulate two different haloes with total masses ≈108 M⊙ at z = 6. We find that the final distribution of [Eu/H] versus [Fe/H] is relatively insensitive to the energy by which the r-process material is ejected into the interstellar medium, but strongly sensitive to the environment in which the NSM event occurs. In one halo, the NSM event takes place at the centre of the stellar distribution, leading to high levels of r-process enrichment such as seen in a local UFD, Reticulum II (Ret II). In a second halo, the NSM event takes place outside of the densest part of the galaxy, leading to a more extended r-process distribution. The subsequent star formation occurs in an interstellar medium with shallow levels of r-process enrichment that results in stars with low levels of [Eu/H] compared to Ret II stars even when the maximum possible r-process mass is assumed to be ejected. This suggests that the natal kicks of neutron stars may also play an important role in determining the r-process abundances in UFD galaxies, a topic that warrants further theoretical investigation.

Deep CCD Photometry of the Rich Galaxy Cluster Abel 1656 Characteristics of the Dwarf Elliptical Galaxy Population in the Cluster Core

NASA Astrophysics Data System (ADS)

Secker, Jeffrey Alan

1995-01-01

We have developed a statistically rigorous and automated method to implement the detection, photometry and classification of faint objects on digital images. We use these methods to analyze deep R- and B-band CCD images of the central ~ 700 arcmin ^2 of the Coma cluster core, and an associated control field. We have detected and measured total R magnitudes and (B-R) colors for a sample of 3741 objects on the galaxy cluster fields, and 1164 objects on a remote control field, complete to a limiting magnitude of R = 22.5 mag. The typical uncertainties are +/- 0.06 and +/-0.12 mag in total magnitude and color respectively. The dwarf elliptical (dE) galaxies are confined to a well-defined sequence in the color range given by 0.7<= (B-R)<= 1.9 mag: within this interval there are 2535 dE candidates on our fields in the cluster core, and 694 objects on the control field. With an image scale of 0.53 arcsec/pixel and seeing near 1.2 arcsec, a large fraction of the dE galaxy candidates are resolved. We find a significant metallicity gradient in the radial distribution of the dwarf elliptical galaxies, which goes as Z~ R^{-0.32 } outwards from the cluster center at NGC 4874. As well, there is a strong color-luminosity correlation, in the sense that more luminous dE galaxies are redder in the mean. These effects give rise to a radial variation in the cluster luminosity function. The spatial distribution of the faint dE galaxies is well fit by a standard King model with a central surface density of Sigma _0 = 1.44 dEs arcmin^{ -2}, a core radius R_{ rm c} = 18.7 arcmin (~eq 0.44 Mpc), and a tidal radius of 1.44 deg ( ~eq 2.05 Mpc). This core is significantly larger than R_{rm c} = 12.3 arcmin (~eq 0.29 Mpc) found for the bright cluster galaxies. The composite luminosity function for Coma galaxies is modeled as the sum of a log -normal distribution for the giant galaxies and a Schechter function for the dwarf elliptical galaxies, with a faint -end slope of alpha = -1.41, consistent with known faint-end slopes for the Virgo and Fornax clusters. The early-type dwarf-to-giant ratio for the Coma cluster core is consistent with that of the Virgo cluster, and thus with the rich Coma cluster being formed as the merger of multiple less-rich galaxy clusters.

THE KEPLER LIGHT CURVE OF THE UNIQUE DA WHITE DWARF BOKS 53856

DOE Office of Scientific and Technical Information (OSTI.GOV)

Holberg, J. B.; Howell, Steve B., E-mail: holberg@argus.lpl.arizona.edu, E-mail: howell@noao.edu

2011-08-15

The faint (g = 16.9) hot white dwarf BOKS 53856 was observed by the Kepler Mission in short cadence mode during mid-2009. Analysis of these observations reveals a highly stable modulation with a period of 6.1375 hr and a 2.46% half-amplitude. The folded light curve has an unusual shape that is difficult to explain in terms of a binary system containing an unseen companion more luminous than an L0 brown dwarf. Optical spectra of BOKS 53856 show a T{sub eff} = 34,000 K, log g = 8.0 DA white dwarf. There are few, if any, known white dwarfs in thismore » temperature range exhibiting photometric variations similar to those we describe. A magnetic spin-modulated white dwarf model can in principle explain the light curve, an interpretation supported by spectral observations of the H{alpha} line showing evidence of Zeeman splitting.« less

A possible formation scenario for dwarf spheroidal galaxies - III. Adding star formation histories to the fiducial model

NASA Astrophysics Data System (ADS)

Alarcón Jara, A. G.; Fellhauer, M.; Matus Carrillo, D. R.; Assmann, P.; Urrutia Zapata, F.; Hazeldine, J.; Aravena, C. A.

2018-02-01

Dwarf spheroidal galaxies are regarded as the basic building blocks in the formation of larger galaxies and are the most dark matter dominated systems in the Universe, known so far. There are several models that attempt to explain their formation and evolution, but they have problems modelling the formation of isolated dwarf spheroidal galaxies. Here, we will explain a possible formation scenario in which star clusters form inside the dark matter halo of a dwarf spheroidal galaxy. These star clusters suffer from low star formation efficiency and dissolve while orbiting inside the dark matter halo. Thereby, they build the faint luminous components that we observe in dwarf spheroidal galaxies. In this paper, we study this model by adding different star formation histories to the simulations and compare the results with our previous work and observational data to show that we can explain the formation of dwarf spheroidal galaxies.

Globular clusters in high-redshift dwarf galaxies: a case study from the Local Group

NASA Astrophysics Data System (ADS)

Zick, Tom O.; Weisz, Daniel R.; Boylan-Kolchin, Michael

2018-06-01

We present the reconstructed evolution of rest-frame ultraviolet (UV) luminosities of the most massive Milky Way dwarf spheroidal satellite galaxy, Fornax, and its five globular clusters (GCs) across redshift, based on analysis of the stellar fossil record and stellar population synthesis modelling. We find that (1) Fornax's (proto-)GCs can generate 10-100 times more UV flux than the field population, despite comprising <˜{5} per cent of the stellar mass at the relevant redshifts; (2) due to their respective surface brightnesses, it is more likely that faint, compact sources in the Hubble Frontier Fields (HFFs) are GCs hosted by faint galaxies, than faint galaxies themselves. This may significantly complicate the construction of a galaxy UV luminosity function at z > 3. (3) GC formation can introduce order-of-magnitude errors in abundance matching. We also find that some compact HFF objects are consistent with the reconstructed properties of Fornax's GCs at the same redshifts (e.g. surface brightness, star formation rate), suggesting we may have already detected proto-GCs in the early Universe. Finally, we discuss the prospects for improving the connections between local GCs and proto-GCs detected in the early Universe.

Tidal stripping and the structure of dwarf galaxies in the Local Group

NASA Astrophysics Data System (ADS)

Fattahi, Azadeh; Navarro, Julio F.; Frenk, Carlos S.; Oman, Kyle A.; Sawala, Till; Schaller, Matthieu

2018-05-01

The shallow faint-end slope of the galaxy mass function is usually reproduced in Λ cold dark matter (ΛCDM) galaxy formation models by assuming that the fraction of baryons that turn into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities Vmax < 20-30 km s-1. Dark-matter-dominated dwarfs should therefore have characteristic velocities of about that value, unless they are small enough to probe only the rising part of the halo circular velocity curve (i.e. half-mass radii, r1/2 ≪ 1 kpc). Many dwarfs have properties in disagreement with this prediction: they are large enough to probe their halo Vmax but their characteristic velocities are well below 20 km s-1. These `cold faint giants' (an extreme example is the recently discovered Crater 2 Milky Way satellite) can only be reconciled with our ΛCDM models if they are the remnants of once massive objects heavily affected by tidal stripping. We examine this possibility using the APOSTLE cosmological hydrodynamical simulations of the Local Group. Assuming that low-velocity-dispersion satellites have been affected by stripping, we infer their progenitor masses, radii, and velocity dispersions, and find them in remarkable agreement with those of isolated dwarfs. Tidal stripping also explains the large scatter in the mass discrepancy-acceleration relation in the dwarf galaxy regime: tides remove preferentially dark matter from satellite galaxies, lowering their accelerations below the amin ˜ 10-11 m s-2 minimum expected for isolated dwarfs. In many cases, the resulting velocity dispersions are inconsistent with the predictions from Modified Newtonian Dynamics, a result that poses a possibly insurmountable challenge to that scenario.

Evidence for a New Class of Extreme Ultraviolet Sources

NASA Technical Reports Server (NTRS)

Maoz, Dan; Ofek, Eran O.; Shemi, Amotz

1997-01-01

Most of the sources detected in the extreme ultraviolet (EUV; 100-600 A) by the ROSAT/WFC and EUVE all-sky surveys have been identified with active late-type stars and hot white dwarfs that are near enough to the Earth to escape absorption by interstellar gas. However, about 15 per cent of EUV sources are as yet unidentified with any optical counterparts. We examine whether the unidentified EUV sources may consist of the same population of late-type stars and white dwarfs. We present B and R photometry of stars in the fields of seven of the unidentified EUV sources. We detect in the optical the entire main-sequence and white dwarf population out to the greatest distances where they could still avoid absorption. We use color-magnitude diagrams to demonstrate that, in most of the fields, none of the observed stars has the colours and magnitudes of late-type dwarfs at distances less than 100 pc. Similarly, none of the observed stars is a white dwarf within 500 pc that is hot enough to be a EUV emitter. The unidentified EUV sources we study are not detected in X-rays, while cataclysmic variables, X-ray binaries, and active galactic nuclei generally are. We conclude that some of the EUV sources may be a new class of nearby objects, which are either very faint at optical bands or which mimic the colours and magnitudes of distant late-type stars or cool white dwarfs. One candidate for optically faint objects is isolated old neutron stars, slowly accreting interstellar matter. Such neutron stars are expected to be abundant in the Galaxy, and have not been unambiguously detected.

The Most Metal-poor Stars in the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Schlaufman, Kevin C.

2018-06-01

The chemical abundances of the most metal-poor stars in a galaxy can be used to investigate the earliest stages of its formation and chemical evolution. Differences between the abundances of the most metal-poor stars in the Milky Way and in its satellite dwarf galaxies have been noted and provide the strongest available constraints on the earliest stages of general galactic chemical evolution models. However, the masses of the Milky Way and its satellite dwarf galaxies differ by four orders of magnitude, leaving a gap in our knowledge of the early chemical evolution of intermediate mass galaxies like the Magellanic Clouds. To close that gap, we have initiated a survey of the metal-poor stellar populations of the Magellanic Clouds using the mid-infrared metal-poor star selection of Schlaufman & Casey (2014). We have discovered the three most metal-poor giant stars known in the Large Magellanic Cloud (LMC) and reobserved the previous record holder. The stars have metallicities in the range -2.70 < [Fe/H] < -2.00 and three show r-process enhancement: one has [Eu II/Fe] = +1.65 and two others have [Eu II/Fe] = +0.65. The probability that four randomly selected very metal-poor stars in the halo of the Milky Way are as r-process enhanced is 0.0002. For that reason, the early chemical enrichment of the heaviest elements in the LMC and Milky Way were qualitatively different. It is also suggestive of a possible chemical link between the LMC and the ultra-faint dwarf galaxies nearby with evidence of r-process enhancement (e.g., Reticulum II and Tucana III). Like Reticulum II, the most metal-poor star in our LMC sample is the only one not enhanced in r-process elements.

The upper bound on the lowest mass halo

NASA Astrophysics Data System (ADS)

Jethwa, P.; Erkal, D.; Belokurov, V.

2018-01-01

We explore the connection between galaxies and dark matter haloes in the Milky Way (MW) and quantify the implications on properties of the dark matter particle and the phenomenology of low-mass galaxy formation. This is done through a probabilistic comparison of the luminosity function of MW dwarf satellite galaxies to models based on two suites of zoom-in simulations. One suite is dark-matter-only, while the other includes a disc component, therefore we can quantify the effect of the MW's baryonic disc on our results. We apply numerous stellar-mass-halo-mass (SMHM) relations allowing for multiple complexities: scatter, a characteristic break scale, and subhaloes which host no galaxy. In contrast to previous works, we push the model/data comparison to the faintest dwarfs by modelling observational incompleteness, allowing us to draw three new conclusions. First, we constrain the SMHM relation for 102 < M*/ M⊙ < 108 galaxies, allowing us to bound the peak halo mass of the faintest MW satellite to Mvir > 2.4 × 108 M⊙ (1σ). Secondly, by translating to a warm dark matter (WDM) cosmology, we bound the thermal relic mass mWDM > 2.9 keV at 95 per cent confidence, on a par with recent constraints from the Lyman-α forest. Lastly, we find that the observed number of ultra-faint MW dwarfs is in tension with the theoretical prediction that reionization prevents galaxy formation in almost all 108 M⊙ haloes. This can be tested with the next generation of deep imaging surveys. To this end, we predict the likely number of detectable satellite galaxies in the Subaru/Hyper Suprime-Cam survey and the Large Synoptic Survey Telescope. Confronting these predictions with future observations will be amongst our strongest tests of WDM and the effect reionization on low-mass systems.

Ultra-high energy cosmic rays from white dwarf pulsars and the Hillas criterion

NASA Astrophysics Data System (ADS)

Lobato, Ronaldo V.; Coelho, Jaziel G.; Malheiro, M.

2017-06-01

The origins of ultra-high-energy cosmic rays (E ≳ 1019 eV) are a mystery and still under debate in astroparticle physics. In recent years some efforts were made to understand their nature. In this contribution we consider the possibility of Some Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) beeing white dwarf pulsars, and show that these sources can achieve large electromagnetic potentials on their surface that accelerate particle almost at the speed of light, with energies E ~ 1020-21 eV. The sources SGRs/AXPs considered as highly magnetized white dwarfs are well described in the Hillas diagram, lying close to the AR Sorpii and AE Aquarii which are understood as white dwarf pulsars.

An Intermediate-Mass Black Hole in the Dwarf Galaxy Pox 52

NASA Astrophysics Data System (ADS)

Barth, Aaron

Do dwarf elliptical and dwarf spiral galaxies contain central black holes with masses below 106 solar masses? Beyond the Local Group dynamical searches for black holes in this mass range are very difficult but the detection of accretion-powered nuclear activity could be used to infer the presence of a black hole. The nearby dwarf spiral galaxy NGC 4395 hosts a faint Seyfert 1 nucleus with a likely black hole mass in the range 104-105 solar masses and for more than a decade it has been the only known example of a Seyfert 1 nucleus in a dwarf galaxy. I will present new Keck spectra of the dwarf galaxy POX 52 which demonstrate that it has a Seyfert 1 spectrum nearly identical to that of NGC 4395. Its velocity dispersion is 37 km/s suggesting a possible black hole mass of order 105 solar masses. I will discuss the prospects for systematic searches for nuclear activity in dwarf galaxies and the implications for black hole demographics.

Quantifying the abundance of faint, low-redshift satellite galaxies in the COSMOS survey

NASA Astrophysics Data System (ADS)

Xi, ChengYu; Taylor, James E.; Massey, Richard J.; Rhodes, Jason; Koekemoer, Anton; Salvato, Mara

2018-06-01

Faint dwarf satellite galaxies are important as tracers of small-scale structure, but remain poorly characterized outside the Local Group, due to the difficulty of identifying them consistently at larger distances. We review a recently proposed method for estimating the average satellite population around a given sample of nearby bright galaxies, using a combination of size and magnitude cuts (to select low-redshift dwarf galaxies preferentially) and clustering measurements (to estimate the fraction of true satellites in the cut sample). We test this method using the high-precision photometric redshift catalog of the COSMOS survey, exploring the effect of specific cuts on the clustering signal. The most effective of the size-magnitude cuts considered recover the clustering signal around low-redshift primaries (z < 0.15) with about two-thirds of the signal and 80% of the signal-to-noise ratio obtainable using the full COSMOS photometric redshifts. These cuts are also fairly efficient, with more than one third of the selected objects being clustered satellites. We conclude that structural selection represents a useful tool in characterizing dwarf populations to fainter magnitudes and/or over larger areas than are feasible with spectroscopic surveys. In reviewing the low-redshift content of the COSMOS field, we also note the existence of several dozen objects that appear resolved or partially resolved in the HST imaging, and are confirmed to be local (at distances of ˜250 Mpc or less) by their photometric or spectroscopic redshifts. This underlines the potential for future space-based surveys to reveal local populations of intrinsically faint galaxies through imaging alone.

A PECULIAR FAINT SATELLITE IN THE REMOTE OUTER HALO OF M31

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mackey, A. D.; Dotter, A.; Huxor, A. P.

2013-06-20

We present Hubble Space Telescope imaging of a newly discovered faint stellar system, PAndAS-48, in the outskirts of the M31 halo. Our photometry reveals this object to be comprised of an ancient and very metal-poor stellar population with age {approx}> 10 Gyr and [Fe/H] {approx}< -2.3. Our inferred distance modulus (m - M){sub 0} = 24.57 {+-} 0.11 confirms that PAndAS-48 is most likely a remote M31 satellite with a three-dimensional galactocentric radius of 149{sup +19}{sub -8} kpc. We observe an apparent spread in color on the upper red giant branch that is larger than the photometric uncertainties should allow,more » and briefly explore the implications of this. Structurally, PAndAS-48 is diffuse, faint, and moderately flattened, with a half-light radius r{sub h}=26{sup +4}{sub -3} pc, integrated luminosity M{sub V} = -4.8 {+-} 0.5, and ellipticity {epsilon}=0.30{sup +0.08}{sub -0.15}. On the size-luminosity plane it falls between the extended globular clusters seen in several nearby galaxies and the recently discovered faint dwarf satellites of the Milky Way; however, its characteristics do not allow us to unambiguously classify it as either type of system. If PAndAS-48 is a globular cluster then it is among the most elliptical, isolated, and metal-poor of any seen in the Local Group, extended or otherwise. Conversely, while its properties are generally consistent with those observed for the faint Milky Way dwarfs, it would be a factor of {approx}2-3 smaller in spatial extent than any known counterpart of comparable luminosity.« less

A faint type of supernova from a white dwarf with a helium-rich companion.

PubMed

Perets, H B; Gal-Yam, A; Mazzali, P A; Arnett, D; Kagan, D; Filippenko, A V; Li, W; Arcavi, I; Cenko, S B; Fox, D B; Leonard, D C; Moon, D-S; Sand, D J; Soderberg, A M; Anderson, J P; James, P A; Foley, R J; Ganeshalingam, M; Ofek, E O; Bildsten, L; Nelemans, G; Shen, K J; Weinberg, N N; Metzger, B D; Piro, A L; Quataert, E; Kiewe, M; Poznanski, D

2010-05-20

Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The 'old' environment near the supernova location, and the very low derived ejected mass ( approximately 0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive (44)Ti.

OGLE-2013-BLG-0132Lb and OGLE-2013-BLG-1721Lb: Two Saturn-mass Planets Discovered around M-dwarfs

NASA Astrophysics Data System (ADS)

Mróz, Przemek; Udalski, A.; Bond, I. A.; Skowron, J.; Sumi, T.; Han, C.; Szymański, M. K.; Soszyński, I.; Poleski, R.; Pietrukowicz, P.; Kozłowski, S.; Wyrzykowski, Ł.; Ulaczyk, K.; OGLE Collaboration; Abe, F.; Asakura, Y.; Barry, R. K.; Bennett, D. P.; Bhattacharya, A.; Donachie, M.; Evans, P.; Fukui, A.; Hirao, Y.; Itow, Y.; Koshimoto, N.; Li, M. C. A.; Ling, C. H.; Masuda, K.; Matsubara, Y.; Muraki, Y.; Nagakane, M.; Ohnishi, K.; Ranc, C.; Rattenbury, N. J.; Saito, To.; Sharan, A.; Sullivan, D. J.; Suzuki, D.; Tristram, P. J.; Yamada, T.; Yamada, T.; Yonehara, A.; The MOA Collaboration

2017-11-01

We present the discovery of two planetary systems consisting of a Saturn-mass planet orbiting an M-dwarf, which were detected in faint microlensing events OGLE-2013-BLG-0132 and OGLE-2013-BLG-1721. The planetary anomalies were covered with high cadence by Optical Gravitational Lensing Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA) photometric surveys. The light curve modeling indicates that the planet-to-host mass ratios are (5.15+/- 0.28)× {10}-4 and (13.18+/- 0.72)× {10}-4, respectively. Both events were too short and too faint to measure a reliable parallax signal and hence the lens mass. We therefore used a Bayesian analysis to estimate the masses of both planets: {0.29}-0.13+0.16 {M}{Jup} (OGLE-2013-BLG-0132Lb) and {0.64}-0.31+0.35 {M}{Jup} (OGLE-2013-BLG-1721Lb). Thanks to a high relative proper motion, OGLE-2013-BLG-0132 is a promising candidate for the high-resolution imaging follow-up. Both planets belong to an increasing sample of sub-Jupiter-mass planets orbiting M-dwarfs beyond the snow line.

DISCOVERY OF A CLOSE PAIR OF FAINT DWARF GALAXIES IN THE HALO OF CENTAURUS A

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crnojević, D.; Sand, D. J.; Caldwell, N.

2014-11-10

As part of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), we report the discovery of a pair of faint dwarf galaxies (CenA-MM-Dw1 and CenA-MM-Dw2) at a projected distance of ∼90 kpc from the nearby elliptical galaxy NGC 5128 (CenA). We measure a tip of the red giant branch distance to each dwarf, finding D = 3.63 ± 0.41 Mpc for CenA-MM-Dw1 and D = 3.60 ± 0.41 Mpc for CenA-MM-Dw2, both of which are consistent with the distance to NGC 5128. A qualitative analysis of the color-magnitude diagrams indicates stellar populations consisting of an old, metal-poor red giant branch (≳12 Gyr, [Fe/H] ∼ –1.7 to –1.9). In addition, CenA-MM-Dw1more » seems to host an intermediate-age population as indicated by its candidate asymptotic giant branch stars. The derived luminosities (M{sub V} = –10.9 ± 0.3 for CenA-MM-Dw1 and –8.4 ± 0.6 for CenA-MM-Dw2) and half-light radii (r{sub h} = 1.4 ± 0.04 kpc for CenA-MM-Dw1 and 0.36 ± 0.08 kpc for CenA-MM-Dw2) are consistent with those of Local Group dwarfs. CenA-MM-Dw1's low central surface brightness (μ {sub V,} {sub 0} = 27.3 ± 0.1 mag arcsec{sup –2}) places it among the faintest and most extended M31 satellites. Most intriguingly, CenA-MM-Dw1 and CenA-MM-Dw2 have a projected separation of only 3 arcmin (∼3 kpc): we are possibly observing the first, faint satellite of a satellite in an external group of galaxies.« less

GHOSTS I: A new faint very isolated dwarf galaxy at D = 12 ± 2 Mpc

DOE Office of Scientific and Technical Information (OSTI.GOV)

Monachesi, Antonela; Bell, Eric F.; Radburn-Smith, David J.

2014-01-10

We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick disk, and Star clusters) fields. Its detected individual stars populate an approximately 1 mag range of its luminosity function (LF). Using synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we find that the colors and magnitudes of GHOSTS I's individual stars are most consistent with being young helium-burning and asymptotic giant branch stars at a distance of ∼12 ± 2 Mpc. Morphologically, GHOSTS I appears to be actively forming stars, so wemore » tentatively classify it as a dwarf irregular (dIrr) galaxy, although future Hubble Space Telescope (HST) observations deep enough to resolve a larger magnitude range in its LF are required to make a more secure classification. GHOSTS I's absolute magnitude is M{sub V}∼−9.85{sub −0.33}{sup +0.40}, making it one of the least luminous dIrr galaxies known, and its metallicity is lower than [Fe/H] = –1.5 dex. The half-light radius of GHOSTS I is 226 ± 38 pc and its ellipticity is 0.47 ± 0.07, similar to Milky Way and M31 dwarf satellites at comparable luminosity. There are no luminous massive galaxies or galaxy clusters within ∼4 Mpc from GHOSTS I that could be considered as its host, making it a very isolated dwarf galaxy in the local universe.« less

A dwarf galaxy near the sight line to PKS 0454+0356 - A fading 'faint blue galaxy'?

NASA Technical Reports Server (NTRS)

Steidel, Charles C.; Dickinson, Mark; Bowen, David V.

1993-01-01

We report the discovery of a dwarf galaxy (MB = -17.2 for H0 = 50 km/s per Mpc) at z = 0.072 which is only 4 arcsec (3.7/h(100) kpc) in projection from the line of sight to the bright quasar PKS 0454+0356 (z(em) = 1.345). The dwarf has very blue optical and optical/IR colors and exhibits line emission indicative of ongoing or recent star formation. However, there is no detection of Ca II 3934 A, 3969 A absorption at z(abs) = 0.072 to equivalent width limits (3 sigma) of about 40 mA, which would suggest an H I column density along the line of sight of less than 5 x 10 exp 19/sq cm, if the Ca II/H I ratio is similar to sight lines in the Galaxy. Based on the absence of Ca II absorption and the unusually weak line emission given the very blue color of the dwarf, we speculate that it may be close to exhausting its supply of gas. As its star formation rate declines, the galaxy's blue magnitude should fade substantially, eventually reaching a quiescent state in accord with its K luminosity of about 0.005 L*. Future observations of the sight line to PKS 0454+0356 using HST in search of Mg II resonance lines, or a search for 21 cm absorption against the quasar radio continuum, could place even more stringent limits on the extent of the gas associated with an intrinsically faint, star-forming dwarf.

Helium ignition in rotating magnetized CO white dwarfs leading to fast and faint rather than classical Type Ia supernovae

NASA Astrophysics Data System (ADS)

Neunteufel, P.; Yoon, S.-C.; Langer, N.

2017-06-01

Context. Based mostly on stellar models that do not include rotation, CO white dwarfs that accrete helium at rates of about 10-8M⊙/ yr have been put forward as candidate progenitors for a number of transient astrophysical phenomena, including Type Ia supernovae and the peculiar and fainter Type Iax supernovae. Aims: Here we study the impact of accretion-induced spin-up including the subsequent magnetic field generation, angular momentum transport, and viscous heating on the white dwarf evolution up to the point of helium ignition. Methods: We resolve the structure of the helium accreting white dwarf models with a one-dimensional Langrangian hydrodynamic code, modified to include rotational and magnetic effects, in 315 model sequences adopting different mass-transfer rates (10-8-10-7M⊙/ yr), and initial white dwarf masses (0.54-1.10 M⊙) and luminosities (0.01-1 L⊙). Results: We find magnetic angular momentum transport, which leads to quasi-solid-body rotation, profoundly impacts the evolution of the white dwarf models, and the helium ignition conditions. Our rotating lower mass (0.54 and 0.82 M⊙) models accrete up to 50% more mass up to ignition than the non-rotating case, while it is the opposite for our more massive models. Furthermore, we find that rotation leads to helium ignition densities that are up to ten times smaller, except for the lowest adopted initial white dwarf mass. Ignition densities on the order of 106 g/cm3 are only found for the lowest accretion rates and for large amounts of accreted helium (≳0.4M⊙). However, correspondingly massive donor stars would transfer mass at much higher rates. We therefore expect explosive He-shell burning to mostly occur as deflagrations and at Ṁ > 2 × 10-8M⊙/ yr, regardless of white dwarf mass. Conclusions: Our results imply that helium accretion onto CO white dwarfs at the considered rates is unlikely to lead to the explosion of the CO core or to classical Type Ia supernovae, but may instead produce events that belong to the recently identified classes of faint and fast hydrogen-free supernovae.

Collecting Brown Dwarfs in the Night Sky

NASA Image and Video Library

2010-11-09

The green dot in the middle of this image might look like an emerald amidst glittering diamonds, but is a dim star belonging to a class called brown dwarfs; it is the first ultra-cool brown dwarf discovered by NASA Wide-field Infrared Survey Explorer.

Dwarf galaxies in the coma cluster: Star formation properties and evolution

NASA Astrophysics Data System (ADS)

Hammer, Derek M.

The infall regions of galaxy clusters are unique laboratories for studying the impact of environment on galaxy evolution. This intermediate region links the low-density field environment and the dense core of the cluster, and is thought to host recently accreted galaxies whose star formation is being quenched by external processes associated with the cluster. In this dissertation, we measure the star formation properties of galaxies at the infall region of the nearby rich cluster of galaxies, Coma. We rely primarily on Ultraviolet (UV) data owing to its sensitivity to recent star formation and we place more emphasis on the properties of dwarf galaxies. Dwarf galaxies are good tracers of external processes in clusters but their evolution is poorly constrained as they are intrinsically faint and hence more challenging to detect. We make use of deep GALEX far-UV and near-UV observations at the infall region of the Coma cluster. This area of the cluster has supporting photometric coverage at optical and IR wavelengths in addition to optical spectroscopic data that includes deep redshift coverage of dwarf galaxies in Coma. Our GALEX observations were the deepest exposures taken for a local galaxy cluster. The depth of these images required alternative data analysis techniques to overcome systematic effects that limit the default GALEX pipeline analysis. Specifically, we used a deblending method that improved detection efficiency by a factor of ˜2 and allowed reliable photometry a few magnitudes deeper than the pipeline catalog. We performed deep measurements of the total UV galaxy counts in our field that were used to measure the source confusion limit for crowded GALEX fields. The star formation properties of Coma members were studied for galaxies that span from starbursts to passive galaxies. Star-forming galaxies in Coma tend to have lower specific star formation rates, on average, as compared to field galaxies. We show that the majority of these galaxies are likely in the process of being quenched or were only recently quenched. We modeled the quenching timescales for transition galaxies, or “green valley” objects, and found that the majority are quenched in less than 1 Gyr. This timescale is consistent with rapid dynamical processes that are active in the cluster environment as opposed to the more gradual quenching mechanisms that exist in the group environment. For the passive galaxy population, we have measured an average stellar age of 6-8 Gyr for the red sequence which is consistent with previous studies based on spectroscopic observations. We note that the star formation properties of Coma member galaxies were established from photometry alone, as opposed to using spectroscopic data which are more challenging to obtain for dwarf galaxies. We have measured the faintest UV luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are 3.5 mag fainter than previous studies in Coma, and are sufficiently deep that we reach the dwarf passive galaxy population for the first time. We have introduced a new technique for measuring the LF which avoids color selection effects associated with previous methods. The UV LFs constructed separately for star-forming and passive galaxies follow a similar distribution at faint magnitudes, which suggests that the recent quenching of infalling dwarf star-forming galaxies is sufficient to build the dwarf passive population in Coma. The Coma UV LFs show a turnover at faint magnitudes as compared to the field, owing to a deficit of dwarf galaxies with stellar masses below M∗ = 108 M⊙ . We show that the UV LFs for the field behind the Coma cluster are nearly identical to the average field environment, and do not show evidence for a turnover at faint magnitudes. We suspect that the missing dwarf galaxies in Coma are severely disrupted by tidal processes as they are accreted onto the cluster, just prior to reaching the infall region studied here.

A far-ultraviolet flare on a Pleiades G dwarf

NASA Technical Reports Server (NTRS)

Ayres, T. R.; Stauffer, J. R.; Simon, Theodore; Stern, R. A.; Antiochos, S. K.; Basri, G. S.; Bookbinder, J. A.; Brown, A.; Doschek, G. A.; Linsky, J. L.

1994-01-01

The Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) recorded a remarkable transient brightening in the C IV lambda lambda 1548,50 emissions of the rapidly rotating Pleiades G dwarf H II 314. On the one hand the 'flare' might be a rare event luckily observed; on the other hand it might be a bellwether of the coronal heating in very young solar-mass stars. If the latter, flaring provides a natural spin-down mechanism through associated sporadic magnetospheric mass loss.

Infrared spectrum of an extremely cool white-dwarf star

PubMed

Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

2000-01-06

White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested to make up much of the 'dark matter' in the halo of the Milky Way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.

AVOCADO: A Virtual Observatory Census to Address Dwarfs Origins

NASA Astrophysics Data System (ADS)

Sánchez-Janssen, Rubén; Sánchez-Janssen

2011-12-01

Dwarf galaxies are by far the most abundant of all galaxy types, yet their properties are still poorly understood-especially due to the observational challenge that their intrinsic faintness represents. AVOCADO aims at establishing firm conclusions on their formation and evolution by constructing a homogeneous, multiwavelength dataset for a statistically significant sample of several thousand nearby dwarfs (-18 < Mi < -14). Using public data and Virtual Observatory tools, we have built GALEX+SDSS+2MASS spectral energy distributions that are fitted by a library of single stellar population models. Star formation rates, stellar masses, ages and metallicities are further complemented with structural parameters that can be used to classify them morphologically. This unique dataset, coupled with a detailed characterization of each dwarf's environment, allows for a fully comprehensive investigation of their origins and to track the (potential) evolutionary paths between the different dwarf types.

Dwarf Hosts of Low-z Supernovae

NASA Astrophysics Data System (ADS)

Pyotr Kolobow, Craig; Perlman, Eric S.; Strolger, Louis

2018-01-01

Hostless supernovae (SNe), or SNe in dwarf galaxies, may serve as excellent beacons for probing the spatial density of dwarf galaxies (M < 10^8M⊙), which themselves are scarcely detected beyond only a few Mpc. Depending on the assumed model for the stellar-mass to halo mass relation for these galaxies, LSST might see 1000s of SNe (of all types) from dwarf galaxies alone. Conversely, one can take the measured rates of these SNe and test the model predictions for the density of dwarf galaxies in the local universe. Current “all-sky” surveys, like PanSTARRS and ASAS-SN, are now finding hostless SNe at a number sufficient to measure their rate. What missing is the appropriate weighting of their host luminosities. Here we seek to continue a successful program to recover the luminosities of these hostless SNe, to z = 0.15, to use their rate to constrain the faint-end slope of the low-z galaxy luminosity function.

ANDROMEDA XXVIII: A DWARF GALAXY MORE THAN 350 kpc FROM ANDROMEDA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Slater, Colin T.; Bell, Eric F.; Martin, Nicolas F.

We report the discovery of a new dwarf galaxy, Andromeda XXVIII, using data from the recently released Sloan Digital Sky Survey Data Release 8. The galaxy is a likely satellite of Andromeda, and, at a separation of 365{sup +17}{sub -1} kpc, would be one of the most distant of Andromeda's satellites. Its heliocentric distance is 650{sup +150}{sub -80} kpc, and analysis of its structure and luminosity shows that it has an absolute magnitude of M{sub V} = -8.5{sup +0.4}{sub -1.0} and half-light radius of r{sub h} = 210{sup +60}{sub -50} pc, similar to many other faint Local Group dwarfs. Withmore » presently available imaging we are unable to determine whether there is ongoing or recent star formation, which prevents us from classifying it as a dwarf spheroidal or a dwarf irregular.« less

Project 1640 observations of the white dwarf HD 114174 B

NASA Astrophysics Data System (ADS)

Bacchus, E.; Parry, I. R.; Oppenheimer, R.; Aguilar, J.; Beichman, C.; Brenner, D.; Burruss, R.; Cady, E.; Luszcz-Cook, S.; Crepp, J.; Dekany, R.; Gianninas, A.; Hillenbrand, L.; Kilic, M.; King, D.; Lockhart, T. G.; Matthews, C. T.; Nilsson, R.; Pueyo, L.; Rice, E. L.; Roberts, L. C.; Sivaramakrishnan, A.; Soummer, R.; Vasisht, G.; Veicht, A.; Zhai, C.; Zimmerman, N. T.

2017-08-01

We present the first near infrared spectrum of the faint white dwarf companion HD 114174 B, obtained with Project 1640. Our spectrum, covering the Y, J and H bands, combined with previous TaRgetting bENchmark-objects with Doppler Spectroscopy (TRENDS) photometry measurements, allows us to place further constraints on this companion. We suggest two possible scenarios; either this object is an old, low-mass, cool H atmosphere white dwarf with Teff ˜ 3800 K or a high-mass white dwarf with Teff > 6000 K, potentially with an associated cool (Teff ˜ 700 K) brown dwarf or debris disc resulting in an infrared excess in the L΄ band. We also provide an additional astrometry point for 2014 June 12 and use the modelled companion mass combined with the radial velocity and direct imaging data to place constraints on the orbital parameters for this companion.

ESO 306-17

NASA Image and Video Library

2017-12-08

View a video clip zoom in on galaxy ESO 306-17 here: www.flickr.com/photos/gsfc/4409589832/ This image from the Advanced Camera for Surveys aboard the NASA/ESA Hubble Space Telescope highlights the large and bright elliptical galaxy called ESO 306-17 in the southern sky. In this image, it appears that ESO 306-17 is surrounded by other galaxies but the bright galaxies at bottom left are thought to be in the foreground, not at the same distance in the sky. In reality, ESO 306-17 lies fairly abandoned in an enormous sea of dark matter and hot gas. Researchers are also using this image to search for nearby ultra-compact dwarf galaxies. Ultra-compact dwarfs are mini versions of dwarf galaxies that have been left with only their core due to interaction with larger, more powerful galaxies. Most ultra-compact dwarfs discovered to date are located near giant elliptical galaxies in large clusters of galaxies, so it will be interesting to see if researchers find similar objects in fossil groups. Credit: NASA, ESA and Michael West (ESO)

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes.

PubMed

García-Berro, Enrique; Torres, Santiago; Althaus, Leandro G; Renedo, Isabel; Lorén-Aguilar, Pablo; Córsico, Alejandro H; Rohrmann, René D; Salaris, Maurizio; Isern, Jordi

2010-05-13

NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age ( approximately 8 Gyr) and the age derived from the termination of the white dwarf cooling sequence ( approximately 6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, (22)Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf ((12)C and (16)O) is expected to occur. This sequence of events is expected to introduce long delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.

An intermediate-mass black hole in the darf galaxy Pox 52

NASA Astrophysics Data System (ADS)

Barth, Aaron

2005-01-01

Do dwarf elliptical and dwarf spiral galaxies contain central black holes with masses below 106 solar masses? Beyond the Local Group dynamical searches for black holes in this mass range are very difficult but the detection of accretion-powered nuclear activity could be used to infer the presence of a black hole. The nearby dwarf spiral galaxy NGC 4395 hosts a faint Seyfert 1 nucleus with a likely black hole mass in the range 104-105 solar masses and for more than a decade it has been the only known example of a Seyfert 1 nucleus in a dwarf galaxy. I will present new Keck spectra of the dwarf galaxy POX 52 which demonstrate that it has a Seyfert 1 spectrum nearly identical to that of NGC 4395. Its velocity dispersion is 37 km/s suggesting a possible black hole mass of order 105 solar masses. I will discuss the prospects for systematic searches for nuclear activity in dwarf galaxies and the implications for black hole demographics.

THE GALAXY LUMINOSITY FUNCTIONS DOWN TO M{sub R} = -10 IN THE COMA CLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yamanoi, Hitomi; Komiyama, Yutaka; Yagi, Masafumi

2012-08-15

We derived the luminosity function (LF) of dwarf galaxies in the Coma Cluster down to M{sub R} = -10 at three fields located at the center, intermediate, and outskirt of the cluster. The LF (-19 < M{sub R} < -10) shows no significant differences among the three fields. It shows a clear dip at M{sub R} {approx} -13 and is composed of two distinct components of different slopes; the bright component with -19 < M{sub R} < -13 has a flatter slope than the faint component with -13 < M{sub R} < -10 which has a steep slope. The brightmore » component (-19 < M{sub R} < -13) consists mostly of red extended galaxies including few blue galaxies whose colors are typical of late-type galaxies. On the other hand, the faint component (-13 < M{sub R} < -10) consists largely of point-spread-function-like compact galaxies. We found that both these compact galaxies and some extended galaxies are present in the center while only compact galaxies are seen in the outskirt. In the faint component, the fraction of blue galaxies is larger in the outskirt than in the center. We suggest that the dwarf galaxies in the Coma Cluster, which make up the two components in the LF, are heterogeneous with some different origins.« less

The Properties of Faint Field Galaxies

NASA Astrophysics Data System (ADS)

Driver, Simon. P.

1994-12-01

One of the current drawbacks of Charge Coupled Devices (CCDs) is their restrictive fields of view. The Hitchhiker CCD camera overcomes this limitation by operating in parallel with existing instrumentation and is able to cover a large area as well as large volumes. Hitchhiker is mounted on the 4.2m William Herschel Telescope and has been operating for two years. The first use of the Hitchhiker data set has been to study the general properties of faint galaxies. The observed trend of how the differential numbers of galaxies vary with magnitude agrees extremely well with those of other groups and covers, for the first time, all four major optical bandpasses. This multi-band capability has also allowed the study of how the colors of galaxies change with magnitude and how the correlation of galaxies on the sky varies between the optical bandpasses. A dwarf dominated model has been developed to explain these observations and challenges our knowledge of the space-density of dwarf galaxies. The model demonstrates that a simple upward turn in the luminosity distribution of galaxies, similar to that observed in clusters, would remain undetected by the field surveys yet can explain many of the observations without recourse to non-passive galaxy evolution. The conclusion is that the field luminosity distribution is not constrained at faint absolute magnitudes. A combination of a high density of dwarf galaxies and mild evolution could explain all the observations. Continuing work with HST and the Medium Deep Survey Team now reveals the morphological mix of galaxies down to mI ~ 24.0. The results confirm that ellipticals and early-type spirals are well fitted by standard no-evolution models whilst the late-type spirals can only be fitted by strong evolution and/or a significant turn-up in the local field LF.

Kinematic, Photometric, and Spectroscopic Properties of Faint White Dwarf Stars Discovered in the HALO7D Survey of the Milky Way Galaxy

NASA Astrophysics Data System (ADS)

Harris, Madison; Cunningham, Emily; Guhathakurta, Puragra; Cheshire, Ishani; Gupta, Nandita

2018-01-01

White dwarf (WD) stars represent the final phase in the life of solar-mass stars. The extreme low luminosity of WDs means that most detailed measurements of such stars are limited to samples in the immediate neighborhood of the Sun in the thin disk of the Milky Way galaxy. We present spectra, line-of-sight (LOS) velocities, and proper motions (PMs) of a sample of faint (m_V ~ 19.0–24.5) white dwarfs (WDs) from the HALO7D survey. HALO7D is a Keck II/DEIMOS spectroscopic survey of unprecedented depth (8–24 hour integrations) in the CANDELS fields of main sequence turnoff stars in the Milky Way's outer halo. Faint WD stars are rare but useful by-products of this survey. We identify the sample of WDs based on their characteristic broad spectral Balmer absorption features, and present a Bayesian method for measuring their LOS velocities. Using their broadband colors, LOS velocities and PMs measured with the Hubble Space Telescope, we identify candidate halo members among the WDs based on the predicted velocity distributions from the Besançon numerical model of stellar populations in the Milky Way galaxy. The WDs found in the HALO7D survey will yield new insights on the old stellar population associated with the Milky Way's thick disk and halo. Funding for this research was provided by the National Science Foundation and NASA/STScI. NG and IC's participation in this research was under the auspices of the Science Internship Program at the University of California Santa Cruz.

Selecting ultra-faint dwarf candidate progenitors in cosmological N-body simulations at high redshifts

NASA Astrophysics Data System (ADS)

Safarzadeh, Mohammadtaher; Ji, Alexander P.; Dooley, Gregory A.; Frebel, Anna; Scannapieco, Evan; Gómez, Facundo A.; O'Shea, Brian W.

2018-06-01

The smallest satellites of the Milky Way ceased forming stars during the epoch of reionization and thus provide archaeological access to galaxy formation at z > 6. Numerical studies of these ultrafaint dwarf galaxies (UFDs) require expensive cosmological simulations with high mass resolution that are carried out down to z = 0. However, if we are able to statistically identify UFD host progenitors at high redshifts with relatively high probabilities, we can avoid this high computational cost. To find such candidates, we analyse the merger trees of Milky Way type haloes from the high-resolution Caterpillar suite of dark matter only simulations. Satellite UFD hosts at z = 0 are identified based on four different abundance matching (AM) techniques. All the haloes at high redshifts are traced forward in time in order to compute the probability of surviving as satellite UFDs today. Our results show that selecting potential UFD progenitors based solely on their mass at z = 12 (8) results in a 10 per cent (20 per cent) chance of obtaining a surviving UFD at z = 0 in three of the AM techniques we adopted. We find that the progenitors of surviving satellite UFDs have lower virial ratios (η), and are preferentially located at large distances from the main MW progenitor, while they show no correlation with concentration parameter. Haloes with favorable locations and virial ratios are ≈3 times more likely to survive as satellite UFD candidates at z = 0.

Organized chaos: scatter in the relation between stellar mass and halo mass in small galaxies

NASA Astrophysics Data System (ADS)

Garrison-Kimmel, Shea; Bullock, James S.; Boylan-Kolchin, Michael; Bardwell, Emma

2017-01-01

We use Local Group galaxy counts together with the ELVIS N-body simulations to explore the relationship between the scatter and slope in the stellar mass versus halo mass relation at low masses, M⋆ ≃ 105-108 M⊙. Assuming models with lognormal scatter about a median relation of the form M_star ∝ M_halo^α, the preferred log-slope steepens from α ≃ 1.8 in the limit of zero scatter to α ≃ 2.6 in the case of 2 dex of scatter in M⋆ at fixed halo mass. We provide fitting functions for the best-fitting relations as a function of scatter, including cases where the relation becomes increasingly stochastic with decreasing mass. We show that if the scatter at fixed halo mass is large enough (≳ 1 dex) and if the median relation is steep enough (α ≳ 2), then the `too-big-to-fail' problem seen in the Local Group can be self-consistently eliminated in about ˜5-10 per cent of realizations. This scenario requires that the most massive subhaloes host unobservable ultra-faint dwarfs fairly often; we discuss potentially observable signatures of these systems. Finally, we compare our derived constraints to recent high-resolution simulations of dwarf galaxy formation in the literature. Though simulation-to-simulation scatter in M⋆ at fixed Mhalo is large among different authors (˜2 dex), individual codes produce relations with much less scatter and usually give relations that would overproduce local galaxy counts.

A systematic search for dwarf counterparts to ultra compact high velocity clouds

NASA Astrophysics Data System (ADS)

Bennet, Paul; Sand, David J.; Crnojevic, Denija; Strader, Jay

2015-01-01

Observations of the Universe on scales smaller than typical, massive galaxies challenge the standard Lambda Cold Dark Matter paradigm for structure formation. It is thus imperative to discover and characterize the faintest dwarf galaxy systems, not just within the Local Group, but in relatively isolated environments as well in order to properly connect them with models of structure formation. Here we report on a systematic search of public ultraviolet and optical archives for dwarf galaxy counterparts to so-called Ultra Compact High Velocity Clouds (UCHVCs), which are compact, isolated HI sources recently found in the Galactic Arecibo L-band Feed Array-HI (GALFA-HI) and Arecibo Legacy Fast ALFA (ALFALFA-HI) surveys. Our search has uncovered at least three strong dwarf galaxy candidates, and we present their inferred star formation rate and structural properties here.

White dwarf stars exceeding the Chandrasekhar mass limit

NASA Astrophysics Data System (ADS)

Tomaschitz, Roman

2018-01-01

The effect of nonlinear ultra-relativistic electron dispersion on the mass-radius relation of high-mass white dwarfs is studied. The dispersion is described by a permeability tensor in the Dirac equation, generated by the ionized high-density stellar matter, which constitutes the neutralizing background of the nearly degenerate electron plasma. The electron dispersion results in a stable mass-radius relation for high-mass white dwarfs, in contrast to a mass limit in the case of vacuum permeabilities. In the ultra-relativistic regime, the dispersion relation is a power law whose amplitude and scaling exponent is inferred from mass and radius estimates of two high-mass white dwarfs, Sirius B and LHS 4033. Evidence for the existence of super-Chandrasekhar mass white dwarfs is provided by several Type Ia supernovae (e.g., SN 2013cv, SN 2003fg, SN 2007if and SN 2009dc), whose mass ejecta exceed the Chandrasekhar limit by up to a factor of two. The dispersive mass-radius relation is used to estimate the radii, central densities, Fermi temperatures, bulk and compression moduli and sound velocities of their white dwarf progenitors.

Discarded candidate companions to low-mass members of Chamaeleon I

NASA Astrophysics Data System (ADS)

Comerón, F.

2012-01-01

Context. Direct detections of brown dwarfs and planetary-mass companions to members of nearby star-forming regions provide important clues about the process of star formation, core fragmentation, and protoplanetary disk evolution. Aims: We study two faint objects at a very small angular distance from the low-mass star ESO-Hα-558 and the possible massive brown dwarf ESO-Hα-566, both of which are members of the Chamaeleon I star-forming region, to establish whether they are physical companions to those sources. If they are, their low luminosities should imply L or T spectral types, which have clearly detectable spectral features. Methods: Adaptive optics-assisted imaging and spectroscopy of both faint candidate companions has been obtained with the NACO instrument at the Very Large Telescope (VLT). Results: Photometry shows that the colors of both objects are compatible with them being moderately reddened, normal stars in the background of the Chamaeleon I clouds. This interpretation is confirmed spectroscopically, as the spectrum between 1.4 and 2.4 μm of both objects has a featureless, monotonic slope lacking the strong H2O absorption features that dominate cool stellar and substellar spectra in that domain. Conclusions: We demonstrate that the two faint sources seen very close to ESO-Hα-558 and ESO-Hα-566 are unrelated background stars, instead of giant planetary-mass companions as might be expected based on their faintness and angular proximity. Based on observations collected with the Very Large Telescope (VLT) at the European Southern Observatory, Paranal, Chile, under observing programmes 075.C-0809(B) and 078.C-0429(C).

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bekki, Kenji

We propose that there is an evolutionary link between ultra-compact blue dwarf galaxies (UCBDs) with active star formation and nucleated dwarfs based on the results of numerical simulations of dwarf–dwarf merging. We consider the observational fact that low-mass dwarfs can be very gas-rich, and thereby investigate the dynamical and chemical evolution of very gas-rich, dissipative dwarf–dwarf mergers. We find that the remnants of dwarf–dwarf mergers can be dominated by new stellar populations formed from the triggered starbursts and consequently can have blue colors and higher metallicities (Z ∼ [0.2–1]Z{sub ⊙}). We also find that the remnants of these mergers canmore » have rather high mass densities (10{sup 4} M{sub ⊙} pc{sup −3}) within the central 10 pc and small half-light radii (40−100 pc). The radial stellar structures of some merger remnants are similar to those of nucleated dwarfs. Star formation can continue in nuclear gas disks (R < 100 pc) surrounding stellar galactic nuclei (SGNs) so that the SGNs can finally have multiple stellar populations with different ages and metallicities. These very compact blue remnants can be identified as UCBDs soon after merging and as nucleated dwarfs after the young stars fade. We discuss these results in the context of the origins of metal-rich ultra-compact dwarfs and SGNs.« less

Galaxy populations in the Antlia cluster - III. Properties of faint early-type galaxies

NASA Astrophysics Data System (ADS)

Smith Castelli, Analía. V.; Cellone, Sergio A.; Faifer, Favio R.; Bassino, Lilia P.; Richtler, Tom; Romero, Gisela A.; Calderón, Juan Pablo; Caso, Juan Pablo

2012-01-01

We present a new analysis of the early-type galaxy population in the central region of the Antlia cluster, focusing on the faint systems such as dwarf ellipticals (dEs) and dwarf spheroidals (dSphs). The colour-magnitude relation (CMR) and the relation between luminosity and mean effective surface brightness for galaxies in the central region of Antlia have been previously studied in Paper I of the present series. Now we confirm 22 early-type galaxies as Antlia members, using Gemini-GMOS and Magellan-MIKE spectra. Among them, 15 are dEs from the FS90 Antlia Group catalogue, two belong to the rare type of compact ellipticals (cEs) and five are new faint dwarfs that had never been catalogued before. In addition, we present 16 newly identified low-surface-brightness galaxy candidates, almost half of them displaying morphologies consistent with being Antlia's counterparts of Local Group dSphs, which extend the faint luminosity limit of our study down to MB=-10.1(BT= 22.6) mag. With these new data, we built an improved CMR in the Washington photometric system, i.e. integrated T1 magnitudes versus (C-T1) colours, which extends ˜4 mag faintwards the limit of spectroscopically confirmed Antlia members. When only confirmed early-type members are considered, this relation extends over 10 mag in luminosity with no apparent change in slope or increase in colour dispersion towards its faint end. The intrinsic colour scatter of the relation is compared with those reported for other clusters of galaxies; we argue that it is likely that the large scatter of the CMR, usually reported at faint magnitudes, is mostly due to photometric errors along with an improper membership/morphological classification. The distinct behaviour of the luminosity versus mean effective surface brightness relation at the bright and faint ends is analysed, while it is confirmed that dE galaxies on the same relation present a very similar effective radius, regardless of their colour. The projected spatial distribution of the member sample confirms the existence of two groups in Antlia, each one dominated by a giant elliptical galaxy and with one cE located close to each giant. Size and position, with respect to massive galaxies, of the dSph candidates are estimated and compared to Local Group counterparts. Based on observations carried out at the Cerro Tololo Inter-American Observatory (Chile), at Las Campanas Observatory (Chile) and at the European Southern Observatory, Paranal (Chile). Also based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministerio da Ciencia e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

The White Dwarf Mass and the Accretion Rate of Recurrent Novae: An X-ray Perspective

NASA Technical Reports Server (NTRS)

Mukai, Koji; Sokoloski, Jennifer L.; Nelson, Thomas; Luna, Gerardo J. M.

2011-01-01

We present recent results of quiescent X-ray observations of recurrent novae (RNe) and related objects. Several RNe are luminous hard X-ray sources in quiescence, consistent with accretion onto a near Chandrasekhar mass white dwarf. Detection of similar hard X-ray emissions in old novae and other cataclysmic variables may lead to identification of additional RN candidates. On the other hand, other RNe are found to be comparatively hard X-ray faint. We present several scenarios that may explain this dichotomy, which should be explored further.

A Search for a Surviving White Dwarf Companion in SN 1006

NASA Astrophysics Data System (ADS)

Kerzendorf, W. E.; Strampelli, G.; Shen, K. J.; Schwab, J.; Pakmor, R.; Do, T.; Buchner, J.; Rest, A.

2018-05-01

Multiple channels have been proposed to produce Type Ia supernovae, with many scenarios suggesting that the exploding white dwarf accretes from a binary companion pre-explosion. In almost all cases, theory suggests that this companion will survive. However, no such companion has been unambiguously identified in ancient supernova remnants - possibly falsifying the accretion scenario. Existing surveys, however, have only looked for stars as faint as ≈0.1L⊙ and thus might have missed a surviving white dwarf companion. In this work, we present very deep DECAM imaging (u, g, r, z) of the Type Ia supernova remnant SN 1006 specifically to search for a potential surviving white dwarf companion. We find no object that is consistent with a relatively young cooling white dwarf within the inner half of the SN 1006 remnant. We find that if there is a companion white dwarf, it must be redder than the standard white dwarf cooling track, or it must have formed long ago and cooled undisturbed for >108 yr. We conclude that our findings are consistent with the complete destruction of the secondary (such as in a merger) or an anomalously red or very dim surviving companion white dwarf.

New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy

NASA Astrophysics Data System (ADS)

Aguado, D. S.; Allende Prieto, C.; González Hernández, J. I.; Rebolo, R.; Caffau, E.

2017-07-01

Context. The first generation of stars formed in the Galaxy left behind the chemical signatures of their nucleosynthesis in the interstellar medium, visible today in the atmospheres of low-mass stars that formed afterwards. Sampling the chemistry of those low-mass provides insight into the first stars. Aims: We aim to increase the samples of stars with extremely low metal abundances, identifying ultra metal-poor stars from spectra with modest spectral resolution and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable metallicities and carbon abundances from such spectra. Methods: We carry out follow-up observations of faint, V > 19, metal-poor candidates selected from SDSS spectroscopy and observed with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at GTC. The SDSS and follow-up OSIRIS spectra were analyzed using the FERRE code to derive effective temperatures, surface gravities, metallicities and carbon abundances. In addition, a well-known extremely metal-poor star has been included in our sample to calibrate the analysis methodology. Results: We observed and analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars in our sample have been confirmed as extremely metal-poor stars, in the [Fe/H] < -3.3 regime. We report the recognition of J173403+644632, a carbon-enhanced ultra metal-poor dwarf star with [Fe/H] = -4.3 and [C/Fe] = + 3.1. Based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. Programme ID GTC2E-16A and ID GTC65-16B.

A CATALOG OF ULTRA-COMPACT HIGH VELOCITY CLOUDS FROM THE ALFALFA SURVEY: LOCAL GROUP GALAXY CANDIDATES?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adams, Elizabeth A. K.; Giovanelli, Riccardo; Haynes, Martha P., E-mail: betsey@astro.cornell.edu, E-mail: riccardo@astro.cornell.edu, E-mail: haynes@astro.cornell.edu

2013-05-01

We present a catalog of 59 ultra-compact high velocity clouds (UCHVCs) extracted from the 40% complete ALFALFA HI-line survey. The ALFALFA UCHVCs have median flux densities of 1.34 Jy km s{sup -1}, median angular diameters of 10', and median velocity widths of 23 km s{sup -1}. We show that the full UCHVC population cannot easily be associated with known populations of high velocity clouds. Of the 59 clouds presented here, only 11 are also present in the compact cloud catalog extracted from the commensal GALFA-HI survey, demonstrating the utility of this separate dataset and analysis. Based on their sky distributionmore » and observed properties, we infer that the ALFALFA UCHVCs are consistent with the hypothesis that they may be very low mass galaxies within the Local Volume. In that case, most of their baryons would be in the form of gas, and because of their low stellar content, they remain unidentified by extant optical surveys. At distances of {approx}1 Mpc, the UCHVCs have neutral hydrogen (H I) masses of {approx}10{sup 5}-10{sup 6} M{sub Sun }, H I diameters of {approx}2-3 kpc, and indicative dynamical masses within the H I extent of {approx}10{sup 7}-10{sup 8} M{sub Sun }, similar to the Local Group ultra-faint dwarf Leo T. The recent ALFALFA discovery of the star-forming, metal-poor, low mass galaxy Leo P demonstrates that this hypothesis is true in at least one case. In the case of the individual UCHVCs presented here, confirmation of their extragalactic nature will require further work, such as the identification of an optical counterpart to constrain their distance.« less

A Peculiar Faint Satellite in the Remote Outer Halo of M31

NASA Astrophysics Data System (ADS)

Mackey, A. D.; Huxor, A. P.; Martin, N. F.; Ferguson, A. M. N.; Dotter, A.; McConnachie, A. W.; Ibata, R. A.; Irwin, M. J.; Lewis, G. F.; Sakari, C. M.; Tanvir, N. R.; Venn, K. A.

2013-06-01

We present Hubble Space Telescope imaging of a newly discovered faint stellar system, PAndAS-48, in the outskirts of the M31 halo. Our photometry reveals this object to be comprised of an ancient and very metal-poor stellar population with age >~ 10 Gyr and [Fe/H] lsim -2.3. Our inferred distance modulus (m - M)0 = 24.57 ± 0.11 confirms that PAndAS-48 is most likely a remote M31 satellite with a three-dimensional galactocentric radius of 149^{+19}_{-8} kpc. We observe an apparent spread in color on the upper red giant branch that is larger than the photometric uncertainties should allow, and briefly explore the implications of this. Structurally, PAndAS-48 is diffuse, faint, and moderately flattened, with a half-light radius r_h=26^{+4}_{-3} pc, integrated luminosity MV = -4.8 ± 0.5, and ellipticity \\epsilon =0.30^{+0.08}_{-0.15}. On the size-luminosity plane it falls between the extended globular clusters seen in several nearby galaxies and the recently discovered faint dwarf satellites of the Milky Way; however, its characteristics do not allow us to unambiguously classify it as either type of system. If PAndAS-48 is a globular cluster then it is among the most elliptical, isolated, and metal-poor of any seen in the Local Group, extended or otherwise. Conversely, while its properties are generally consistent with those observed for the faint Milky Way dwarfs, it would be a factor of ~2-3 smaller in spatial extent than any known counterpart of comparable luminosity. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO 12515.

The luminosity function for different morphological types in the CfA Redshift Survey

NASA Technical Reports Server (NTRS)

Marzke, Ronald O.; Geller, Margaret J.; Huchra, John P.; Corwin, Harold G., Jr.

1994-01-01

We derive the luminosity function for different morphological types in the original CfA Redshift Survey (CfA1) and in the first two slices of the CfA Redshift Survey Extension (CfA2). CfA1 is a complete sample containing 2397 galaxies distributed over 2.7 steradians with m(sub z) less than or equal 14.5. The first two complete slices of CfA2 contain 1862 galaxies distributed over 0.42 steradians with m(sub z)=15.5. The shapes of the E-S0 and spiral luminosity functions (LF) are indistinguishable. We do not confirm the steeply decreasing faint end in the E-S0 luminosity function found by Loveday et al. for an independent sample in the southern hemisphere. We demonstrate that incomplete classification in deep redshift surveys can lead to underestimates of the faint end of the elliptical luminosity function and could be partially responsible for the difference between the CfA survey and other local field surveys. The faint end of the LF for the Magellanic spirals and irregulars is very steep. The Sm-Im luminosity function is well fit by a Schechter function with M*=-18.79, alpha=-1.87, and phi*=0.6x10(exp -3) for M(sub z) less than or equal to -13. These galaxies are largely responsible for the excess at the faint end of the general CfA luminosity function. The abundance of intrinsically faint, blue galaxies nearby affects the interpretation of deep number counts. The dwarf population increases the expected counts at B=25 in a no-evolution, q(sub 0)=0.05 model by a factor of two over standard no-evolution estimates. These dwarfs change the expected median redshift in deep redshift surveys by less than 10 percent . Thus the steep Sm-Im LF may contribute to the reconciliation of deep number counts with deep redshift surveys.

Stellar properties of dwarf galaxies and their connections with the Milky Way halo

NASA Astrophysics Data System (ADS)

Revaz, Yves; Pascale Jablonka

2018-06-01

In this talk, relying on recent chemo-dynamical simulations, I will describe the stellar properties and in particular the abundances ratios of dwarf galaxies emerging from a LCDM framework. Faint systems quenched by the UV-background as well as luminous ones exhibiting an extended star formation history nicely reproduce observations, without necessary requiring a strong interaction with the Milky Way. However, dwarf galaxies with complex star formation histories like Carina and Fornax are much more difficult to reproduce. Those systems are often believed to result from an interaction with the Milky Way. I will show that when such interaction is taken into account in our high resolution simulations through ram pressure stripping, a much more complex reality appears.

A search for HI in some peculiar faint dwarf galaxies

NASA Astrophysics Data System (ADS)

Begum, Ayesha; Chengalur, Jayaram N.

2005-09-01

We present a deep Giant Metrewave Radio Telescope (GMRT) search for HI 21-cm emission from three dwarf galaxies, viz. POX 186, SC 24 and KKR 25. Based, in part, on previous single-dish HI observations, these galaxies have been classified as a blue compact dwarf (BCD), a dwarf irregular and a transition galaxy, respectively. However, in conflict with previous single-dish detections, we do not detect HI in SC 24 or KKR 25. We suggest that the previous single-dish measurements were probably confused with the local Galactic emission. In the case of POX 186, we confirm the previous non-detection of HI but with substantially improved limits on its HI mass. Our derived upper limits on the HI mass of SC 24 and KKR 25 are similar to the typical HI mass limit for dwarf spheroidal (dSph) galaxies, whereas in the case of POX 186, we find that its gas content is somewhat smaller than is typical of BCD galaxies.

Deadly Dark Matter Cusps versus Faint and Extended Star Clusters: Eridanus II and Andromeda XXV

NASA Astrophysics Data System (ADS)

Amorisco, Nicola C.

2017-07-01

The recent detection of two faint and extended star clusters in the central regions of two Local Group dwarf galaxies, Eridanus II and Andromeda XXV, raises the question of whether clusters with such low densities can survive the tidal field of cold dark matter halos with central density cusps. Using both analytic arguments and a suite of collisionless N-body simulations, I show that these clusters are extremely fragile and quickly disrupted in the presence of central cusps ρ ˜ {r}-α with α ≳ 0.2. Furthermore, the scenario in which the clusters were originally more massive and sank to the center of the halo requires extreme fine tuning and does not naturally reproduce the observed systems. In turn, these clusters are long lived in cored halos, whose central regions are safe shelters for α ≲ 0.2. The only viable scenario for hosts that have preserved their primordial cusp to the present time is that the clusters formed at rest at the bottom of the potential, which is easily tested by measurement of the clusters proper velocity within the host. This offers means to readily probe the central density profile of two dwarf galaxies as faint as {L}V˜ 5× {10}5 {L}⊙ and {L}V˜ 6× {10}4 {L}⊙ , in which stellar feedback is unlikely to be effective.

Discovery of a New Photometric Sub-class of Faint and Fast Classical Novae

NASA Astrophysics Data System (ADS)

Kasliwal, M. M.; Cenko, S. B.; Kulkarni, S. R.; Ofek, E. O.; Quimby, R.; Rau, A.

2011-07-01

We present photometric and spectroscopic follow-up of a sample of extragalactic novae discovered by the Palomar 60 inch telescope during a search for "Fast Transients In Nearest Galaxies" (P60-FasTING). Designed as a fast cadence (1 day) and deep (g < 21 mag) survey, P60-FasTING was particularly sensitive to short-lived and faint optical transients. The P60-FasTING nova sample includes 10 novae in M 31, 6 in M 81, 3 in M 82, 1 in NGC 2403, and 1 in NGC 891. This significantly expands the known sample of extragalactic novae beyond the Local Group, including the first discoveries in a starburst environment. Surprisingly, our photometry shows that this sample is quite inconsistent with the canonical maximum-magnitude-rate-of-decline (MMRD) relation for classical novae. Furthermore, the spectra of the P60-FasTING sample are indistinguishable from classical novae. We suggest that we have uncovered a sub-class of faint and fast classical novae in a new phase space in luminosity-timescale of optical transients. Thus, novae span two orders of magnitude in both luminosity and time. Perhaps the MMRD, which is characterized only by the white dwarf mass, was an oversimplification. Nova physics appears to be characterized by a relatively rich four-dimensional parameter space in white dwarf mass, temperature, composition, and accretion rate.

The white dwarf luminosity function - A possible probe of the galactic halo

NASA Technical Reports Server (NTRS)

Tamanaha, Christopher M.; Silk, Joseph; Wood, M. A.; Winget, D. E.

1990-01-01

The dynamically inferred dark halo mass density, amounting to above 0.01 solar masses/cu pc at the sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/solar L) = -4.4, due to the onset of star formation in the disk. By using a narrow range for the initial mass function and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time, all the halo dark matter is allowed to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Although it is unlikely that all the dark matter is in these dim white dwarfs, a definite signature in the low-luminosity end of the white dwarf luminosity function is predicted even if they comprise only 1 percent of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years.

The UV Spectrum of the Ultracool Dwarf LSR J1835+3259 Observed with the Hubble Space Telescope

NASA Astrophysics Data System (ADS)

Saur, Joachim; Fischer, Christian; Wennmacher, Alexandre; Feldman, Paul; Roth, Lorenz; Strobel, D.; Reiners, Ansgar

2018-01-01

An interesting question about ultracool dwarfs is whether their emission is purely internally driven or partially powered by external processes similar to auroral emission known from planetary bodies of the solar system. Here we present Hubble Space Telescope observations of the energy fluxes of the M8.5 ultracool dwarf LSR J1835+3259 throughout the UV. The dwarf’s UV emission is generally weaker compared to younger-type M-dwarfs. We detect the Mg II doublet at 2800 A and constrain an average flux throughout the Near-UV. In the Far-UV without Lyman alpha, the ultracool dwarf is extremely faint with an energy output of at least a factor of 1000 smaller than expected from auroral emission physically similar to that on Jupiter. We also detect the red wing of the Lyman alpha emission. Our overall finding is that the observed UV spectrum of LSR J1835+3259 resembles the spectrum of M-dwarf stars much closer than the spectrum expected from Jupiter-like auroral processes.

The UV Spectrum of the Ultracool Dwarf LSR J1835+3259 Observed with the Hubble Space Telescope

NASA Astrophysics Data System (ADS)

Saur, Joachim; Fischer, Christian; Wennmacher, Alexandre; Feldman, Paul D.; Roth, Lorenz; Strobel, Darrell F.; Reiners, Ansgar

2018-05-01

An interesting question about ultracool dwarfs recently raised in the literature is whether their emission is purely internally driven or partially powered by external processes similar to planetary aurora known from the solar system. In this work, we present Hubble Space Telescope observations of the energy fluxes of the M8.5 ultracool dwarf LSR J1835+3259 throughout the ultraviolet (UV). The obtained spectra reveal that the object is generally UV-fainter compared with other earlier-type dwarfs. We detect the Mg II doublet at 2800 Å and constrain an average flux throughout the near-UV. In the far-UV without Lyα, the ultracool dwarf is extremely faint with an energy output at least a factor of 250 smaller as expected from auroral emission physically similar to that on Jupiter. We also detect the red wing of the Lyα emission. Our overall finding is that the observed UV spectrum of LSR J1835+3259 resembles the spectrum of mid/late-type M-dwarf stars relatively well, but it is distinct from a spectrum expected from Jupiter-like auroral processes.

a Faint and Lonely Brown Dwarf in the Solar Vicinity

NASA Astrophysics Data System (ADS)

1997-04-01

Discovery of KELU-1 Promises New Insights into Strange Objects Brown Dwarfs are star-like objects which are too small to become real stars, yet too large to be real planets. Their mass is too small to ignite those nuclear processes which are responsible for the large energies and high temperatures of stars, but it is much larger than that of the planets we know in our solar system. Until now, very few Brown Dwarfs have been securely identified as such. Two are members of double-star systems, and a few more are located deep within the Pleiades star cluster. Now, however, Maria Teresa Ruiz of the Astronomy Department at Universidad de Chile (Santiago de Chile), using telescopes at the ESO La Silla observatory, has just discovered one that is all alone and apparently quite near to us. Contrary to the others which are influenced by other objects in their immediate surroundings, this new Brown Dwarf is unaffected and will thus be a perfect object for further investigations that may finally allow us to better understand these very interesting celestial bodies. It has been suggested that Brown Dwarfs may constitute a substantial part of the unseen dark matter in our Galaxy. This discovery may therefore also have important implications for this highly relevant research area. Searching for nearby faint stars The story of this discovery goes back to 1987 when Maria Teresa Ruiz decided to embark upon a long-term search (known as the Calan-ESO proper-motion survey ) for another type of unusual object, the so-called White Dwarfs , i.e. highly evolved, small and rather faint stars. Although they have masses similar to that of the Sun, such stars are no larger than the Earth and are therefore extremely compact. They are particularly interesting, because they most probably represent the future end point of evolution of our Sun, some billions of years from now. For this project, the Chilean astronomer obtained large-field photographic exposures with the 1-m ESO Schmidt telescope at La Silla, each covering a sky area of 5 o.5 x 5 o.5. When comparing plates of the same sky field obtained at time intervals of several years [1] , she was able to detect, among the hundreds of thousands of stellar images on the plates, a few faint ones whose positions had changed a little in the meantime. The search technique is based on the fact that such a shift is a good indicator of the object being relatively nearby. It must therefore also be intrinsically faint, i.e. a potential White Dwarf candidate. On every pair of plates, approximately twenty faint moving objects were detected with proper motions [2] of more than 0.25 arcsec per year. Indeed, follow-up spectroscopic observations showed that about 20 percent of these or about four per plate were White Dwarfs. Until now, a total of forty new White Dwarfs have been discovered during this very successful project, i.e. over ten times more than originally expected. And then - a Brown Dwarf! Caption to ESO PR Photo 11/97 [JPEG, 144k] ESO Press Photo 11/97 When checking two plates with a time inverval of 11 years, Maria Teresa Ruiz earlier this year discovered a very faint object in the southern constellation of Hydra (The Water-Snake), moving at 0.35 arcsec per year (cf. ESO Press Photo 11/97). In order to establish its true nature, she obtained its spectrum (in the visual to near-infrared region from wavelengths 450-1000 nm) on March 15 using the ESO 3.6-m telescope and the EFOSC1 spectrograph. Caption to ESO PR Photo 12/97 [GIF, 35k] ESO Press Photo 12/97 To her great surprise, the spectrum was of a type never seen before and certainly not that of a White Dwarf or any other easily identifiable type of star (cf. ESO Press Photo 12/97). In particular, there were no signs of spectral bands of titanium oxide (TiO) or vanadium oxide (VO) which are common in very cool stars, nor of the spectral lines seen in White Dwarfs. On the other hand, an absorption line of the short-lived element lithium was identified, as well as a hydrogen line in emission. However, when the colour of this mysterious object was measured in different wavebands, it was found to be very red and quite similar to that of one of the two known Brown Dwarfs in double star systems. The presence of the lithium line in the spectrum is also an indication that it might be of that type. The astronomer now decided to give the new object the name KELU-1 ; this word means `red' in the language of the Mapuche people, the ancient population in the central part of Chile. Its visual magnitude is 22.3, i.e. more than 3 million times fainter than what can be seen with the unaided eye. In early April, additional infrared observations with the UKIRT (UK Infrared Telescope) on Mauna Kea (Hawaii) by Sandra K. Leggett (Joint Astrophysical Centre, Hilo, Hawaii, USA) confirmed the Brown Dwarf nature of KELU-1, in particular through the unambiguous detection of Methane (CH 4 ) bands in its spectrum. The nature of Brown Dwarfs Brown Dwarfs are first of all characterised by their low mass. When a body of such a small mass is formed in an interstellar cloud and subsequently begins to contract, its temperature at the centre will rise, but it will never reach a level that is sufficient to ignite the nuclear burning of hydrogen to helium, the process that it is main source of energy in the Sun and most other stars. The Brown Dwarf will just continue to contract, more and more slowly, and it will eventually fade from view. This is also the reason that some astronomers consider Brown Dwarfs in the Milky Way and other galaxies as an important component of the `dark matter' whose presence is infered from other indirect measurements but has never been directly observed. It is assumed that the mass limit that separates nuclear-burning stars and slowly contracting Brown Dwarfs is at about 90 times the mass of the giant planet Jupiter, or 8 percent of that of the Sun. KELU-1: a great opportunity for Brown Dwarf studies Assuming that KELU-1 is identical to other known Brown Dwarfs, its measured characteristics indicate that it must be located at a distance of only 10 parsecs, that is about 33 light-years, from the solar system. Its temperature is obviously below 1700 degrees C (where TiO and VO condense as dust grains [3] so that the spectral lines of these molecules are no longer seen). Its mass can be no more than 75 times that of Jupiter, or 6 percent of that of the Sun. During recent years, several Brown Dwarf candidates have been de-masked as low-mass stars and only recently a few Brown Dwarfs were identified in the Pleiades star cluster. Those Brown Dwarfs are quite young and therefore comparatively hotter and brighter. Contrarily, KELU-1 is most probably somewhat older and its unique location so close to us greatly facilitates future investigations. Moreover, it is not at all `disturbed' by the presence of other objects in its immediate surroundings, as this is the case for all other known objects of this type. It will now be important to obtain accurate measurements of KELU-1's parallax , that is, the small annual change of its position in the sky that is caused by the Earth's motion around the Sun and thus the viewing angle of an Earth-based observer. This should be possible within the next year. Moreover, high resolution spectral investigations with large telescope facilities, soon to include the ESO Very Large Telescope at the Paranal observatory in northern Chile, will now for the first time enable us to investigate the processes that take place in the relatively cold upper layers of Brown Dwarfs. For instance, the observed presence of lithium shows that its atmosphere must be different from that of low-mass stars. KELU-1 and the `Dark Matter' From the fact that KELU-1 is so faint that it was barely detectable on the ESO Schmidt plates, it is possible to estimate that the total volume so far surveyed for this type of objects by this research programme is rather small, only about 23 cubic parsecs (800 cubic light-years). A further consideration of the search statistics indicates that less than 10 percent of the Brown Dwarfs present in the surveyed volume would have been found. This translates into a local density of about 0.4 such objects per cubic parsec. Although the mass density of Brown Dwarfs derived from this estimate is insufficient to constitute all the `dark matter' in the Milky Way Galaxy, it is consistent with the most recent estimates of the local mass density, both observed and as infered from dynamical considerations of the motions of stars in the solar neighborhood. Notes: [1] This is done by means of a so-called blink-comparator , an optical device in which the two plates are placed. A tilting mirror allows to view the same sky field alternately on the two plates. Any celestial object that has changed its position will appear to `jump' back and forth and can thus be identified. [2] A proper motion in the sky of 0.25 arcsec/year corresponds to a transversal speed of about 12 km/sec if the object is located at a distance of 10 parsec, or 32.6 light-years. The largest known proper motion of an object outside the solar system is that of Barnard's Star at about 10 arcsec/year. [3] For instance, as the mineral perovskite . How to obtain ESO Press Information ESO Press Information is made available on the World-Wide Web (URL: http://www.eso.org../). ESO Press Photos may be reproduced, if credit is given to the European Southern Observatory.

Looking Deep with Infrared Eyes

NASA Astrophysics Data System (ADS)

2006-07-01

Today, British astronomers are releasing the first data from the largest and most sensitive survey of the heavens in infrared light to the ESO user community. The UKIRT Infrared Deep Sky Survey (UKIDSS) has completed the first of seven years of data collection, studying objects that are too faint to see at visible wavelengths, such as very distant or very cool objects. New data on young galaxies is already challenging current thinking on galaxy formation, revealing galaxies that are massive at a much earlier stage of development than expected. These first science results already show how powerful the full survey will be at finding rare objects that hold vital clues to how stars and galaxies in our Universe formed. UKIDSS will make an atlas of large areas of the sky in the infrared. The data become available to the entire ESO user community immediately after they are entered into the archive [2]. Release to the world follows 18 months after each release to ESO. "Astronomers across Europe will jump on these exciting new data. We are moving into new territory - our survey is both wide and deep, so we are mapping huge volumes of space. That's how we will locate rare objects - the very nearest and smallest stars, and young galaxies at the edge of the universe," said Andy Lawrence from the University of Edinburgh, UKIDSS Principal Investigator. The UKIDSS data are collected by the United Kingdom Infrared Telescope [3] situated near the summit of Mauna Kea in Hawaii using the Wide Field Camera (WFCAM) built by the United Kingdom Astronomy Technology Centre (UKATC) in Edinburgh. WFCAM is the most powerful infrared imager in the world, generating enormous amounts of data - 150 gigabytes per night (equivalent to more than 200 CDs) - and approximately 10.5 Terabytes in total so far (or 15,000 CDs). Mark Casali, now at ESO, was the Project Scientist in charge of the WFCAM instrument construction at the UKATC. "WFCAM was a bold technological undertaking," said Mark Casali. "Nothing quite like it has ever been built before. The fact that it is working reliably and reaching its theoretical sensitivity is a testament to the hard work and skill of the engineering team at the UKATC." ESO PR Photo 24a/06 ESO PR Photo 26a/06 Faint Red Galaxy in the UKIDSS Ultra-Deep Survey A small amount of data was released in January 2006 and already teams led by Omar Almaini at the University of Nottingham and Nigel Hambly of the Institute for Astronomy at the University of Edinburgh are beginning to reveal some of the secrets of star and galaxy formation. Omar Almaini, Ross McLure and the Ultra Deep Survey team have been looking at distant galaxies by surveying the same region of sky night after night to see deeper and to find these very faint objects. This survey will be one hundred times larger than any similar survey attempted to date and will cover an area four times the size of the full Moon. So far several hundred thousand galaxies have been detected and among the early discoveries, nine remarkable galaxies have been found that appear to be 12 billion light years away. As it has taken 12 billion years for the light to travel from these galaxies to Earth, we are seeing them as they were when they were very young - only a billion years after the Big Bang. The newly discovered galaxies are unusual as they appear to be very massive for their age. This challenges thinking on how galaxies form, since it was thought that large galaxies form gradually over billions of years as smaller components merge together. "We're surveying an enormous volume of the distant Universe, which allows us to discover rare massive galaxies that were previously almost impossible to find. Understanding how these galaxies form is one of the Holy Grails of modern astronomy, and now we can trace them back to the edge of the known Universe" said Omar Almaini. ESO PR Photo 26b/06 ESO PR Photo 26b/06 Brown Dwarf Candidates in the Pleiades Cluster (UKIDSS) Nigel Hambly and Nicolas Lodieu have been using the UKIDSS data to discover more about very cold objects in our Galaxy called brown dwarfs. Brown dwarfs are formed in the same way as stars but have typically less than 8% of the mass of the Sun (or approximately 80 times the mass of Jupiter). This is not large enough for core nuclear reactions to occur, and so brown dwarfs do not shine like normal stars. Brown dwarfs give off less than one ten thousandth of the radiation of a star like our Sun. This relatively tiny amount of heat can be detected by WFCAM and the UKIDSS survey hopes to find out how many of these "failed stars" there are in our Galaxy. Nigel Hambly, of the UKIDSS Galactic Clusters Survey said: "With UKIDSS, we will find many thousands of brown dwarfs in many different star formation environments within our own Galaxy; furthermore we expect to find even cooler and much dimmer objects than are currently known. This will tell us how significant a role the brown dwarfs have in the overall scheme of Galactic structure and evolution."

[The inheritance of an ultra-dwarf plant mutant from upland cotton].

PubMed

Chen, Xu-Sheng; DI, Jia-Chun; Xu, Nai-Yin; Xiao, Song-Hua; Liu, Jian-Guang

2007-04-01

The inheritance of an ultra-dwarf plant mutant from upland cotton (Gossypium hirsutum L.) was studied, which showed that the mutant was controlled by single recessive quality gene. This gene was denominated as du tentatively. No similar mutant has been found in upland cotton. The mutation could not normally flower and produce bolls under natural conditions, and its mature height was only 10.5 cm. When treated with exogenous GA3, it could normally flower and boll, and plant height could reach 57.8 cm finally.

Confirming Planetary Mass Candidate Companions in Ophiuchus

NASA Astrophysics Data System (ADS)

Fontanive, Clemence

2016-10-01

We propose for follow-up observations to confirm common proper motion for two candidate planetary mass companions, identified as part of our GO 12944 (PI Allers) search for companions to the youngest ( 0.5 Myr) brown dwarfs in the nearby Ophiuchus star-forming region. If confirmed to be co-moving, these would be among the lowest mass planetary mass companions imaged to date, with estimated masses <5 Jupiter Masses and would be vital benchmark objects for evolutionary models at these young ages. With our multi-band optical and IR photometric approach based on the SpT-Q relation seen for Ophiuchus brown dwarfs (Allers in prep.), we have already estimated the spectral type of our candidate companions. This approach distinguishes substellar objects from background interlopers based on the strength of the 1.4 um water feature robustly observed in MLTY objects but not in reddened background stars - both our candidates show clear evidence of absorption at 1.4 um. If confirmed, these candidate companions would significantly increase the census of young planetary mass companions around extremely young brown dwarfs. These candidate companions are too faint to be observed with ground-based laser guide star adaptive optics (LGS AO) nor is the 1.4 um water feature observable from the ground for such faint objects due to telluric absorption, thus HST is the only telescope in the world suitable for these observations.

Two dwarf galaxies in Orion with low radial velocities.

NASA Astrophysics Data System (ADS)

Karachentsev, I.; Musella, I.

1996-11-01

Two relatively faint (B=15.7 and B=18.4mag, respectively), low velocity (+276 and +322km/s) galaxies were imaged with a CCD in the B, V, I bands. By means of the brightest stars we estimated their distances to be 6.4 and 5.5(+/-2)Mpc, assuming a galactic extinction of 2.7 and 2.9mag, respectively. We note that these isolated irregular dwarfs are located to a high Supergalactic latitude, -63deg, and their low radial velocities may be the result of a retarded expansion along the polar axis of the Local cloud of galaxies.

Nearest neighbor: The low-mass Milky Way satellite Tucana III

DOE PAGES

Simon, J. D.; Li, T. S.; Drlica-Wagner, A.; ...

2017-03-17

Here, we present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite Tucana III (Tuc III). We identify 26 member stars in Tuc III from which we measure a mean radial velocity of v hel = -102.3 ± 0.4 (stat.) ± 2.0 (sys.)more » $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$, a velocity dispersion of $${0.1}_{-0.1}^{+0.7}$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$, and a mean metallicity of $${\\rm{[Fe/H]}}=-{2.42}_{-0.08}^{+0.07}$$. The upper limit on the velocity dispersion is σ < 1.5 $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$ at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0 × 10 4 M ⊙. We cannot rule out mass-to-light ratios as large as 240 M ⊙/L ⊙ for Tuc III, but much lower mass-to-light ratios that would leave the system baryon-dominated are also allowed. We measure an upper limit on the metallicity spread of the stars in Tuc III of 0.19 dex at 95.5% confidence. Tuc III has a smaller metallicity dispersion and likely a smaller velocity dispersion than any known dwarf galaxy, but a larger size and lower surface brightness than any known globular cluster. Its metallicity is also much lower than those of the clusters with similar luminosity. We therefore tentatively suggest that Tuc III is the tidally stripped remnant of a dark matter-dominated dwarf galaxy, but additional precise velocity and metallicity measurements will be necessary for a definitive classification. If Tuc III is indeed a dwarf galaxy, it is one of the closest external galaxies to the Sun. Because of its proximity, the most luminous stars in Tuc III are quite bright, including one star at V = 15.7 that is the brightest known member star of an ultra-faint satellite.« less

Impact of NLTE on research of early chemical enrichment of the dwarf galaxies

NASA Astrophysics Data System (ADS)

Mashonkina, Lyudmila

2015-08-01

The individual stars observed in the dwarf galaxies orbiting the Milky Way are presumably red giants. Their chemical abundances are commonly determined under the classical LTE assumption, despite its validity is questionable for atmospheres of giant, in particular, metal-poor stars. Exactly metal-poor objects are important for understanding the early chemical enrichment processes of the host galaxy and the onset of star formation. We selected a sample of the -4 < [Fe/H] < -2 stars in the dwarf spheroidal (dSph) galaxies Sculptor, Sextans, and Fornax and the ultra-faint galaxies Bootes I and Segue I, with the high-resolution observational data available, and revised abundances of up to 12 chemical species based on the non-local thermodynamic equilibrium (NLTE) line formation. Stellar parameters taken from the literature were checked through the NLTE analysis of lines of iron observed in the two ionisation stages, Fe I and Fe II. For the Scl, Sex, and Fnx stars, with effective temperatures and surface gravities derived from the photometry and known distance (Jablonka et al. 2015; Tafelmeyer et al. 2010), the Fe I/Fe II ionisation equilibrium was found to be fulfilled, when applying a scaling factor of SH = 0.5 to the Drawinian rates of Fe+H collisions. Pronounced NLTE effects were calculated for lines of Na I and Al I resulting in up to 0.5 dex lower [Na/Fe] ratios and up to 0.65 dex higher [Al/Fe] ratios compared with the corresponding LTE values. For the six Scl stars, the scatter of data on Mg/Na is much smaller in NLTE, with the mean [Mg/Na] = 0.61 +- 0.11, than LTE, where [Mg/Na] = 0.42 +- 0.21. We computed a grid of the NLTE abundance corrections for an extensive list of the Ca I, Ti I-Ti II, and Fe I lines in the MARCS models of cool giants, 4000 K <= Teff <= 4750 K, 0.5 <= log g <= 2.5, -4 <= [M/H] <= 0.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simon, J. D.; Li, T. S.; Drlica-Wagner, A.

Here, we present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite Tucana III (Tuc III). We identify 26 member stars in Tuc III from which we measure a mean radial velocity of v hel = -102.3 ± 0.4 (stat.) ± 2.0 (sys.)more » $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$, a velocity dispersion of $${0.1}_{-0.1}^{+0.7}$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$, and a mean metallicity of $${\\rm{[Fe/H]}}=-{2.42}_{-0.08}^{+0.07}$$. The upper limit on the velocity dispersion is σ < 1.5 $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$ at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0 × 10 4 M ⊙. We cannot rule out mass-to-light ratios as large as 240 M ⊙/L ⊙ for Tuc III, but much lower mass-to-light ratios that would leave the system baryon-dominated are also allowed. We measure an upper limit on the metallicity spread of the stars in Tuc III of 0.19 dex at 95.5% confidence. Tuc III has a smaller metallicity dispersion and likely a smaller velocity dispersion than any known dwarf galaxy, but a larger size and lower surface brightness than any known globular cluster. Its metallicity is also much lower than those of the clusters with similar luminosity. We therefore tentatively suggest that Tuc III is the tidally stripped remnant of a dark matter-dominated dwarf galaxy, but additional precise velocity and metallicity measurements will be necessary for a definitive classification. If Tuc III is indeed a dwarf galaxy, it is one of the closest external galaxies to the Sun. Because of its proximity, the most luminous stars in Tuc III are quite bright, including one star at V = 15.7 that is the brightest known member star of an ultra-faint satellite.« less

STELLAR ARCHAEOLOGY IN THE GALACTIC HALO WITH THE ULTRA-FAINT DWARFS. VI. URSA MAJOR II

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dall'Ora, M.; Ripepi, Vincenzo; Marconi, Marcella

2012-06-10

We present a B, V color-magnitude diagram (CMD) of the Milky Way dwarf satellite Ursa Major II (UMa II), spanning the magnitude range from V {approx} 15 to V {approx} 23.5 mag and extending over an 18 Multiplication-Sign 18 arcmin{sup 2} area centered on the Galaxy. Our photometry goes down to about 2 mag below the Galaxy's main-sequence turnoff that we detected at V {approx} 21.5 mag. We have discovered a bona fide RR Lyrae variable star in UMa II, which we use to estimate a conservative dereddened distance modulus for the galaxy of (m - M){sub 0} = 17.70more » {+-} 0.04 {+-} 0.12 mag, where the first error accounts for the uncertainties of the calibrated photometry, and the second reflects our lack of information on the metallicity of the star. The corresponding distance to UMa II is 34.7{sup +0.6}{sub -0.7}({sup +2.0}{sub -1.9}) kpc. Our photometry shows evidence of a spread in the Galaxy's subgiant branch, compatible with a spread in metal abundance in the range between Z = 0.0001 and Z = 0.001. Based on our estimate of the distance, a comparison of the fiducial lines of the Galactic globular clusters M68 and M5 ([Fe/H] = -2.27 {+-} 0.04 dex and -1.33 {+-} 0.02 dex, respectively), with the position on the CMD of spectroscopically confirmed Galaxy members, may suggest the existence of stellar populations of different metal abundance/age in the central region of UMa II.« less

The Resolved Stellar Populations Early Release Science Program

NASA Astrophysics Data System (ADS)

Weisz, Daniel; Anderson, J.; Boyer, M.; Cole, A.; Dolphin, A.; Geha, M.; Kalirai, J.; Kallivayalil, N.; McQuinn, K.; Sandstrom, K.; Williams, B.

2017-11-01

We propose to obtain deep multi-band NIRCam and NIRISS imaging of three resolved stellar systems within 1 Mpc (NOI 104). We will use this broad science program to optimize observational setups and to develop data reduction techniques that will be common to JWST studies of resolved stellar populations. We will combine our expertise in HST resolved star studies with these observations to design, test, and release point spread function (PSF) fitting software specific to JWST. PSF photometry is at the heart of resolved stellar populations studies, but is not part of the standard JWST reduction pipeline. Our program will establish JWST-optimized methodologies in six scientific areas: star formation histories, measurement of the sub-Solar mass stellar IMF, extinction maps, evolved stars, proper motions, and globular clusters, all of which will be common pursuits for JWST in the local Universe. Our observations of globular cluster M92, ultra-faint dwarf Draco II, and star-forming dwarf WLM, will be of high archival value for other science such as calibrating stellar evolution models, measuring properties of variable stars, and searching for metal-poor stars. We will release the results of our program, including PSF fitting software, matched HST and JWST catalogs, clear documentation, and step-by-step tutorials (e.g., Jupyter notebooks) for data reduction and science application, to the community prior to the Cycle 2 Call for Proposals. We will host a workshop to help community members plan their Cycle 2 observations of resolved stars. Our program will provide blueprints for the community to efficiently reduce and analyze JWST observations of resolved stellar populations.

The Smallest Galaxies in the Universe: Investigating the Origins of Ultra-faint Galaxies

NASA Astrophysics Data System (ADS)

Qi, Yuewen; Graus, Andrew; Bullock, James

2018-01-01

One outstanding question in cosmology is, what are the smallest galaxies that can form? The answer to this question can tell us much about galaxy formation, and even of the properties of dark matter itself. A candidate for the smallest galaxies that can form are the ultrafaint galaxies. The star formation of ultrafaints appears to have been shut off during the epoch of reionization, when radiation from the first stars ionized all the free hydrogen in the universe. This would imply ultrafaints should exist everywhere in the universe. However, we can only observe ultrafaints as satellites of the Milky Way, due to their low brightness. This will change with the next generation of telescopes such as the Large Synoptic Survey Telescope (LSST). The focus of this work is to predict the number of ultrafaints that should be seen with future surveys. To that end, we use the ELVIS suite, which contains 14 dark matter only simulations of Local Group like systems containing a Milky Way and Andromeda-like galaxy and the substructure out to around 1 Mpc of the barycenter. We mock observe the simulations in order to mimic current surveys such as the Sloan Digital Sky Survey (SDSS), and the Dark Energy Survey (DES), and use the population of galaxies found by those surveys to project the population of dwarf galaxies out beyond the virial radius of either galaxy. This number will depend sensitively on the formation mechanism of ultrafaint dwarfs, and comparisons of future surveys to this work could help rule out certain formation scenarios.

Studies of the Coldest Brown Dwarfs With The James Webb Space Telescope

NASA Astrophysics Data System (ADS)

Roellig, Thomas L.; Greene, Thomas P.; Beichman, Charles; Meyer, Michael; Rieke, Marcia

2016-07-01

The coolest T and Y-class Brown Dwarf objects are very faint and are therefore very poorly understood, since they are barely detectable with the current astronomical instrumentation. The upcoming James Webb Space Telescope now in development for a launch in the Fall of 2018 will have vastly increased sensitivity in the near and mid-infrared compared to any current facilities and will not be affected by telluric absorption over its entire wavelength range of operations. As a result it will be an ideal tool to obtain information about the composition and temperature-pressure structure in these objects' atmospheres. This presentation outlines the JWST guaranteed time observing plans for these studies. These plans comprise both spectro-photometric and spectroscopic observations of a selection of late T and Y-dwarf targets.

Probing LSST's Ability to Detect Planets Around White Dwarfs

NASA Astrophysics Data System (ADS)

Cortes, Jorge; Kipping, David

2018-01-01

Over the last four years more than 2,000 planets outside our solar system have been discovered, motivating us to search for and characterize potentially habitable worlds. Most planets orbit Sun-like stars, but more exotic stars can also host planets. Debris disks and disintegrating planetary bodies have been detected around white dwarf stars, the inert, Earth-sized cores of once-thriving stars like our Sun. These detections are clues that planets may exist around white dwarfs. Due to the faintness of white dwarfs and the potential rarity of planets around them, a vast survey is required to have a chance at detecting these planetary systems. The Large Synoptic Survey Telescope (LSST), scheduled to commence operations in 2023, will image the entire southern sky every few nights for 10 years, providing our first real opportunity to detect planets around white dwarfs. We characterized LSST’s ability to detect planets around white dwarfs through simulations that incorporate realistic models for LSST’s observing strategy and the white dwarf distribution within the Milky Way galaxy. This was done through the use of LSST's Operations Simulator (OpSim) and Catalog Simulator (CatSim). Our preliminary results indicate that, if all white dwarfs were to possess a planet, LSST would yield a detection for every 100 observed white dwarfs. In the future, a larger set of ongoing simulations will help us quantify the number of planets LSST could potentially find.

KECK/LRIS SPECTROSCOPIC CONFIRMATION OF COMA CLUSTER DWARF GALAXY MEMBERSHIP ASSIGNMENTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chiboucas, Kristin; Tully, R. Brent; Marzke, Ronald O.

2010-11-01

Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy candidates in the core region of the Coma Cluster. These spectra are used to measure redshifts and establish membership for these faint dwarf populations. The primary goal of the low surface brightness sample is to test our ability to use morphological and surface brightness criteria to distinguish between Coma Cluster members and background galaxies using high resolution Hubble Space Telescope/Advanced Camera for Surveys images. Candidates were rated as expected members, uncertain, or expected background.more » From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are identified. Our morphological membership estimation success rate is {approx}100% for objects expected to be members and better than {approx}90% for galaxies expected to be in the background. We confirm that low surface brightness is a very good indicator of cluster membership. High surface brightness galaxies are almost always background with confusion arising only from the cases of the rare compact elliptical (cE) galaxies. The more problematic cases occur at intermediate surface brightness. Many of these galaxies are given uncertain membership ratings, and these were found to be members about half of the time. Including color information will improve membership determination but will fail for some of the same objects that are already misidentified when using only surface brightness and morphology criteria. cE galaxies with B-V colors {approx}0.2 mag redward of the red sequence in particular require spectroscopic follow up. In a sample of 47 high surface brightness, ultracompact dwarf candidates, 19 objects have redshifts which place them in the Coma Cluster, while another 6 have questionable redshift measurements but may also prove to be members. Redshift measurements are presented and the use of indirect means for establishing cluster membership is discussed.« less

Dwarf Galaxies in the Coma Cluster. II. Photometry and Analysis

NASA Astrophysics Data System (ADS)

Secker, J.; Harris, W. E.; Plummer, J. D.

1997-12-01

We use the data set derived in our previous paper (Secker & Harris 1997) to study the dwarf galaxy population in the central =~ 700 arcmin(2) of the Coma cluster, the majority of which are early-type dwarf elliptical (dE) galaxies. Analysis of the statistically-decontaminated dE galaxy sequence in the color-magnitude diagram reveals that the mean dE color at R = 18.0 mag is (B-R) =~ 1.4 mag, but that a highly significant trend of color with magnitude exists (Delta (B-R)/Delta R = -0.056+/-0.002 mag) in the sense that fainter dEs are bluer and thus presumably more metal-poor. The mean color of the faintest dEs in our sample is (B-R) =~ 1.15 mag, consistent with a color measurement of the diffuse intracluster light in the Coma core. This intracluster light could then have originated from the tidal disruption of faint dEs in the cluster core. The total galaxy luminosity function (LF) is well modeled as the sum of a log-normal distribution for the giant galaxies, and a Schechter function for the dE galaxies with a faint-end slope alpha = -1.41+/-0.05. This value of alpha is consistent with those measured for the Virgo and Fornax clusters. The spatial distribution of the faint dE galaxies (19.0 < R <= 22.5 mag) is well fit by a standard King model with a central surface density of Sigma_0 = 1.17 dEs arcmin(-2) and a core radius R_c = 22.15 arcmin ( =~ 0.46h(-1) Mpc). This core is significantly larger than the R_c = 13.71 arcmin ( =~ 0.29h(-1) Mpc) found for the cluster giants and the brighter dEs (R <= 19.0 mag), again consistent with the idea that faint dEs in the dense core have been disrupted. Finally, we find that most dEs belong to the general Coma cluster potential rather than as satellites of individual giant galaxies: An analysis of the number counts around 10 cluster giants reveals that they each have on average 4+/- 1 dE companions within a projected radius of 13.9h(-1) kpc. (SECTION: Galaxies)

The Next Generation Fornax Survey (NGFS). II. The Central Dwarf Galaxy Population

NASA Astrophysics Data System (ADS)

Eigenthaler, Paul; Puzia, Thomas H.; Taylor, Matthew A.; Ordenes-Briceño, Yasna; Muñoz, Roberto P.; Ribbeck, Karen X.; Alamo-Martínez, Karla A.; Zhang, Hongxin; Ángel, Simón; Capaccioli, Massimo; Côté, Patrick; Ferrarese, Laura; Galaz, Gaspar; Grebel, Eva K.; Hempel, Maren; Hilker, Michael; Lançon, Ariane; Mieske, Steffen; Miller, Bryan; Paolillo, Maurizio; Powalka, Mathieu; Richtler, Tom; Roediger, Joel; Rong, Yu; Sánchez-Janssen, Ruben; Spengler, Chelsea

2018-03-01

We present a photometric study of the dwarf galaxy population in the core region (≲r vir/4) of the Fornax galaxy cluster based on deep u‧g‧i‧ photometry from the Next Generation Fornax Cluster Survey. All imaging data were obtained with the Dark Energy Camera mounted on the 4 m Blanco telescope at the Cerro Tololo Interamerican Observatory. We identify 258 dwarf galaxy candidates with luminosities ‑17 ≲ M g‧ ≲ ‑8 mag, corresponding to typical stellar masses of 9.5≳ {log}{{ \\mathcal M }}\\star /{M}ȯ ≳ 5.5, reaching ∼3 mag deeper in point-source luminosity and ∼4 mag deeper in surface brightness sensitivity compared to the classic Fornax Cluster Catalog. Morphological analysis shows that the dwarf galaxy surface-brightness profiles are well represented by single-component Sérsic models with average Sérsic indices of < n{> }u\\prime ,g\\prime ,i\\prime =(0.78{--}0.83)+/- 0.02 and average effective radii of < {r}e{> }u\\prime ,g\\prime ,i\\prime =(0.67{--}0.70)+/- 0.02 {kpc}. Color–magnitude relations indicate a flattening of the galaxy red sequence at faint galaxy luminosities, similar to the one recently discovered in the Virgo cluster. A comparison with population synthesis models and the galaxy mass–metallicity relation reveals that the average faint dwarf galaxy is likely older than ∼5 Gyr. We study galaxy scaling relations between stellar mass, effective radius, and stellar mass surface density over a stellar mass range covering six orders of magnitude. We find that over the sampled stellar mass range several distinct mechanisms of galaxy mass assembly can be identified: (1) dwarf galaxies assemble mass inside the half-mass radius up to {log}{{ \\mathcal M }}\\star ≈ 8.0, (2) isometric mass assembly occurs in the range 8.0 ≲ {log}{{ \\mathcal M }}\\star /{M}ȯ ≲ 10.5, and (3) massive galaxies assemble stellar mass predominantly in their halos at {log}{{ \\mathcal M }}\\star ≈ 10.5 and above.

White Dwarfs in the SDSS Photometric Footprint

NASA Astrophysics Data System (ADS)

Gentile Fusillo, N. P.; Girven, J.; Gänsicke, B.

2013-01-01

Attempts to create a homogeneous catalogue of white dwarfs have always been faced with the challenge posed by the intrinsic faintness of these objects. In recent years, thanks to large area surveys like the Sloan Digital Sky Survey (SDSS), the size of the known white dwarf population has increased dramatically, but, in order to carry out a statical study on the population of white dwarfs, it is necessary to have a reliable and well-defined selection method. We present a method which uses cuts in colour-colour space to select from DR7 16785 bright (g ≤ 19) photometric DA white dwarf candidates (Girven et al. 2011). The selection is 62% efficient in returning DA white dwarfs and produces a DA sample which is 95% complete for Teff > 8000 K. This sample contains 4636 spectroscopically confirmed DA white dwarfs; i.e. a ˜70% increase compared to Eisenstein et al.'s sample. As a first application of the SDSS DR7 DA candidates sample we cross correlated it with Data Release 8 of UKIDSS Large Area Survey with the aim of identifying white dwarfs which exhibit an infrared excess consistent with the presence of low mass stellar companions or dusty debris discs. Our current work aims to extend the photometric selection to all types of white dwarfs, using reduced proper motion as a further constrain. We expect to find a total of ˜20 000 photometric white dwarf candidates with g ≤ 19 in the footprint of SDSS DR8.

The origin of ultra-compact binaries

NASA Technical Reports Server (NTRS)

Hachisu, Izumi; Miyaji, Shigeki; Saio, Hideyuki

1987-01-01

The origin of ultra-compact binaries composed of a neutron star and a low-mass (about 0.06 solar mass) white dwarf is considered. Taking account of the systemic losses of mass and angular momentum, it was found that a serious difficulty exists in the scenarios which involve tidal captures of a normal star (a main sequence star or a red giant) by a neutron star. This difficulty can be avoided if a red giant star is captured by a massive white dwarf (M is approx. greater than 1.2 solar masses), which becomes a neutron star through the accretion induced collapse.

A Survey of Stellar Populations in Ultra-Diffuse Galaxies

NASA Astrophysics Data System (ADS)

Romanowsky, Aaron; Laine, Seppo; Pandya, Viraj; Brodie, Jean; Glaccum, Bill; van Dokkum, Pieter; Alabi, Busola; Cohen, Yotam; Danieli, Shany; Abraham, Bob; Martinez-Delgado, David; Greco, Johnny; Greene, Jenny

2018-05-01

Ultra-diffuse galaxies (UDGs) are a recently identified, mysterious class of galaxies with luminosities like dwarfs, but sizes like giants. Quiescent UDGs are found in all environments from cluster to isolated, and intensive study has revealed three very distinctive sub-types: low surface brightness dwarfs, 'failed galaxies', and low-dark-matter UDGs. Following up on our recent, successful Spitzer pilot work to characterize the stellar populations (ages and metallicities) of UDGs, we propose a survey of 25 UDGs with a range of optical properties and environments, in order to understand the formation histories of different the different UDG sub-types.

New Metallicty Calibration for Dwarfs for the RGU-Photometry

NASA Astrophysics Data System (ADS)

Karaali, Salih; Bilir, Selçuk

2002-10-01

We adopted the procedure of Carney to obtain a metallicity calibration for dwarfs for the RGU photometry. For this purpose we selected 76 dwarfs of different metallicities from Carney, and Strobel et al., and evaluated their δ(U-G) ultra-violet excess relative to Hyades by transforming their UBV magnitudes to RGU via metallicity dependent equations of Ak-Güngör. The δ0.6/ΔM normalized factors of Sandage transform Δ(U-G) excess at any G-R to δ=δ1.08, i.e.: the ultra-violet excess at G-R = 1.08 mag, corresponding to B-V = 0.60 mag in the UBV-system. Finally, the (δ, [Fe/H]) couples were fitted by the equation [Fe/H] = 0.11-2.22δ-7.95δ2. This calibration covers the metallicity interval (-2.20, +0.20) dex.

Discovery of a Dwarf Poststarburst Galaxy near a High Column Density Local Lyα Absorber

NASA Astrophysics Data System (ADS)

Stocke, John T.; Keeney, Brian A.; McLin, Kevin M.; Rosenberg, Jessica L.; Weymann, R. J.; Giroux, Mark L.

2004-07-01

We report the discovery of a dwarf (MB=-13.9) poststarburst galaxy coincident in recession velocity (within uncertainties) with the highest column density absorber (NHI=1015.85 cm-2 at cz=1586 km s- 1) in the 3C 273 sight line. This galaxy is by far the closest galaxy to this absorber, projected just 71h-170 kpc on the sky from the sight line. The mean properties of the stellar populations in this galaxy are consistent with a massive starburst ~3.5 Gyr ago, whose attendant supernovae, we argue, could have driven sufficient gas from this galaxy to explain the nearby absorber. Beyond its proximity on the sky and in recession velocity, the further evidence in favor of this conclusion includes both a match in the metallicities of absorber and galaxy and the fact that the absorber has an overabundance of Si/C, suggesting recent Type II supernova enrichment. Thus, this galaxy and its ejecta are in the expected intermediate stage in the fading dwarf evolutionary sequence envisioned by Babul & Rees to explain the abundance of faint blue galaxies at intermediate redshifts. While this one instance of a QSO metal-line absorber and a nearby dwarf galaxy is not proof of a trend, a similar dwarf galaxy would be too faint to be observed by galaxy surveys around more distant metal-line absorbers. Thus, we cannot exclude the possibility that dwarf galaxies are primarily responsible for weak (NHI=1014-1017 cm-2) metal-line absorption systems in general. If a large fraction of the dwarf galaxies expected to exist at high redshift had a similar history (i.e., they had a massive starburst that removed all or most of their gas), these galaxies could account for at least several hundred high-z metal-line absorbers along the line of sight to a high-z QSO. The volume-filling factor for this gas, however, would be less than 1%. ID="FN1"> 1Based on observations made with the Apache Point 3.5 m telescope, operated by the Astronomical Research Consortium, and the 2.6 m du Pont telescope of the Las Campanas Observatory, operated by the Carnegie Institution of Washington, DC, and Pasadena, CA.

HUBBLE PINPOINTS WHITE DWARFS IN GLOBULAR CLUSTER

NASA Technical Reports Server (NTRS)

2002-01-01

Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope uncovered the oldest burned-out stars in our Milky Way Galaxy. Located in the globular cluster M4, these small, dying stars - called white dwarfs - are giving astronomers a fresh reading on one of the biggest questions in astronomy: How old is the universe? The ancient white dwarfs in M4 are about 12 to 13 billion years old. After accounting for the time it took the cluster to form after the big bang, astronomers found that the age of the white dwarfs agrees with previous estimates for the universe's age. In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's 0.9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope. The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles pinpoint the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars. Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the oldest stars puts astronomers within arm's reach of the universe's age. M4 is 7,000 light-years away in the constellation Scorpius. Hubble's Wide Field and Planetary Camera 2 made the observations from January through April 2001. These optical observations were combined to create the above images. Spectral data were also taken. Credit for Hubble telescope photos: NASA and H. Richer (University of British Columbia) Credit for ground-based photo: NOAO/AURA/NSF

Polluted White Dwarf (Artist's Concept)

NASA Image and Video Library

2017-11-01

This artist's concept shows an exoplanet and debris disk orbiting a polluted white dwarf. White dwarfs are dim, dense remnants of stars similar to the Sun that have exhausted their nuclear fuel and blown off their outer layers. By "pollution," astronomers mean heavy elements invading the photospheres -- the outer atmospheres -- of these stars. The leading explanation is that exoplanets could push small rocky bodies toward the star, whose powerful gravity would pulverize them into dust. That dust, containing heavy elements from the torn-apart body, would then fall on the star. NASA's Spitzer Space Telescope has been instrumental in expanding the field of polluted white dwarfs orbited by hot, dusty disks. Since launch in 2004, Spitzer has confirmed about 40 of these special stars. Another space telescope, NASA's Wide-field Infrared Survey Explorer (WISE), also detected a handful, bringing the total up to about four dozen known today. Because these objects are so faint, infrared light is crucial to identifying them. https://photojournal.jpl.nasa.gov/catalog/PIA22084

Resolving the extended stellar halos of nearby galaxies: the wide-field PISCeS survey

NASA Astrophysics Data System (ADS)

Crnojevic, Denija; Sand, David; Spekkens, Kristine; Caldwell, Nelson; Guhathakurta, Puragra; McLeod, Brian; Seth, Anil; Simon, Joshua D.; Strader, Jay; Toloba, Elisa

2018-01-01

I will present results from the wide-field Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS): the resolved stellar halos of two nearby galaxies (the spiral NGC253 and the elliptical Centaurus A, D~3.7 Mpc) are investigated out to a galactocentric radius of 150 kpc with Magellan/Megacam. The survey pushes the limits of near-field cosmology beyond the Local Group, by characterizing the stellar content (ages, metallicities, gradients) of extended halos and their substructures in two environments substantially different from the Local Group, i.e. the loose Sculptor group of galaxies and the Centaurus A group dominated by an elliptical. PISCeS has to date led to the discovery of 11 confirmed satellites as faint as M_V=-8 (including Ultra Diffuse Galaxies), streams and tidal substructures with surface brigthness limits as low as ~32 mag/arcsec^2, and hundreds of globular cluster/ultra-compact dwarf candidates. The unique strength of PISCeS is the exquisite synergy between the wide-field, ground-based survey and its extensive imaging and spectroscopic follow-up (HST, Keck, VLT, Magellan, AAT), which constitute the first accurate characterization of the past and ongoing accretion processes shaping the halos of these nearby galaxies. Our observational campaign will not only provide crucial constraints to quantitatively inform theoretical models of galaxy formation and evolution, but it also represents a necessary testbed in preparation for future very large datasets stemming from the next generation of ground-based (LSST, TMT, GMT) as well as space-borne (JWST, WFIRST) telescopes.

Analyzing the Formation of Ultra-compact Dwarfs through Stellar Populations

NASA Astrophysics Data System (ADS)

Seshadri, Anish; Wang, Carolyn; Romanowsky, Aaron J.; Martin-navarro, Ignacio

2017-01-01

Since their discovery in 1999, ultra-compact dwarfs (UCDs) have been the subjects of intense study. Their small size, yet tremendous mass, brings into question their place among celestial objects. Are they galaxies or globular clusters? The answer to this question could come from analyzing how they formed. Thus, the goal of this project is to test one of the theories for the formation of UCDs, the theory of tidal stripping.This project approaches the issue by looking at dwarf galaxies currently in the process of stripping to understand formation history. Over twenty such dwarf galaxies were identified and their stellar populations analyzed. Using modeling techniques on spectroscopic and photometric data, the age, metallicity, and color of each object was identified. By objectively categorizing each object into a stage of evolution in the process of tidal stripping, a virtual timeline was built for the formation of UCDs. Data for each object were plotted vs. stage of formation, with pristine dwarfs and UCDs signifying the endpoints. Trends in the data revealed a natural progression over all stages of evolution, showing that tidally stripped dwarfs likely represent an intermediate stage in the formation of UCDs.This research was supported by NSF Grant AST-1515084. Most of this work was carried out by high school students working under the auspices of the Science Internship Program at UC Santa Cruz.

An observer's guide to the (Local Group) dwarf galaxies: predictions for their own dwarf satellite populations

NASA Astrophysics Data System (ADS)

Dooley, Gregory A.; Peter, Annika H. G.; Yang, Tianyi; Willman, Beth; Griffen, Brendan F.; Frebel, Anna

2017-11-01

A recent surge in the discovery of new ultrafaint dwarf satellites of the Milky Way has inspired the idea of searching for faint satellites, 103 M⊙ 99 per cent chance that at least one satellite with stellar mass M* > 105 M⊙ exists around the combined five Local Group field dwarf galaxies with the largest stellar mass. When considering satellites with M* > 104 M⊙, we predict a combined 5-25 satellites for the five largest field dwarfs, and 10-50 for the whole Local Group field dwarf population. Because of the relatively small number of predicted dwarfs, and their extended spatial distribution, a large fraction each Local Group dwarf's virial volume will need to be surveyed to guarantee discoveries. We compute the predicted number of satellites in a given field of view of specific Local Group galaxies, as a function of minimum satellite luminosity, and explicitly obtain such values for the Solitary Local dwarfs survey. Uncertainties in abundance-matching and reionization models are large, implying that comprehensive searches could lead to refinements of both models.

Estimating the Mass of the Milky Way Using the Ensemble of Classical Satellite Galaxies

NASA Astrophysics Data System (ADS)

Patel, Ekta; Besla, Gurtina; Mandel, Kaisey; Sohn, Sangmo Tony

2018-04-01

High precision proper motion (PM) measurements are available for approximately 20% of all known dwarf satellite galaxies of the Milky Way (MW). Here we extend the Bayesian framework of Patel et al. to include all MW satellites with measured 6D phase-space information and apply it with the Illustris-Dark simulation to constrain the MW’s mass. Using the properties of each MW satellite individually, we find that the scatter among mass estimates is reduced when the magnitude of specific orbital angular momentum (j) is adopted, rather than their combined instantaneous positions and velocities. We also find that high j satellites (i.e., Leo II) constrain the upper limits for the MW’s mass and low j satellites, rather than the highest speed satellites (i.e., Leo I and Large Magellanic Cloud), set the lower mass limits. When j of all classical satellites is used to simultaneously estimate the MW’s mass, we conclude the halo mass is 0.85+0.23 ‑0.26 × 1012 {M}ȯ (including Sagittarius dSph) and 0.96+0.29 ‑0.28 × 1012 {M}ȯ (excluding Sagittarius dSph), cautioning that low j satellites on decaying orbits like Sagittarius dSph may bias the distribution. These estimates markedly reduce the current factor of two spread in the mass range of the MW. We also find a well-defined relationship between host halo mass and satellite j distribution, which yields the prediction that upcoming PMs for ultra-faint dwarfs should reveal j within 5 × 103–104 kpc km s‑1. This is a promising method to significantly constrain the cosmologically expected mass range for the MW and eventually M31 as more satellite PMs become available.

A MegaCam Survey of Outer Halo Satellites. I. Description of the Survey

NASA Astrophysics Data System (ADS)

Muñoz, Ricardo R.; Côté, Patrick; Santana, Felipe A.; Geha, Marla; Simon, Joshua D.; Oyarzún, Grecco A.; Stetson, Peter B.; Djorgovski, S. G.

2018-06-01

We describe a deep, systematic imaging study of satellites in the outer halo of the Milky Way. Our sample consists of 58 stellar overdensities—i.e., substructures classified as either globular clusters, classical dwarf galaxies, or ultra-faint dwarf galaxies—that are located at Galactocentric distances of R GC ≥ 25 kpc (outer halo) and out to ∼400 kpc. This includes 44 objects for which we have acquired deep, wide-field, g- and r-band imaging with the MegaCam mosaic cameras on the 3.6 m Canada–France–Hawaii Telescope and the 6.5 m Magellan-Clay telescope. These data are supplemented by archival imaging, or published gr photometry, for an additional 14 objects, most of which were discovered recently in the Dark Energy Survey (DES). We describe the scientific motivation for our survey, including sample selection, observing strategy, data reduction pipeline, calibration procedures, and the depth and precision of the photometry. The typical 5σ point-source limiting magnitudes for our MegaCam imaging—which collectively covers an area of ≈52 deg2—are g lim ≃ 25.6 and r lim ≃ 25.3 AB mag. These limits are comparable to those from the coadded DES images and are roughly a half-magnitude deeper than will be reached in a single visit with the Large Synoptic Survey Telescope. Our photometric catalog thus provides the deepest and most uniform photometric database of Milky Way satellites available for the foreseeable future. In other papers in this series, we have used these data to explore the blue straggler populations in these objects, their density distributions, star formation histories, scaling relations, and possible foreground structures.

Using photometrically selected metal-poor stars to study dwarf galaxies and the Galactic stellar halo

NASA Astrophysics Data System (ADS)

Youakim, Kris; Starkenburg, Else; Martin, Nicolas; Pristine Team

2018-06-01

The Pristine survey is a narrow-band photometric survey designed to efficiently search for extremely metal-poor (EMP) stars. In the first three years of the survey, it has demonstrated great efficiency at finding EMP stars, and also great promise for increasing the current, small sample of the most metal-poor stars. The present sky coverage is ~2500 square degrees in the Northern Galactic Halo, including several individual fields targeting dwarf galaxies. By efficiently identifying member stars in the outskirts of known faint dwarf galaxies, the dynamical histories and chemical abundance patterns of these systems can be understood in greater detail. Additionally, with reliable photometric metallicities over a large sky coverage it is possible to perform a large scale clustering analysis in the Milky Way halo, and investigate the characteristic scale of substructure at different metallicities. This can reveal important details about the process of building up the halo through dwarf galaxy accretion, and offer insight into the connection between dwarf galaxies and the Milky Way halo. In this talk I will outline our results on the search for the most pristine stars, with a focus on how we are using this information to advance our understanding of dwarf galaxies and their contribution to the formation of the Galactic stellar halo.

Evidence for dwarf stars at D of about 100 kiloparsecs near the Sextans dwarf spheroidal galaxy

NASA Technical Reports Server (NTRS)

Gould, Andrew; Guhathakurta, Puragra; Richstone, Douglas; Flynn, Chris

1992-01-01

A method is presented for detecting individual, metal-poor, dwarf stars at distances less than about 150 kpc - a method specifically designed to filter out stars from among the much more numerous faint background field galaxies on the basis of broad-band colors. This technique is applied to two fields at high Galactic latitude, for which there are deep CCD data in four bands ranging from 3600 to 9000 A. The field in Sextans probably contains more than about five dwarf stars with BJ not greater than 25.5. These are consistent with being at a common distance about 100 kpc and lie about 1.7 deg from the newly discovered dwarf galaxy in Sextans whose distance is about 85 +/- 10 kpc. The stars lie near the major axis of the galaxy and are near or beyond the tidal radius. The second field, toward the south Galactic pole, may contain up to about five extra-Galactic stars, but these show no evidence for being at a common distance. Possible applications of this type technique are discussed, and it is shown that even very low surface brightness star clusters or dwarf galaxies may be detected at distances less than about 1 Mpc.

Detection of Double White Dwarf Binaries with Gaia, LSST and eLISA

NASA Astrophysics Data System (ADS)

Korol, V.; Rossi, E. M.; Groot, P. J.

2017-03-01

According to simulations around 108 double degenerate white dwarf binaries are expected to be present in the Milky Way. Due to their intrinsic faintness, the detection of these systems is a challenge, and the total number of detected sources so far amounts only to a few tens. This will change in the next two decades with the advent of Gaia, the LSST and eLISA. We present an estimation of how many compact DWDs with orbital periods less than a few hours we will be able to detect 1) through electromagnetic radiation with Gaia and LSST and 2) through gravitational wave radiation with eLISA. We find that the sample of simultaneous electromagnetic and gravitational waves detections is expected to be substantial, and will provide us a powerful tool for probing the white dwarf astrophysics and the structure of the Milky Way, letting us into the era of multi-messenger astronomy for these sources.

The Gas in Virgo’s “Red and Dead” Dwarf Elliptical Galaxies

NASA Astrophysics Data System (ADS)

Hallenbeck, Gregory L.; Koopmann, Rebecca A.

2017-01-01

As star-forming dwarf irregulars and faint spirals fall onto a cluster, their gas content is easily and quickly removed by ram-pressure stripping or other cluster forces. Residual signs of star formation cease within 100 Myr, and only after approximately 1 Gyr do their optical features transition to elliptical.Despite this, ALFALFA has uncovered a population of three “red and dead” dwarf ellipticals in the Virgo Cluster which still have detectable reservoirs of HI. These dwarf ellipticals are extremely gas-rich—as gas-rich as the cluster’s star-forming dwarf irregulars (Hallenbeck et al. 2012). Where does this gas come from? We consider two possibilities. First, that the gas is recently acquired, and has not yet had time to form stars. Second, that the gas is primordial, and has been disrupted from being able to form stars during the current epoch.We present deep optical (using CFHT and KPNO) and HI (Arecibo and VLA) observations of this sample to demonstrate that this gas is primordial. These observations show that all three galaxies have exponentially decreasing profiles characteristic of dwarf ellipticals and that their rotation velocities are extremely low. However, like more massive elliptical galaxies with HI, these dwarf galaxies show irregular optical morphology. For one target, VCC 190, we additionally observe an HI tail consistent with a recent interaction with the massive spiral galaxy NGC 4224.

Early gas stripping as the origin of the darkest galaxies in the Universe.

PubMed

Mayer, L; Kazantzidis, S; Mastropietro, C; Wadsley, J

2007-02-15

The known galaxies most dominated by dark matter (Draco, Ursa Minor and Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies. They are members of a class of faint galaxies, devoid of gas, known as dwarf spheroidals, and have by far the highest ratio of dark to luminous matter. None of the models proposed to unravel their origin can simultaneously explain their exceptional dark matter content and their proximity to a much larger galaxy. Here we report simulations showing that the progenitors of these galaxies were probably gas-dominated dwarf galaxies that became satellites of a larger galaxy earlier than the other dwarf spheroidals. We find that a combination of tidal shocks and ram pressure swept away the entire gas content of such progenitors about ten billion years ago because heating by the cosmic ultraviolet background kept the gas loosely bound: a tiny stellar component embedded in a relatively massive dark halo survived until today. All luminous galaxies should be surrounded by a few extremely dark-matter-dominated dwarf spheroidal satellites, and these should have the shortest orbital periods among dwarf spheroidals because they were accreted early.

Radial velocity studies of cool stars.

PubMed

Jones, Hugh R A; Barnes, John; Tuomi, Mikko; Jenkins, James S; Anglada-Escude, Guillem

2014-04-28

Our current view of exoplanets is one derived primarily from solar-like stars with a strong focus on understanding our Solar System. Our knowledge about the properties of exoplanets around the dominant stellar population by number, the so-called low-mass stars or M dwarfs, is much more cursory. Based on radial velocity discoveries, we find that the semi-major axis distribution of M dwarf planets appears to be broadly similar to those around more massive stars and thus formation and migration processes might be similar to heavier stars. However, we find that the mass of M dwarf planets is relatively much lower than the expected mass dependency based on stellar mass and thus infer that planet formation efficiency around low-mass stars is relatively impaired. We consider techniques to overcome the practical issue of obtaining good quality radial velocity data for M dwarfs despite their faintness and sustained activity and emphasize (i) the wavelength sensitivity of radial velocity signals, (ii) the combination of radial velocity data from different experiments for robust detection of small amplitude signals, and (iii) the selection of targets and radial velocity interpretation of late-type M dwarfs should consider Hα behaviour.

TWO LOCAL VOLUME DWARF GALAXIES DISCOVERED IN 21 cm EMISSION: PISCES A AND B

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tollerud, Erik J.; Geha, Marla C.; Grcevich, Jana

2015-01-01

We report the discovery of two dwarf galaxies, Pisces A and B, from a blind 21 cm H I search. These were the only two galaxies found via optical imaging and spectroscopy of 22 H I clouds identified in the GALFA-H I survey as dwarf galaxy candidates. They have properties consistent with being in the Local Volume (<10 Mpc), and one has resolved stellar populations such that it may be on the outer edge of the Local Group (∼1 Mpc from M31). While the distance uncertainty makes interpretation ambiguous, these may be among the faintest star-forming galaxies known. Additionally, rough estimatesmore » comparing these galaxies to ΛCDM dark matter simulations suggest consistency in number density, implying that the dark matter halos likely to host these galaxies are primarily H I-rich. The galaxies may thus be indicative of a large population of dwarfs at the limit of detectability that are comparable to the faint satellites of the Local Group. Because they are outside the influence of a large dark matter halo to alter their evolution, these galaxies can provide critical anchors to dwarf galaxy formation models.« less

HOW THE FIRST STARS SHAPED THE FAINTEST GAS-DOMINATED DWARF GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Verbeke, R.; Vandenbroucke, B.; Rijcke, S. De, E-mail: robbert.verbeke@UGent.be

2015-12-20

Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account has existed of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of themore » first population of stars on a dwarf’s star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully–Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques.« less

Finding the elusive substellar members of young moving groups

NASA Astrophysics Data System (ADS)

Aller, Kimberly Mei

Young moving groups (YMGs) consist of coeval, comoving stars, with ages between 10-100Myrs, that have migrated from their origins after formation. They provide a valuable link between ongoing star formation in molecular clouds (˜1Myr) and old field stars (≥1Gyr). However, previous searches based on optical surveys such as Hipparcos and the Palomar Sky Survey were insensitive to these very faint cool dwarfs. More recent surveys with GALEX have begun to reveal the nearby (<25 pc) low-mass members (≥ 0.1 M solar massses) but the cool, substellar members have remained elusive. We have increased the search volume by a factor of ˜10 using a novel combination of photometry and proper motions from Pan-STARRS, WISE, and 2MASS in order to uncover the missing substellar members down to ≥ 00.1 M solar massses (at 10Myr). We have obtained NIR low-resolution spectroscopy and confirmed the youth of 65 new ultracool dwarf YMG candidates. We also obtained high-resolution NIR spectroscopy to determine radial velocities for our young brown dwarfs. With our RVs and PS1 parallaxes, we have nearly doubled the number of confirmed bona fide substellar YMG members, which are also brown dwarf age benchmarks. Our new young brown dwarfs empirically define the substellar spectral evolution with age and provide us with a snapshot of brown dwarf evolution. Finally, our resulting young brown dwarfs will be valuable targets for future surveys of brown dwarf binarity and young exoplanet characterization.

Weather on Other Worlds. IV. Hα Emission and Photometric Variability Are Not Correlated in L0–T8 Dwarfs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miles-Páez, Paulo A.; Metchev, Stanimir A.; Heinze, Aren

Recent photometric studies have revealed that surface spots that produce flux variations are present on virtually all L and T dwarfs. Their likely magnetic or dusty nature has been a much-debated problem, the resolution to which has been hindered by paucity of diagnostic multi-wavelength observations. To test for a correlation between magnetic activity and photometric variability, we searched for H α emission among eight L3–T2 ultra-cool dwarfs with extensive previous photometric monitoring, some of which are known to be variable at 3.6 μ m or 4.5 μ m. We detected H α only in the non-variable T2 dwarf 2MASS J12545393−0122474.more » The remaining seven objects do not show H α emission, even though six of them are known to vary photometrically. Combining our results with those for 86 other L and T dwarfs from the literature show that the detection rate of H α emission is very high (94%) for spectral types between L0 and L3.5 and much smaller (20%) for spectral types ≥L4, while the detection rate of photometric variability is approximately constant (30%–55%) from L0 to T8 dwarfs. We conclude that chromospheric activity, as evidenced by H α emission, and large-amplitude photometric variability are not correlated. Consequently, dust clouds are the dominant driver of the observed variability of ultra-cool dwarfs at spectral types, at least as early as L0.« less

The Origin of Dwarf Ellipticals in the Virgo Cluster

NASA Astrophysics Data System (ADS)

Boselli, A.; Boissier, S.; Cortese, L.; Gavazzi, G.

2008-02-01

We study the evolution of dwarf (LH < 109.6 LH⊙) star-forming and quiescent galaxies in the Virgo Cluster by comparing their UV to radio centimetric properties to the predictions of multizone chemospectrophotometric models of galaxy evolution especially tuned to take into account the perturbations induced by the interaction with the cluster intergalactic medium. Our models simulate one or multiple ram pressure stripping events and galaxy starvation. Models predict that all star-forming dwarf galaxies entering the cluster for the first time loose most, if not all, of their atomic gas content, quenching on short timescales (<=150 Myr) their activity of star formation. These dwarf galaxies soon become red and quiescent, gas metal-rich objects with spectrophotometric and structural properties similar to those of dwarf ellipticals. Young, low-luminosity, high surface brightness star-forming galaxies such as late-type spirals and BCDs are probably the progenitors of relatively massive dwarf ellipticals, while it is likely that low surface brightness Magellanic irregulars evolve into very low surface brightness quiescent objects hardly detectable in ground-based imaging surveys. The small number of dwarf galaxies with physical properties intermediate between those of star-forming and quiescent systems is consistent with a rapid (<1 Gyr) transitional phase between the two dwarf galaxy populations. These results, combined with statistical considerations, are consistent with the idea that most of the dwarf ellipticals dominating the faint end of the Virgo luminosity function were initially star-forming systems, accreted by the cluster and stripped of their gas by one or subsequent ram pressure stripping events.

Weighing Ultra-Cool Stars

NASA Astrophysics Data System (ADS)

2004-05-01

Large Ground-Based Telescopes and Hubble Team-Up to Perform First Direct Brown Dwarf Mass Measurement [1] Summary Using ESO's Very Large Telescope at Paranal and a suite of ground- and space-based telescopes in a four-year long study, an international team of astronomers has measured for the first time the mass of an ultra-cool star and its companion brown dwarf. The two stars form a binary system and orbit each other in about 10 years. The team obtained high-resolution near-infrared images; on the ground, they defeated the blurring effect of the terrestrial atmosphere by means of adaptive optics techniques. By precisely determining the orbit projected on the sky, the astronomers were able to measure the total mass of the stars. Additional data and comparison with stellar models then yield the mass of each of the components. The heavier of the two stars has a mass around 8.5% of the mass of the Sun and its brown dwarf companion is even lighter, only 6% of the solar mass. Both objects are relatively young with an age of about 500-1,000 million years. These observations represent a decisive step towards the still missing calibration of stellar evolution models for very-low mass stars. PR Photo 19a/04: Orbit of the ultra-cool stars in 2MASSW J0746425+2000321. PR Photo 19b/04: Animated Gif of the orbital motion. Telephone number star Even though astronomers have found several hundreds of very low mass stars and brown dwarfs, the fundamental properties of these extreme objects, such as masses and surface temperatures, are still not well known. Within the cosmic zoo, these ultra-cool stars represent a class of "intermediate" objects between giant planets - like Jupiter - and "normal" stars less massive than our Sun, and to understand them well is therefore crucial to the field of stellar astrophysics. The problem with these ultra-cool stars is that contrary to normal stars that burn hydrogen in their central core, no unique relation exists between the luminosity of the star and its mass. Indeed, luminosities and surface temperatures of ultra-cool dwarf stars depend both on their age and their mass. An older, somewhat more massive ultra-cool dwarf can thus have exactly the same temperature as a younger, less massive one. It is therefore a basic goal of modern astrophysics to obtain independently the masses of an ultra-cool dwarf star. This is in principle possible by studying such objects that are members in a binary system. This is precisely what an international team of astronomers [2] has now done in a four-year long study of a binary stellar system with an ultra-cool dwarf star, using a plethora of top telescopic facilities, including ESO's Very Large Telescope, as well as Keck I and Gemini North in Hawaii and also the Hubble Space Telescope. This system - with the telephone number name of 2MASSW J0746425+2000321 [3]- is located at a distance of 40 light-years. Beating the seeing ESO PR Photo 19a/04 ESO PR Photo 19a/04 Orbit of the ultra-cool stars in 2MASSW J0746425+2000321 [Preview - JPEG: 400 x 548 pix - 121k] [Normal - JPEG: 800 x 1095 pix - 320k] [Hires - JPEG: 2591 x 3546 pix - 1.8M] [Hires - TIFF: 2591 x 3546 pix - 36.8M] ESO PR Photo 19b/04 ESO PR Photo 19b/04 Animated GIF showing the orbital motion (size: 416 kb) Caption: ESO PR Photo 19a/04 shows the orbit of the brown dwarf around the ultra-cool dwarf. Each red dot on the orbit corresponds to one observation made with a ground- or space-based telescope. The observations cover 60% of the whole orbit. ESO PR Photo 19b/04 is an animated Gif showing the motion of the brown dwarf and the various high-resolution images obtained by the astronomers. The astronomers used high-angular-resolution imaging to see both stars in the binary system and to measure their motion over a four-year period. However, this is more easily said than done, as the separation on the sky between the two stars is quite small: between 0.13 and 0.22 arcsec. This corresponds to the size of a 1-Euro coin, seen at a distance of about 25 km. This separation is so small that it is normally not possible to differentiate the two stars due to the blurring effect of atmospheric turbulence (the "seeing"). It is therefore necessary to use the technique of adaptive optics. This wonderful method is based on the measurement of the image quality in real-time and sending corresponding corrective signals up to 100 times every second to a small deformable mirror, located in front of the detector. As the mirror continuously modifies its shape, the disturbing effect of the turbulence is neutralised. Applied at the VLT, this technique has resulted in images which are at least ten times sharper than the "seeing" and which therefore show many more details in the observed objects. At the Very Large Telescope, the astronomers used the state-of-the-art adaptive optics NACO instrument [4]. Says Hervé Bouy, principal author of the paper presenting the results described here: "NACO offers the possibility to work in the infrared and is therefore ideally suited for the study of ultra-cool stars, which emit most of their light in this wavelength range. With the combination of the high efficiency of NACO and the VLT, and the excellent atmospheric conditions prevailing at Paranal, we were able to achieve very sharp images of this binary stellar system, almost as good as if the telescope were located in space." Ultra-cool and on diet During their four-year long study, seven different relative positions of the two components of the binary system were measured and Hervé Bouy and his co-workers were able to determine with good precision the stellar orbits. They find that the two stars revolve around each other once every 10 years and that their physical separation is only 2.5 times the distance of the Earth to the Sun - as astronomers say, 2.5 Astronomical Units. Using Kepler's laws, it is then straightforward to derive the total mass of the system. The obtained value is less than 15 % of the mass of the Sun. The astronomers then used the photometric data of each star obtained in several wavebands, as well as spectra obtained with the Hubble Space Telescope to study the two objects in more detail. Using the latest stellar models of the group of the Ecole Normale Supérieure de Lyon, they found that both stars have roughly the same surface temperature, around 1500 °C (1800 K). For a star, this is ultra-cool indeed - by comparison, the surface temperature of the Sun is more than three times higher. Using theoretical models, the team also found that the two stars are rather young (in astrophysical terms) - their age is between 500 and 1,000 million years only. The more massive of the two has a mass between 7.5 and 9.5% the mass of the Sun, while its companion has a mass between 5 and 7% of the solar mass. Objects weighing less than about 7% of our Sun have been variously called "Brown Dwarfs", "Failed Stars" or "Super Planets". Indeed, since they have no sustained energy generation by thermal nuclear reactions in their interior, many of their properties are more similar to those of giant gas planets in our own solar system such as Jupiter, than to stars like the Sun. The system 2MASSW J0746425+2000321 is thus apparently made up of a brown dwarf orbiting a slightly more massive ultra-cool dwarf star. It is a true "Rosetta stone" in the new field of low-mass stellar astrophysics and further studies will surely provide more valuable information about these objects in the transitional zone between stars and planets. More information The research described in this press release is published in the research journal Astronomy & Astrophysics ("First determination of the dynamical mass of a binary L1.5 dwarf" by H. Bouy et al.). The paper is available in PDF format on the publisher web site.

WFPC2 Observations of Astrophysically Important Visual Binaries

NASA Astrophysics Data System (ADS)

Bond, Howard

1997-07-01

We recently used WFPC2 images of Procyon A and B to measure an extremely accurate separation of the bright F star and its much fainter white-dwarf companion. Combined with ground-based astrometry of the bright star, our observation significantly revises downward the derived masses, and brings Procyon A into excellent agreement with theoretical evolutionary tracks for the first time. We now propose to begin a modest but long-term program of WFPC2 measurements of astrophysically important visual binaries, working in a regime of large magnitude differences and/or faint stars where ground-based speckle interferometry cannot compete. We have selected three systems: Procyon {P=40 yr}, for which continued monitoring will even further refine the very accurate masses; Mu Cas {P=21 yr}, a famous metal-deficient G dwarf for which accurate masses will lead to the star's helium content with cosmological implications; and G 107-70, a close double white dwarf {P=18 yr} that promises to add two accurate masses to the tiny handful of white-dwarf masses that are directly known from dynamical measurements.

WFPC2 Observations of Astrophysically Important Visual Binaries - Continued

NASA Astrophysics Data System (ADS)

Bond, Howard

1999-07-01

We recently used WFPC2 images of Procyon A and B to measure an extremely accurate separation of the bright F star and its much fainter white-dwarf companion. Combined with ground-based astrometry of the bright star, our observation significantly revises downward the derived masses, and brings Procyon A into excellent agreement with theoretical evolutionary tracks for the first time. We now propose to begin a modest but long-term program of WFPC2 measurements of astrophysically important visual binaries, working in a regime of large magnitude differences and/or faint stars where ground-based speckle interferometry cannot compete. We have selected three systems: Procyon {P=40 yr}, for which continued monitoring will even further refine the very accurate masses; Mu Cas {P=21 yr}, a famous metal-deficient G dwarf for which accurate masses will lead to the star's helium content with cosmological implications; and G 107-70, a close double white dwarf {P=18 yr} that promises to add two accurate masses to the tiny handful of white-dwarf masses that are directly known from dynamical measurements.

Disks around Failed Stars - a Question of Age

NASA Astrophysics Data System (ADS)

2002-08-01

First Ground-Based Mid-Infrared Observations of Brown Dwarfs [1] Summary A team of European astronomers [2] have observed eight Brown Dwarfs, i.e., small and faint objects also known as "failed stars", with the TIMMI2 infrared sensitive instrument at the ESO 3.6-m telescope on La Silla. From two of these, mid-infrared radiation is detected - for the first time ever from such objects with a ground-based telescope . While the younger Brown Dwarf, aged a few million years, is found to be surrounded by a dusty disk, no warm dust is present around the older ones. The new observations support the following formation hypothesis for Brown Dwarfs: they are born in the same way as "real" stars, by contraction in interstellar clouds of gas and dust . During the later stages of this process, the infalling material is transferred onto the star via a gas and dust disk . This disk - in which planets may possibly form - then disperses with time. PR Photo 17a/02 : Image of Brown Dwarf LP 944-20 PR Photo 17b/02 : Models of the disk around Brown Dwarf Cha HA 2 Brown Dwarfs are faint and cool objects Astronomical objects known as "Brown Dwarfs" are "failed stars" . Their comparatively small mass, less than about 7% of that of our Sun (or about 75 times the mass of planet Jupiter), is too small to achieve sufficiently high pressure and temperature at their centre to ignite energy-producing nuclear processes. Some astronomers also refer to Brown Dwarfs as a "missing link" between planets and stars, being neither one nor the other, yet with similarities to both. They do not burn hydrogen to helium as "real" stars do, but continue to emit faint light as they slowly contract and cool during millions of years. They end their inglorious life with a whimper and finally fade into eternal insignificance. Although Brown Dwarfs were theoretically predicted already in 1963, astronomers had to wait until 1995 for the first one to be discovered. This was mainly due to their extreme faintness as compared to normal stars - even the most nearby Brown Dwarfs shine so faintly that they can only be observed with relatively large telescopes. As they are rather cool objects, they emit mostly in the infrared spectral region; hence they are best observed with astronomical instruments that operate at those wavelengths. With improved techniques, however, more and more Brown Dwarfs have been found and the count has now reached several hundred. Many of these are located in the well-known Orion Nebula. Others move through interstellar space, like the lonely KELU-1 first discovered in 1997 at the ESO La Silla Observatory by Chilean astronomers, cf. ESO PR 07/97.With a distance of only 33 light-years from the Sun, it was one of the closest Brown Dwarfs known at that time. Formation of Brown Dwarfs Astronomers are still doubtful about the way Brown Dwarfs form. Among the numerous suggestions are the star-like contraction from an interstellar cloud of gas and dust and also another based on "ejected stellar embryos" . This latter scenario says that very young stars that are still accreting material are "kicked out of the nest" by their more massive brothers in multiple stellar systems. In this dramatic process, the unlucky objects are stripped of their surrounding disks. This effectively halts their further growth by accretion and they end up as underweight Brown Dwarfs. Recent observations at ESO have shown that the Brown Dwarfs in the Orion Nebula most likely have formed as stars do, i.e. by contraction in a cloud of dust and gas, cf. ESO PR 14/01. The clue to this was the observation of an excess of near-infrared radiation from many of these objects, interpreted as the presence of dusty disks around them. The astronomers then argued that if the young Brown Dwarfs possess such disks exactly like real stars do, then they must also form in the same way. Infrared observations of Brown Dwarfs Those observations were carried out in the near-infrared spectral region (in the 1.2 - 2.2 µm wavelength interval) with the ESO 3.5-m New Technology Telescope (NTT) . However, dusty disks around young stars (and presumably, those around Brown Dwarfs) radiate mostly at longer wavelengths. A detailed study of those disks is therefore best done with instruments that are sensitive to even longer wavelengths, e.g., in the mid-infrared to the sub-millimetre spectral region (10 - 1000 µm). This is in fact the only spectral interval where emission emanating from solid particles can be directly observed and their (mineral) composition thus be analysed. Pioneering observations in this wavelength interval of some Brown Dwarfs were made in mid-1995 by the ESA Infrared Space Observatory. However, the ISO instruments provided comparatively low image sharpness and these observations were hampered by confusion with the radiation from other objects in the same sky field. And the ISO mission was over before Brown Dwarf objects were discovered in larger numbers. Astronomers have therefore long wanted to observe Brown Dwarfs with large ground-based telescopes in the mid-infrared spectral region. But these objects are faint and few suitable instruments that work at these wavelengths are available at the world's large astronomical telescopes. Long exposures are necessary to record the faint emissions and until now, it had not been possible to perform such highly demanding observations of Brown Dwarfs. TIMMI2 observes Brown Dwarfs ESO PR Photo 17a/02 ESO PR Photo 17a/02 [Preview - JPEG: 874 x 400 pix - 75k [Normal - JPEG: 1747 x 800 pix - 752k] ESO PR Photo 17b/02 ESO PR Photo 17b/02 [Preview - JPEG: 400 x 451 pix - 48k] [Normal - JPEG: 800 x 901 pix - 200k] Caption : PR Photo 17a/02 shows the sky field with the nearby Brown Dwarf LP 944-20 at the centre, as photographed in blue (B), red (R) and near-infrared (IR) light (reproduced from the Digital Sky Survey [STScI Digitized Sky Survey, (C) 1993, 1994, AURA, Inc. all rights reserved - cf. http://archive.eso.org/dss/dss]). The object (at the arrow) is obviously very red. Observations with the TIMMI2 instrument at the ESO 3.6-m telescope on La Silla have shown that this comparatively old object does not possess a surrounding disk of dust and gas. Another much younger Brown Dwarf, Cha HA 2 , has one. The new measurements show that this object has a flat, dense disk (lower diagramme in PR Photo 17b/02 ), unlike the hotter (solar-like), young stars, that harbour "flared" disks with a diluted, very hot top layer (upper diagramme). Now, however, the first ground-based detection of mid-infrared radiation from two Brown Dwarfs has been achieved by a team of European astronomers [2], using the Thermal Infrared Multimode Instrument (TIMMI2) on the ESO 3.6-m telescope at the La Silla Observatory (Chile). They pointed the telescope towards a total of eight Brown Dwarf objects and recorded the emission at three different mid-infrared wavelengths (5, 9.8 and 11.9 µm). "We were delighted" , says team leader Daniel Apai, "to detect radiation from two of these with TIMMI2. These are the first observations of their kind with a ground-based instrument. And although we could only establish upper limits for the radiation from the five other objects, these results are highly significant for our attempts to understand the formation and evolution of Brown Dwarfs." One of the objects, known as Cha HA 2 and located in the southern constellation Chamaeleon [3], had earlier been observed with ISO. It is a bona-fide Brown Dwarf object and an image obtained with the Hubble Space Telescope indicates that it may possibly be double. It is a relatively young Brown Dwarf and is a member of the very young Cha I star-forming region - the age has been estimated at 2 - 4.5 million years. The ISO observations hinted at the presence of a dust disk around this object - this is fully confirmed by the new TIMMI2 observations. Moreover, the mid-IR radiation measured with this instrument interestingly shows the absence of a strong emission feature from silicates (at about 10 µm wavelength). According to the astronomers, this indicates that the disk around Cha HA 2 is comparatively dense and flat, and without a heated outer layer, cf. PR Photo 17b/02 . The other Brown Dwarf from which TIMMI2 has now detected mid-infrared radiation is one of the closest of its type. Designated LP 944-20 , it is located in the southern constellation Fornax (The Oven) at a distance of only ~15 light-years. It is much older than Cha HA 2 , though, probably 500 - 650 million years. In this case, the age was determined by measuring the strengths of spectral lines of the element Lithium; the older the object, the less is the content of Lithium. The observations show that the radiation from LP 944-20 comes from the cool star itself - it does not possess a surrounding disk as does the much younger Cha HA 2 [4]. Evolution of Brown Dwarfs Daniel Apai explains: "This all fits very nicely into the current picture of the evolution of Brown Dwarfs. They are born like stars by contraction in an interstellar cloud of gas and dust. At least some of them acquire a surrounding disk during this process. But that disk disperses after some time and we therefore only find it around relatively young Brown Dwarfs, not around older ones." . Nobody knows yet whether planets form in those disks around young Brown Dwarfs (as this was the case in the disk around the young Sun and other stars), but it might happen. Only future observations with much more sensitive instruments will be able to cast more light on this intriguing question. Future observations ESO's Very Large Telescope (VLT) will soon be equipped with the VLT Mid Infrared Spectrometer/Imager (VISIR) , an extremely powerful mid-infrared sensitive instrument that is well suited for this kind of studies. Further into the future, the Atacama Large Millimeter Array (ALMA) will provide excellent opportunities for in-depth investigations of Brown Dwarfs. With unequalled sensitivity and very good image sharpness, ALMA will be able to image disks around the nearest Brown Dwarfs and possibly, to detect signs of (forming) planets in them. More information The information presented in this Press Release is based on a Letter to the Editor in the research journal "Astrophysical Journal" ("Probing Dust around Brown Dwarfs: The Naked LP 944-20 and the Disk of Chamaeleon H-alpha 2" by D. Apai and co-authors (Vol. 573, pp. L115-L117; July 10, 2002). It is available on the web at http://arXiv.org/abs/astro-ph/0206210. Notes [1]: This press release is issued in coordination between ESO and the Max-Planck-Institut für Astronomie (Heidelberg, Germany). A German version is available at the MPIA website ( http://www.mpia.de/Public/Aktuelles/index_de.html ). [2]: The team consists of Daniel Apai , Ilaria Pascucci and Thomas Henning (all at Astrophysikalisches Institut und Universitätssternwarte, Jena, Germany, and Max-Planck-Institut für Astronomie, Heidelberg, Germany), Michael Sterzik (ESO-Chile), Randolf Klein and Dimitri Semenov (Astrophysikalisches Institut und Universitätssternwarte, Jena, Germany), Eike Günther and Bringfried Stecklum (Thüringer Landessternwarte Tautenburg, Germany). [3]: Cha HA 2 stands for "H-alpha emitting object no. 2 in the Chamaeleon I Dark Cloud". [4]: The photosphere of the young Brown Dwarf Cha HA 2 also emits mid-IR radiation. However, it is quite far away - about 500 light-years, or more than 30 times more distant than LP 944-20 - and that radiation is too weak to be detected with TIMMI2.

Cool Companions of White Dwarfs from 2MASS

NASA Astrophysics Data System (ADS)

Hoard, D. W.; Wachter, S.; Sturch, L. K.; Widhalm, A. M.; Weiler, K. P.; Wellhouse, J. W.; Gibiansky, M.

2006-12-01

Detecting low mass stellar companions to white dwarfs (WDs) offers many advantages compared to main sequence primaries. In the latter case, faint low mass companions are often hidden in the glare of the more luminous main sequence primary, and radial velocity variations are small and, therefore, difficult to detect. Since WDs are less luminous than main sequence stars, the brightness contrast compared to a potential faint companion is significantly reduced. Most importantly, the markedly different spectral energy distributions of the WDs and their low mass companions makes the detection and separation of the two components relatively straightforward even with simple broad-band multi-color photometry. We have shown in Wachter et al. (2003) that the 2MASS near-IR color-color diagram can easily and efficiently identify candidates for unresolved WD + red dwarf binaries. Our follow-up observations (e.g., Farihi et al. 2006) have shown that a large fraction of these candidates are confirmed as previously unknown binary stars. Here, we present results from our full survey of the 2235 WDs from the McCook & Sion (1999) Catalog using the 2MASS All-Sky Data Release. We have identified an additional large sample of candidate WD + red dwarf binaries, as well as a number of systems that may contain extremely low mass stellar or substellar companions. Support for this work was provided by the National Aeronautics and Space Administration (NASA) under an Astrophysics Data Program grant issued through the Office of Space Science. This research made use of the NASA/Infrared Processing and Analysis Center (IPAC) Infrared Science Archive, which is operated by the Jet Propulsion Laboratory/California Institute of Technology (CIT), under contract with NASA, and data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and IPAC/CIT, funded by NASA and the National Science Foundation.

Detection of Escaping Lyman Continuum Radiation in Two Local Starbursts Using FUSE

NASA Astrophysics Data System (ADS)

Leitet, E.; Bergvall, N.; Andersson, B.-G.; Zackrisson, E.

2007-05-01

Dwarf galaxies may play a significant role in the reionization history of the universe, and as such also for the history of structure formation. These galaxies are however too faint to be observed at high redshifts, and it is therefore important to establish the amount of Lyman continuum (LyC) radiation that escape local starbursting dwarf galaxies. The amount of leakage is important to know also in order to improve models of star formation and spectral evolution of galaxies, which if neglected, might lead to false conclusions about the properties of the galaxies themself. Previous attempts to directly observe the leakage of hydrogen-ionizing radiation from local galaxies has before this resulted only in one successful case. In Bergvall et al. (2006 A&A 448, 513) an escape fraction of 4-10 % for the blue compact galaxy Haro 11, was found. In this work the detection and quantification of the LyC escape fractions for two additional local starburst galaxies using the Far Ultraviolet Spectroscopic Explorer, FUSE, is presented. The detections were made using FUSE archival data reduced with the latest, and much improved, pipeline. The LyC continuum can be seen as a faint structure between the Lyman limit of the target galaxies and the Milky Way. From line profile fitting it was found that both galaxies have high column densities of neutral gas, indicating that the LyC radiation is escaping through holes in the ISM. The escape fractions are calculated using spectral evolutionary models, based on the f(900Å), f(960Å) and f(Ha) fluxes. The fact that LyC leakage now is detected in three local starbursting dwarf galaxies is going to have a deep impact on future research on structure formation and galaxy evolution. The next challenge would be to statistically determine the escape fraction in extended surveys of starbursting dwarf galaxies.

The Next Generation Fornax Survey (NGFS). III. Revealing the Spatial Substructure of the Dwarf Galaxy Population Inside Half of Fornax's Virial Radius

NASA Astrophysics Data System (ADS)

Ordenes-Briceño, Yasna; Eigenthaler, Paul; Taylor, Matthew A.; Puzia, Thomas H.; Alamo-Martínez, Karla; Ribbeck, Karen X.; Muñoz, Roberto P.; Zhang, Hongxin; Grebel, Eva K.; Ángel, Simón; Côté, Patrick; Ferrarese, Laura; Hilker, Michael; Lançon, Ariane; Mieske, Steffen; Miller, Bryan W.; Rong, Yu; Sánchez-Janssen, Ruben

2018-05-01

We report the discovery of 271 previously undetected dwarf galaxies in the outer Fornax cluster regions at radii r vir/4 < r < r vir/2 using data from the Next Generation Fornax Survey (NGFS) with deep coadded u‧, g‧, and i‧ images obtained with Blanco/DECam at Cerro Tololo Interamerican Observatory. From the 271 dwarf candidates, we find 39 to be nucleated. Together with our previous study of the central Fornax region, the new dwarfs detected with NGFS data number 392, of which 56 are nucleated. The total Fornax dwarf galaxy population from NGFS and other catalogs rises, therefore, to a total of 643 with 181 being nucleated, yielding an overall nucleation fraction of 28%. The absolute i‧-band magnitudes for the outer NGFS dwarfs are in the range ‑18.80 ≤ M i‧ ≤ ‑8.78 with effective radii r eff,i‧ = 0.18–2.22 kpc and an average Sérsic index < n{> }i\\prime =0.81. Nonnucleated dwarfs are found to be fainter and smaller by {{Δ }}< {M}i\\prime > =2.25 mag and {{Δ }}< {r}eff,i\\prime }> =0.4 {kpc} than the nucleated dwarfs. We demonstrate a significant clustering of dwarf galaxies on scales ≲100 kpc, and projected surface number density profile estimates, Σ N (r), show a concentration of dwarfs in the Fornax core region within r ≲ 350 kpc. Σ N (r) has a flat distribution up to ∼350 kpc, beyond which it declines for the nonnucleated dwarfs. The nucleated dwarfs have a steeper Σ N (r) distribution, are more concentrated toward NGC 1399, and are decreasing rapidly outwards. This is the first time the transition from cluster to field environment has been established for the very faint dwarf galaxy population with robust sample statistics.

Early chemical enrichment of the Galactic dwarf satellites from a homogeneous and NLTE abundance analysis

NASA Astrophysics Data System (ADS)

Mashonkina, Lyudmila; Jablonka, Pascale; Sitnova, Tatyana; Pakhomov, Yuri; North, Pierre

2018-06-01

We review recent abundance results for very metal-poor (VMP, -4 ≤ [Fe/H] ≤ -2) stars in seven dwarf spheroidal galaxies (dSphs) and in the Milky Way (MW) halo comparison sample that were obtained based on high-resolution spectroscopic datasets, homogeneous and accurate atmospheric parameters, and the non-local thermodynamic equilibrium (NLTE) line formation for 10 chemical species. A remarkable gain of using such an approach is the reduction, compared to a simple compilation of the literature data, of the spread in abundance ratios at given metallicity within each galaxy and from one to the other. We show that all massive galaxies in our sample, that is, the MW halo and the classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax, reveal a similar plateau at [α/Fe] \\simeq 0.3 for each of the α-process elements: Mg, Ca, and Ti. We put on a firm ground the evidence for a decline in α/Fe with increasing metallicity in the Boötes I ultra-faint dwarf galaxy (UFD), that is most probably due to the ejecta of type Ia supernovae. In our classical dSphs, we observe the dichotomy in the [Sr/Ba] versus [Ba/H] diagram, similarly to the MW halo, calling for two different nucleosynthesis channels for Sr at the earliest evolution stages of these galaxies. Our three UFDs, that is Boötes I, UMa II, and Leo IV, are depleted in Sr and Ba relative to Fe and Mg, with very similar ratios of [Sr/Mg] ≈ -1.3 and [Ba/Mg] ≈ -1 on the entire range of their Mg abundances. The subsolar Sr/Ba ratios of Boötes I and UMa II indicate a common r-process origin of their neutron-capture elements. For Na/Fe, Na/Mg, and Al/Mg, the MW halo and all dSphs reveal indistinguishable trends with metallicity, suggesting that the processes of Na and Al synthesis are identical in all systems, independent of their mass. Sculptor remains the classical dSph, in which the evidence for inhomogeneous mixing in the early evolution stage, at [Fe/H] < -2, is the strongest.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bramante, Joseph; Linden, Tim

Recent observations of Reticulum II have uncovered an overabundance of r -process elements compared to similar ultra-faint dwarf spheroidal galaxies (UFDs). Because the metallicity and star formation history of Reticulum II appear consistent with all known UFDs, the high r -process abundance of Reticulum II suggests enrichment through a single, rare event, such as a double neutron star (NS) merger. However, we note that this scenario is extremely unlikely, as binary stellar evolution models require significant supernova natal kicks to produce NS–NS or NS–black hole (BH) mergers, and these kicks would efficiently remove compact binary systems from the weak gravitationalmore » potentials of UFDs. We examine alternative mechanisms for the production of r -process elements in UFDs, including a novel mechanism wherein NSs in regions of high dark matter (DM) density implode after accumulating a BH-forming mass of DM. We find that r -process proto-material ejection by tidal forces, when a single NS implodes into a BH, can occur at a rate matching the r -process abundance of both Reticulum II and the Milky Way. Remarkably, DM models which collapse a single NS in observed UFDs also solve the missing pulsar problem in the Milky Way Galactic Center. We propose tests specific to DM r -process production which may uncover or rule out this model.« less

Discovery of two neighbouring satellites in the Carina constellation with MagLiteS

NASA Astrophysics Data System (ADS)

Torrealba, G.; Belokurov, V.; Koposov, S. E.; Bechtol, K.; Drlica-Wagner, A.; Olsen, K. A. G.; Vivas, A. K.; Yanny, B.; Jethwa, P.; Walker, A. R.; Li, T. S.; Allam, S.; Conn, B. C.; Gallart, C.; Gruendl, R. A.; James, D. J.; Johnson, M. D.; Kuehn, K.; Kuropatkin, N.; Martin, N. F.; Martinez-Delgado, D.; Nidever, D. L.; Noël, N. E. D.; Simon, J. D.; Stringfellow, G. S.; Tucker, D. L.

2018-04-01

We report the discovery of two ultra-faint satellites in the vicinity of the Large Magellanic Cloud (LMC) in data from the Magellanic Satellites Survey (MagLiteS ). Situated 18 deg (˜20 kpc) from the LMC and separated from each other by only 18 arcmin, Carina II and III form an intriguing pair. By simultaneously modelling the spatial and the colour-magnitude stellar distributions, we find that both Carina II and Carina III are likely dwarf galaxies, although this is less clear for Carina III. There are in fact several obvious differences between the two satellites. While both are well described by an old and metal poor population, Carina II is located at ˜36 kpc from the Sun, with MV ˜ -4.5 and rh ˜ 90 pc, and it is further confirmed by the discovery of 3 RR Lyrae at the right distance. In contrast, Carina III is much more elongated, measured to be fainter (MV ˜ -2.4), significantly more compact (rh ˜ 30 pc), and closer to the Sun, at ˜28 kpc, placing it only 8 kpc away from Car II. Together with several other systems detected by the Dark Energy Camera, Carina II and III form a strongly anisotropic cloud of satellites in the vicinity of the Magellanic Clouds.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Best, William M. J.; Liu, Michael C.; Magnier, Eugene A.

We present initial results from a wide-field (30,000 deg{sup 2}) search for L/T transition brown dwarfs within 25 pc using the Pan-STARRS1 and Wide-field Infrared Survey Explorer (WISE) surveys. Previous large-area searches have been incomplete for L/T transition dwarfs, because these objects are faint in optical bands and have near-infrared (near-IR) colors that are difficult to distinguish from background stars. To overcome these obstacles, we have cross-matched the Pan-STARRS1 (optical) and WISE (mid-IR) catalogs to produce a unique multi-wavelength database for finding ultracool dwarfs. As part of our initial discoveries, we have identified seven brown dwarfs in the L/T transitionmore » within 9-15 pc of the Sun. The L9.5 dwarf PSO J140.2308+45.6487 and the T1.5 dwarf PSO J307.6784+07.8263 (both independently discovered by Mace et al.) show possible spectroscopic variability at the Y and J bands. Two more objects in our sample show evidence of photometric J-band variability, and two others are candidate unresolved binaries based on their spectra. We expect our full search to yield a well-defined, volume-limited sample of L/T transition dwarfs that will include many new targets for study of this complex regime. PSO J307.6784+07.8263 in particular may be an excellent candidate for in-depth study of variability, given its brightness (J = 14.2 mag) and proximity (11 pc)« less

The PAndAS View of the Andromeda Satellite System. I. A Bayesian Search for Dwarf Galaxies Using Spatial and Color-Magnitude Information

NASA Astrophysics Data System (ADS)

Martin, Nicolas F.; Ibata, Rodrigo A.; McConnachie, Alan W.; Mackey, A. Dougal; Ferguson, Annette M. N.; Irwin, Michael J.; Lewis, Geraint F.; Fardal, Mark A.

2013-10-01

We present a generic algorithm to search for dwarf galaxies in photometric catalogs and apply it to the Pan-Andromeda Archaeological Survey (PAndAS). The algorithm is developed in a Bayesian framework and, contrary to most dwarf galaxy search codes, makes use of both the spatial and color-magnitude information of sources in a probabilistic approach. Accounting for the significant contamination from the Milky Way foreground and from the structured stellar halo of the Andromeda galaxy, we recover all known dwarf galaxies in the PAndAS footprint with high significance, even for the least luminous ones. Some Andromeda globular clusters are also recovered and, in one case, discovered. We publish a list of the 143 most significant detections yielded by the algorithm. The combined properties of the 39 most significant isolated detections show hints that at least some of these trace genuine dwarf galaxies, too faint to be individually detected. Follow-up observations by the community are mandatory to establish which are real members of the Andromeda satellite system. The search technique presented here will be used in an upcoming contribution to determine the PAndAS completeness limits for dwarf galaxies. Although here tuned to the search of dwarf galaxies in the PAndAS data, the algorithm can easily be adapted to the search for any localized overdensity whose properties can be modeled reliably in the parameter space of any catalog.

Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey

NASA Astrophysics Data System (ADS)

Leggett, S. K.; Lodieu, N.; Tremblay, P.-E.; Bergeron, P.; Nitta, A.

2011-07-01

We present the results of a search for cool white dwarfs in the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky Survey to identify cool hydrogen-rich white dwarf candidates by their neutral optical colors and blue near-infrared colors, as well as faint reduced proper motion magnitudes. Optical spectroscopy was obtained at Gemini Observatory and showed the majority of the candidates to be newly identified cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants. Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with effective temperature T eff ≈ 6000 K. The current follow-up of 1400 deg2 of sky has produced 13 new white dwarfs. Model fits to the photometry show that seven of the newly identified white dwarfs have 4120 K <=T eff <= 4480 K, and cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km s-1 <= v tan <= 85 km s-1 and are likely to be thick disk 10-11 Gyr-old objects. The other half of the sample has 4610 K <=T eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr, and 60 km s-1 <= v tan <= 100 km s-1. These are either thin disk remnants with unusually high velocities, or lower-mass remnants of thick disk or halo late-F or G stars.

THE PAndAS VIEW OF THE ANDROMEDA SATELLITE SYSTEM. I. A BAYESIAN SEARCH FOR DWARF GALAXIES USING SPATIAL AND COLOR-MAGNITUDE INFORMATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Martin, Nicolas F.; Ibata, Rodrigo A.; McConnachie, Alan W.

We present a generic algorithm to search for dwarf galaxies in photometric catalogs and apply it to the Pan-Andromeda Archaeological Survey (PAndAS). The algorithm is developed in a Bayesian framework and, contrary to most dwarf galaxy search codes, makes use of both the spatial and color-magnitude information of sources in a probabilistic approach. Accounting for the significant contamination from the Milky Way foreground and from the structured stellar halo of the Andromeda galaxy, we recover all known dwarf galaxies in the PAndAS footprint with high significance, even for the least luminous ones. Some Andromeda globular clusters are also recovered and,more » in one case, discovered. We publish a list of the 143 most significant detections yielded by the algorithm. The combined properties of the 39 most significant isolated detections show hints that at least some of these trace genuine dwarf galaxies, too faint to be individually detected. Follow-up observations by the community are mandatory to establish which are real members of the Andromeda satellite system. The search technique presented here will be used in an upcoming contribution to determine the PAndAS completeness limits for dwarf galaxies. Although here tuned to the search of dwarf galaxies in the PAndAS data, the algorithm can easily be adapted to the search for any localized overdensity whose properties can be modeled reliably in the parameter space of any catalog.« less

A spectroscopic search for faint secondaries in cataclysmic variables

NASA Astrophysics Data System (ADS)

Vande Putte, D.; Smith, Robert Connon; Hawkins, N. A.; Martin, J. S.

2003-06-01

The secondary in cataclysmic variables (CVs) is usually detected by cross-correlation of the CV spectrum with that of a K or M dwarf template, to produce a radial velocity curve. Although this method has demonstrated its power, it has its limits in the case of noisy spectra, such as are found when the secondary is faint. A method of coadding spectra, called skew mapping, has been proposed in the past. Gradually, examples of its application are being published; none the less, so far no journal article has described the technique in detail. To answer this need, this paper explores in detail the capabilities of skew mapping when determining the amplitude of the radial velocity for faint secondaries. It demonstrates the power of the method over techniques that are more conventional, when the signal-to-noise ratio is poor. The paper suggests an approach to assessing the quality of results. This leads in the case of the investigated objects to a first tier of results, where we find K2= 127 +/- 23 km s-1 for SY Cnc, K2= 144 +/- 18 km s-1 for RW Sex and K2= 262 +/- 14 km s-1 for UX UMa. These we believe to be the first direct determinations of K2 for these objects. Furthermore, we also obtain K2= 263 +/- 30 km s-1 for RW Tri, close to a skew mapping result obtained elsewhere. In the first three cases, we use these results to derive the mass of the white dwarf companion. A second tier of results includes UU Aqr, EX Hya and LX Ser, for which we propose more tentative values of K2. Clear failures of the method are also discussed (EF Eri, VV Pup and SW Sex).

HST Observations of Astrophysically Important Visual Binaries

NASA Astrophysics Data System (ADS)

Bond, Howard

2013-10-01

We propose to continue our long-term program of astrometry of close visual binaries, with the primary goal of determining purely dynamical masses for 3 important main-sequence stars and 9 white dwarfs {WDs}. A secondary aim is to set limits on third bodies in the systems down to planetary mass. Three of our targets are naked-eye stars with much fainter companions that are extremely difficult to image from the ground. Our other 2 targets are double WDs, whose small separations and faintness likewise make them difficult to measure using ground-based techniques. Observations have been completed for a 3rd double WD.The bright stars, to be imaged with WFC3, are: {1} Procyon {P = 40.83 yr}, containing a bright F star and a much fainter WD companion. With the continued monitoring proposed here, we will obtain masses to an accuracy of better than 1%, providing a testbed for theories of both Sun-like stars and WDs. {2} Sirius {P = 50.14 yr}, an A-type star also having a faint WD companion, Sirius B, the nearest and brightest of all WDs. {3} Mu Cas {P = 21.08 yr}, a nearby metal-deficient G dwarf for which accurate masses will lead to the stars' helium contents, with cosmological implications. The faint double WDs, to be observed with FGS, are: {1} G 107-70 {P = 18.84 yr}, and {2} WD 1818+126 {P = 12.19 yr}. Our astrometry of these systems will add 4 accurate masses to the handful of WD masses that are directly known from dynamical measurements. The FGS measurements will also provide precise parallaxes for the systems, a necessary ingredient in the mass determinations.

HST Observations of Astrophysically Important Visual Binaries

NASA Astrophysics Data System (ADS)

Bond, Howard

2015-10-01

We propose to continue our long-term program of astrometry of close visual binaries, with the primary goal of determining purely dynamical masses for 3 important main-sequence stars and 9 white dwarfs (WDs). A secondary aim is to set limits on third bodies in the systems down to planetary mass. Three of our targets are naked-eye stars with much fainter companions that are extremely difficult to image from the ground. Our other 2 targets are double WDs, whose small separations and faintness likewise make them difficult to measure using ground-based techniques. Observations have been completed for a 3rd double WD.The bright stars, to be imaged with WFC3, are: (1) Procyon (P = 40.83 yr), containing a bright F star and a much fainter WD companion. With the continued monitoring proposed here, we will obtain masses to an accuracy of better than 1%, providing a testbed for theories of both Sun-like stars and WDs. (2) Sirius (P = 50.14 yr), an A-type star also having a faint WD companion, Sirius B, the nearest and brightest of all WDs. (3) Mu Cas (P = 21.08 yr), a nearby metal-deficient G dwarf for which accurate masses will lead to the stars' helium contents, with cosmological implications. The faint double WDs, to be observed with FGS, are: (1) G 107-70 (P = 18.84 yr), and (2) WD 1818+126 (P = 12.19 yr). Our astrometry of these systems will add 4 accurate masses to the handful of WD masses that are directly known from dynamical measurements. The FGS measurements will also provide precise parallaxes for the systems, a necessary ingredient in the mass determinations.

HST Observations of Astrophysically Important Visual Binaries

NASA Astrophysics Data System (ADS)

Bond, Howard

2014-10-01

We propose to continue our long-term program of astrometry of close visual binaries, with the primary goal of determining purely dynamical masses for 3 important main-sequence stars and 9 white dwarfs (WDs). A secondary aim is to set limits on third bodies in the systems down to planetary mass. Three of our targets are naked-eye stars with much fainter companions that are extremely difficult to image from the ground. Our other 2 targets are double WDs, whose small separations and faintness likewise make them difficult to measure using ground-based techniques. Observations have been completed for a 3rd double WD.The bright stars, to be imaged with WFC3, are: (1) Procyon (P = 40.83 yr), containing a bright F star and a much fainter WD companion. With the continued monitoring proposed here, we will obtain masses to an accuracy of better than 1%, providing a testbed for theories of both Sun-like stars and WDs. (2) Sirius (P = 50.14 yr), an A-type star also having a faint WD companion, Sirius B, the nearest and brightest of all WDs. (3) Mu Cas (P = 21.08 yr), a nearby metal-deficient G dwarf for which accurate masses will lead to the stars' helium contents, with cosmological implications. The faint double WDs, to be observed with FGS, are: (1) G 107-70 (P = 18.84 yr), and (2) WD 1818+126 (P = 12.19 yr). Our astrometry of these systems will add 4 accurate masses to the handful of WD masses that are directly known from dynamical measurements. The FGS measurements will also provide precise parallaxes for the systems, a necessary ingredient in the mass determinations.

The Search for RR Lyrae Variables in the Dark Energy Survey

NASA Astrophysics Data System (ADS)

Nielsen, Chandler; Marshall, Jennifer L.; Long, James

2017-01-01

RR Lyrae variables are stars with a characteristic relationship between magnitude and phase and whose distances can be easily determined, making them extremely valuable in mapping and analyzing galactic substructure. We present our method of searching for RR Lyrae variable stars using data extracted from the Dark Energy Survey (DES). The DES probes for stars as faint as i = 24.3. Finding such distant RR Lyrae allows for the discovery of objects such as dwarf spheroidal tidal streams and dwarf galaxies; in fact, at least one RR Lyrae has been discovered in each of the probed dwarf spheroidal galaxies orbiting the Milky Way (Baker & Willman 2015). In turn, these discoveries may ultimately resolve the well-known missing satellite problem, in which theoretical simulations predict many more dwarf satellites than are observed in the local Universe. Using the Lomb-Scargle periodogram to determine the period of the star being analyzed, we could display the relationship between magnitude and phase and visually determine if the star being analyzed was an RR Lyrae. We began the search in frequently observed regions of the DES footprint, known as the supernova fields. We then moved our search to known dwarf galaxies found during the second year of the DES. Unfortunately, we did not discover RR Lyrae in the probed dwarf galaxies; this method should be tried again once more observations are taken in the DES.

The Stellar Populations of Two Ultra-diffuse Galaxies from Optical and Near-infrared Photometry

NASA Astrophysics Data System (ADS)

Pandya, Viraj; Romanowsky, Aaron J.; Laine, Seppo; Brodie, Jean P.; Johnson, Benjamin D.; Glaccum, William; Villaume, Alexa; Cuillandre, Jean-Charles; Gwyn, Stephen; Krick, Jessica; Lasker, Ronald; Martín-Navarro, Ignacio; Martinez-Delgado, David; van Dokkum, Pieter

2018-05-01

We present observational constraints on the stellar populations of two ultra-diffuse galaxies (UDGs) using optical through near-infrared (NIR) spectral energy distribution (SED) fitting. Our analysis is enabled by new Spitzer-IRAC 3.6 and 4.5 μm imaging, archival optical imaging, and the prospector fully Bayesian SED fitting framework. Our sample contains one field UDG (DGSAT I), one Virgo cluster UDG (VCC 1287), and one Virgo cluster dwarf elliptical for comparison (VCC 1122). We find that the optical–NIR colors of the three galaxies are significantly different from each other. We infer that VCC 1287 has an old (≳7.7 Gyr) and surprisingly metal-poor ([Z/Z ⊙] ≲ ‑1.0) stellar population, even after marginalizing over uncertainties on diffuse interstellar dust. In contrast, the field UDG DGSAT I shows evidence of being younger than the Virgo UDG, with an extended star formation history and an age posterior extending down to ∼3 Gyr. The stellar metallicity of DGSAT I is sub-solar but higher than that of the Virgo UDG, with [Z/{Z}ȯ ]=-{0.63}-0.62+0.35; in the case of exactly zero diffuse interstellar dust, DGSAT I may even have solar metallicity. With VCC 1287 and several Coma UDGs, a general picture is emerging where cluster UDGs may be “failed” galaxies, but the field UDG DGSAT I seems more consistent with a stellar feedback-induced expansion scenario. In the future, our approach can be applied to a large and diverse sample of UDGs down to faint surface brightness limits, with the goal of constraining their stellar ages, stellar metallicities, and circumstellar and diffuse interstellar dust content.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kasliwal, M. M.; Kulkarni, S. R.; Ofek, E. O.

We present photometric and spectroscopic follow-up of a sample of extragalactic novae discovered by the Palomar 60 inch telescope during a search for 'Fast Transients In Nearest Galaxies' (P60-FasTING). Designed as a fast cadence (1 day) and deep (g < 21 mag) survey, P60-FasTING was particularly sensitive to short-lived and faint optical transients. The P60-FasTING nova sample includes 10 novae in M 31, 6 in M 81, 3 in M 82, 1 in NGC 2403, and 1 in NGC 891. This significantly expands the known sample of extragalactic novae beyond the Local Group, including the first discoveries in a starburstmore » environment. Surprisingly, our photometry shows that this sample is quite inconsistent with the canonical maximum-magnitude-rate-of-decline (MMRD) relation for classical novae. Furthermore, the spectra of the P60-FasTING sample are indistinguishable from classical novae. We suggest that we have uncovered a sub-class of faint and fast classical novae in a new phase space in luminosity-timescale of optical transients. Thus, novae span two orders of magnitude in both luminosity and time. Perhaps the MMRD, which is characterized only by the white dwarf mass, was an oversimplification. Nova physics appears to be characterized by a relatively rich four-dimensional parameter space in white dwarf mass, temperature, composition, and accretion rate.« less

The Extreme Ultraviolet Flux of Very Low Mass Stars

NASA Astrophysics Data System (ADS)

Drake, Jeremy

2017-09-01

The X-ray and EUV emission of stars is vital for understanding the atmospheres and evolution of their planets. The coronae of dwarf stars later than M6 behave differently to those of earlier spectral types and are more X-ray dim and radio bright. Too faint to have been observed by EUVE, their EUV behavior is currently highly uncertain. We propose to observe a small sample of late M dwarfs using the off-axis HRC-S thin Al" filter that is sensitive to EUV emission in the 50-200 A range. The measured fluxes will be used to understand the amount of cooler coronal plasma present, and extend X-ray-EUV flux relations to the latest stellar types.

The formation of Dwarf Spheroidal galaxies by the dissolving star cluster model.

NASA Astrophysics Data System (ADS)

Alarcon, Alex; Theory and Star Formation Group

2018-01-01

Dwarf spheroidal (dSph) galaxies are regarded as key object in the formation of larger galaxies and are believed to be the most dark matter dominated systems known. There are several model that attempt to explain their formation, but they have problems to model the formation of isolated dSph. Here we will explain a possible formation scenario in which star clusters form in the dark matter halo of a dSph. these cluster suffer from low star formation efficiency and dissolve while orbiting inside the halo. Thereby they build the faint luminous components that we observe in dSph galaxies. Here we will show the main results of this simulations and how they would be corroborated using observational data.

A Sample of Fast Moving M Dwarfs in the Milky Way

NASA Astrophysics Data System (ADS)

Favia, Andrej; West, Andrew A.

2014-06-01

In the past decade, several high-mass stars have been discovered to have high enough velocities to escape the Milky Way (dubbed hypervelocity stars), yet until recently, stars with similar velocities were not observed for Solar- and lower-mass stars. There has been an observational paucity of hypervelocity M dwarfs, which account for ~70% of the stars in the Milky Way. While some of the shortage of low-mass, high-velocity stars may be due to the specific mechanisms accelerating these stars, it is also possible that the M dwarfs have been overlooked due to their faint luminosities. We present results from a study that uses the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) M Dwarf Spectroscopic Catalog (70,841 M dwarfs) to identify and characterize several hundred M dwarfs with velocities greater than 400 km/s relative to the Galactic center. Our study marks the first step in demonstrating that there is a significant sample of low-mass, high-velocity stars. We examined the 3D kinematics of M dwarfs in the SDSS DR7 catalog with velocities > 400 km/s relative to the Galactic center. Stars with poor photometry or a SNR (near H-alpha) < 3 were excluded, as well as stars that were flagged in the original data set as being possible M dwarf-white dwarf binaries. We confirmed the radial velocities reported by West et al. (2011) by manually examining the remaining stars, specifically the locations of the sodium absorption lines (two at 5891/5897 Å, and two at 8185/8197 Å). We present the final catalog of high velocity candidates and a preliminary analysis of their spectroscopically derived properties, including 3D kinematics, magnetic activity and metallicity distributions.

Faint dwarf galaxies in Hickson Compact Group 90*

NASA Astrophysics Data System (ADS)

Ordenes-Briceño, Yasna; Taylor, Matthew A.; Puzia, Thomas H.; Muñoz, Roberto P.; Eigenthaler, Paul; Georgiev, Iskren Y.; Goudfrooij, Paul; Hilker, Michael; Lançon, Ariane; Mamon, Gary; Mieske, Steffen; Miller, Bryan W.; Peng, Eric W.; Sánchez-Janssen, Rubén

2016-12-01

We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U- and I-band images obtained with Very Large Telescope/Visible Multi-Object Spectrograph. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies. We measure spheroid half-light radii in the range 0.7 ≲ reff/kpc ≲ 1.5 with luminosities of -11.65 ≲ MU ≲ -9.42 and -12.79 ≲ MI ≲ -10.58 mag, corresponding to a colour range of (U - I)0 ≃ 1.1-2.2 mag and surface brightness levels of μU ≃ 28.1 mag arcsec-2 and μI ≃ 27.4 mag arcsec-2. Their colours and luminosities are consistent with a diverse set of stellar population properties. Assuming solar and 0.02 Z⊙ metallicities we obtain stellar masses in the range M*|Z⊙ ≃ 105.7 - 6.3 M⊙ and M_{*}|_{0.02 Z_{⊙} ≃ 10^{6.3-8} M_{⊙}. Three dwarfs are older than 1 Gyr, while the other two significantly bluer dwarfs are younger than ˜2 Gyr at any mass/metallicity combination. Altogether, the new LSB dwarf galaxy candidates share properties with dwarf galaxies found throughout the Local Volume and in nearby galaxy clusters such as Fornax. We find a pair of candidates with ˜2 kpc projected separation, which may represent one of the closest dwarf galaxy pairs found. We also find a nucleated dwarf candidate, with a nucleus size of reff ≃ 46-63 pc and magnitude MU, 0 = -7.42 mag and (U - I)0 = 1.51 mag, which is consistent with a nuclear stellar disc with a stellar mass in the range 104.9 - 6.5 M⊙.

A VLT Large Programme to Study Galaxies at z ~ 2: GMASS — the Galaxy Mass Assembly Ultra-deep Spectroscopic Survey

NASA Astrophysics Data System (ADS)

Kurk, Jaron; Cimatti, Andrea; Daddi, Emanuele; Mignoli, Marco; Bolzonella, Micol; Pozzetti, Lucia; Cassata, Paolo; Halliday, Claire; Zamorani, Gianni; Berta, Stefano; Brusa, Marcella; Dickinson, Mark; Franceschini, Alberto; Rodighiero, Guilia; Rosati, Piero; Renzini, Alvio

2009-03-01

We report on the motivation, sample selection and first results of our VLT FORS2 Large Programme (173.A-0687), which has obtained the longest targeted spectra of distant galaxies obtained so far with the VLT. These long exposures, up to 77 hours for objects included in three masks, were required to detect spectral features of extremely faint galaxies, such as absorption lines of passive galaxies at z > 1.4, a population that had previously escaped attention due to its faintness in the optical wavelength regime, but which represents a critical phase in the evolution of massive galaxies. The ultra-deep spectroscopy allowed us to estimate the stellar metallicity of star-forming galaxies at z ~ 2, to trace colour bimodality up to z = 2 and to characterise a galaxy cluster progenitor at z = 1.6. The approximately 200 spectra produced by GMASS constitute a lasting legacy, populating the “redshift desert” in GOODS-S.

A class of compact dwarf galaxies from disruptive processes in galaxy clusters.

PubMed

Drinkwater, M J; Gregg, M D; Hilker, M; Bekki, K; Couch, W J; Ferguson, H C; Jones, J B; Phillipps, S

2003-05-29

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails and intra-cluster stars being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster. This revealed a new population of compact members, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weisz, Daniel R.; Johnson, Benjamin D.; Conroy, Charlie, E-mail: drw@ucsc.edu

We present a new technique to estimate the evolution of the very faint end of the UV luminosity function (LF) out to z ∼ 5. Measured star formation histories (SFHs) from the fossil record of Local Group (LG) galaxies are used to reconstruct the LF down to M {sub UV} ∼–5 at z ∼ 5 and M {sub UV} ∼–1.5 at z < 1. Such faint limits are well beyond the current observational limits and are likely to remain beyond the limits of next-generation facilities. The reconstructed LFs, when combined with direct measurements of the LFs at higher luminosity, aremore » well-fit by a standard Schechter function with no evidence of a break to the faintest limits probed by this technique. The derived faint-end slope, α, steepens from ≈ – 1.2 at z < 1 to ≈ – 1.6 at 4 < z < 5. We test the effects of burstiness in the SFHs and find the recovered LFs to be only modestly affected. Incompleteness corrections for the faintest LG galaxies and the (unlikely) possibility of significant luminosity-dependent destruction of dwarf galaxies between high redshift and the present epoch are important uncertainties. These and other uncertainties can be mitigated with more detailed modeling and future observations. The reconstructed faint end LF from the fossil record can therefore be a powerful and complementary probe of the high-redshift faint galaxies believed to play a key role in the reionization of the universe.« less

The Star, the Dwarf and the Planet

NASA Astrophysics Data System (ADS)

2006-10-01

Astronomers have detected a new faint companion to the star HD 3651, already known to host a planet. This companion, a brown dwarf, is the faintest known companion of an exoplanet host star imaged directly and one of the faintest T dwarfs detected in the Solar neighbourhood so far. The detection yields important information on the conditions under which planets form. "Such a system is an interesting example that might prove that planets and brown dwarfs can form around the same star", said Markus Mugrauer, lead author of the paper presenting the discovery. ESO PR Photo 39a/06 ESO PR Photo 39a/06 The Companion to HD 3651 HD 3651 is a star slightly less massive than the Sun, located 36 light-years away in the constellation Pisces (the "Fish"). For several years, it has been known to harbour a planet less massive than Saturn, sitting closer to its parent star than Mercury is from the Sun: the planet accomplishes a full orbit in 62 days. Mugrauer and his colleagues first spotted the faint companion in 2003 on images from the 3.8-m United Kingdom Infrared Telescope (UKIRT) in Hawaii. Observations in 2004 and 2006 using ESO's 3.6 m New Technology Telescope (NTT) at La Silla provided the crucial confirmation that the speck of light is not a spurious background star, but indeed a true companion. The newly found companion, HD 3651B, is 16 times further away from HD 3651 than Neptune is from the Sun. HD 3651B is the dimmest directly imaged companion of an exoplanet host star. Furthermore, as it is not detected on the photographic plates of the Palomar All Sky Survey, the companion must be even fainter in the visible spectral range than in the infrared, meaning it is a very cool low-mass sub-stellar object. Comparing its characteristics with theoretical models, the astronomers infer that the object has a mass between 20 and 60 Jupiter masses, and a temperature between 500 and 600 degrees Celsius. It is thus ten times colder and 300 000 less luminous than the Sun. These properties place it in the category of cool T-type brown dwarfs. ESO PR Photo 38b/06 ESO PR Photo 39b/06 The Relative Position of the Companion to HD 3651 "Due to their faintness even in the infrared, these cool T dwarfs are very difficult to find", said Mugrauer. "Only two other brown dwarfs with similar brightness are presently known. Their study will provide important insights into the atmospheric properties of cool sub-stellar objects." More than 170 stars are currently known to host exoplanets. In some cases, these stars were also found to have one or several stellar companions, showing that planet formation can also take place in a dynamically more complex environment than our own Solar System where planet formation occurred around an isolated single star. In 2001, Mugrauer and his colleagues started an observational programme to find out whether exoplanet host stars are single or married. In this programme, known exoplanet host stars are systematically imaged at two different epochs, at least several months apart. True companions can be distinguished from coincidental background objects as only they move together with the stars over time. With this effective search strategy several new companions of exoplanet host stars have been detected. Most of the detected companions are low-mass stars in the same evolutionary state as the Sun. In two cases, however, the astronomers found the companions to be white dwarfs, that is, stars at the end of their life. These intriguing systems bear evidence that planets can even survive the troubled last moments in the life of a nearby star. The planet host star HD 3651 is thus surrounded by two sub-stellar objects. The planet, HD 3651b, is very close, while the newly found brown dwarf companion revolves around the star 1500 times farther away than the planet. This system is the first imaged example that planets and brown dwarfs can form around the same star.

NIFTE: The Near Infrared Faint-Object Telescope Experiment

NASA Technical Reports Server (NTRS)

Bock, James J.; Lange, Andrew E.; Matsumoto, T.; Eisenhardt, Peter B.; Hacking, Perry B.; Schember, Helene R.

1994-01-01

The high sensitivity of large format InSb arrays can be used to obtain deep images of the sky at 3-5 micrometers. In this spectral range cool or highly redshifted objects (e.g. brown dwarfs and protogalaxies) which are not visible at shorter wavelengths may be observed. Sensitivity at these wavelengths in ground-based observations is severly limited by the thermal flux from the telescope and from the earth's atmosphere. The Near Infrared Faint-Object Telescope Experiment (NIFTE), a 50 cm cooled rocket-borne telescope combined with large format, high performance InSb arrays, can reach a limiting flux less than 1 micro-Jy(1-sigma) over a large field-of-view in a single flight. In comparison, the Infrared Space Observatory (ISO) will require days of observation to reach a sensitivity more than one order of magnitude worse over a similar area of the sky. The deep 3-5 micrometer images obtained by the rocket-borne telescope will assist in determining the nature of faint red objects detected by ground-based telescopes at 2 micrometers, and by ISO at wavelengths longer than 5 micrometers.

FIGGS 2: An HI survey of extremely faint irregular galaxies

NASA Astrophysics Data System (ADS)

Patra, N. N.; Chengalur, J. N.; Karachentsev, I. D.; Sharina, M. E.

2016-10-01

We present observations and first results from the FIGGS2 survey. FIGGS2 is an extension of the earlier Faint Irregular Galaxies GMRT survey (FIGGS) towards faint luminosity end. The sample consists of 20 galaxies, 15 of which were detected in HI 21 cm line using the Giant Meterwave Radio Telescope (GMRT). The median blue band magnitude of our sample is approximately -11.m 6, which is more than one magnitude fainter than earlier FIGGS survey. From our GMRT observations we found that, for many of our sample galaxies, the HI disks are offset from their optical disks. The HI diameters of the FIGGS2 galaxies show a tight correlation with their HI mass. The slope of the correlation is 2.08 ± 0.20 similar to what is found for FIGGS galaxies. We also found that for almost all galaxies, the HI disks are larger than the optical disks which is a common trend for dwarf or spiral galaxies. The mean value of the ratio of HI to optical diameter is about 1.54.

TIMING OF FIVE MILLISECOND PULSARS DISCOVERED IN THE PALFA SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Scholz, P.; Kaspi, V. M.; Ferdman, R. D.

2015-02-20

We present the discovery of five millisecond pulsars (MSPs) from the PALFA Galactic plane survey using Arecibo. Four of these (PSRs J0557+1551, J1850+0244, J1902+0300, and J1943+2210) are binary pulsars whose companions are likely white dwarfs, and one (PSR J1905+0453) is isolated. Phase-coherent timing solutions, ranging from ∼1 to ∼3 yr in length, and based on observations from the Jodrell Bank and Arecibo telescopes, provide precise determinations of spin, orbital, and astrometric parameters. All five pulsars have large dispersion measures (>100 pc cm{sup –3}, within the top 20% of all known Galactic field MSPs) and are faint (1.4 GHz flux densitymore » ≲0.1 mJy, within the faintest 5% of all known Galactic field MSPs), illustrating PALFA's ability to find increasingly faint, distant MSPs in the Galactic plane. In particular, PSR J1850+0244 has a dispersion measure of 540 pc cm{sup –3}, the highest of all known MSPs. Such distant, faint MSPs are important input for accurately modeling the total Galactic MSP population.« less

HI-bearing Ultra Diffuse Galaxies in the ALFALFA Survey

NASA Astrophysics Data System (ADS)

Leisman, Lukas; Janowiecki, Steven; Jones, Michael G.; ALFALFA Almost Darks Team

2018-01-01

The Arecibo Legacy Fast ALFA (Arecibo L-band Feed Array) extragalactic HI survey, with over 30,000 high significance extragalactic sources, is well positioned to locate gas-bearing, low surface brightness sources missed by optical detection algorithms. We investigate the nature of a population of HI-bearing sources in ALFALFA with properties similar to "ultra-diffuse" galaxies (UDGs): galaxies with stellar masses of dwarf galaxies, but radii of L* galaxies. These "HI-bearing ultra-diffuse" sources (HUDS) constitute a small, but pertinent, fraction of the dwarf-mass galaxies in ALFALFA. They are bluer and have more irregular morphologies than the optically-selected UDGs found in clusters, and they appear to be gas-rich for their stellar mass, indicating low star formation efficiency. To illuminate potential explanations for the extreme properties of these sources we explore their environments and estimate their halo properties. We conclude that environmental mechanism are unlikely the cause of HUDS' properties, as they exist in environments equivalent to that of the other ALFALFA sources of similar HI-masses, however, we do find some suggestion that these HUDS may reside in high spin parameter halos, a potential explanation for their "ultra-diffuse" nature.

Formation of ultra-compact dwarf galaxies from supergiant molecular clouds

NASA Astrophysics Data System (ADS)

Goodman, Morgan; Bekki, Kenji

2018-05-01

The origin of ultra-compact dwarf galaxies (UCDs) is not yet clear. One possible formation path of UCDs is the threshing of a nucleated elliptical dwarf galaxy (dE, N), however, it remains unclear how such massive nuclear stellar systems were formed in dwarf galaxies. To better establish the early history of UCDs, we investigate the formation of UCD progenitor clusters from super giant molecular clouds (SGMCs), using hydrodynamical simulations. In this study we focus on SGMCs with masses 107 - 108 M_{\\odot } that can form massive star clusters that display physical properties similar to UCDs. We find that the clusters have extended star formation histories with two phases, producing multiple distinct stellar populations, and that the star formation rate is dependent on the feedback effects of SNe and AGB stars. The later generations of stars formed in these clusters are more compact, leading to a clearly nested structure, and these stars will be more He-rich than those of the first generation, leading to a slight colour gradient. The simulated clusters demonstrate scaling relations between Reff and M and σv and M consistent with those observed in UCDs and strongly consistent with those of the original SGMC. We discuss whether SGMCs such as these can be formed through merging of self-gravitating molecular clouds in galaxies at high-z.

Space Motions of Low-Mass Stars. III.

NASA Astrophysics Data System (ADS)

Upgren, A. R.; Sperauskas, J.; Boyle, R. P.

Radial velocity observations are presented for 149 stars taken from the McCormick lists of dwarf K and M stars in a continuing program of radial velocities of faint nearby stars. The data will serve to derive a total stellar density of these kinds of stars in the solar neighborhood. These data were obtained with the spectrometer of the Vilnius University Observatory mounted on the 1.6 m Kuiper Telescope of the Steward Observatory.

Search for faint nearby stars

NASA Astrophysics Data System (ADS)

Ruiz, M. T.; Maza, J.; Mendez, R.; Wischnjewsky, M.

1988-09-01

The possibility that the 'missing mass' in the solar neighborhood may be accounted for by the existence in sufficiently great numbers of such very low mass stars as brown dwarfs, as well as very old dead stars now observable as cold, low-luminosity degenerates, is presently addressed observationally with a search through ESO R Survey plates using a stereocomparator. Attention is given to ESO area 439, where four low-luminosity degenerates have been discovered by the present study.

The StEllar Counterparts of COmpact high velocity clouds (SECCO) survey. II. Sensitivity of the survey and the atlas of synthetic dwarf galaxies

NASA Astrophysics Data System (ADS)

Beccari, G.; Bellazzini, M.; Battaglia, G.; Ibata, R.; Martin, N.; Testa, V.; Cignoni, M.; Correnti, M.

2016-06-01

The searching for StEllar Counterparts of COmpact high velocity clouds (SECCO) survey is devoted to the search for stellar counterparts within ultra compact high velocity clouds that are candidate low-mass, low-luminosity galaxies. We present the results of a set of simulations aimed at the quantitative estimate of the sensitivity of the survey as a function of the total luminosity, size, and distance of the stellar systems we are looking for. For all of our synthetic galaxies we assumed an exponential surface brightness profile and an old and metal-poor population. The synthetic galaxies are simulated both on the images and on the photometric catalogues, taking all the observational effects into account. In the fields where the available observational material is of top quality (≃36% of the SECCO fields), we detect synthetic galaxies as ≥5σ over-densities of resolved stars down to μV,h ≃ 30.0 mag/arcsec2, for D ≤ 1.5 Mpc, and down to μV,h ≃ 29.5 mag/arcsec2, for D ≤ 2.5 Mpc. In the field with the worst observational material of the whole survey, we detect synthetic galaxies with μV,h ≤ 28.8 mag/arcsec2 out to D ≤ 1.0 Mpc, and those with μV,h ≤ 27.5 mag/arcsec2 out to D ≤ 2.5 Mpc. Dwarf galaxies with MV = -10.0, with sizes in the range spanned by known dwarfs, are detected by visual inspection of the images up to D = 5 Mpc independent of the image quality. In the best quality images, dwarfs are partially resolved into stars up to D = 3.0 Mpc and completely unresolved at D = 5 Mpc. As an independent test of the sensitivity of our images to low surface brightness galaxies, we report on the detection of several dwarf spheroidal galaxies probably located in the Virgo cluster with MV ≲ -8.0 and μV,h ≲ 26.8 mag/arcsec2. The nature of the previously discovered SECCO 1 stellar system, also likely located in the Virgo cluster, is rediscussed in comparison with these dwarfs. While specific for the SECCO survey, our study may also provide general guidelines for the detection of faint stellar systems with 8 m class telescopes. Based on data acquired using the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the United States, Italy, and Germany. The LBT Corporation partners are The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.

International Ultraviolet Explorer observations of the white dwarf nucleus of the very old, diffuse planetary nebula, IW-2

NASA Technical Reports Server (NTRS)

Bruhweiler, F. C.; Feibelman, Walter A.

1993-01-01

UV low-dispersion spectra of the central star of the faint planetary nebula, IW-2, were obtained with the IUE. The apparent large diameter of the very diffuse nebula, about half that of the moon, as seen on the Palomar Sky Survey plates by Ishida and Weinberger (1987), indicates this object to be potentially quite evolved, and nearby. The IUE spectra clearly reveal a hot stellar continuum extending over the entire wavelength range of the short-wavelength prime camera (1200-2000 A). This object with V = 17.7 +/- 0.4 is definitely one of the faintest stars ever successfully observed with the IUE. Comparisons of the IUE observed fluxes with those from white dwarf model atmospheres suggest extinction near E(B - V) = 0.45 for a white dwarf of T(eff) roughly 100,000 K. Constraints from estimates of the nebular emission measure and observed visual magnitude also argue for a white dwarf of T(eff) roughly 100,000 K at a distance of 300 to 350 pc. The nucleus of IW-2 is one of the most evolved stars to be identified with a planetary nebula.

Feige 7 - A hot, rotating magnetic white dwarf

NASA Technical Reports Server (NTRS)

Liebert, J.; Angel, J. R. P.; Stockman, H. S.; Spinrad, H.; Beaver, E. A.

1977-01-01

Results are reported for image-tube-scanner and digicon observations of Feige 7, a faint blue star identified as a probable white dwarf. It is found that this star is a magnetic white dwarf showing a very rich spectrum with Zeeman subcomponents of both hydrogen and neutral helium as well as periodic spectrum and circular-polarization variations. A polarization period of 2.2 hr is computed, and a surface magnetic-field strength of about 18 MG is determined by matching features of the absorption spectrum to Zeeman components. It is suggested that the only reasonable explanation for the periodic variations in circular polarization is an oblique rotator with the spin axis approximately in the plane of the sky and tilted by about 24 deg to the magnetic axis. An effective temperature in the range from 20,000 to 25,000 K is estimated, an absolute magnitude of about 10.5 is derived, and the atmosphere is shown to be helium-dominated. The evolution of Feige 7 is discussed in terms of possible magnetic-field effects on atmospheric composition, rotation velocity (5.5 km/s for a radius of 7000 km), and the origin of white-dwarf magnetic fields.

THE AROMATIC FEATURES IN VERY FAINT DWARF GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu Ronin; Hogg, David W.; Moustakas, John

2011-04-01

We present optical and mid-infrared photometry of a statistically complete sample of 29 dwarf galaxies (M{sub r} > - 15 mag) selected from the Sloan Digital Sky Survey (SDSS) spectroscopic sample and observed in the mid-infrared with Spitzer IRAC. This sample contains nearby (redshift {approx}<0.005) galaxies 3 mag fainter than previously studied samples. We compare our sample with other star-forming galaxies that have been observed with both IRAC and SDSS. We examine the relationship of the infrared color, [3.6]-[7.8], sensitive to polycyclic aromatic hydrocarbon (PAH) abundance and also hot dust and stellar continuum, with star formation rates (SFRs), oxygen abundances,more » and radiation hardness, all estimated by optical emission lines. Consistent with studies of more luminous dwarfs, we find that these dwarf galaxies show much redder [3.6]-[7.8] color than luminous galaxies with similar specific SFRs. Unlike luminous galaxies, we find that these dwarf galaxies show no significant dependence at all of the [3.6]-[7.8] color on SFR, oxygen abundance, or radiation hardness, despite the fact that the sample spans a significant range in all of these quantities. When the dwarfs in our sample are compared with more luminous dwarfs, we find that the [3.6]-[7.8] color, potentially tracing the PAH emission, depends on oxygen abundance and radiation hardness. However, these two parameters are correlated with one another as well; we break this degeneracy by looking at the PAH-oxygen abundance relation at a fixed radiation hardness and the PAH-hardness relation at a fixed oxygen abundance. This test shows that the [3.6]-[7.8] color in dwarf galaxies appears to depend more directly on oxygen abundance based on the data currently available.« less

PHYSICAL PROPERTIES OF THE CURRENT CENSUS OF NORTHERN WHITE DWARFS WITHIN 40 pc OF THE SUN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Limoges, M.-M.; Bergeron, P.; Lépine, S., E-mail: limoges@astro.umontreal.ca, E-mail: bergeron@astro.umontreal.ca, E-mail: slepine@chara.gsu.edu

We present a detailed description of the physical properties of our current census of white dwarfs within 40 pc of the Sun, based on an exhaustive spectroscopic survey of northern hemisphere candidates from the SUPERBLINK proper motion database. Our method for selecting white dwarf candidates is based on a combination of theoretical color–magnitude relations and reduced proper motion diagrams. We reported in an earlier publication the discovery of nearly 200 new white dwarfs, and we present here the discovery of an additional 133 new white dwarfs, among which we identify 96 DA, 3 DB, 24 DC, 3 DQ, and 7more » DZ stars. We further identify 178 white dwarfs that lie within 40 pc of the Sun, representing a 40% increase of the current census, which now includes 492 objects. We estimate the completeness of our survey at between 66% and 78%, allowing for uncertainties in the distance estimates. We also perform a homogeneous model atmosphere analysis of this 40 pc sample and find a large fraction of massive white dwarfs, indicating that we are successfully recovering the more massive, and less luminous objects often missed in other surveys. We also show that the 40 pc sample is dominated by cool and old white dwarfs, which populate the faint end of the luminosity function, although trigonometric parallaxes will be needed to shape this part of the luminosity function more accurately. Finally, we identify 4 probable members of the 20 pc sample, 4 suspected double degenerate binaries, and we also report the discovery of two new ZZ Ceti pulsators.« less

PEERING INTO THE CORE OF A GLOBULAR CLUSTER

NASA Technical Reports Server (NTRS)

2002-01-01

Astronomers have used NASA's Hubble Space Telescope to peer into the center of a dense swarm of stars called Omega Centauri. Located some 17,000 light-years from Earth, Omega Centauri is a massive globular star cluster, containing several million stars swirling in locked orbits around a common center of gravity. The stars are packed so densely in the cluster's core that it is difficult for ground-based telescopes to make out individual stars. Hubble's high resolution is able to pick up where ground-based telescopes leave off, capturing distinct points of light from stars at the very center of the cluster. Omega Centauri is so large in our sky that only a small part of it fits within the field of view of the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. Yet even this tiny patch contains some 50,000 stars, all packed into a region only about 13 light-years wide. For comparison, a similarly sized region centered on the Sun would contain about a half dozen stars. The vast majority of stars in this Hubble image are faint, yellow-white dwarf stars similar to our Sun. The handful of bright yellow-orange stars are red giants that have begun to exhaust their nuclear fuel and have expanded to diameters about a hundred times that of the Sun. A number of faint blue stars are also visible in the image. These are in a brief phase of evolution between the dwarf stage and the red-giant stage, during which the surface temperature is high. The stars in Omega Centauri are all very old, about 12 billion years. Stars with a mass as high as that of our Sun have already completed their evolution and have faded away as white dwarfs, too faint to be seen even in the Hubble image. The stars in the core of Omega Centauri are so densely packed that occasionally one of them will actually collide with another one. Even in the dense center of Omega Centauri, stellar collisions will be infrequent. But the cluster is so old that many thousands of collisions have occurred. What happens when stars collide? These Hubble images were taken to help answer that question. When stars collide head-on, they probably just merge together and make one bigger star. But if the collision is a near miss, they may go into orbit around each other, forming a close binary star system. Searching for a needle in a haystack, scientists have found two binary star systems in these Hubble images that may have had such an origin. Both of them are close pairs in which once component is a white dwarf that pulls gas off of its companion. When the gas falls onto the surface of the white dwarf, it is heated to the point that it emits ultraviolet light. These unusual emissions enabled scientists to pinpoint these two faint stars among the myriad of other faint stars in the cluster. Omega Centauri is the most luminous and massive globular star cluster in the Milky Way. It is one of the few globular clusters that can be seen with the unaided eye. Named by Johann Bayer in 1603 as the 24th brightest object in the constellation Centaurus, it resembles a small cloud in the southern sky and might easily be mistaken for a comet. This Hubble WFPC2 image was taken on June 11, 1997 in ultraviolet, red, and H-alpha filters. The science team, led by Dr. Adrienne Cool of San Francisco State University includes Jennifer Carson, a former SFSU student who is now at UCLA, Charles Bailyn at Yale and Jonathan Grindlay at Harvard. These data are currently being used by Jeff Carlin and Daryl Haggard, two SFSU students, to look for optical counterparts of X-ray sources recently discovered with the Chandra Observatory. This image was produced by the Hubble Heritage Team (STScI/AURA). Credits: NASA and The Hubble Heritage Team (STScI/AURA) Acknowledgment: A. Cool (SFSU)

Title: Characterizing a Frozen Extrasolar World

NASA Technical Reports Server (NTRS)

Skemer, Andrew J.; Morley, Caroline V.; Allers, Katelyn N.; Geballe, Thomas R.; Marley, Mark S.; Fortney, Jonathan J.; Faherty, Jacqueline K.; Bjoraker, Gordon L.

2016-01-01

The recently discovered brown dwarf WISE 0855 presents our first opportunity to study an object outside the Solar System that is nearly as cold as our own gas giant planets. However the traditional methodology for characterizing brown dwarfs-near infrared spectroscopy-is not currently feasible as WISE 0855 is too cold and faint. To characterize this frozen extrasolar world we obtained a 4.5-5.2 micrometers spectrum, the same bandpass long used to study Jupiter's deep thermal emission. Our spectrum reveals the presence of atmospheric water vapor and clouds, with an absorption profile that is strikingly similar to Jupiter. The spectrum is high enough quality to allow the investigation of dynamical and chemical processes that have long been studied in Jupiter's atmosphere, but this time on an extrasolar world.

THE SUBARU HIGH-z QUASAR SURVEY: DISCOVERY OF FAINT z ∼ 6 QUASARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kashikawa, Nobunari; Furusawa, Hisanori; Niino, Yuu

2015-01-01

We present the discovery of one or two extremely faint z ∼ 6 quasars in 6.5 deg{sup 2} utilizing a unique capability of the wide-field imaging of the Subaru/Suprime-Cam. The quasar selection was made in (i'-z{sub B} ) and (z{sub B} -z{sub R} ) colors, where z{sub B} and z{sub R} are bandpasses with central wavelengths of 8842 Å and 9841 Å, respectively. The color selection can effectively isolate quasars at z ∼ 6 from M/L/T dwarfs without the J-band photometry down to z{sub R} < 24.0, which is 3.5 mag deeper than the Sloan Digital Sky Survey (SDSS). Wemore » have selected 17 promising quasar candidates. The follow-up spectroscopy for seven targets identified one apparent quasar at z = 6.156 with M {sub 1450} = –23.10. We also identified one possible quasar at z = 6.041 with a faint continuum of M {sub 1450} = –22.58 and a narrow Lyα emission with HWHM =427 km s{sup –1}, which cannot be distinguished from Lyman α emitters. We derive the quasar luminosity function at z ∼ 6 by combining our faint quasar sample with the bright quasar samples by SDSS and CFHQS. Including our data points invokes a higher number density in the faintest bin of the quasar luminosity function than the previous estimate employed. This suggests a steeper faint-end slope than lower z, though it is yet uncertain based on a small number of spectroscopically identified faint quasars, and several quasar candidates still remain to be diagnosed. The steepening of the quasar luminosity function at the faint end does increase the expected emission rate of the ionizing photon; however, it only changes by a factor of approximately two to six. This was found to still be insufficient for the required photon budget of reionization at z ∼ 6.« less

Adaptive optics for high-contrast imaging of faint substellar companions

NASA Astrophysics Data System (ADS)

Morzinski, Katie M.

Direct imaging of faint objects around bright stars is challenging because the primary star's diffracted light can overwhelm low-mass companions. Nevertheless, advances in adaptive optics (AO) and high-contrast imaging have revealed the first pictures of extrasolar planets. In this dissertation I employ today's high-contrast AO techniques to image brown dwarfs around stars in the nearby Hyades cluster. Furthermore, I prepare for the next generation of high-contrast AO instrumentation, by qualifying MEMS deformable mirrors for wavefront control in the Gemini Planet Imager. In Part I, I present discovery of 3 new brown dwarfs and 36 low-mass stellar companions to 85 stars in the Hyades, imaged with AO at Keck and Lick Observatories. The "locally-optimized combination of images" (LOCI) image-diversity technique filters out the primary star to reveal faint companions. This survey is complete to the hydrogen-burning limit at separations beyond 20 AU. In the complete sample, multiplicity increases as primary star mass decreases. Additionally, the brown dwarfs are at wide >150 AU separations. Finding this preference for low binding-energy systems is an unexpected result, as the Hyades is 625 Myr old and dynamically relaxed. Future work will continue to explore this trend to understand the dynamical and star formation history of the Hyades. The brown dwarfs are near interesting transition regimes for low-mass objects; therefore, characterizing their atmospheres with spectrophotometry will serve as an important benchmark for our understanding of these cool objects. In Part II, I demonstrate micro-electro-mechanical systems (MEMS) deformable mirrors for high-order wavefront control in the Gemini Planet Imager (GPI). MEMS micromirrors have thousands of degrees of freedom and represent a significant cost efficiency over conventional glass deformable mirrors, making them ideal for high-contrast AO. In Chapter 7, I present experimental evidence that MEMS actuators function well and are stable and repeatable at the sub-nm level over the course of an hour. In Chapter 8, I prove MEMS ability to correct high-order Kolmogorov turbulence and maintain the high-contrast "dark hole" in the GPI woofer-tweeter architecture. Finally, in Chapter 9, I analyze MEMS performance on sky with Villages, a telescope testbed for MEMS technology, visible-light AO, and open-loop control. The MEMS remains repeatably flat and controllable over ˜4 years and ˜800 hours of operation. Open loop control of the hysteresis-free MEMS produces a diffraction-limited core in I-band, while internal static errors dominate the on-sky error budget. This work establishes MEMS deformable mirrors as excellent wavefront correctors for high-order AO. The MEMS in GPI will produce a deeper, broader dark hole, allowing for detection and characterization of directly-imaged planets in a fainter, wider search space.

A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies

NASA Astrophysics Data System (ADS)

Martin, N. F.; Ibata, R. A.; Chapman, S. C.; Irwin, M.; Lewis, G. F.

2007-09-01

We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Boötes, Ursa Major I, Ursa Major II and Willman 1 (Wil1). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have obtained samples that contain from ~15 to ~85 probable members of these satellites for which we derive radial velocities precise to a few kms-1 down to i ~ 21-22. About half of these stars are observed with a high enough signal-to-noise ratio to estimate their metallicity to within +/-0.2 dex. The characteristics of all the observed stars are made available, along with those of the Canes Venatici I dwarf galaxy that have been analysed in a companion paper. From this data set, we show that Ursa Major II is the only object that does not show a clear radial velocity peak. However, the measured systemic radial velocity (vr = 115 +/- 5kms-1) is in good agreement with simulations in which this object is the progenitor of the recently discovered Orphan Stream. The three other satellites show velocity dispersions that make them highly dark matter dominated systems (under the usual assumptions of symmetry and virial equilibrium). In particular, we show that despite its small size and faintness, the Wil1 object is not a globular cluster given its metallicity scatter over -2.0 <~ [Fe/H] <~ -1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity dispersion of only 4.3+2.3-1.3kms-1 around a systemic velocity of -12.3 +/- 2.3kms-1 which implies a mass-to-light ratio of ~700 and a total mass of ~5 × 105Msolar for this satellite, making it the least massive satellite galaxy known to date. Such a low mass could mean that the 107Msolar limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter, less massive systems exist within the Local Group. However, more modelling and an extended search for potential extratidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation. E-mail: martin@mpia-hd.mpg.de ‡ Canadian Space Agency Fellow.

Laboratory measurements of resistivity in warm dense plasmas relevant to the microphysics of brown dwarfs

DOE PAGES

Booth, N.; Robinson, A. P. L.; Hakel, P.; ...

2015-11-06

Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. Furthermore, themore » inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs.« less

Double-lined M dwarf eclipsing binaries from Catalina Sky Survey and LAMOST

NASA Astrophysics Data System (ADS)

Lee, Chien-Hsiu; Lin, Chien-Cheng

2017-02-01

Eclipsing binaries provide a unique opportunity to determine fundamental stellar properties. In the era of wide-field cameras and all-sky imaging surveys, thousands of eclipsing binaries have been reported through light curve classification, yet their basic properties remain unexplored due to the extensive efforts needed to follow them up spectroscopically. In this paper we investigate three M2-M3 type double-lined eclipsing binaries discovered by cross-matching eclipsing binaries from the Catalina Sky Survey with spectroscopically classified M dwarfs from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey data release one and two. Because these three M dwarf binaries are faint, we further acquire radial velocity measurements using GMOS on the Gemini North telescope with R˜ 4000, enabling us to determine the mass and radius of individual stellar components. By jointly fitting the light and radial velocity curves of these systems, we derive the mass and radius of the primary and secondary components of these three systems, in the range between 0.28-0.42M_⊙ and 0.29-0.67R_⊙, respectively. Future observations with a high resolution spectrograph will help us pin down the uncertainties in their stellar parameters, and render these systems benchmarks to study M dwarfs, providing inputs to improving stellar models in the low mass regime, or establishing an empirical mass-radius relation for M dwarf stars.

Searching for Dwarf H Alpha Emission-line Galaxies within Voids III: First Spectra

NASA Astrophysics Data System (ADS)

Moody, J. Ward; Draper, Christian; McNeil, Stephen; Joner, Michael D.

2017-02-01

The presence or absence of dwarf galaxies with {M}r\\prime > -14 in low-density voids is determined by the nature of dark matter halos. To better understand what this nature is, we are conducting an imaging survey through redshifted Hα filters to look for emission-line dwarf galaxies in the centers of two nearby galaxy voids called FN2 and FN8. Either finding such dwarfs or establishing that they are not present is a significant result. As an important step in establishing the robustness of the search technique, we have observed six candidates from the survey of FN8 with the Gillett Gemini telescope and GMOS spectrometer. All of these candidates had emission, although none was Hα. The emission in two objects was the [O III]λ4959, 5007 doublet plus Hβ, and the emission in the remaining four was the [O II]λ3727 doublet, all from objects beyond the void. While no objects were within the void, these spectra show that the survey is capable of finding emission-line dwarfs in the void centers that are as faint as {M}r\\prime ˜ -12.4, should they be present. These spectra also show that redshifts estimated from our filtered images are accurate to several hundred km s-1 if the line is identified correctly, encouraging further work in finding ways to conduct redshift surveys through imaging alone.

Dwarfs in the Deepest Fields at Noon: Studying Size and Shape of Low-mass Galaxies out to z 3 in Five HST Legacy Fields

NASA Astrophysics Data System (ADS)

Guo, Yicheng

2017-08-01

Galaxies with stellar mass 100x-1000x times smaller than our Milky Way (hereafter dwarf galaxies or DGs) are important for understanding galaxy formation and evolution by being the most sensitive probes of both the macro-physics of dark matter halos and the micro-physics of the different physical mechanisms that regulate star formation and shape galaxies. Currently, however, observations of distant DGs have been hampered by small samples and poor quality due to their faintness. We propose an archival study of the size, morphology, and structures of DGs out to z 3.0 by combining the archived data from five of the deepest regions that HST has ever observed: eXtreme Deep Field (XDF, updated from HUDF) and the Hubble Legacy Fields (HLFs). Our program would be the first to advance the morphology studies of DGs to the Cosmic Noon (z 2), and hence place unprecedented constraints on models of galaxy structure formation. Equally important is the data product of our program: multi-wavelength photometry and morphology catalogs for all detected galaxies in these fields. These catalogs would be a timely treasure for the public to prepare for the coming JWST era by providing detailed information of small, faint, but important objects in some deepest HST fields for JWST observations.

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites AndIX, AndXI, AndXII and AndXIII†

NASA Astrophysics Data System (ADS)

Collins, M. L. M.; Chapman, S. C.; Irwin, M. J.; Martin, N. F.; Ibata, R. A.; Zucker, D. B.; Blain, A.; Ferguson, A. M. N.; Lewis, G. F.; McConnachie, A. W.; Peñarrubia, J.

2010-10-01

We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, as well as a re-analysis of existing spectroscopic data for two further faint companions, AndIX (correcting for an error in earlier geometric modelling that caused a misclassification of member stars in previous work) and AndXII. By combining data obtained using the Deep Imaging Multi-Object Spectrograph (DEIMOS) mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have identified the most probable members for each of the satellites based on their radial velocities (precise to several down to i ~ 22), distance from the centre of the dwarf spheroidal galaxies (dSphs) and their photometric [Fe/H]. Using both the photometric and spectroscopic data, we have also calculated global properties for the dwarfs, such as systemic velocities, metallicities and half-light radii. We find each dwarf to be very metal poor ([Fe/H] ~ -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve dispersion for AndXI due to small sample size and low signal-to-noise ratio, but we set a 1σ upper limit of σv < 4.5kms-1. For AndIX, AndXII and AndXIII we resolve velocity dispersions of σv = 4.5+3.6-3.4, 2.6+5.1-2.6 and 9.7+8.9-4.5kms-1, though we note that the dispersion for AndXIII is based on just three stars. We derive masses within the half-light radii for these galaxies of 6.2+5.3-5.1 × 106, 2.4+6.5-2.4 × 106 and 1.1+1.4-0.7 × 107Msolar, respectively. We discuss each satellite in the context of the Mateo relations for dSphs, and in reference to the universal halo profiles established for Milky Way dwarfs. Both AndIX and AndXII fall below the universal halo profiles of Walker et al., indicating that they are less massive than would be expected for objects of their half-light radius. When combined with the findings of McConnachie & Irwin, which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit haloes that with regard to their central densities are significantly different from those of the Milky Way. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. ‡ E-mail: mlmc2@ast.cam.ac.uk

Eight new Milky Way companions discovered in first-year Dark Energy Survey data

DOE PAGES

Bechtol, K.

2015-06-30

We report the discovery of eight new Milky Way companions inmore » $$\\sim 1800\\;{\\mathrm{deg}}^{2}$$ of optical imaging data collected during the first year of the Dark Energy Survey (DES). Each system is identified as a statistically significant over-density of individual stars consistent with the expected isochrone and luminosity function of an old and metal-poor stellar population. The objects span a wide range of absolute magnitudes (MV from $-2.2$ to $$-7.4\\;\\mathrm{mag}$$), physical sizes ($$10-170\\;\\mathrm{pc}$$), and heliocentric distances ($$30-330\\;\\mathrm{kpc}$$). Based on the low surface brightnesses, large physical sizes, and/or large Galactocentric distances of these objects, several are likely to be new ultra-faint satellite galaxies of the Milky Way and/or Magellanic Clouds. We introduce a likelihood-based algorithm to search for and characterize stellar over-densities, as well as identify stars with high satellite membership probabilities. As a result, we also present completeness estimates for detecting ultra-faint galaxies of varying luminosities, sizes, and heliocentric distances in the first-year DES data.« less

Metal abundances in hot white dwarfs with signatures of a superionized wind

NASA Astrophysics Data System (ADS)

Werner, K.; Rauch, T.; Kruk, J. W.

2018-01-01

About a dozen hot white dwarfs with effective temperatures Teff = 65 000-120 000 K exhibit unusual absorption features in their optical spectra. These objects were tentatively identified as Rydberg lines of ultra-high excited metals in ionization stages v-x, indicating line formation in a dense environment with temperatures near 106 K. Since some features show blueward extensions, it was argued that they stem from a superionized wind. A unique assignment of the lines to particular elements is not possible, although they probably stem from C, N, O, and Ne. To further investigate this phenomenon, we analyzed the ultraviolet spectra available from only three stars of this group; that is, two helium-rich white dwarfs, HE 0504-2408 and HS 0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148 with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with generally subsolar abundances and heavy elements from the iron group (Cr, Mn, Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not unusual for hot WDs and can be interpreted as the result of gravitational settling and radiative levitation of elements. As to the origin of the ultra-high ionized metals lines, we discuss the possible presence of a multicomponent radiatively driven wind that is frictionally heated.

Origins of ultra-diffuse galaxies in the Coma cluster - II. Constraints from their stellar populations

NASA Astrophysics Data System (ADS)

Ferré-Mateu, Anna; Alabi, Adebusola; Forbes, Duncan A.; Romanowsky, Aaron J.; Brodie, Jean; Pandya, Viraj; Martín-Navarro, Ignacio; Bellstedt, Sabine; Wasserman, Asher; Stone, Maria B.; Okabe, Nobuhiro

2018-06-01

In this second paper of the series we study, with new Keck/DEIMOS spectra, the stellar populations of seven spectroscopically confirmed ultra-diffuse galaxies (UDGs) in the Coma cluster. We find intermediate to old ages (˜ 7 Gyr), low metallicities ([Z/H]˜ - 0.7 dex) and mostly super-solar abundance patterns ([Mg/Fe] ˜ 0.13 dex). These properties are similar to those of low-luminosity (dwarf) galaxies inhabiting the same area in the cluster and are mostly consistent with being the continuity of the stellar mass scaling relations of more massive galaxies. These UDGs' star formation histories imply a relatively recent infall into the Coma cluster, consistent with the theoretical predictions for a dwarf-like origin. However, considering the scatter in the resulting properties and including other UDGs in Coma, together with the results from the velocity phase-space study of the Paper I in this series, a mixed-bag of origins is needed to explain the nature of all UDGs. Our results thus reinforce a scenario in which many UDGs are field dwarfs that become quenched through their later infall onto cluster environments, whereas some UDGs could be be genuine primordial galaxies that failed to develop due to an early quenching phase. The unknown proportion of dwarf-like to primordial-like UDGs leaves the enigma of the nature of UDGs still open.

Triangulum II. Not Especially Dense After All

NASA Astrophysics Data System (ADS)

Kirby, Evan N.; Cohen, Judith G.; Simon, Joshua D.; Guhathakurta, Puragra; Thygesen, Anders O.; Duggan, Gina E.

2017-04-01

Among the Milky Way satellites discovered in the past three years, Triangulum II has presented the most difficulty in revealing its dynamical status. Kirby et al. identified it as the most dark-matter-dominated galaxy known, with a mass-to-light ratio within the half-light radius of {3600}-2100+3500 {M}⊙ {L}⊙ -1. On the other hand, Martin et al. measured an outer velocity dispersion that is 3.5 ± 2.1 times larger than the central velocity dispersion, suggesting that the system might not be in equilibrium. From new multi-epoch Keck/DEIMOS measurements of 13 member stars in Triangulum II, we constrain the velocity dispersion to be {σ }v< 3.4 km s-1 (90% C.L.). Our previous measurement of {σ }v, based on six stars, was inflated by the presence of a binary star with variable radial velocity. We find no evidence that the velocity dispersion increases with radius. The stars display a wide range of metallicities, indicating that Triangulum II retained supernova ejecta and therefore possesses, or once possessed, a massive dark matter halo. However, the detection of a metallicity dispersion hinges on the membership of the two most metal-rich stars. The stellar mass is lower than galaxies of similar mean stellar metallicity, which might indicate that Triangulum II is either a star cluster or a tidally stripped dwarf galaxy. Detailed abundances of one star show heavily depressed neutron-capture abundances, similar to stars in most other ultra-faint dwarf galaxies but unlike stars in globular clusters. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Fifteen DO, PG 1159 and related white dwarf stars in the SDSS, including two DO stars with ultra-high excitation ion lines

NASA Astrophysics Data System (ADS)

Krzesiński, J.; Nitta, A.; Kleinman, S. J.; Harris, H. C.; Liebert, J.; Schmidt, G.; Lamb, D. Q.; Brinkmann, J.

2004-04-01

We report on observations of 15 spectroscopically-identified DO stars from the Sloan Digital Sky Survey (SDSS) database, 13 of which are new discoveries. There are four PG 1159 type stars, two DO stars showing ultra-high excitation ion features (CVI, NVII, OVII, OVIII, NeIX, NeX) likely formed in stellar winds, 6 normal DO stars, one DBO and DBAO star, and one DAO star, which may also be magnetic. Since roughly 60 DO stars were known up to now, this new finding substantially increases the number of known DO white dwarf stars and we expect to at least double the current number of known DO stars by the end of the SDSS. We present each spectrum and provide catalog information (magnitudes, proper motion, coordinates) for each star.

Ultrafaint dwarfs—star formation and chemical evolution in the smallest galaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webster, David; Bland-Hawthorn, Joss; Sutherland, Ralph, E-mail: d.webster@physics.usyd.edu.au

2014-11-20

In earlier work, we showed that a dark matter halo with a virial mass of 10{sup 7} M {sub ☉} can retain a major part of its baryons in the face of the pre-ionization phase and supernova (SN) explosion from a 25 M {sub ☉} star. Here, we expand on the results of that work, investigating the star formation and chemical evolution of the system beyond the first SN. In a galaxy with a mass M {sub vir} = 10{sup 7} M {sub ☉}, sufficient gas is retained by the potential for a second period of star formation to occur.more » The impact of a central explosion is found to be much stronger than that of an off-center explosion both in blowing out the gas and in enriching it, as in the off-center case most of the SN energy and metals escape into the intergalactic medium. We model the star formation and metallicity, given the assumption that stars form for 100, 200, 400, and 600 Myr, and discuss the results in the context of recent observations of very low-mass galaxies. We show that we can account for most features of the observed relationship between [α/Fe] and [Fe/H] in ultra-faint dwarf galaxies with the assumption that the systems formed at a low mass, rather than being remnants of much larger systems.« less

Discovery of two neighbouring satellites in the Carina constellation with MagLiteS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Torrealba, G.; Belokurov, V.; Koposov, S. E.

Here, we report the discovery of two ultra-faint satellites in the vicinity of the Large Magellanic Cloud (LMC) in data from the Magellanic Satellites Survey (MagLiteS). Situated 18more » $$^{\\circ}$$ ($$\\sim 20$$ kpc) from the LMC and separated from each other by only $$18^\\prime$$, Carina~II and III form an intriguing pair. By simultaneously modeling the spatial and the color-magnitude stellar distributions, we find that both Carina~II and Carina~III are likely dwarf galaxies, although this is less clear for Carina~III. There are in fact several obvious differences between the two satellites. While both are well described by an old and metal poor population, Carina~II is located at $$\\sim 36$$ kpc from the Sun, with $$M_V\\sim-4.5$$ and $$r_h\\sim 90$$ pc, and it is further confirmed by the discovery of 3 RR Lyrae at the right distance. In contrast, Carina~III is much more elongated, measured to be fainter ($$M_V\\sim-2.4$$), significantly more compact ($$r_h\\sim30$$ pc), and closer to the Sun, at $$\\sim 28$$ kpc, placing it only 8 kpc away from Car~II. Together with several other systems detected by the Dark Energy Camera, Carina~II and III form a strongly anisotropic cloud of satellites in the vicinity of the Magellanic Clouds.« less

Discovery of two neighbouring satellites in the Carina constellation with MagLiteS

DOE PAGES

Torrealba, G.; Belokurov, V.; Koposov, S. E.; ...

2018-01-23

Here, we report the discovery of two ultra-faint satellites in the vicinity of the Large Magellanic Cloud (LMC) in data from the Magellanic Satellites Survey (MagLiteS). Situated 18more » $$^{\\circ}$$ ($$\\sim 20$$ kpc) from the LMC and separated from each other by only $$18^\\prime$$, Carina~II and III form an intriguing pair. By simultaneously modeling the spatial and the color-magnitude stellar distributions, we find that both Carina~II and Carina~III are likely dwarf galaxies, although this is less clear for Carina~III. There are in fact several obvious differences between the two satellites. While both are well described by an old and metal poor population, Carina~II is located at $$\\sim 36$$ kpc from the Sun, with $$M_V\\sim-4.5$$ and $$r_h\\sim 90$$ pc, and it is further confirmed by the discovery of 3 RR Lyrae at the right distance. In contrast, Carina~III is much more elongated, measured to be fainter ($$M_V\\sim-2.4$$), significantly more compact ($$r_h\\sim30$$ pc), and closer to the Sun, at $$\\sim 28$$ kpc, placing it only 8 kpc away from Car~II. Together with several other systems detected by the Dark Energy Camera, Carina~II and III form a strongly anisotropic cloud of satellites in the vicinity of the Magellanic Clouds.« less

Ring Structure and Warp of NGC 5907: Interaction with Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Shang, Zhaohui; Zheng, Zhongyuan; Brinks, Elias; Chen, Jiansheng; Burstein, David; Su, Hongjun; Byun, Yong-ik; Deng, Licai; Deng, Zugan; Fan, Xiaohui; Jiang, Zhaoji; Li, Yong; Lin, Weipeng; Ma, Feng; Sun, Wei-hsin; Wills, Beverley; Windhorst, Rogier A.; Wu, Hong; Xia, Xiaoyang; Xu, Wen; Xue, Suijian; Yan, Haojing; Zhou, Xu; Zhu, Jin; Zou, Zhenlong

1998-09-01

The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a ``noninteracting'' warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring's foci. This suggests that the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Hα images or in Very Large Array H I 21 cm line maps. The visible material in the ring has an integrated luminosity <=108 Lsolar, and its brightest part has a color R-I~0.9. All of these properties are consistent with the ring being a tidally disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, which is projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity (ΔV=45 km s-1) to the giant spiral galaxy. This dwarf is seen at the tip of the H I warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered noninteracting but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies. Partially based on observations taken with the Very Large Array of the National Radio Astronomy Observatory is a facility of the National Science Foundation operated by a cooperative agreement with Associated Universities, Inc.

Infrared observations of white dwarfs and the implications for the accretion of dusty planetary material

NASA Astrophysics Data System (ADS)

Bonsor, Amy; Farihi, Jay; Wyatt, Mark C.; van Lieshout, Rik

2017-06-01

Infrared excesses around metal-polluted white dwarfs have been associated with the accretion of dusty planetary material. This work analyses the available infrared data for an unbiased sample of white dwarfs and demonstrates that no more than 3.3 per cent can have a wide, flat, opaque dust disc, extending to the Roche radius, with a temperature at the disc inner edge of Tin = 1400 K, the standard model for the observed excesses. This is in stark contrast to the incidence of pollution of about 30 per cent. We present four potential reasons for the absence of an infrared excess in polluted white dwarfs, depending on their stellar properties and inferred accretion rates: (I) their dust discs are opaque, but narrow, thus evading detection if more than 85 per cent of polluted white dwarfs have dust discs narrower than δr < 0.04r, (II) their dust discs have been fully consumed, which only works for the oldest white dwarfs with sinking time-scales longer than hundreds of years, (III) their dust is optically thin, which can supply low accretion rates of <107 gs-1 if dominated by (Poynting-Robertson) PR-drag, and higher accretion rates, if inwards transport of material is enhanced, e.g. due to the presence of gas, (IV) their accretion is supplied by a pure gas disc, which could result from the sublimation of optically thin dust for T* > 20 000 K. Future observations sensitive to faint infrared excesses or the presence of gas can test the scenarios presented here, thereby better constraining the nature of the material fuelling accretion in polluted white dwarfs.

Gas, Stars, and Star Formation in Alfalfa Dwarf Galaxies

NASA Technical Reports Server (NTRS)

Huang, Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G.

2012-01-01

We examine the global properties of the stellar and Hi components of 229 low H i mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H i masses <10(sup 7.7) solar mass and Hi line widths <80 kilometers per second. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M*) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M* approximately less than10(exp 8)M(sub 0) is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper Hi mass limit yields the selection of a sample with lower gas fractions for their M* than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H i depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that Hi disks are more extended than stellar ones.

THE FIRST SPECTRUM OF THE COLDEST BROWN DWARF

DOE Office of Scientific and Technical Information (OSTI.GOV)

Skemer, Andrew J.; Morley, Caroline V.; Fortney, Jonathan J.

2016-08-01

The recently discovered brown dwarf WISE 0855 presents the first opportunity to directly study an object outside the solar system that is nearly as cold as our own gas giant planets. However, the traditional methodology for characterizing brown dwarfs—near-infrared spectroscopy—is not currently feasible, as WISE 0855 is too cold and faint. To characterize this frozen extrasolar world we obtained a 4.5–5.2 μ m spectrum, the same bandpass long used to study Jupiter’s deep thermal emission. Our spectrum reveals the presence of atmospheric water vapor and clouds, with an absorption profile that is strikingly similar to Jupiter’s. The spectrum quality ismore » high enough to allow for the investigation of dynamical and chemical processes that have long been studied in Jupiter’s atmosphere, but now on an extrasolar world.« less

Dwarf Galaxies and the Cosmic Web

NASA Astrophysics Data System (ADS)

Benítez-Llambay, Alejandro; Navarro, Julio F.; Abadi, Mario G.; Gottlöber, Stefan; Yepes, Gustavo; Hoffman, Yehuda; Steinmetz, Matthias

2013-02-01

We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram pressure. The loss of gas is especially pronounced in low-mass halos due to their lower binding energy and has a dramatic effect on the star formation history of affected systems. This "cosmic web stripping" may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in ΛCDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.

Astrophysics of brown dwarfs; Proceedings of the Workshop, George Mason University, Fairfax, VA, Oct. 14, 15, 1985

NASA Technical Reports Server (NTRS)

Kafatos, Minas C. (Editor); Harrington, Robert S. (Editor); Maran, Stephen P. (Editor)

1986-01-01

Various reports on theoretical and observational studies of brown dwarfs (BDs) are presented. The topics considered include: astrometric detection of BDs, search for substellar companions to nearby stars using IR imaging, constraints on BD mass function from optical and IR searches, properties of stellar objects near the main sequence mass limit, search for low-mass stellar companions with the HF precision velocity technique, dynamical search for substellar objects, search for BDs in the IRAS data base, deep CCD survey for low mass stars in the disk and halo, the Berkeley search for a faint solar companion, the luminosity function for late M stars, astronomic search for IR dwarfs, and the role of the Space Telescope in the detection of BDs. Also addressed are: theoretical significance of BDs, evolution of super-Jupiters, compositional indicators in IR spectra of BDs, evolution of BDs and the evolutionary status of VB8B, the position of BDs on universal diagrams, theoretical determination of the minimum protostellar mass, Population II BDs and dark halos.

A spectroscopic binary in the Hercules dwarf spheroidal galaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koch, Andreas; Hansen, Terese; Feltzing, Sofia

2014-01-01

We present the radial velocity curve of a single-lined spectroscopic binary in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual spectra covering more than 2 yr of observations. This is the first time that orbital elements could be derived for a binary in a dSph. The system consists of a metal-poor red giant and a low-mass companion, possibly a white dwarf, with a 135 day period in a moderately eccentric (e = 0.18) orbit. Its period and eccentricity are fully consistent with metal-poor binaries in the Galactic halo, while the projected semimajor axis is small, at a{submore » p} sin i = 38 R {sub ☉}. In fact, a very close orbit could inhibit the production of heavier elements through s-process nucleosynthesis, leading to the very low abundances of neutron-capture elements that are found in this star. We discuss the further implications for the chemical enrichment history of the Hercules dSph, but find no compelling binary scenario that could reasonably explain the full, peculiar abundance pattern of the Hercules dSph galaxy.« less

DENIS J081730.0-615520: AN OVERLOOKED MID-T DWARF IN THE SOLAR NEIGHBORHOOD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Artigau, Etienne; Lafreniere, David; Doyon, Rene

2010-07-20

Recent wide-field near-infrared surveys have uncovered a large number of cool brown dwarfs (BDs), extending the temperature sequence down to less than 500 K and constraining the faint end of the luminosity function (LF). One interesting implication of the derived LF is that the BD census in the immediate (<10 pc) solar neighborhood is still largely incomplete, and some bright (J < 16) BDs remain to be identified in existing surveys. These objects are especially interesting as they are the ones that can be studied in most detail, especially with techniques that require large fluxes (e.g., time-variability, polarimetry, and high-resolutionmore » spectroscopy) that cannot realistically be applied to objects uncovered by deep surveys. By cross-matching the DEep Near-Infrared Survey of the Southern sky (DENIS) and the Two Micron All Sky Survey point-source catalogs, we have identified an overlooked BD-DENIS J081730.0-615520-that is the brightest field mid-T dwarf in the sky (J = 13.6). We present astrometry and spectroscopy follow-up observations of this BD. Our data indicate a spectral type T6 and a distance-from parallax measurement-of 4.9 {+-} 0.3 pc, placing this mid-T dwarf among the three closest isolated BDs to the Sun.« less

Ultra high-definition video: convergence toward 100Gbps and beyond for digital A/V connectivity with fiber optics

NASA Astrophysics Data System (ADS)

Parekh, Devang; Nguyen, Nguyen X.

2018-02-01

The recent advent of Ultra-high-definition television (also known as Ultra HD television, Ultra HD, UHDTV, UHD and Super Hi-Vision) has accelerated a demand for a Fiber-in-the-Premises video communication (VCOM) solution that converges toward 100Gbps and Beyond. Hybrid Active-Optical-Cables (AOC) is a holistic connectivity platform well suited for this "The Last Yard" connectivity; as it combines both copper and fiber optics to deliver a high data-rate and power transmission needed. While technically feasible yet challenging to manufacture, hybrid-AOC could be a holygrail fiber-optics solution that dwarfs the volume of both telecom and datacom connection in the foreseeable future.

On the RR Lyrae Stars in Globulars. IV. ω Centauri Optical UBVRI Photometry

NASA Astrophysics Data System (ADS)

Braga, V. F.; Stetson, P. B.; Bono, G.; Dall'Ora, M.; Ferraro, I.; Fiorentino, G.; Freyhammer, L. M.; Iannicola, G.; Marengo, M.; Neeley, J.; Valenti, E.; Buonanno, R.; Calamida, A.; Castellani, M.; da Silva, R.; Degl'Innocenti, S.; Di Cecco, A.; Fabrizio, M.; Freedman, W. L.; Giuffrida, G.; Lub, J.; Madore, B. F.; Marconi, M.; Marinoni, S.; Matsunaga, N.; Monelli, M.; Persson, S. E.; Piersimoni, A. M.; Pietrinferni, A.; Prada-Moroni, P.; Pulone, L.; Stellingwerf, R.; Tognelli, E.; Walker, A. R.

2016-12-01

New accurate and homogeneous optical UBVRI photometry has been obtained for variable stars in the Galactic globular cluster ω Cen (NGC 5139). We secured 8202 CCD images covering a time interval of 24 years and a sky area of 84 × 48 arcmin. The current data were complemented with data available in the literature and provided new, homogeneous pulsation parameters (mean magnitudes, luminosity amplitudes, periods) for 187 candidate ω Cen RR Lyrae (RRLs). Among them we have 101 RRc (first overtone) and 85 RRab (fundamental) variables, and a single candidate RRd (double-mode) variable. Candidate Blazhko RRLs show periods and colors that are intermediate between the RRc and RRab variables, suggesting that they are transitional objects. A comparison of the period distribution and the Bailey diagram indicates that RRLs in ω Cen show a long-period tail not present in typical Oosterhoff II (OoII) globulars. The RRLs in dwarf spheroidals and in ultra-faint dwarfs have properties between Oosterhoff intermediate and OoII clusters. Metallicity plays a key role in shaping the above evidence. These findings do not support the hypothesis that ω Cen is the core remnant of a spoiled dwarf galaxy. Using optical period-Wesenheit relations that are reddening-free and minimally dependent on metallicity we find a mean distance to ω Cen of 13.71 ± 0.08 ± 0.01 mag (semi-empirical and theoretical calibrations). Finally, we invert the I-band period-luminosity-metallicity relation to estimate individual RRLs’ metal abundances. The metallicity distribution agrees quite well with spectroscopic and photometric metallicity estimates available in the literature. Based in part on proprietary data and on data obtained from the ESO Science Archive Facility under multiple requests by the authors; and in part upon data distributed by the NOAO Science Archive. NOAO is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. This research also benefited from the Digitized Sky Survey service provided by the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. A detailed description of the log of the observations used in this investigation is given in Table 1.

An Investigation into the Periodic Optical Variability of Radio Detected Ultracool Dwarfs using the GUFI Photometer

NASA Astrophysics Data System (ADS)

Boyle, Richard P.; Harding, L. K.; Hallinan, G.; Butler, R. F.; Golden, A.

2011-05-01

In the past ten years or so, radio observations of ultracool dwarfs have yielded the detection of both quiescent and time-variable radio emission in the late-M and L dwarf regime. Four of these dwarfs have been found to produce periodic pulses, determined to be associated with the dwarf's rotation. More recently, two of these radio pulsing dwarfs have been shown to be periodically variable in broadband optical photometry, where the detected periods match the periods of the radio pulses. For one of these dwarfs in particular, it has been established that the mechanism which is driving the optical and radio periodic variability are possibly linked, being a consequence of a magnetically-driven auroral process. We therefore undertook a campaign to investigate the ubiquity of optical periodicity for known radio detected ultracool dwarfs, via multi-color photometric monitoring. To facilitate this research, the GUFI instrument (Galway Ultra Fast Imager) was commissioned on the 1.8m VATT observatory, on Mt. Graham, Arizona. We present the recently published results from this observation campaign, where we have confirmed periodic variability for five of these dwarfs, three of which have been detected for the first time by GUFI. These data provide an insight into the cause of this optical emission, its connection to the radio processes, and most importantly determine whether optical periodic signals are present only in radio pulsing dwarfs.

The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits. II. Survey description, results, and performances

NASA Astrophysics Data System (ADS)

Chauvin, G.; Vigan, A.; Bonnefoy, M.; Desidera, S.; Bonavita, M.; Mesa, D.; Boccaletti, A.; Buenzli, E.; Carson, J.; Delorme, P.; Hagelberg, J.; Montagnier, G.; Mordasini, C.; Quanz, S. P.; Segransan, D.; Thalmann, C.; Beuzit, J.-L.; Biller, B.; Covino, E.; Feldt, M.; Girard, J.; Gratton, R.; Henning, T.; Kasper, M.; Lagrange, A.-M.; Messina, S.; Meyer, M.; Mouillet, D.; Moutou, C.; Reggiani, M.; Schlieder, J. E.; Zurlo, A.

2015-01-01

Context. Young, nearby stars are ideal targets for direct imaging searches for giant planets and brown dwarf companions. After the first-imaged planet discoveries, vast efforts have been devoted to the statistical analysis of the occurence and orbital distributions of giant planets and brown dwarf companions at wide (≥5-6 AU) orbits. Aims: In anticipation of the VLT/SPHERE planet-imager, guaranteed-time programs, we have conducted a preparatory survey of 86 stars between 2009 and 2013 to identify new faint comoving companions to ultimately analyze the occurence of giant planets and brown dwarf companions at wide (10-2000 AU) orbits around young, solar-type stars. Methods: We used NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs between typically 0.1 and 8''. Diffraction-limited observations in H-band combined with angular differential imaging enabled us to reach primary star-companion brightness ratios as small as 10-6 at 1.5''. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects. Results: During our survey, twelve systems were resolved as new binaries, including the discovery of a new white dwarf companion to the star HD 8049. Around 34 stars, at least one companion candidate was detected in the observed field of view. More than 400 faint sources were detected; 90% of them were in four crowded fields. With the exception of HD 8049 B, we did not identify any new comoving companions. The survey also led to spatially resolved images of the thin debris disk around HD 61005 that have been published earlier. Finally, considering the survey detection limits, we derive a preliminary upper limit on the frequency of giant planets for the semi-major axes of [10, 2000] AU: typically less than 15% between 100 and 500 AU and less than 10% between 50 and 500 AU for exoplanets that are more massive than 5 MJup and 10 MJup respectively, if we consider a uniform input distribution and a confidence level of 95%. Conclusions: The results from this survey agree with earlier programs emphasizing that massive, gas giant companions on wide orbits around solar-type stars are rare. These results will be part of a broader analysis of a total of ~210 young, solar-type stars to bring further statistical constraints for theoretical models of planetary formation and evolution. Based on observations collected at the European Southern Observatory, Chile (ESO Large Program 184.C-0157 and Open Time 089.C-0137A and 090.C-0252A).Tables 2 and 6 are available in electronic form at http://www.aanda.org

Panchromatic observations of dwarf starburst galaxies: Infant super star clusters and a low-luminosity AGN

NASA Astrophysics Data System (ADS)

Reines, Amy Ellen

2011-01-01

Globular star clusters and supermassive black holes are fundamental components of today's massive galaxies, with origins dating back to the very early universe. Both globular clusters and the seeds of supermassive black holes are believed to have formed in the progenitors of modern massive galaxies, although the details are poorly understood. Direct observations of these low-mass, distant, and hence faint systems are unobtainable with current capabilities. However, gas-rich dwarf starburst galaxies in the local universe, analogous in many ways to protogalaxies at high-redshift, can provide critical insight into the early stages of galaxy evolution including the formation of globular clusters and massive black holes. This thesis presents a panchromatic study of nearby dwarf starburst galaxies harboring nascent globular clusters still embedded in their birth material. Infant clusters are identified via their production of thermal radio emission at centimeter wavelengths, which comes from dense gas ionized by young massive stars. By combining radio observations with complementary data at ultraviolet, optical and infrared wavelengths, we obtain a comprehensive view of massive clusters emerging from their gaseous and dusty birth cocoons. This thesis also presents the first example of a nearby dwarf starburst galaxy hosting an actively accreting massive central black hole. The black hole in this dwarf galaxy is unusual in that it is not associated with a bulge, a nuclear star cluster, or any other well-defined nucleus, likely reflecting an early phase of black hole and galaxy evolution that has not been previously observed.

A progenitor binary and an ejected mass donor remnant of faint type Ia supernovae

NASA Astrophysics Data System (ADS)

Geier, S.; Marsh, T. R.; Wang, B.; Dunlap, B.; Barlow, B. N.; Schaffenroth, V.; Chen, X.; Irrgang, A.; Maxted, P. F. L.; Ziegerer, E.; Kupfer, T.; Miszalski, B.; Heber, U.; Han, Z.; Shporer, A.; Telting, J. H.; Gänsicke, B. T.; Østensen, R. H.; O'Toole, S. J.; Napiwotzki, R.

2013-06-01

Type Ia supernovae (SN Ia) are the most important standard candles for measuring the expansion history of the universe. The thermonuclear explosion of a white dwarf can explain their observed properties, but neither the progenitor systems nor any stellar remnants have been conclusively identified. Underluminous SN Ia have been proposed to originate from a so-called double-detonation of a white dwarf. After a critical amount of helium is deposited on the surface through accretion from a close companion, the helium is ignited causing a detonation wave that triggers the explosion of the white dwarf itself. We have discovered both shallow transits and eclipses in the tight binary system CD-30°11223 composed of a carbon/oxygen white dwarf and a hot helium star, allowing us to determine its component masses and fundamental parameters. In the future the system will transfer mass from the helium star to the white dwarf. Modelling this process we find that the detonation in the accreted helium layer is sufficiently strong to trigger the explosion of the core. The helium star will then be ejected at such high velocity that it will escape the Galaxy. The predicted properties of this remnant are an excellent match to the so-called hypervelocity star US 708, a hot, helium-rich star moving at more than 750 km s-1, sufficient for it to leave the Galaxy. The identification of both progenitor and remnant provides a consistent picture of the formation and evolution of underluminous SNIa.

The galaxy UV luminosity function at z ≃ 2-4; new results on faint-end slope and the evolution of luminosity density

NASA Astrophysics Data System (ADS)

Parsa, Shaghayegh; Dunlop, James S.; McLure, Ross J.; Mortlock, Alice

2016-03-01

We present a new, robust measurement of the evolving rest-frame ultraviolet (UV) galaxy luminosity function (LF) over the key redshift range from z ≃ 2 to z ≃ 4. Our results are based on the high dynamic range provided by combining the Hubble Ultra Deep Field (HUDF), CANDELS/GOODS-South, and UltraVISTA/COSMOS surveys. We utilize the unparalleled multifrequency photometry available in this survey `wedding cake' to compile complete galaxy samples at z ≃ 2, 3, 4 via photometric redshifts (calibrated against the latest spectroscopy) rather than colour-colour selection, and to determine accurate rest-frame UV absolute magnitudes (M1500) from spectral energy distribution (SED) fitting. Our new determinations of the UV LF extend from M1500 ≃ -22 (AB mag) down to M1500 = -14.5, -15.5 and -16 at z ≃ 2, 3 and 4, respectively (thus, reaching ≃ 3-4 mag fainter than previous blank-field studies at z ≃ 2,3). At z ≃ 2, 3, we find a much shallower faint-end slope (α = -1.32 ± 0.03) than reported in some previous studies (α ≃ -1.7), and demonstrate that this new measurement is robust. By z ≃ 4, the faint-end slope has steepened slightly, to α = -1.43 ± 0.04, and we show that these measurements are consistent with the overall evolutionary trend from z = 0 to 8. Finally, we find that while characteristic number density (φ*) drops from z ≃ 2 to z ≃ 4, characteristic luminosity (M*) brightens by ≃ 1 mag. This, combined with the new flatter faint-end slopes, has the consequence that UV luminosity density (and hence unobscured star formation density) peaks at z ≃ 2.5-3, when the Universe was ≃ 2.5 Gyr old.

Serendipitous discovery of a dwarf Nova in the Kepler field near the G dwarf KIC 5438845

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, Alexander; Ayres, Thomas R.; Neff, James E.

2015-02-01

The Kepler satellite provides a unique window into stellar temporal variability by observing a wide variety of stars with multi-year, near-continuous, high precision, optical photometric time series. While most Kepler targets are faint stars with poorly known physical properties, many unexpected discoveries should result from a long photometric survey of such a large area of sky. During our Kepler Guest Observer programs that monitored late-type stars for starspot and flaring variability, we discovered a previously unknown dwarf nova that lies within a few arcseconds of the mid-G dwarf star KIC 5438845. This dwarf nova underwent nine outbursts over a 4more » year time span. The two largest outbursts lasted ∼17–18 days and show strong modulations with a 110.8 minute period and a declining amplitude during the outburst decay phase. These properties are characteristic of an SU UMa-type cataclysmic variable. By analogy with other dwarf nova light curves, we associate the 110.8 minute (1.847 hr) period with the superhump period, close to but slightly longer than the orbital period of the binary. No precursor outbursts are seen before the super-outbursts and the overall super-outburst morphology corresponds to Osaki and Meyer “Case B” outbursts, which are initiated when the outer edge of the disk reaches the tidal truncation radius. “Case B” outbursts are rare within the Kepler light curves of dwarf novae. The dwarf nova is undergoing relatively slow mass transfer, as evidenced by the long intervals between outbursts, but the mass transfer rate appears to be steady, because the smaller “normal” outbursts show a strong correlation between the integrated outburst energy and the elapsed time since the previous outburst. At super-outburst maximum the system was at V ∼ 18, but in quiescence it is fainter than V ∼ 22, which will make any detailed quiescent follow-up of this system difficult.« less

New Ultra-Compact Dwarf Galaxies in Clusters

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2017-02-01

How do ultra-compact dwarf galaxies (UCDs) galaxies that are especially small and dense form and evolve? Scientists have recently examined distant galaxy clusters, searching for more UCDs to help us answer this question.Origins of DwarfsIn recent years we have discovered a growing sample of small, very dense galaxies. Galaxies that are tens to hundreds of light-years across, with masses between a million and a billion solar masses, fall into category of ultra-compact dwarfs (UCDs).An example of an unresolved compact object from the authors survey that is likely an ultra-compact dwarf galaxy. [Adapted from Zhang Bell 2017]How do these dense and compact galaxies form? Two possibilities are commonly suggested:An initially larger galaxy was tidally stripped during interactions with other galaxies in a cluster, leaving behind only its small, dense core as a UCD.UCDs formed as compact galaxies at very early cosmic times. The ones living in a massive dark matter halo may have been able to remain compact over time, evolving into the objectswe see today.To better understand which of these formation scenarios applies to which galaxies, we need a larger sample size! Our census of UCDs is fairly limited and because theyare small and dim, most of the ones weve discovered are in the nearby universe. To build a good sample, we need to find UCDs at higher redshifts as well.A New SampleIn a recent study, two scientists from University of Michigan have demonstrated how we might find more UCDs. Yuanyuan Zhang (also affiliated with Fermilab) and Eric Bell used the Cluster Lensing and Supernova Survey with Hubble (CLASH) to search 17 galaxy clusters at intermediate redshifts of 0.2 z 0.6, looking for unresolved objects that might be UCDs.The mass and size distributions of the UCD candidates reported in this study, in the context of previously known nuclear star clusters, globular clusters (GCs), UCDs, compact elliptical galaxies (cEs), and dwarf galaxies. [Zhang Bell 2017]Zhang and Bell discovered a sample of compact objects grouped around the central galaxies of the clusters that are consistent with ultra-compact galaxies. The inferred sizes (many around 600 light-years in radius) and masses (roughly one billion solar masses) of these objects suggest that this sample may contain some of the densest UCDs discovered to date.The properties of this new set of UCD candidates arent enough to distinguish between formation scenarios yet, but the authors argue that if we find more such galaxies, we will be able to use the statistics of their spatial and color distributions to determine how they were formed.Zhang and Bell estimate that the 17 CLASH clusters studied in this work each contain an average of 2.7 of these objects in the central million light-years of the cluster. The authors work here suggests that searching wide-field survey data for similar discoveries is a plausible way to increase our sample of UCDs. This will allow us to statistically characterize these dense, compact galaxies and better understand their origins.CitationYuanyuan Zhang and Eric F. Bell 2017 ApJL 835 L2. doi:10.3847/2041-8213/835/1/L2

A possible brown dwarf companion to Gliese 569

NASA Technical Reports Server (NTRS)

Forrest, W. J.; Shure, Mark; Skrutskie, M. F.

1988-01-01

A faint cool companion to Gliese 569, discovered during an IR imaging survey of nearby stars, may be the lowest-mass stellar object yet found. The companion is somewhat cooler in its 1.65-3.75-micron energy distribution than the coolest known main-sequence stars, indicating a low mass. Despite its lower temperature, it is more luminous than similar extremely low-mass stars, suggesting that it is either a young low-mass star evolving toward the main sequence or a cooling substellar brown dwarf. The primary star has emission lines and a low space velocity and exhibits flaring, all of which imply youth for this system. Observations of Gliese 569 and its companion over a period of 2 yr confirm the common proper motion expected of a true binary. The 5-arcsec apparent separation (50 AU) implies an orbital period of roughly 500 yr, which will permit an eventual direct determination of the mass of the companion.

A deep view on the Virgo cluster core

NASA Astrophysics Data System (ADS)

Lieder, S.; Lisker, T.; Hilker, M.; Misgeld, I.; Durrell, P.

2012-02-01

Studies of dwarf spheroidal (dSph) galaxies with statistically significant sample sizes are still rare beyond the Local Group, since these low surface brightness objects can only be identified with deep imaging data. In galaxy clusters, where they constitute the dominant population in terms of number, they represent the faint end slope of the galaxy luminosity function and provide important insight on the interplay between galaxy mass and environment. In this study we investigate the optical photometric properties of early-type galaxies (dwarf ellipticals (dEs) and dSphs) in the Virgo cluster core region, by analysing their location on the colour magnitude relation (CMR) and the structural scaling relations down to faint magnitudes, and by constructing the luminosity function to compare it with theoretical expectations. Our work is based on deep CFHT V- and I-band data covering several square degrees of the Virgo cluster core that were obtained in 1999 using the CFH12K instrument. We visually select potential cluster members based on morphology and angular size, excluding spiral galaxies. A photometric analysis has been carried out for 295 galaxies, using surface brightness profile shape and colour as further criteria to identify probable background contaminants. 216 galaxies are considered to be certain or probable Virgo cluster members. Our study reveals 77 galaxies not catalogued in the VCC (with 13 of them already found in previous studies) that are very likely Virgo cluster members because they follow the Virgo CMR and exhibit low Sérsic indices. Those galaxies reach MV = -8.7 mag. The CMR shows a clear change in slope from dEs to dSphs, while the scatter of the CMR in the dSph regime does not increase significantly. Our sample might, however, be somewhat biased towards redder colours. The scaling relations given by the dEs appear to be continued by the dSphs indicating a similar origin. The observed change in the CMR slope may mark the point at which gas loss prevented significant metal enrichment. The almost constant scatter around the CMR possibly indicates a short formation period, resulting in similar stellar populations. The luminosity function shows a Schechter function's faint end slope of α = -1.50 ± 0.17, implying a lack of galaxies related to the expected number of low-mass dark matter haloes from theoretical models. Our findings could be explained by suppressed star formation in low-mass dark matter halos or by tidal disruption of dwarfs in the dense core region of the cluster. Tables 3 and 4 are available in electronic form at http://www.aanda.org

NTT Observations Indicate that Brown Dwarfs Form Like Stars

NASA Astrophysics Data System (ADS)

2001-06-01

Dusty Disks Detected around Very Young Substellar Objects in the Orion Nebula Summary An international team of astronomers [2] is announcing today the discovery of dusty disks surrounding numerous very faint objects that are believed to be recently formed Brown Dwarfs in the Orion Nebula [3]. This finding is based on detailed observations with SOFI, a specialised infrared-sensitive instrument at the ESO 3.5-m New Technology Telescope at the La Silla Observatory. It is of special interest because it sheds light on the origin and nature of substellar objects, known as "Brown Dwarfs" . In particular, these results suggest that Brown Dwarfs share a common origin with stars and that Brown Dwarfs are more similar in nature to stars than to planets and, like stars, have the potential to form with accompanying systems of planets. Moreover, the presence of dusty protoplanetary disks around the faintest objects in the Orion Nebula cluster confirms both the membership of these faint stars in the cluster and their nature as bona-fide substellar objects, making this the largest population of Brown Dwarf objects yet known . These important results are being reported today to the American Astronomical Society Meeting in Pasadena (California, USA). PR Photo 22a/01 : Infrared picture of the Orion Nebula (NTT + SOFI). PR Photo 22b/01 : "Finding Chart" for Very Young Brown Dwarfs in the Orion Nebula. PR Photo 22c/01 : Animated GIF presentation of PR Photos 22a+b/01. Faint substellar objects in the Milky Way Over the past 5 years, several groups of astronomers have identified a type of very faint, substellar objects within our Milky Way galaxy. These gaseous objects have very low masses and will never shine like normal stars because they cannot achieve central temperatures high enough for sustained thermal nuclear reactions to occur in their cores. Such objects weigh less than about 7% of our Sun and have been variously called "Brown Dwarfs" , "Failed Stars" or "Super Planets" . Indeed, since they have no sustained energy generation by thermal nuclear reactions, many of their properties are more similar to those of giant gas planets in our own solar system such as Jupiter, than to stars like the Sun. For example, even though their masses range between 10-70 times that of Jupiter (the largest and most massive planet in our solar system), the sizes of Brown Dwarfs are still comparable to that of Jupiter, approximately 140,000 km, or roughly 10 times smaller than the Sun. Are Brown Dwarfs giant planets or failed stars? Among the most fundamental issues raised by the existence of Brown Dwarfs is the question of their origin and genetic relationship to planets and stars. Are Brown Dwarfs giant planets or small, failed stars, or perhaps something completely different? The critical test needed to resolve this very basic question is to learn whether Brown Dwarfs form by a process similar to what produces stars or rather to one which produces planets. Stars are thought to form when gravity causes a cold, dusty and rarefied cloud of gas to contract. Such clouds are inevitably rotating so the gas naturally collapses into a rotating disk before it falls onto the forming star. These disks are called circumstellar or protoplanetary disks . They have been found around virtually all young stars and are considered to be sites of planet formation. Gravity helps planets form too, but this occurs by condensation and agglomeration of material contained in the circumstellar disk around a young star. Thus, stars form with a disk around them while planets form within disks around young stars . The planets in our own solar system were formed in such a circumstellar disk around the young Sun about 4.6 billion years ago. To date, the most important observations bearing on the question of Brown Dwarf origin have been: * the observed lack of Brown Dwarf companions to normal stars (something astronomers have called the "Brown Dwarf desert"), and * the existence of free-floating Brown Dwarfs in the Milky Way galaxy. Both facts would appear to imply a stellar, rather than a planet-like origin for these objects. However, one might also explain these observations if most Brown Dwarfs initially formed as companions to stars (within circumstellar disks), but were later ejected from the systems, e.g., because of gravitational effects during encounters with other stars. So the issue of Brown Dwarf origin is still unsettled. NTT observations of substellar objects in the Orion Nebula ESO PR Photo 22a/01 ESO PR Photo 22a/01 [Preview - JPEG: 400 x 434 pix - 192k] [Normal - JPEG: 800 x 877 pix - 496k] [Full Resolution - JPEG: 1772 x 1943 pix - 1.2Mb Caption : PR Photo 22a/01 shows a colour composite of near-infrared images of the central regions of the Orion Nebula, obtained on March 14, 2000, with the SOFI instrument at the ESO 3.5-m New Technology Telescope (NTT) at La Silla. Three exposures were made through J- (wavelength 1.25 µm here colour-coded as "blue"), H- (1.65 µm; "green") and Ks-filters (2.16 µm; "red"), respectively. The central group of bright stars is the famous "Trapezium" . The total effective exposure time was 86.4 seconds per band. The sky field measures about 4.9 x 4.9 arcmin 2 (1024 x 1024 pix 2 ). North is up and East is left. ESO PR Photo 22b/01 ESO PR Photo 22b/01 [Preview - JPEG: 400 x 439 pix - 35k] [Normal - JPEG: 800 x 877 pix - 90k] Caption : PR Photo 22b/01 contains the corresponding "finding chart" with the positions of the very young Brown Dwarfs in the Orion Nebula that were studied during the present investigation. The starlike symbols represent the brightest stars in PR Photo 22a/01 and are plotted for reference. In this chart, very young Brown Dwarfs are represented by a double open circle (if a dusty disk was detected) or with a single open circle (if no dusty disk was detected). The scale is exactly as in PR Photo 22a/01 . ESO PR Photo 22c/01 ESO PR Photo 22c/01 [Animated GIF: 482 x 465 pix - 248k] Caption : PR Photo 22c/01 is an animated GIF-composite of PR Photo 22a/01 and PR Photo 22b/01 for easy comparison. To resolve this mystery, an international team of astronomers [2] has obtained sensitive near-infrared observations of young Brown Dwarf candidates in the Trapezium cluster , at the centre of the Orion Nebula. For this, they used the state-of-the-art near-infrared SOFI instrument on the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory (Chile). The Trapezium Cluster is a group of young stars that appears to the unaided eye as a faint central 'star' in the Orion Nebula . This cluster is located at a distance of about 1200 light-years and contains nearly 1000 stars, most of which are younger than 1 million years. The stars in this cluster are in their infancy when compared to our middle-aged Sun that is about 4.6 billion years old (reduced to a human timescale, they would be just 3 days old, compared to the Sun's 40 years). Among the hundreds of normal stars in the Trapezium Cluster, astronomers have previously identified a population of objects so faint that they have been considered as prime candidates for very young Brown Dwarfs. The observations obtained with the NTT benefitted from superb atmospheric conditions (e.g., a seeing of 0.5 arcsec) and allowed the astronomers to examine the near-infrared light of more than 100 of the Brown Dwarf candidates in the cluster. More than half of them were found to have excess near-infrared light , compared to that a normal young Brown Dwarf should emit. The only plausible explanation is that this extra light originates from glowing, hot dust within protoplanetary disks surrounding these objects . It was the same method, albeit at longer infrared wavelengths, that first led to the discovery of dust disks around several normal stars, some of which have later been studied in much detail, e.g., that at the southern star Beta Pictoris. In fact, and strongly supporting this explanation, twenty-one of the Brown Dwarf candidates detected via the NTT observations are also identified with optical "proplyds" , the famous dusty disks first imaged in 1994 by the Hubble Space Telescope (HST) at optical wavelengths, cf. the corresponding HST Press Release and images [4]. Dusty disks The presence of such hot and dusty disks around these objects is a clear sign of their extreme youth - this in turn confirms both their membership in the young cluster and their nature as bona-fide substellar objects . Thus, the Trapezium Cluster contains the largest population (approximately 100) of Brown Dwarfs yet known. Indeed, only about 80 freely floating Brown Dwarfs have so far been positively identified outside this cluster. " Brown Dwarfs are considerably easier to detect and study when they are young, because they are ten times larger and thousands of times brighter during their early youth than during their middle age " says Elizabeth Lada from the University of Florida and a member of the team. Her colleague August Muench explains that " even at their brightest, however, most Brown Dwarfs are still 100 or more times intrinsically fainter than our Sun, explaining why astronomers have great difficulties in detecting such objects ". A common origin of normal stars and Brown Dwarfs " The high incidence of disks around both young stars and Brown Dwarfs in this cluster strongly suggests that both stars and Brown Dwarfs trace their origin to a common physical process and that Brown Dwarfs are more similar in nature to stars than to planets " says Charles Lada from the Smithsonian Astrophysical Observatory. Moreover, as is the case for stars, the disks that surround Brown Dwarfs may be capable of forming systems of planets. According to João Alves from ESO, " it is entirely possible that the Milky Way Galaxy contains numerous planetary systems that orbit cold and dark, failed stars. Whether these disks can indeed form planetary systems, however, still remains to be determined ". Even if Brown Dwarfs do have planetary systems, their planets would not have a stable climate and thus would be inhospitable to life as we know it. This is because Brown Dwarfs do not generate their own energy for any substantial period of time but instead fade rapidly as they age. The next steps For the moment being, the detection of disks around the Brown Dwarf candidates in the Trapezium Cluster rests entirely on the measurements of the near-infrared colours of these objects. Additional confirmation of the presence of such dust disks can be obtained with sensitive infrared observations made at longer wavelengths. Such observations are possible with the largest ground-based telescopes like the VLT [5] or with the upcoming NASA infrared satellite mission ( SIRTF ). Notes [1]: This ESO Press Release is issued in parallel with a Press Release on the same subject by the American Astronomical Society (AAS). The indicated embargo corresponds to the time of release at the AAS meting in Pasadena. [2]: The team consists of João F. Alves (ESO, Garching, Germany), Charles J. Lada (Smithsonian Astrophysical Observatory, Cambridge MA, USA), Elizabeth A. Lada and August A. Muench (both Department of Astronomy, University of Florida, Gainesville FL, USA). The research reported here was supported in part by the US National Science Foundation. [3]: Other ESO Press Communications about Brown Dwarfs include PR 07/97 , PR 14/99 and PR 16/00. Discoveries of exoplanets and other small objects, some of which have masses near the borderline between Brown Dwarfs and planets, are reported in PR 18/98 , PR 13/00 and PR 07/01. A spectacular infrared image of the Orion Nebula with the VLT and the ISAAC instrument was published earlier this year ( PR Photo 03a/01 ) with a discussion about small objects within this nebula. [4]: More information about "proplyds" (PROto-PLanetarY DiskS) is available in ESO PR 06/97 that discusses the discovery of the first such object outside the Orion Nebula. [5]: The VLT is already equipped with one instrument suited for such measurements, the Infrared Spectrometer And Array Camera (ISAAC) - examples of mid-infrared observations of the giant planet Jupiter have just been published as ESO PR Photos 21a-f/01. The NAOS-CONICA adaptive optics multi-mode instrument will enter into operation later in 2001, to be followed by the VLT Mid Infrared Spectrometer/Imager (VISIR). Another powerful mid-infrared facility at ESO is the Thermal Infrared Multimode Instrument (TIMMI2) , now in operation at the ESO 3.6-m telescope on La Silla and with which observations of the central part of the Orion Nebula were recently made, cf. PR Photos 12a-e/01.

Morphological evidence for a past minor merger in the Seyfert galaxy NGC 1068†

NASA Astrophysics Data System (ADS)

Tanaka, Ichi; Yagi, Masafumi; Taniguchi, Yoshiaki

2017-12-01

Deep optical imaging with both Hyper Suprime-Cam and Suprime-Cam on the 8.2 m Subaru Telescope reveals a number of outer faint structures around the archetypical Seyfert galaxy NGC 1068 (M 77). We find three ultra diffuse objects (UDOs) around NGC 1068. Since these UDOs are located within the projected distance of 45 kpc from the center of NGC 1068, they appear to be associated with NGC 1068. Hereafter, we call them UDO-SW, UDO-NE, and UDO-SE where UDO = Ultra Diffuse Object, SW = southwest, NE = northwest, and SE = southeast; note that UDO-SE was already found in the SDSS Stripe 82 data. Among them, UDO-NE and UDO-SW appear to be a part of a loop or stream structure around the main body of NGC 1068, providing evidence for the physical connection to NGC 1068. We consider that UDO-SE may be a tidal dwarf galaxy. We also find another UDO-like object that is two magnitudes fainter and smaller by a factor of 3 to 5 than those of the three UDOs. This object may belong to a class of low surface brightness galaxy. Since this object is located along the line connecting UDO-NE and UDO-SW, it is suggested that this object is related to the past interaction event that formed the loop by UDO-NE and UDO-SW, thus implying the physical connection to NGC 1068. Another newly discovered feature is an asymmetric outer one-arm structure emanating from the western edge of the outermost disk of NGC 1068 together with a ripple-like structure at the opposite side. These structures are expected to arise in a late phase of a minor merger, according to published numerical simulations of minor mergers. All these lines of evidence show that NGC 1068 experienced a minor merger several billion years ago. We then discuss the minor-merger driven triggering of nuclear activity in the case of NGC 1068.

SEARCHING FOR BINARY Y DWARFS WITH THE GEMINI MULTI-CONJUGATE ADAPTIVE OPTICS SYSTEM (GeMS)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Opitz, Daniela; Tinney, C. G.; Faherty, Jacqueline K.

The NASA Wide-field Infrared Survey Explorer (WISE) has discovered almost all the known members of the new class of Y-type brown dwarfs. Most of these Y dwarfs have been identified as isolated objects in the field. It is known that binaries with L- and T-type brown dwarf primaries are less prevalent than either M-dwarf or solar-type primaries, they tend to have smaller separations and are more frequently detected in near-equal mass configurations. The binary statistics for Y-type brown dwarfs, however, are sparse, and so it is unclear if the same trends that hold for L- and T-type brown dwarfs alsomore » hold for Y-type ones. In addition, the detection of binary companions to very cool Y dwarfs may well be the best means available for discovering even colder objects. We present results for binary properties of a sample of five WISE Y dwarfs with the Gemini Multi-Conjugate Adaptive Optics System. We find no evidence for binary companions in these data, which suggests these systems are not equal-luminosity (or equal-mass) binaries with separations larger than ∼0.5–1.9 AU. For equal-mass binaries at an age of 5 Gyr, we find that the binary binding energies ruled out by our observations (i.e., 10{sup 42} erg) are consistent with those observed in previous studies of hotter ultra-cool dwarfs.« less

The PAndAS View of the Andromeda Satellite System. II. Detailed Properties of 23 M31 Dwarf Spheroidal Galaxies

NASA Astrophysics Data System (ADS)

Martin, Nicolas F.; Ibata, Rodrigo A.; Lewis, Geraint F.; McConnachie, Alan; Babul, Arif; Bate, Nicholas F.; Bernard, Edouard; Chapman, Scott C.; Collins, Michelle M. L.; Conn, Anthony R.; Crnojević, Denija; Fardal, Mark A.; Ferguson, Annette M. N.; Irwin, Michael; Mackey, A. Dougal; McMonigal, Brendan; Navarro, Julio F.; Rich, R. Michael

2016-12-01

We present a comprehensive analysis of the structural properties and luminosities of the 23 dwarf spheroidal galaxies that fall within the footprint of the Pan-Andromeda Archaeological Survey (PAndAS). These dwarf galaxies represent the large majority of Andromeda’s known satellite dwarf galaxies and cover a wide range in luminosity (-11.6≲ {M}V≲ -5.8 or {10}4.2≲ L≲ {10}6.5 {L}⊙ ) and surface brightness (25.1≲ {μ }0≲ 29.3 mag arcsec-2). We confirm most previous measurements, but we find And XIX to be significantly larger than before ({r}h={3065}-935+1065 {pc}, {M}V=-{10.1}-0.4+0.8) and cannot derive parameters for And XXVII as it is likely not a bound stellar system. We also significantly revise downward the luminosities of And XV and And XVI, which are now {M}V˜ -7.5 or L˜ {10}5 {L}⊙ . Finally, we provide the first detailed analysis of Cas II/And XXX, a fairly faint system ({M}V=-{8.0}-0.3+0.4) of typical size ({r}h=270+/- 50 {pc}), located in close proximity to the two bright elliptical dwarf galaxies NGC 147 and NGC 185. Combined with the set of homogeneous distances published in an earlier contribution, our analysis dutifully tracks all relevant sources of uncertainty in the determination of the properties of the dwarf galaxies from the PAndAS photometric catalog. We further publish the posterior probability distribution functions of all the parameters we fit for in the form of MCMC chains available online; these inputs should be used in any analysis that aims to remain truthful to the data and properly account for covariance between parameters.

The Ages of the Thin Disk, Thick Disk, and the Halo from Nearby White Dwarfs

NASA Astrophysics Data System (ADS)

Kilic, Mukremin; Munn, Jeffrey A.; Harris, Hugh C.; von Hippel, Ted; Liebert, James W.; Williams, Kurtis A.; Jeffery, Elizabeth; DeGennaro, Steven

2017-03-01

We present a detailed analysis of the white dwarf luminosity functions derived from the local 40 pc sample and the deep proper motion catalog of Munn et al. Many previous studies have ignored the contribution of thick disk white dwarfs to the Galactic disk luminosity function, which results in an erroneous age measurement. We demonstrate that the ratio of thick/thin disk white dwarfs is roughly 20% in the local sample. Simultaneously fitting for both disk components, we derive ages of 6.8-7.0 Gyr for the thin disk and 8.7 ± 0.1 Gyr for the thick disk from the local 40 pc sample. Similarly, we derive ages of 7.4-8.2 Gyr for the thin disk and 9.5-9.9 Gyr for the thick disk from the deep proper motion catalog, which shows no evidence of a deviation from a constant star formation rate in the past 2.5 Gyr. We constrain the time difference between the onset of star formation in the thin disk and the thick disk to be {1.6}-0.4+0.3 Gyr. The faint end of the luminosity function for the halo white dwarfs is less constrained, resulting in an age estimate of {12.5}-3.4+1.4 Gyr for the Galactic inner halo. This is the first time that ages for all three major components of the Galaxy have been obtained from a sample of field white dwarfs that is large enough to contain significant numbers of disk and halo objects. The resultant ages agree reasonably well with the age estimates for the oldest open and globular clusters.

The RSA survey of dwarf galaxies, 1: Optical photometry

NASA Technical Reports Server (NTRS)

Vader, J. Patricia; Chaboyer, Brian

1994-01-01

We present detailed surface photometry, based on broad B-band charge coupled device (CCD) images, of about 80 dwarf galaxies. Our sample represents approximately 10% of all dwarf galaxies identified in the vicinity of Revised Shapley-Ames (RSA) galaxies on high resolution blue photographic plates, referred to as the RSA survey of dwarf galaxies. We derive global properties and radial surface brightness profiles, and examine the morphologies. The radial surface brightness profiles of dwarf galaxies, whether early or late type, display the same varieties in shape and complexity as those of classical giant galaxies. Only a few are well described by a pure r(exp 1/4) law. Exponential profiles prevail. Features typical of giant disk galaxies, such as exponential profiles with a central depression, lenses, and even, in one case (IC 2041), a relatively prominent bulge are also found in dwarf galaxies. Our data suggest that the central region evolves from being bulge-like, with an r(exp 1/4) law profile, in bright galaxies to a lens-like structure in dwarf galaxies. We prove detailed surface photometry to be a helpful if not always sufficient tool in investigating the structure of dwarf galaxies. In many cases kinematic information is needed to complete the picture. We find the shapes of the surface brightness profiles to be loosely associated with morphological type. Our sample contains several new galaxies with properties intermediate between those of giant and dwarf ellipticals (but no M32-like objects). This shows that such intermediate galaxies exist so that at least a fraction of early-type dwarf ellipticals is structurally related to early-type giants instead of belonging to a totally unrelated, disjunct family. This supports an origin of early-type dwarf galaxies as originally more massive systems that acquired their current morphology as a result of substantial, presumable supernova-driven, mass loss. On the other hand, several early-type dwarfs in our sample are merger candidates. Merger events may lead to anisotropic velocity distributions in systems of any luminosity, including dwarfs. The RSA sample of dwarf galaxies is more likely to contain mergers because, in contrast to earlier dwarf galaxy surveys that have focused on clusters and rich groups of galaxies, the RSA dwarfs are typically located in low density environments. The occurrence of mergers among dwarf galaxies is of interest in connection with the rapid evolution of faint blue galaxy counts at redshift z less than 1 which suggests that dwarf galaxies were about five times more numerous in the recent past. Finally, our sample contains several examples of late-type dwarfs and 'transition' types that are potential precursors of nucleated early-type dwarfs. All the above processes--mass loss, mergers, astration--are likely to have contributed to the formation of the current population of diffuse early-type dwarfs. A few new redshifts of dwarf galaxies are reported in this paper.

A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics

NASA Astrophysics Data System (ADS)

Savcheva, Antonia; West, Andrew A.; Bochanski, John J.

2014-06-01

We present a new sample of M subdwarfs compiled from the 7th data re- lease of the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample significantly increases the number the existing sample of low-mass subdwarfs. This catalog includes unprecedentedly large numbers of extreme and ultra sudwarfs. Here, we present the catalog and the statistical analysis we perform. Subdwarf template spectra are derived. We show color-color and reduced proper motion diagrams of the three metallicity classes, which are shown to separate from the disk dwarf population. The extreme and ultra subdwarfs are seen at larger values of reduced proper motion as expected for more dynamically heated populations. We determine 3D kinematics for all of the stars with proper motions. The color-magnitude diagrams show a clear separation of the three metallicity classes with the ultra and extreme subdwarfs being significantly closer to the main sequence than the ordinary subdwarfs. All subdwarfs lie below and to the blue of the main sequence. Based on the average (U, V, W ) velocities and their dispersions, the extreme and ultra subdwarfs likely belong to the Galactic halo, while the ordinary subdwarfs are likely part of the old Galactic (or thick) disk. An extensive activity analy- sis of subdwarfs is performed using chromospheric Hα emission and 208 active subdwarfs are found. We show that while the activity fraction of subdwarfs rises with spectral class and levels off at the latest spectral classes, consistent with the behavior of M dwarfs, the extreme and ultra subdwarfs are basically flat.

A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics

NASA Astrophysics Data System (ADS)

Savcheva, Antonia S.; West, Andrew A.; Bochanski, John J.

2014-10-01

We present a new sample of M subdwarfs compiled from the seventh data release of the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample significantly increases the number of spectroscopically confirmed low-mass subdwarfs. This catalog also includes 905 extreme and 534 ultra sudwarfs. We present the entire catalog, including observed and derived quantities, and template spectra created from co-added subdwarf spectra. We show color-color and reduced proper motion diagrams of the three metallicity classes, which are shown to separate from the disk dwarf population. The extreme and ultra subdwarfs are seen at larger values of reduced proper motion, as expected for more dynamically heated populations. We determine 3D kinematics for all of the stars with proper motions. The color-magnitude diagrams show a clear separation of the three metallicity classes with the ultra and extreme subdwarfs being significantly closer to the main sequence than the ordinary subdwarfs. All subdwarfs lie below (fainter) and to the left (bluer) of the main sequence. Based on the average (U, V, W) velocities and their dispersions, the extreme and ultra subdwarfs likely belong to the Galactic halo, while the ordinary subdwarfs are likely part of the old Galactic (or thick) disk. An extensive activity analysis of subdwarfs is performed using Hα emission, and 208 active subdwarfs are found. We show that while the activity fraction of subdwarfs rises with spectral class and levels off at the latest spectral classes, consistent with the behavior of M dwarfs, the extreme and ultra subdwarfs are basically flat.

CoRoT 101186644: A transiting low-mass dense M-dwarf on an eccentric 20.7-day period orbit around a late F-star. Discovered in the CoRoT lightcurves

NASA Astrophysics Data System (ADS)

Tal-Or, L.; Mazeh, T.; Alonso, R.; Bouchy, F.; Cabrera, J.; Deeg, H. J.; Deleuil, M.; Faigler, S.; Fridlund, M.; Hébrard, G.; Moutou, C.; Santerne, A.; Tingley, B.

2013-05-01

We present the study of the CoRoT transiting planet candidate 101186644, also named LRc01_E1_4780. Analysis of the CoRoT lightcurve and the HARPS spectroscopic follow-up observations of this faint (mV = 16) candidate revealed an eclipsing binary composed of a late F-type primary (Teff = 6090 ± 200 K) and a low-mass, dense late M-dwarf secondary on an eccentric (e = 0.4) orbit with a period of ~20.7 days. The M-dwarf has a mass of 0.096 ± 0.011 M⊙, and a radius of 0.104-0.006+0.026 R⊙, which possibly makes it the smallest and densest late M-dwarf reported so far. Unlike the claim that theoretical models predict radii that are 5-15% smaller than measured for low-mass stars, this one seems to have a radius that is consistent and might even be below the radius predicted by theoretical models. Based on observations made with the 1-m telescope at the Wise Observatory, Israel, the Swiss 1.2-m Leonhard Euler telescope at La Silla Observatory, Chile, the IAC-80 telescope at the Observatory del Teide, Canarias, Spain, and the 3.6-m telescope at La Silla Observatory (ESO), Chile (program 184.C-0639).

Meeting contribution: Low mass stars, brown dwarfs and hot Jupiters

NASA Astrophysics Data System (ADS)

Viti, S.

2007-02-01

Dr Viti opened with an apology that hers was not a field which could yet produce nice images; the objects she would be describing were too faint to be meaningfully imaged. Yet she would be arguing that even without images, a tremendous amount had been learnt from such objects in the past decade, and that there were exciting prospects for the future. The primary subject of this talk would be low mass stars (LMSs), defined as those stars of around half the mass of the Sun or less.

Infrared Astronomy at Extremely Faint Light Levels in Support of the LAIRTS Program.

DTIC Science & Technology

1987-09-01

Elliptical and Irregular Galaxies (T. X. Thuan), Ap. J., 299, 881-895 (1985). 2. IC 3475: A Stripped Dwarf Galaxy in the Virgo Cluster (L. Vigroux, T. X...imply a metallicity range for BCDs and dis between 1/30 and 1/3 of the sun’s metallicity, while the near-IR colors of the dEs (in the Virgo cluster of... clusters . A paper is in preparation which will study the stellar populations in these first brightest galaxies, as a function of the cluster

Searching for brown dwarfs from submotions of binaries with speckle observations

NASA Astrophysics Data System (ADS)

Fu, Hsieh-Hai

1994-01-01

The search for brown dwarfs in binary systems is of great scientific interest and is a quest that pushes observing accuracy to its limit. The study of brown dwarfs is related to the search for dark matter, the initial mass function for stars of all masses, and theories of stellar formation. On the other hand, searching for brown dwarfs is a challenge because of their faintness and very low mass. Although many techniques have been used to detect brown dwarfs, a direct measurement of mass is the only criterion for distinguishing a brown dwarf from a star, and binary observation is still the best way for determining the accurate masses of celestial objects through Kepler's third law. Since 1976, CHARA has accumulated thousands of binary star speckle observations with high precision that can be used to find masses of possible unseen companions in binary systems through astrometrically measured submotions. A modified discrete Fourier transform was used to detect periodicity in data sets having uneven temporal distributions. This dissertation, an extension of work initiated by Dr. Ali Al-Shukri in 1991, uses the CHARA speckle measurements to evaluate their limiting accuracy and then to search for unseen companions from submotions of binary orbital motions. The successful detection of the previously known 1.83-year period sub-motion of the astrometric system ADS 8119 Aa demonstrates that this analysis can be used to find other systems in future investigations, even though no convincing evidence was found for the existence of a brown dwarf. Four possible companions were found to the binaries ADS 8197, ADS 9392, ADS 9494, and ADS 14073 with periods of 3.3, 2.6, 0.3, and 3.78 years and minimum masses in the ranges of 0.015-0.019, 0.11-0.65, 0.04-0.19, and 0.14-0.16 solar masses, respectively. The overall null result for detecting brown dwarfs may be partially explained as a real lack of massive brown dwarfs as members of multiple systems.

DWARF GALAXIES AND THE COSMIC WEB

DOE Office of Scientific and Technical Information (OSTI.GOV)

Benitez-Llambay, Alejandro; Abadi, Mario G.; Navarro, Julio F.

2013-02-01

We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram pressure. The loss of gas is especially pronounced in low-mass halos due tomore » their lower binding energy and has a dramatic effect on the star formation history of affected systems. This 'cosmic web stripping' may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in {Lambda}CDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Suk; Rey, Soo-Chang; Lisker, Thorsten

We present ultraviolet (UV) color-magnitude relations (CMRs) of early-type dwarf galaxies in the Virgo cluster, based on Galaxy Evolution Explorer (GALEX) UV and Sloan Digital Sky Survey (SDSS) optical imaging data. We find that dwarf lenticular galaxies (dS0s), including peculiar dwarf elliptical galaxies (dEs) with disk substructures and blue centers, show a surprisingly distinct and tight locus separated from that of ordinary dEs, which is not clearly seen in previous CMRs. The dS0s in UV CMRs follow a steeper sequence than dEs and show bluer UV-optical color at a given magnitude. We also find that the UV CMRs of dEsmore » in the outer cluster region are slightly steeper than that of their counterparts in the inner region, due to the existence of faint, blue dEs in the outer region. We explore the observed CMRs with population models of a luminosity-dependent delayed exponential star formation history. We confirm that the feature of delayed star formation of early-type dwarf galaxies in the Virgo cluster is strongly correlated with their morphology and environment. The observed CMR of dS0s is well matched by models with relatively long delayed star formation. Our results suggest that dS0s are most likely transitional objects at the stage of subsequent transformation of late-type progenitors to ordinary red dEs in the cluster environment. In any case, UV photometry provides a powerful tool to disentangle the diverse subpopulations of early-type dwarf galaxies and uncover their evolutionary histories.« less

A Rogues’ Gallery of Andromeda's Dwarf Galaxies. I. A Predominance of Red Horizontal Branches

NASA Astrophysics Data System (ADS)

Martin, Nicolas F.; Weisz, Daniel R.; Albers, Saundra M.; Bernard, Edouard; Collins, Michelle L. M.; Dolphin, Andrew E.; Ferguson, Annette M. N.; Ibata, Rodrigo A.; Laevens, Benjamin; Lewis, Geraint F.; Mackey, A. Dougal; McConnachie, Alan; Rich, R. Michael; Skillman, Evan D.

2017-11-01

We present homogeneous, sub-horizontal branch photometry of 20 dwarf spheroidal satellite galaxies of M31 observed with the Hubble Space Telescope. Combining our new data for 16 systems with archival data in the same filters for another four, we show that Andromeda dwarf spheroidal galaxies favor strikingly red horizontal branches or red clumps down to ˜104.2 L ⊙ (M V ˜ -5.8). The age-sensitivity of horizontal branch stars implies that a large fraction of the M31 dwarf galaxies have extended star formation histories (SFHs), and appear inconsistent with early star formation episodes that were rapidly shutdown. Systems fainter than ˜105.5 L ⊙ show the widest range in the ratios and morphologies of red and blue horizontal branches, indicative of both complex SFHs and a diversity in quenching timescales and/or mechanisms, which is qualitatively different from what is currently known for faint Milky Way (MW) satellites of comparable luminosities. Our findings bolster similar conclusions from recent deeper data for a handful of M31 dwarf galaxies. We discuss several sources for diversity of our data such as varying halo masses, patchy reionization, mergers/accretion, and the environmental influence of M31 and the Milky Way on the early evolution of their satellite populations. A detailed comparison between the histories of M31 and MW satellites would shed signifiant insight into the processes that drive the evolution of low-mass galaxies. Such a study will require imaging that reaches the oldest main-sequence turnoffs for a significant number of M31 companions.

Dead Star Warps Light of Red Star Artist Animation

NASA Image and Video Library

2013-04-04

This artist concept depicts an ultra-dense dead star, called a white dwarf, passing in front of a small red star. NASA planet-hunting Kepler was able to detect gravitational lensing by measuring a strangely subtle dip in the star brightness.

The WFCAM Transit Survey: a search for rocky planets around cool stars

NASA Astrophysics Data System (ADS)

Birkby, Jayne

2010-09-01

The theory of core accretion makes two intriguing, observable predictions: i) that the formation of rocky/icy planets is common around M-dwarfs, and ii) that hot-Jupiters are extremely difficult to produce around low-mass stars. Furthermore, due to their small physical size and lower bolometric luminosity, M-dwarfs are up to 300? more sensitive to planetary transits in their habitable zones than solar-type stars. We present here the WFCAM Transit Survey (WTS); an ambitious, near- infrared photometric monitoring campaign of ˜6000 M-dwarfs across four 1.5 sq deg fields situated >5 degrees above and below the galactic plane. We utilise a unique opportunity provided by the highly efficient queue-scheduled operational mode of the UKIRT to observe our fields, with at least one visible at any time, when atmospheric conditions and RA coverage are unsuitable for other ongoing UKIRT programs. By probing the peak of the M-dwarf spectral energy distribution (13<17), we obtain a statistically significant sample of low-mass stars, which allows us to place meaningful constraints on the occurrence and formation of planets around M-dwarfs. The WTS has achieved one thousand epochs after 2 years in one of our target fields and will continue until April 2012. Our light curves have a per datapoint photometric precision of ˜3-4 mmag for the brightest objects, with RMS scatter < 1% for J<16, sufficient to detect Earth-like transits around M-dwarfs. I report here on the goals of our survey, our most recent results and the properties of our M-dwarf target sample. I also discuss our processing methods and how we combat the challenges encountered when observing occultations of faint red stars and the spectroscopic follow-up required to confirm them. (http://www.ast.cam.ac.uk/˜sth/wts/index.html)

On The Missing Dwarf Problem In Clusters And Around The Nearby Galaxy M33

NASA Astrophysics Data System (ADS)

Keenan, Olivia Charlotte

2017-08-01

This thesis explores possible solutions to the dwarf galaxy problem. This is a discrepancy between the number of dwarf galaxies we observe, and the number predicted from cosmological computer simulations. Simulations predict around ten times more dwarf galaxy satellites than are currently observed. I have investigated two possible solutions: dark galaxies and the low surface brightness universe. Dark galaxies are dark matter halos which contain gas, but few or no stars, hence are optically dark. As part of the Arecibo Galaxy Environment Survey I surveyed the neutral hydrogen gas around the nearby galaxy M33. I found 32 gas clouds, 11 of which are new detections. Amongst these there was one particularly interesting cloud. AGESM33-32 is ring shaped and larger than M33 itself, if at the same distance. It has a velocity width which is similar to the velocity dispersion of gas in a disk galaxy, as well as having a clear velocity gradient across it which may be due to rotation. The fact that it also currently has no observed associated stars means it is a dark galaxy candidate. Optically, dwarf galaxies may be out there, but too faint for us to detect. This means that with newer, deeper, images we may be able to unveil a large, low surface brightness, population of dwarf galaxies. However, the question remains as to how these can be distinguished from background galaxies. I have used Next Generation Virgo Survey (NGVS) data to carry out photometry on 852 Virgo galaxies in four bands. I also measured the photometric properties of galaxies on a background (non-cluster) NGVS frame. I discovered that a combination of colour, magnitude and surface brightness information could be used to identify cluster dwarf galaxies from background field galaxies. The most effective method is to use the surface brightness-magnitude relation.

Follow-up spectroscopic observations of HD 107148 B: A new white dwarf companion of an exoplanet host star

NASA Astrophysics Data System (ADS)

Mugrauer, M.; Dinçel, B.

2016-06-01

We report on our follow-up spectroscopy of HD 1071478 B, a recently detected faint co-moving companion of the exoplanet host star HD 107148 A. The companion is separated from its primary star by about 35 arcsec (or 1790 AU of projected separation) and its optical and near infrared photometry is consistent with a white dwarf, located at the distance of HD 107148 A. In order to confirm the white dwarf nature of the co-moving companion, we obtained follow-up spectroscopic observations of HD 107148 B with CAFOS at the CAHA 2.2 m telescope. According to our CAFOS spectroscopy HD 107148 B is a DA white dwarf with an effective temperature in the range between 5900 and 6400 K. The properties of HD 107148 B can further be constrained with the derived effective temperature and the known visual and infrared photometry of the companion, using evolutionary models of DA white dwarfs. We obtain for HD 107148 B a mass of 0.56±0.05 M_⊙, a luminosity of (2.0±0.2)×10-4 L_⊙, log g [cm s-2])=7.95±0.09, and a cooling age of 2100±270 Myr. With its white dwarf companion the exoplanet host star HD 107148 A forms an evolved stellar system, which hosts at least one exoplanet. So far, only few of these evolved systems are known, which represent only about 5 % of all known exoplanet host multiple stellar systems. HD 107148 B is the second confirmed white dwarf companion of an exoplanet host star with a projected separation to its primary star of more than 1000 AU. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).

The CARMENES search for exoplanets around M dwarfs. High-resolution optical and near-infrared spectroscopy of 324 survey stars

NASA Astrophysics Data System (ADS)

Reiners, A.; Zechmeister, M.; Caballero, J. A.; Ribas, I.; Morales, J. C.; Jeffers, S. V.; Schöfer, P.; Tal-Or, L.; Quirrenbach, A.; Amado, P. J.; Kaminski, A.; Seifert, W.; Abril, M.; Aceituno, J.; Alonso-Floriano, F. J.; Ammler-von Eiff, M.; Antona, R.; Anglada-Escudé, G.; Anwand-Heerwart, H.; Arroyo-Torres, B.; Azzaro, M.; Baroch, D.; Barrado, D.; Bauer, F. F.; Becerril, S.; Béjar, V. J. S.; Benítez, D.; Berdinas˜, Z. M.; Bergond, G.; Blümcke, M.; Brinkmöller, M.; del Burgo, C.; Cano, J.; Cárdenas Vázquez, M. C.; Casal, E.; Cifuentes, C.; Claret, A.; Colomé, J.; Cortés-Contreras, M.; Czesla, S.; Díez-Alonso, E.; Dreizler, S.; Feiz, C.; Fernández, M.; Ferro, I. M.; Fuhrmeister, B.; Galadí-Enríquez, D.; Garcia-Piquer, A.; García Vargas, M. L.; Gesa, L.; Galera, V. Gómez; González Hernández, J. I.; González-Peinado, R.; Grözinger, U.; Grohnert, S.; Guàrdia, J.; Guenther, E. W.; Guijarro, A.; Guindos, E. de; Gutiérrez-Soto, J.; Hagen, H.-J.; Hatzes, A. P.; Hauschildt, P. H.; Hedrosa, R. P.; Helmling, J.; Henning, Th.; Hermelo, I.; Hernández Arabí, R.; Hernández Castaño, L.; Hernández Hernando, F.; Herrero, E.; Huber, A.; Huke, P.; Johnson, E. N.; Juan, E. de; Kim, M.; Klein, R.; Klüter, J.; Klutsch, A.; Kürster, M.; Lafarga, M.; Lamert, A.; Lampón, M.; Lara, L. M.; Laun, W.; Lemke, U.; Lenzen, R.; Launhardt, R.; López del Fresno, M.; López-González, J.; López-Puertas, M.; López Salas, J. F.; López-Santiago, J.; Luque, R.; Magán Madinabeitia, H.; Mall, U.; Mancini, L.; Mandel, H.; Marfil, E.; Marín Molina, J. A.; Maroto Fernández, D.; Martín, E. L.; Martín-Ruiz, S.; Marvin, C. J.; Mathar, R. J.; Mirabet, E.; Montes, D.; Moreno-Raya, M. E.; Moya, A.; Mundt, R.; Nagel, E.; Naranjo, V.; Nortmann, L.; Nowak, G.; Ofir, A.; Oreiro, R.; Pallé, E.; Panduro, J.; Pascual, J.; Passegger, V. M.; Pavlov, A.; Pedraz, S.; Pérez-Calpena, A.; Medialdea, D. Pérez; Perger, M.; Perryman, M. A. C.; Pluto, M.; Rabaza, O.; Ramón, A.; Rebolo, R.; Redondo, P.; Reffert, S.; Reinhart, S.; Rhode, P.; Rix, H.-W.; Rodler, F.; Rodríguez, E.; Rodríguez-López, C.; Rodríguez Trinidad, A.; Rohloff, R.-R.; Rosich, A.; Sadegi, S.; Sánchez-Blanco, E.; Sánchez Carrasco, M. A.; Sánchez-López, A.; Sanz-Forcada, J.; Sarkis, P.; Sarmiento, L. F.; Schäfer, S.; Schmitt, J. H. M. M.; Schiller, J.; Schweitzer, A.; Solano, E.; Stahl, O.; Strachan, J. B. P.; Stürmer, J.; Suárez, J. C.; Tabernero, H. M.; Tala, M.; Trifonov, T.; Tulloch, S. M.; Ulbrich, R. G.; Veredas, G.; Vico Linares, J. I.; Vilardell, F.; Wagner, K.; Winkler, J.; Wolthoff, V.; Xu, W.; Yan, F.; Zapatero Osorio, M. R.

2018-04-01

The CARMENES radial velocity (RV) survey is observing 324 M dwarfs to search for any orbiting planets. In this paper, we present the survey sample by publishing one CARMENES spectrum for each M dwarf. These spectra cover the wavelength range 520-1710 nm at a resolution of at least R >80 000, and we measure its RV, Hα emission, and projected rotation velocity. We present an atlas of high-resolution M-dwarf spectra and compare the spectra to atmospheric models. To quantify the RV precision that can be achieved in low-mass stars over the CARMENES wavelength range, we analyze our empirical information on the RV precision from more than 6500 observations. We compare our high-resolution M-dwarf spectra to atmospheric models where we determine the spectroscopic RV information content, Q, and signal-to-noise ratio. We find that for all M-type dwarfs, the highest RV precision can be reached in the wavelength range 700-900 nm. Observations at longer wavelengths are equally precise only at the very latest spectral types (M8 and M9). We demonstrate that in this spectroscopic range, the large amount of absorption features compensates for the intrinsic faintness of an M7 star. To reach an RV precision of 1 m s-1 in very low mass M dwarfs at longer wavelengths likely requires the use of a 10 m class telescope. For spectral types M6 and earlier, the combination of a red visual and a near-infrared spectrograph is ideal to search for low-mass planets and to distinguish between planets and stellar variability. At a 4 m class telescope, an instrument like CARMENES has the potential to push the RV precision well below the typical jitter level of 3-4 m s-1.

GAS, STARS, AND STAR FORMATION IN ALFALFA DWARF GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang Shan; Haynes, Martha P.; Giovanelli, Riccardo

2012-06-15

We examine the global properties of the stellar and H I components of 229 low H I mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H I masses <10{sup 7.7} M{sub Sun} and H I line widths <80 km s{sup -1}. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M{sub *}) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; onlymore » 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M{sub *} obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M{sub *} {approx}< 10{sup 8} M{sub Sun} is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper H I mass limit yields the selection of a sample with lower gas fractions for their M{sub *} than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H I depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that H I disks are more extended than stellar ones.« less

MUCHFUSS - Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS

NASA Astrophysics Data System (ADS)

Geier, S.; Schaffenroth, V.; Hirsch, H.; Tillich, A.; Heber, U.; Maxted, P. F. L.; Østensen, R. H.; Barlow, B. N.; O'Toole, S. J.; Kupfer, T.; Marsh, T.; Gänsicke, B.; Napiwotzki, R.; Cordes, O.; Müller, S.; Classen, L.; Ziegerer, E.; Drechsel, H.

2012-06-01

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions (white dwarfs with masses M>1.0 M⊙, neutron stars or black holes). The existence of such systems is predicted by binary evolution calculations and some candidate systems have been found. We identified ≃1100 hot subdwarf stars from the Sloan Digital Sky Survey (SDSS). Stars with high velocities have been reobserved and individual SDSS spectra have been analysed. About 70 radial velocity variable subdwarfs have been selected as good candidates for follow-up time resolved spectroscopy to derive orbital parameters and photometric follow-up to search for features like eclipses in the light curves. Up to now we found nine close binary sdBs with short orbital periods ranging from ≃0.07 d to 1.5 d. Two of them are eclipsing binaries with companions that are most likely of substellar nature.

Galaxy NGC 247

NASA Technical Reports Server (NTRS)

2003-01-01

This image of the dwarf spiral galaxy NGC 247 was taken by Galaxy Evolution Explorer on October 13, 2003, in a single orbit exposure of 1600 seconds. The region that looks like a 'hole' in the upper part of the galaxy is a location with a deficit of gas and therefore a lower star formation rate and ultraviolet brightness. Optical images of this galaxy show a bright star on the southern edge. This star is faint and red in the Galaxy Evolution Explorer ultraviolet image, revealing that it is a foreground star in our Milky Way galaxy. The string of background galaxies to the North-East (upper left) of NGC 247 is 355 million light years from our Milky Way galaxy whereas NGC 247 is a mere 9 million light years away. The faint blue light that can be seen in the Galaxy Evolution Explorer image of the upper two of these background galaxies may indicate that they are in the process of merging together.

Galaxy NGC 247

NASA Image and Video Library

2003-12-10

This image of the dwarf spiral galaxy NGC 247 was taken by Galaxy Evolution Explorer on October 13, 2003, in a single orbit exposure of 1600 seconds. The region that looks like a "hole" in the upper part of the galaxy is a location with a deficit of gas and therefore a lower star formation rate and ultraviolet brightness. Optical images of this galaxy show a bright star on the southern edge. This star is faint and red in the Galaxy Evolution Explorer ultraviolet image, revealing that it is a foreground star in our Milky Way galaxy. The string of background galaxies to the North-East (upper left) of NGC 247 is 355 million light years from our Milky Way galaxy whereas NGC 247 is a mere 9 million light years away. The faint blue light that can be seen in the Galaxy Evolution Explorer image of the upper two of these background galaxies may indicate that they are in the process of merging together. http://photojournal.jpl.nasa.gov/catalog/PIA04922

Discovery of an Ultra-diffuse Galaxy in the Pisces--Perseus Supercluster

NASA Astrophysics Data System (ADS)

Martínez-Delgado, David; Läsker, Ronald; Sharina, Margarita; Toloba, Elisa; Fliri, Jürgen; Beaton, Rachael; Valls-Gabaud, David; Karachentsev, Igor D.; Chonis, Taylor S.; Grebel, Eva K.; Forbes, Duncan A.; Romanowsky, Aaron J.; Gallego-Laborda, J.; Teuwen, Karel; Gómez-Flechoso, M. A.; Wang, Jie; Guhathakurta, Puragra; Kaisin, Serafim; Ho, Nhung

2016-04-01

We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located 10.°4 in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy (μV = 24.8 mag arcsec-2), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius (Re(V) = 12″) and proximity (15‧) to the well-known dwarf spheroidal galaxy And II. Its red color (V - I = 1.0), shallow Sérsic index (nV = 0.68), and the absence of detectable Hα emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6 m telescope (Vh = 5450 ± 40 km s-1) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (˜78 Mpc), DGSAT I would have an Re ˜ 4.7 kpc and MV ˜ -16.3. Its properties resemble those of the ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy.

DISCOVERY OF AN ULTRA-DIFFUSE GALAXY IN THE PISCES-PERSEUS SUPERCLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Martínez-Delgado, David; Grebel, Eva K.; Läsker, Ronald

We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located 10.°4 in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy (μ{sub V} = 24.8 mag arcsec{sup −2}), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius (R{sub e}(V) = 12″) and proximity (15′) to the well-known dwarf spheroidal galaxy And II. Its red color (V − I = 1.0), shallow Sérsic index (n{sub V} = 0.68), and the absence of detectable Hα emission aremore » typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6 m telescope (V{sub h} = 5450 ± 40 km s{sup −1}) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (∼78 Mpc), DGSAT I would have an R{sub e} ∼ 4.7 kpc and M{sub V} ∼ −16.3. Its properties resemble those of the ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy.« less

White Dwarf Stars

NASA Technical Reports Server (NTRS)

1999-01-01

Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy, giving astronomers a fresh reading on the age of the universe.

Located in the globular cluster M4, these small, burned-out stars -- called white dwarfs -- are about 12 to 13 billion years old. By adding the one billion years it took the cluster to form after the Big Bang, astronomers found that the age of the white dwarfs agrees with previous estimates that the universe is 13 to 14 billion years old.

The images, including some taken by Hubble's Wide Field and Planetary Camera 2, are available online at

http://oposite.stsci.edu/pubinfo/pr/2002/10/ or

http://www.jpl.nasa.gov/images/wfpc .

The camera was designed and built by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's .9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope.

The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles indicate the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars.

Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the oldest stars puts astronomers within arm's reach of the universe's age.

Hubble's Wide Field and Planetary Camera 2 made the observations from January through April 2001. These optical observations were combined to create the above images. Spectral data were also taken. M4 is 7,000 light-years away in the constellation Scorpius.

The full press release on the latest findings is online at

http://oposite.stsci.edu/pubinfo/pr/2002/10/pr.html .

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract with the Goddard Space Flight Center, Greenbelt, Md. The Hubble Space Telescope is a project of international cooperation between the European Space Agency and NASA. The California Institute of Technology in Pasadena manages JPL for NASA.

Direct imaging and new technologies to search for substellar companions around MGs cool dwarfs

NASA Astrophysics Data System (ADS)

Gálvez-Ortiz, M. C.; Clarke, J. R. A.; Pinfield, D. J.; Folkes, S. L.; Jenkins, J. S.; García Pérez, A. E.; Burningham, B.; Day-Jones, A. C.; Jones, H. R. A.

2011-07-01

We describe here our project based in a search for sub-stellar companions (brown dwarfs and exo-planets) around young ultra-cool dwarfs (UCDs) and characterise their properties. We will use current and future technology (high contrast imaging, high-precision Doppler determinations) from the ground and space (VLT, ELT and JWST), to find companions to young objects. Members of young moving groups (MGs) have clear advantages in this field. We compiled a catalogue of young UCD objects and studied their membership to five known young moving groups: Local Association (Pleiades moving group, 20-150 Myr), Ursa Mayor group (Sirius supercluster, 300 Myr), Hyades supercluster (600 Myr), IC 2391 supercluster (35 Myr) and Castor moving group (200 Myr). To assess them as members we used different kinematic and spectroscopic criteria.

VizieR Online Data Catalog: Spectroscopically Identified Hot Subdwarf Stars (Kilkenny+ 1988)

NASA Astrophysics Data System (ADS)

Kilkenny, D.; Heber, U.; Drilling, J. S.

1996-05-01

Prior to 1986 there were around 200 spectroscopically classified hot subdwarf stars. The Palomar-Green survey (Green et al., 1986ApJS...61..305G) detected over 900 hot subdwarfs, mostly in the North Galactic Cap and mostly previously unknown objects; the Kitt-Peak_Downes survey found another 60 near the Galactic Plane (Downes, 1986ApJS...61..569D). These form the basis of the present catalog but new subdwarfs are continually being found by spectroscopic surveys of photographically discovered faint blue star samples; examples are the work of Wegner and his co-workers on the Kiso survey (Wegner et al., 1985AJ.....90.1511W, 1986AJ.....91..139W, 1987AJ.....94.1271W) and of Kilkenny and Muller (1987) on southern discoveries by Luyten and collaborators (e.g. Haro and Luyten, 1962, Cat. III/74; Luyten and Anderson, 1958, 1959, 1967, "A Search for Faint Blue Stars"). Only stars for which a spectroscopic classification exists have been included. There is a significant probability that stars with only photometric classifications can be normal high-latitude B stars, white dwarfs or cataclysmic variable, for example. Hot subdwarfs in binary systems have been included but not planetary nebulae nuclei classified 'sd' since the latter have been catalogued elsewhere. Although there is not a universally accepted classification scheme for hot subdwarfs, it is fairly clear that the main criterion is a surface gravity higher than that of hot main sequence stars but less than that of hot white dwarfs. Also, hot subdwarf stars typically show helium abundance anomalies. (3 data files).

The formation of ultra compact dwarf galaxies and massive globular clusters. Quasar-like objects to test for a variable stellar initial mass function

NASA Astrophysics Data System (ADS)

Jeřábková, T.; Kroupa, P.; Dabringhausen, J.; Hilker, M.; Bekki, K.

2017-12-01

The stellar initial mass function (IMF) has been described as being invariant, bottom-heavy, or top-heavy in extremely dense star-burst conditions. To provide usable observable diagnostics, we calculate redshift dependent spectral energy distributions of stellar populations in extreme star-burst clusters, which are likely to have been the precursors of present day massive globular clusters (GCs) and of ultra compact dwarf galaxies (UCDs). The retention fraction of stellar remnants is taken into account to assess the mass to light ratios of the ageing star-burst. Their redshift dependent photometric properties are calculated as predictions for James Webb Space Telescope (JWST) observations. While the present day GCs and UCDs are largely degenerate concerning bottom-heavy or top-heavy IMFs, a metallicity- and density-dependent top-heavy IMF implies the most massive UCDs, at ages < 100 Myr, to appear as objects with quasar-like luminosities with a 0.1-10% variability on a monthly timescale due to core collapse supernovae.

A Virtual Observatory Census to Address Dwarfs Origins (AVOCADO). I. Science goals, sample selection, and analysis tools

NASA Astrophysics Data System (ADS)

Sánchez-Janssen, R.; Amorín, R.; García-Vargas, M.; Gomes, J. M.; Huertas-Company, M.; Jiménez-Esteban, F.; Mollá, M.; Papaderos, P.; Pérez-Montero, E.; Rodrigo, C.; Sánchez Almeida, J.; Solano, E.

2013-06-01

Context. Even though they are by far the most abundant of all galaxy types, the detailed properties of dwarf galaxies are still only poorly characterised - especially because of the observational challenge that their intrinsic faintness and weak clustering properties represent. Aims: AVOCADO aims at establishing firm conclusions on the formation and evolution of dwarf galaxies by constructing and analysing a homogeneous, multiwavelength dataset for a statistically significant sample of approximately 6500 nearby dwarfs (Mi - 5 log h100 > - 18 mag). The sample is selected to lie within the 20 < D < 60 h100-1 Mpc volume covered by the SDSS-DR7 footprint, and is thus volume-limited for Mi - 5 log h100 < -16 mag dwarfs - but includes ≈1500 fainter systems. We will investigate the roles of mass and environment in determining the current properties of the different dwarf morphological types - including their structure, their star formation activity, their chemical enrichment history, and a breakdown of their stellar, dust, and gas content. Methods: We present the sample selection criteria and describe the suite of analysis tools, some of them developed in the framework of the Virtual Observatory. We use optical spectra and UV-to-NIR imaging of the dwarf sample to derive star formation rates, stellar masses, ages, and metallicities - which are supplemented with structural parameters that are used to classify them morphologically. This unique dataset, coupled with a detailed characterisation of each dwarf's environment, allows for a fully comprehensive investigation of their origins and enables us to track the (potential) evolutionary paths between the different dwarf types. Results: We characterise the local environment of all dwarfs in our sample, paying special attention to trends with current star formation activity. We find that virtually all quiescent dwarfs are located in the vicinity (projected distances ≲ 1.5 h100-1 Mpc) of ≳ L∗ companions, consistent with recent results. While star-forming dwarfs are preferentially found at separations of the order of 1 h100-1 Mpc, there appears to be a tail towards low separations (≲ 100 h100-1 kpc) in the distribution of projected distances. We speculate that, modulo projection effects, this probably represents a genuine population of late-type dwarfs caught upon first infall about their host and before environmental quenching has fully operated. In this context, these results suggest that internal mechanisms - such as gas exhaustion via star formation or feedback effects - are not sufficient to completely cease the star formation activity in dwarf galaxies, and that becoming the satellite of a massive central galaxy appears to be a necessary condition to create a quiescent dwarf.

Chemistry and structure of giant molecular clouds in energetic environments

NASA Astrophysics Data System (ADS)

Anderson, Crystal Nicole

2016-09-01

Throughout the years many studies on Galactic star formation have been conducted. This resulted in the idea that giant molecular clouds (GMCs) are hierarchical in nature with substructures spanning a large range of sizes. The physical processes that determine how molecular clouds fragment, form clumps/cores and then stars depends strongly on both recent radiative and mechanical feed- back from massive stars and, on longer term, from enhanced cooling due to the buildup of metals. Radiative and mechanical energy input from stellar populations can alter subsequent star formation over a large part of a galaxy and hence is relevant to the evolution of galaxies. Much of our knowledge of star formation on galaxy wide scales is based on scaling laws and other parametric descriptions. But to understand the overall evolution of star formation in galaxies we need to watch the feedback processes at work on giant molecular cloud (GMC) scales. By doing this we can begin to answer how strong feedback environments change the properties of the substructure in GMCs. Tests of Galactic star formation theory to other galaxies has been a challenging process due to the lack of resolution with current instruments. Thus, only the nearest galaxies allow us to resolve GMCs and their substructures. The Large Magellanic Cloud (LMC), is one of the closest low metallicity dwarf galaxies (D˜ 50 kpc) and is close enough that current instruments can resolve the sub- structure of its GMCs to <1pc. The LMC has a star cluster located near the GMC, 30Doradus, producing high levels of far ultra violet (FUV) radiation in the inter- stellar medium (ISM). The dwarf galaxy, NGC 5253, is also a close low metallicity galaxy (3.8 Mpc) with a super star cluster, which appears to be composed of several newborn globular clusters, located within the center of the galaxy. These huge, compact collections of massive stars and their supernovae have the potential to dump large amounts of FUV radiation and momentum into the ISM. Under such hostile conditions, we cannot expect star formation to evolve in the same fashion as it does across much of the Galaxy. With the advancement of radio interferometry instruments like ALMA and the ATCA, we are able to observe nearby dwarf galaxies at 1.5-40 pc scales. Also, with the advancement of the instruments, astrochemistry is becoming an exciting and dominant field in studying star forming regions at varying densities and evolutionary stages outside the Galaxy. In this dissertation, I discuss observations of molecular gas tracers (e.g. HCO+, HCN, HNC, CS, C2H, N2H+) detected in the LMC at 1.5-40 pc scales and in NGC 5253 at 40 pc scales. I then compare the molecular gas detections to the Central Molecular Zone in our Galaxy. Dense molecular gas was detected in all of the sources. For the regions in the LMC, molecular lines of CS, N2H+, C 2H, HNC, HCO+ and HCN were all detected in N159W and N113 while only HCN, HCO+, HNC, and C2H were detected in 30Dor-10. Toward NGC 5253 only HCO+, HCN, C2H and CS were detected. I observe anomalously large HCO+/HCN line ratios of >5 for the NGC 5253 SSC, 30Dor-10 and N159W clumps. However the ratio is <2 for N113, the least energetic source, on clump scales. NGC 5253, 30Dor-10 and N159W have anomalously faint HCN. The CMZ however, does not have anomalously faint HCN it actually has HCO+/HCN common of high metallicity environments, active galactic nuclei and ultra luminous infrared galaxies. These observations suggest the reason HCN has fainter emission than HCO+ must be a combination of low metallicity and energetics contributing to the change in the HCO+/HCN ratio. I find that the impact of the massive star forming regions on surrounding gas in different galaxies from small to large scales changes the chemistry within these regions. A more energetic region's chemistry seems to be different from a less energetic region. There is a richer chemistry within a less energetic region; which may suggest that the chemistry in an energetic environment is quenched due to increased photodissociation.

Faint submillimeter galaxies revealed by multifield deep ALMA observations: number counts, spatial clustering, and a dark submillimeter line emitter

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ono, Yoshiaki; Ouchi, Masami; Momose, Rieko

2014-11-01

We present the statistics of faint submillimeter/millimeter galaxies (SMGs) and serendipitous detections of a submillimeter/millimeter line emitter (SLE) with no multi-wavelength continuum counterpart revealed by the deep ALMA observations. We identify faint SMGs with flux densities of 0.1-1.0 mJy in the deep Band-6 and Band-7 maps of 10 independent fields that reduce cosmic variance effects. The differential number counts at 1.2 mm are found to increase with decreasing flux density down to 0.1 mJy. Our number counts indicate that the faint (0.1-1.0 mJy, or SFR{sub IR} ∼ 30-300 M {sub ☉} yr{sup –1}) SMGs contribute nearly a half of themore » extragalactic background light (EBL), while the remaining half of the EBL is mostly contributed by very faint sources with flux densities of <0.1 mJy (SFR{sub IR} ≲ 30 M {sub ☉} yr{sup –1}). We conduct counts-in-cells analysis with multifield ALMA data for the faint SMGs, and obtain a coarse estimate of galaxy bias, b {sub g} < 4. The galaxy bias suggests that the dark halo masses of the faint SMGs are ≲ 7 × 10{sup 12} M {sub ☉}, which is smaller than those of bright (>1 mJy) SMGs, but consistent with abundant high-z star-forming populations, such as sBzKs, LBGs, and LAEs. Finally, we report the serendipitous detection of SLE-1, which has no continuum counterparts in our 1.2 mm-band or multi-wavelength images, including ultra deep HST/WFC3 and Spitzer data. The SLE has a significant line at 249.9 GHz with a signal-to-noise ratio of 7.1. If the SLE is not a spurious source made by the unknown systematic noise of ALMA, the strong upper limits of our multi-wavelength data suggest that the SLE would be a faint galaxy at z ≳ 6.« less

Characterizing Dw1335-29, a Recently Discovered Dwarf Satellite of M83

NASA Astrophysics Data System (ADS)

Carrillo, Andreia Jessica; Bell, Eric F.; Bailin, Jeremy; Monachesi, Antonela

2016-01-01

Simulations of galaxy formation in a cosmological context predict that galaxies should be surrounded by hundreds of relatively massive dark matter subhalos, each of which was expected to host a dwarf satellite galaxy. Large numbers of luminous dwarf galaxies do not exist around the Milky Way or M31 - this has been termed the missing satellite problem. There are a number of possible physical drivers of this discrepancy, some of which might predict significant differences from galaxy to galaxy. Accordingly, there are a number of efforts whose goal is to solidify and augment the census of dwarf satellites of external galaxies, outside the Local Group. Recently, Mueller, Jergen & Bingelli (2015; arXiv.1509.04931) presented 16 dwarf galaxy candidates in the vicinity of M83 using the Dark Energy CAMera (DECAM). With a field from the HST/GHOSTS survey that partly covers dw1335-29 (Radburn-Smith et al. 2011; ApJS, 195, 18) in conjunction with complementary ground-based images from VIMOS that cover the whole dwarf, we confirm that one of the candidates dw1335-29 is a dwarf satellite of M83, at a projected distance from M83 of 26 kpc and a with distance modulus of m-M = 28.5-0.1+0.3, placing it in the M83 group. From our VIMOS imaging that covers the entire dwarf, we estimate an absolute magnitude of MV = -9.8-0.1+0.3, show that it is elongated with an ellipticity of 0.35+/-0.15, and has a half light radius of 500+/-50pc. Dw1335-29 has both a somewhat irregular shape and has superimposed young stars in the resolved stellar population maps, leading us to classify this galaxy as a faint dwarf irregular or transition dwarf. This is especially curious, as with a projected distance of only 26kpc from M83, our prior expectation from study of the Local Group (following e.g., Grebel et al. 2003; AJ, 125, 1926, Slater & Bell 2013; ApJ, 772, 15) would be that dw1335-29 would lack recent star formation. Further study of M83's dwarf population will reveal if star formation in its dwarfs is commonplace (suggesting a lack of a hot gas envelope for M83 that would quench star formation) or rare (suggesting that dw1335-29 is at much larger 3D distance from M83, and is fortuitously projected to small radii).

K-KIDS: K Dwarfs and Their Companions. First Results from Radial Velocity Survey with CHIRON Spectrograph

NASA Astrophysics Data System (ADS)

Paredes, Leonardo; Henry, Todd; Nusdeo, Daniel; Winters, J.; Dincer, Tolga

2018-01-01

We present the K-KIDS project, an effort to survey a large sample of K dwarfs and their companions, the KIDS. We are observing a carefully vetted equatorial sample (DEC = -30 to +30) of more than 1000 K dwarfs within 50 pc to make a comprehensive assessment of stellar, substellar and planetary companions with separations of 0.1 to 10,000 AU.The initial sample of 1048 stars has been compiled using astrometric data from Hipparcos and photometric data from Tycho-2 and 2MASS. Four different imaging and spectroscopic surveys are underway. Here we present the strategy and initial results for our high-precision radial velocity survey for the closest companions using the CHIRON spectrograph on the CTIO/SMARTS 1.5m telescope. Individual measurements with CHIRON at R = 80,000 using ThAr wavelength calibration, indicate that for K dwarf radial velocity standards with V = 5.8, 7.0 and 8.0 yield precisions over 6 weeks of observing of 7.4 m/s, 9.8 m/s and 5.7 m/s. In the first two months, a core sample of 42 K dwarfs, including carefully selected calibration systems as well as previously unobserved stars, was observed every few nights to detect the radial velocity signals of close companions. In our calibration stellar systems, we have confirmed the suitability of CHIRON for our studies, by having found periodic radial velocity perturbations consistent with hot Jupiter and stellar companions previously detected. This set forms the foundation of our one-year survey of 100 K dwarfs with magnitudes as faint as V = 11.5, for which we should detect companions with masses as low as Jupiter.In light of the promising performance and efficiency of the CHIRON spectrograph for a long-term radial velocity survey, we have expanded our initial sample using Gaia Data Release 1 to 1824 K dwarfs within 50 pc. Ultimately, the combination of all four surveys will provide an unprecedented portrait of K dwarfs and their kids.This effort has been supported by the NSF through grant AST-1517413, and via observations made possible by the SMARTS Consortium.

The Next Generation Virgo Cluster Survey. VI. The Kinematics of Ultra-compact Dwarfs and Globular Clusters in M87

NASA Astrophysics Data System (ADS)

Zhang, Hong-Xin; Peng, Eric W.; Côté, Patrick; Liu, Chengze; Ferrarese, Laura; Cuillandre, Jean-Charles; Caldwell, Nelson; Gwyn, Stephen D. J.; Jordán, Andrés; Lançon, Ariane; Li, Biao; Muñoz, Roberto P.; Puzia, Thomas H.; Bekki, Kenji; Blakeslee, John P.; Boselli, Alessandro; Drinkwater, Michael J.; Duc, Pierre-Alain; Durrell, Patrick; Emsellem, Eric; Firth, Peter; Sánchez-Janssen, Rubén

2015-03-01

The origin of ultra-compact dwarfs (UCDs; rh >~ 10 pc)—objects larger and more massive than typical globular clusters (GCs), but more compact than typical dwarf galaxies—has been hotly debated in the 15 years since their discovery. Even whether UCDs should be considered galactic in origin, or simply the most extreme star clusters, is not yet settled. We present the dynamical properties of 97 spectroscopically confirmed UCDs and 911 GCs associated with the central cD galaxy of the Virgo cluster, M87. Our UCDs, of which 89% have M sstarf gsim 2× 106 M ⊙ and 92% are as blue as the classic blue GCs, nearly triple the confirmed sample of Virgo UCDs, providing by far the best opportunity for studying global dynamics of a UCD system. We found that (1) UCDs have a surface number density profile that is shallower than that of blue GCs in the inner ~70 kpc and as steep as that of red GCs at larger radii; (2) UCDs exhibit a significantly stronger rotation than GCs, and blue GCs seem to have a velocity field that is more consistent with that of the surrounding dwarf ellipticals than with that of UCDs; (3) UCDs have an orbital anisotropy profile that is tangentially biased at radii lsim40 kpc and radially biased farther out, whereas blue GCs become more tangentially biased at larger radii beyond ~40 kpc (4) GCs with M sstarf gsim 2 × 106 M ⊙ have rotational properties indistinguishable from the less massive ones, suggesting that it is the size, instead of mass, that differentiates UCDs from GCs as kinematically distinct populations. We conclude that most UCDs in M87 are not consistent with being merely the most luminous and extended examples of otherwise normal GCs. The radially biased orbital structure of UCDs at large radii is in general agreement with the "tidally threshed dwarf galaxy" scenario.

Characterizing the evolving K -band luminosity function using the UltraVISTA, CANDELS and HUDF surveys

NASA Astrophysics Data System (ADS)

Mortlock, Alice; McLure, Ross J.; Bowler, Rebecca A. A.; McLeod, Derek J.; Mármol-Queraltó, Esther; Parsa, Shaghayegh; Dunlop, James S.; Bruce, Victoria A.

2017-02-01

We present the results of a new study of the K-band galaxy luminosity function (KLF) at redshifts z ≤ 3.75, based on a nested combination of the UltraVISTA, Cosmic Assembly Near-infrared Deep Legacy Extragalactic Survey and HUDF surveys. The large dynamic range in luminosity spanned by this new data set (3-4 dex over the full redshift range) is sufficient to clearly demonstrate for the first time that the faint-end slope of the KLF at z ≥ 0.25 is relatively steep (-1.3 ≤ α ≤ -1.5 for a single Schechter function), in good agreement with recent theoretical and phenomenological models. Moreover, based on our new data set, we find that a double Schechter function provides a significantly improved description of the KLF at z ≤ 2. At redshifts z ≥ 0.25, the evolution of the KLF is remarkably smooth, with little or no evolution evident at faint (MK ≥ -20.5) or bright magnitudes (MK ≤ -24.5). Instead, the KLF is seen to evolve rapidly at intermediate magnitudes, with the number density of galaxies at MK ≃-23 dropping by a factor of ≃5 over the redshift interval 0.25 ≤ z ≤ 3.75. Motivated by this, we explore a simple description of the evolving KLF based on a double Schechter function with fixed faint-end slopes (α1 = -0.5, α2 = -1.5) and a shared characteristic magnitude (MK^{star }). According to this parametrization, the normalization of the component which dominates the faint end of the KLF remains approximately constant, with φ ^{star }2 decreasing by only a factor of ≃2 between z ≃0 and 3.25. In contrast, the component which dominates the bright end of the KLF at low redshifts evolves dramatically, becoming essentially negligible by z ≃3. Finally, we note that within this parametrization, the observed evolution of MK^{star } between z ≃0 and 3.25 is entirely consistent with MK^{star } corresponding to a constant stellar mass of M⋆ ≃5 × 1010 M⊙ at all redshifts.

Dark-Matter Halos of Tenuous Galaxies

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2016-03-01

A series of recent deep-imaging surveys has revealed dozens of lurking ultra-diffuse galaxies (UDGs) in nearby galaxy clusters. A new study provides key information to help us understand the origins of these faint giants.What are UDGs?There are three main possibilities for how UDGs galaxies with the sizes of giants, but luminosities no brighter than those of dwarfs formed:They are tidal dwarfs, created in galactic collisions when streams of matter were pulled away from the parent galaxies and halos to form dwarfs.They are descended from normal galaxies and were then altered by tidal interactions with the galaxy cluster.They are ancient remnant systems large galaxies whose gas was swept away, putting an early halt to star formation. The gas removal did not, however, affect their large dark matter halos, which permitted them to survive in the cluster environment.The key to differentiating between these options is to obtain mass measurements for the UDGs how large are their dark matter halos? In a recent study led by Michael Beasley (Institute of Astrophysics of the Canary Islands, University of La Laguna), a team of astronomers has determined a clever approach for measuring these galaxies masses: examine their globular clusters.Masses from Globular ClustersVCC 1287s mass measurements put it outside of the usual halo-mass vs. stellar-mass relationships for nearby galaxies: it has a significantly higher halo mass than is normal, given its stellar mass. [Adapted from Beasley et al. 2016]Beasley and collaborators selected one UDG, VCC 1287, from the Virgo galaxy cluster, and they obtained spectra of the globular clusters around it using the OSIRIS spectrograph on the Great Canary Telescope. They then determined VCC 1287s total halo mass in two ways: first by using the dynamics of the globular clusters, and then by relying on a relation between total globular cluster mass and halo mass.The two masses they found are in good agreement with each other; both are around 80 billion solar masses. This is an unprecedented factor of 3,000 larger than the stellar mass for the galaxy (obtained from the galaxys luminosity) which means that VCC 1287 has an unusually large dark matter halo given its stellar population.Clues to OriginsThis result makes it unlikely that VCC 1287 is a tidal-dwarf system, since these usually have dark-matter fractions of less than 10%. The authors also dont believe it is a tidally stripped system, since no obvious tidal features were revealed in their imaging. Instead, they think the most probable scenario is that VCC 1287 is a massive dwarf galaxy that had its star formation quenched by gas starvation as it fell into the Virgo cluster long ago.To learn whether VCC 1287 is typical of UDGs, the authors encourage finding additional UDG masses using the same techniques outlined in this study. Additional observations of the globular-cluster populations for UDGs will significantly help understand these unusual galaxies.CitationMichael A. Beasley et al 2016 ApJ 819 L20. doi:10.3847/2041-8205/819/2/L20

The formation of the Milky Way halo and its dwarf satellites; a NLTE-1D abundance analysis. II. Early chemical enrichment

NASA Astrophysics Data System (ADS)

Mashonkina, L.; Jablonka, P.; Sitnova, T.; Pakhomov, Yu.; North, P.

2017-12-01

We present the non-local thermodynamic equilibrium (NLTE) abundances of up to 10 chemical species in a sample of 59 very metal-poor (VMP, -4 ≤ [Fe/H] ≾-2) stars in seven dwarf spheroidal galaxies (dSphs) and in the Milky Way (MW) halo. Our results are based on high-resolution spectroscopic datasets and homogeneous and accurate atmospheric parameters determined in Paper I. We show that once the NLTE effects are properly taken into account, all massive galaxies in our sample, that is, the MW halo and the classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax, reveal a similar plateau at [α/Fe] ≃ 0.3 for each of the α-process elements: Mg, Ca, and Ti. We put on a firm ground the evidence for a decline in α/Fe with increasing metallicity in the Boötes I ultra-faint dwarf galaxy (UFD), that is most probably due to the ejecta of type Ia supernovae. For Na/Fe, Na/Mg, and Al/Mg, the MW halo and all dSphs reveal indistinguishable trends with metallicity, suggesting that the processes of Na and Al synthesis are identical in all systems, independent of their mass. The dichotomy in the [Sr/Ba] versus [Ba/H] diagram is observed in the classical dSphs, similarly to the MW halo, calling for two different nucleosynthesis channels for Sr. We show that Sr in the massive galaxies is well correlated with Mg suggesting a strong link to massive stars and that its origin is essentially independent of Ba, for most of the [Ba/H] range. Our three UFDs, that is Boötes I, UMa II, and Leo IV, are depleted in Sr and Ba relative to Fe and Mg, with very similar ratios of [Sr/Mg] ≃-1.3 and [Ba/Mg] ≃-1 on the entire range of their Mg abundances. The subsolar Sr/Ba ratios of Boötes I and UMa II indicate a common r-process origin of their neutron-capture elements. Sculptor remains the classical dSph, in which the evidence for inhomogeneous mixing in the early evolution stage, at [Fe/H] <-2, is the strongest. Full Tables 3 and 4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/608/A89

Exploring Substellar Evolution with the Coldest Brown Dwarfs

NASA Astrophysics Data System (ADS)

Dupuy, Trent J.

2017-01-01

The coldest brown dwarfs are our best analogs to extrasolar gas-giant planets, representing the lowest mass products of star formation. Our view of such objects has been transformed over the last few years as new observations have revealed that the solar neighborhood is populated by much colder objects than previously recognized. At the center of efforts to discover and characterize these coldest substellar objects have been observations from NASA missions (WISE, Spitzer, HST) and the Keck Telescopes. I will review the tremendous progress made in this field over just the last few years thanks to major community efforts to overcome observational challenges in obtaining spectroscopy, photometry, and astrometry of these infrared-faint, optically invisible objects. Spectra from HST and Keck were key in establishing the much anticipated "Y" spectral type, extending the classic stellar classification scheme to atmospheres as cool as 300-400 K. Parallaxes and photometry from Spitzer and Keck have provided absolute fluxes, enabling robust temperature determinations and critical tests of model atmopheres. High-resolution imaging with Keck laser guide star adaptive optics (LGS AO) has been the most prolific resource for revealing tight companions among the coldest brown dwarfs. In fact, with continued orbit monitoring with Keck LGS AO and HST, these binary systems will ultimately provide dynamical masses that will allow the strongest tests of models and reveal if the coldest brown dwarfs are indeed "planetary mass" (less than about 13 Jupiter masses) as is currently thought.

A Proper-Motion Corrected, Cross-Matched Catalog Of M Dwarfs In SDSS And FIRST

NASA Astrophysics Data System (ADS)

Arai, Erin; West, A. A.; Thyagarajan, N.; Agüeros, M.; Helfand, D.

2011-05-01

We present a preliminary analysis of M dwarfs identified in both the Sloan Digital Sky Survey (SDSS) and the Very Large Array's (VLA) Faint Images of the Radio Sky at Twenty-centimeters survey (FIRST). The presence of magnetic fields is often associated with indirect magnetic activity measurements, such as H-alpha or X-ray emission. Radio emission, in contrast, is directly proportional to the magnetic field strength in addition to being another measure of activity. We search for stellar radio emission by cross-matching the SDSS DR7 M dwarf sample with the FIRST catalog. The SDSS data allow us to examine the spectra of our objects and correlate the magnetic activity (H-alpha) with the magnetic field strength (radio emission). Accurate positions and proper motions are important for obtaining a complete list of overlapping targets. Positions in FIRST and SDSS need to be proper motion corrected in order to ensure unique target matches since nearby M dwarfs can have significant proper motions (up to 1'' per year). Some previous studies have neglected the significance of proper motions in identifying overlapping targets between SDSS and FIRST; we correct for some of these previous oversights. In addition the FIRST data were taken in multiple epochs; individual images need to be proper motion corrected before the images can be co-added. Our cross-match catalog puts important constraints on models of magnetic field generation in low-mass stars in addition to the true habitability of attending planets.

New Y and T Dwarfs from WISE Identified by Methane Imaging

NASA Astrophysics Data System (ADS)

Tinney, C. G.; Kirkpatrick, J. Davy; Faherty, Jacqueline K.; Mace, Gregory N.; Cushing, Mike; Gelino, Christopher R.; Burgasser, Adam J.; Sheppard, Scott S.; Wright, Edward L.

2018-06-01

We identify new Y- and T-type brown dwarfs from the WISE All Sky data release using images obtained in filters that divide the traditional near-infrared H and J bands into two halves—specifically {CH}}4{{s}} and CH4l in the H and J2, and J3 in the J. This proves to be very effective at identifying cool brown dwarfs via the detection of their methane absorption, as well as providing preliminary classification using methane colors and WISE -to-near-infrared colors. New and updated calibrations between T/Y spectral types and {CH}}4{{s}}–CH4l J3–W2, and {CH}}4{{s}}–W2 colors are derived, producing classification estimates good to a few spectral sub-types. We present photometry for a large sample of T and Y dwarfs in these filters, together with spectroscopy for 23 new ultra-cool dwarfs—2 Y dwarfs and 21 T dwarfs. We identify a further 8 new cool brown dwarfs, which we have high confidence are T dwarfs based on their methane photometry. We find that, for objects observed on a 4 m class telescope at J-band magnitudes of ∼20 or brighter, {CH}}4{{s}}–CH4l is the more powerful color for detecting objects and then estimating spectral types. Due to the lower sky background in the J-band, the J3 and J2 bands are more useful for identifying fainter cool dwarfs at J ≳ 22. The J3–J2 color is poor at estimating spectral types. But fortunately, once J3–J2 confirms that an object is a cool dwarf, the J3–W2 color is very effective at estimating approximate spectral types.

Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina

NASA Astrophysics Data System (ADS)

Li, T. S.; Simon, J. D.; Pace, A. B.; Torrealba, G.; Kuehn, K.; Drlica-Wagner, A.; Bechtol, K.; Vivas, A. K.; van der Marel, R. P.; Wood, M.; Yanny, B.; Belokurov, V.; Jethwa, P.; Zucker, D. B.; Lewis, G.; Kron, R.; Nidever, D. L.; Sánchez-Conde, M. A.; Ji, A. P.; Conn, B. C.; James, D. J.; Martin, N. F.; Martinez-Delgado, D.; Noël, N. E. D.; MagLiteS Collaboration

2018-04-01

We present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocity {v}hel}=477.2+/- 1.2 {km} {{{s}}}-1 and a velocity dispersion of {σ }v={3.4}-0.8+1.2 {km} {{{s}}}-1. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of {1.0}-0.4+0.8× {10}6 {M}ȯ , indicating a mass-to-light ratio of {369}-161+309 {M}ȯ /{L}ȯ . From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of {{[Fe/H]}}=-2.44+/- 0.09 with dispersion {σ }{{[Fe/H]}}={0.22}-0.07+0.10. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of {v}hel}={284.6}-3.1+3.4 {km} {{{s}}}-1. The brightest RGB member of Car III has a metallicity of {{[Fe/H]}} =-1.97+/- 0.12. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation ({{Δ }}d∼ 10 {kpc}) among Local Group satellites, the large difference in their systemic velocities, ∼ 200 {km} {{{s}}}-1, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of γ-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.

On the Formation of Ultra-Difuse Galaxies as Tidally-Stripped Systems

NASA Astrophysics Data System (ADS)

Carleton, Timothy; Cooper, Michael; Kaplinghat, Manoj; Errani, Raphael; Penarrubia, Jorge

2018-01-01

The recent identification of a large population of so-called 'Ultra-Diffuse' Galaxies (UDGs), with stellar masses ~108 M⊙, but half light radii over 1.5 kpc, has challenged our understanding of galaxy evolution. Motivated by the environmental dependence of UDG properties and abundance, I present a model for the formation of UDGs through tidal-stripping of dwarf galaxies in cored dark matter halos. To test this scenario, I utilize results from simulations of tidal stripping, which demonstrate that changes in the stellar profile of a tidally stripped galaxy can be written as a function of the amount of tidal stripping experienced by the halo (tidal tracks). These tracks, however, are different for cored and cuspy halos. Additional simulations show how the halo responds to tidal interactions given the halo orbit within a cluster.In particular, dwarf elliptical galaxies, born in 1010-10.5 M⊙ halos, expand significantly as a result of tidal stripping and produce UDGs. Applying these models to the population of halos in the Bolshoi simulation, I am able to follow the effects of tidal stripping on the dwarf galaxy population in clusters. Using tidal tracks for cuspy halos does not reproduce the observed properties of UDGs. However, using the tidal tracks for cored halos, I reproduce the distribution of sizes, stellar masses, and abundance of UDGs in clusters remarkably well.

Reconciling Dwarf Galaxies with ΛCDM Cosmology: Simulating a Realistic Population of Satellites around a Milky Way-mass Galaxy

NASA Astrophysics Data System (ADS)

Wetzel, Andrew R.; Hopkins, Philip F.; Kim, Ji-hoon; Faucher-Giguère, Claude-André; Kereš, Dušan; Quataert, Eliot

2016-08-01

Low-mass “dwarf” galaxies represent the most significant challenges to the cold dark matter (CDM) model of cosmological structure formation. Because these faint galaxies are (best) observed within the Local Group (LG) of the Milky Way (MW) and Andromeda (M31), understanding their formation in such an environment is critical. We present first results from the Latte Project: the Milky Way on Feedback in Realistic Environments (FIRE). This simulation models the formation of an MW-mass galaxy to z=0 within ΛCDM cosmology, including dark matter, gas, and stars at unprecedented resolution: baryon particle mass of 7070 {M}⊙ with gas kernel/softening that adapts down to 1 {pc} (with a median of 25{--}60 {pc} at z=0). Latte was simulated using the GIZMO code with a mesh-free method for accurate hydrodynamics and the FIRE-2 model for star formation and explicit feedback within a multi-phase interstellar medium. For the first time, Latte self-consistently resolves the spatial scales corresponding to half-light radii of dwarf galaxies that form around an MW-mass host down to {M}{star}≳ {10}5 {M}⊙ . Latte’s population of dwarf galaxies agrees with the LG across a broad range of properties: (1) distributions of stellar masses and stellar velocity dispersions (dynamical masses), including their joint relation; (2) the mass-metallicity relation; and (3) diverse range of star formation histories, including their mass dependence. Thus, Latte produces a realistic population of dwarf galaxies at {M}{star}≳ {10}5 {M}⊙ that does not suffer from the “missing satellites” or “too big to fail” problems of small-scale structure formation. We conclude that baryonic physics can reconcile observed dwarf galaxies with standard ΛCDM cosmology.

Dwarf elliptical galaxies

NASA Technical Reports Server (NTRS)

Ferguson, Henry C.; Binggeli, Bruno

1994-01-01

Dwarf elliptical (dE) galaxies, with blue absolute magnitudes typically fainter than M(sub B) = -16, are the most numerous type of galaxy in the nearby universe. Tremendous advances have been made over the past several years in delineating the properties of both Local Group satellite dE's and the large dE populations of nearby clusters. We review some of these advances, with particular attention to how well currently availiable data can constrain (a) models for the formation of dE's, (b) the physical and evolutionary connections between different types of galaxies that overlap in the same portion of the mass-spectrum of galaxies, (c) the contribution of dE's to the galaxy luminosity functions in clusters and the field, (d) the star-forming histories of dE's and their possible contribution to faint galaxy counts, and (e) the clustering properties of dE's. In addressing these issues, we highlight the extent to which selection effects temper these constraints, and outline areas where new data would be particularly valuable.

First confirmed ultra-compact dwarf galaxy in the NGC 5044 group

NASA Astrophysics Data System (ADS)

Faifer, Favio R.; Escudero, Carlos G.; Scalia, María C.; Smith Castelli, Analía V.; Norris, Mark; De Rossi, María E.; Forte, Juan C.; Cellone, Sergio A.

2017-03-01

Context. Ultra-compact dwarfs (UCDs) are stellar systems displaying colours and metallicities between those of globular clusters (GCs) and early-type dwarf galaxies, as well as sizes of Reff ≲ 100 pc and luminosities in the range -13.5

First Light LBT AO Images of HR 8799 bcde at 1.6 and 3.3 μm: New Discrepancies between Young Planets and Old Brown Dwarfs

NASA Astrophysics Data System (ADS)

Skemer, Andrew J.; Hinz, Philip M.; Esposito, Simone; Burrows, Adam; Leisenring, Jarron; Skrutskie, Michael; Desidera, Silvano; Mesa, Dino; Arcidiacono, Carmelo; Mannucci, Filippo; Rodigas, Timothy J.; Close, Laird; McCarthy, Don; Kulesa, Craig; Agapito, Guido; Apai, Daniel; Argomedo, Javier; Bailey, Vanessa; Boutsia, Konstantina; Briguglio, Runa; Brusa, Guido; Busoni, Lorenzo; Claudi, Riccardo; Eisner, Joshua; Fini, Luca; Follette, Katherine B.; Garnavich, Peter; Gratton, Raffaele; Guerra, Juan Carlos; Hill, John M.; Hoffmann, William F.; Jones, Terry; Krejny, Megan; Males, Jared; Masciadri, Elena; Meyer, Michael R.; Miller, Douglas L.; Morzinski, Katie; Nelson, Matthew; Pinna, Enrico; Puglisi, Alfio; Quanz, Sascha P.; Quiros-Pacheco, Fernando; Riccardi, Armando; Stefanini, Paolo; Vaitheeswaran, Vidhya; Wilson, John C.; Xompero, Marco

2012-07-01

As the only directly imaged multiple planet system, HR 8799 provides a unique opportunity to study the physical properties of several planets in parallel. In this paper, we image all four of the HR 8799 planets at H band and 3.3 μm with the new Large Binocular Telescope adaptive optics system, PISCES, and LBTI/LMIRCam. Our images offer an unprecedented view of the system, allowing us to obtain H and 3.3 μm photometry of the innermost planet (for the first time) and put strong upper limits on the presence of a hypothetical fifth companion. We find that all four planets are unexpectedly bright at 3.3 μm compared to the equilibrium chemistry models used for field brown dwarfs, which predict that planets should be faint at 3.3 μm due to CH4 opacity. We attempt to model the planets with thick-cloudy, non-equilibrium chemistry atmospheres but find that removing CH4 to fit the 3.3 μm photometry increases the predicted L' (3.8 μm) flux enough that it is inconsistent with observations. In an effort to fit the spectral energy distribution of the HR 8799 planets, we construct mixtures of cloudy atmospheres, which are intended to represent planets covered by clouds of varying opacity. In this scenario, regions with low opacity look hot and bright, while regions with high opacity look faint, similar to the patchy cloud structures on Jupiter and L/T transition brown dwarfs. Our mixed-cloud models reproduce all of the available data, but self-consistent models are still necessary to demonstrate their viability. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are as follows: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di AstroÞsica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.

The red and blue galaxy populations in the GOODS field: evidence for an excess of red dwarfs

NASA Astrophysics Data System (ADS)

Salimbeni, S.; Giallongo, E.; Menci, N.; Castellano, M.; Fontana, A.; Grazian, A.; Pentericci, L.; Trevese, D.; Cristiani, S.; Nonino, M.; Vanzella, E.

2008-01-01

Aims: We study the evolution of the galaxy population up to z˜ 3 as a function of its colour properties. In particular, luminosity functions and luminosity densities were derived as a function of redshift for the blue/late and red/early populations. Methods: We use data from the GOODS-MUSIC catalogue, which have typical magnitude limits z850≤ 26 and K_s≤ 23.5 for most of the sample. About 8% of the galaxies have spectroscopic redshifts; the remaining have well calibrated photometric redshifts derived from the extremely wide multi-wavelength coverage in 14 bands (from the U band to the Spitzer 8~ μm band). We have derived a catalogue of galaxies complete in the rest-frame B-band, which has been divided into two subsamples according to their rest-frame U-V colour (or derived specific star formation rate) properties. Results: We confirm a bimodality in the U-V colour and specific star formation rate of the galaxy sample up to z˜ 3. This bimodality is used to compute the luminosity functions of the blue/late and red/early subsamples. The luminosity functions of the blue/late and total samples are well represented by steep Schechter functions evolving in luminosity with increasing redshifts. The volume density of the luminosity functions of the red/early populations decreases with increasing redshift. The shape of the red/early luminosity functions shows an excess of faint red dwarfs with respect to the extrapolation of a flat Schechter function and can be represented by the sum of two Schechter functions. Our model for galaxy formation in the hierarchical clustering scenario, which also includes external feedback due to a diffuse UV background, shows a general broad agreement with the luminosity functions of both populations, the larger discrepancies being present at the faint end for the red population. Hints on the nature of the red dwarf population are given on the basis of their stellar mass and spatial distributions.

DEEP NEAR-IR OBSERVATIONS OF THE GLOBULAR CLUSTER M4: HUNTING FOR BROWN DWARFS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dieball, A.; Bedin, L. R.; Knigge, C.

2016-01-20

We present an analysis of deep Hubble Space Telescope (HST)/Wide Field Camera 3 near-IR (NIR) imaging data of the globular cluster (GC) M4. The best-photometry NIR color–magnitude diagram (CMD) clearly shows the main sequence extending toward the expected end of the hydrogen-burning limit and going beyond this point toward fainter sources. The white dwarf (WD) sequence can be identified. As such, this is the deepest NIR CMD of a GC to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the WDs from brown dwarf (BD) candidates. Detection limits in the NIR aremore » around F110W ≈ 26.5 mag and F160W ≈ 27 mag, and in the optical around F775W ≈ 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical–NIR CMDs. Thus, any faint NIR sources that have no optical counterpart are potential BD candidates, since the optical data are not deep enough to detect them. We visually inspected the positions of NIR sources that are fainter than the H-burning limit in F110W and for which the optical photometry did not return a counterpart. We found in total five sources for which we did not get an optical measurement. For four of these five sources, a faint optical counterpart could be visually identified, and an upper optical magnitude was estimated. Based on these upper optical magnitude limits, we conclude that one source is likely a WD, one source could be either a WD or BD candidate, and the remaining two sources agree with being BD candidates. No optical counterpart could be detected for just one source, which makes this source a good BD candidate. We conclude that we found in total four good BD candidates.« less

Chandra ACIS Sub-pixel Resolution

NASA Astrophysics Data System (ADS)

Kim, Dong-Woo; Anderson, C. S.; Mossman, A. E.; Allen, G. E.; Fabbiano, G.; Glotfelty, K. J.; Karovska, M.; Kashyap, V. L.; McDowell, J. C.

2011-05-01

We investigate how to achieve the best possible ACIS spatial resolution by binning in ACIS sub-pixel and applying an event repositioning algorithm after removing pixel-randomization from the pipeline data. We quantitatively assess the improvement in spatial resolution by (1) measuring point source sizes and (2) detecting faint point sources. The size of a bright (but no pile-up), on-axis point source can be reduced by about 20-30%. With the improve resolution, we detect 20% more faint sources when embedded on the extended, diffuse emission in a crowded field. We further discuss the false source rate of about 10% among the newly detected sources, using a few ultra-deep observations. We also find that the new algorithm does not introduce a grid structure by an aliasing effect for dithered observations and does not worsen the positional accuracy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Janowiecki, Steven; Salzer, John J.; Zee, Liese van

We discuss and test possible evolutionary connections between blue compact dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low-mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each galaxy with a grid of theoretical models to determine their stellar massesmore » and star formation properties. We compare our results for BCDs with the LVL galaxies to put BCDs in the context of normal galaxy evolution. The SED fits demonstrate that the star formation events currently underway in BCDs are at the extreme of the continuum of normal dwarf galaxies, both in terms of the relative mass involved and in the relative increase over previous star formation rates. Today’s BCDs are distinctive objects in a state of extreme star formation that is rapidly transforming them. This study also suggests ways to identify former BCDs whose star formation episodes have since faded.« less

VARIABLE AND POLARIZED RADIO EMISSION FROM THE T6 BROWN DWARF WISEP J112254.73+255021.5

DOE Office of Scientific and Technical Information (OSTI.GOV)

Williams, P. K. G.; Berger, E.; Gizis, J. E., E-mail: pwilliams@cfa.harvard.edu

2017-01-10

Route and Wolszczan recently detected five radio bursts from the T6 dwarf WISEP J112254.73+255021.5 and used the timing of these events to propose that this object rotates with an ultra-short period of ∼17.3 minutes. We conducted follow-up observations with the Very Large Array and Gemini-North but found no evidence for this periodicity. We do, however, observe variable, highly circularly polarized radio emission. Assuming that the radio emission of this T dwarf is periodically variable on ∼hour timescales, like other radio-active ultracool dwarfs, we infer a likely period of 116 minutes. However, our observation lasted only 162 minutes and so more data are needed to test thismore » hypothesis. The handedness of the circular polarization switches twice and there is no evidence for any unpolarized emission component, the first time such a phenomenology has been observed in radio studies of very low-mass stars and brown dwarfs. We suggest that the object’s magnetic dipole axis may be highly misaligned relative to its rotation axis.« less

A Search for Periodic Optical Variability in Radio Detected Ultracool Dwarfs: A Consequence of a Magnetically-Driven Auroral Process?

NASA Astrophysics Data System (ADS)

Harding, L. K.; Hallinan, G.; Boyle, R. P.; Butler, R. F.; Sheehan, B.; Golden, A.

2011-12-01

A number of ultracool dwarfs have been unexpectedly detected as radio sources in the last decade, four of which have been found to be producing periodic pulses. More recently, two of these pulsing dwarfs have also been found to be periodically variable in broadband optical photometry. The detected periods match the periods of the radio pulses which have previously been associated with the rotation period of the dwarf. For one of these objects, it has also been established that the optical and radio periodic variability are possibly linked, being a consequence of magnetically-driven auroral processes. In order to investigate the ubiquity of the periodic optical variability in radio detected sources, the GUFI instrument (Galway Ultra Fast Imager) was commissioned on the 1.8m Vatican Advanced Technology Telescope, on Mt. Graham, Arizona, and has been obtaining data for the past eighteen months. More than two hundred hours of multi-epoch photometric monitoring observations of radio detected ultracool dwarfs have been completed. We present initial results confirming optical periodic variability for four of this sample, three of which have been newly confirmed using GUFI.

A normal abundance of faint satellites in the fossil group NGC 6482

NASA Astrophysics Data System (ADS)

Lieder, S.; Mieske, S.; Sánchez-Janssen, R.; Hilker, M.; Lisker, T.; Tanaka, M.

2013-11-01

A fossil group is considered the end product in a galaxy group's evolution. It is a massive central galaxy that dominates the luminosity budget of the group, and is the outcome of efficient merging between intermediate-luminosity members. Little is known, however, about the faint satellite systems of fossil groups. Here we present a Subaru/Suprime-Cam wide-field, deep imaging study in the B - and R -bands of the nearest fossil group NGC 6482 (Mtot ~ 4 × 1012M⊙), covering the virial radius out to 310 kpc. We performed detailed completeness estimations and selected group member candidates by a combination of automated object detection and visual inspection. A fiducial sample of 48 member candidates down to MR ~ -10.5 mag is detected, making this study the deepest of a fossil group to now. We investigate the photometric scaling relations, the color-magnitude relation, and the luminosity function of our galaxy sample. We find evidence of recent and ongoing merger events among bright group galaxies. The color-magnitude relation is comparable to that of nearby galaxy clusters, and it exhibits significant scatter at the faintest luminosities. The completeness-corrected luminosity function is dominated by early-type dwarfs and is characterized by a faint end slope α = -1.32 ± 0.05. We conclude that the NGC 6482 fossil group shows photometric properties consistent with those of regular galaxy clusters and groups, including a normal abundance of faint satellites. Appendix A is available in electronic form at http://www.aanda.orgThe reduced data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/559/A76

Proper motion of the Draco dwarf galaxy from Subaru Suprime-Cam data

NASA Astrophysics Data System (ADS)

Casetti-Dinescu, Dana I.; Girard, Terrence M.

2016-09-01

We have measured the absolute proper motion of the Draco dwarf spheroidal galaxy using Subaru Suprime-Cam images taken at three epochs, with time baselines of 4.4 and 7 yr. The magnitude limit of the proper-motion study is I = 25, thus allowing for thousands of background galaxies and Draco stars to be used to perform extensive astrometric tests and to derive the correction to an inertial reference frame. The derived proper motion is (μα, μδ) = (-0.284 ± 0.047, -0.289 ± 0.041) mas yr-1. This motion implies an orbit that takes Draco to a pericentre of ˜20 kpc; a somewhat disruptive orbit suggesting that tides might account for the rising velocity-dispersion profile of Draco seen in line-of-sight velocity studies. The orbit is only marginally consistent with Draco's membership to the vast polar structure of Galactic satellites, in contrast to a recent Hubble Space Telescope proper-motion measurement that finds alignment very likely. Our study is a test case to demonstrate that deep imaging with mosaic cameras of appropriate resolution can be used for high-accuracy, ground-based proper-motion measurement. As a useful by-product of the study, we also identify two faint brown-dwarf candidates in the foreground field.

How to Reconcile the Observed Velocity Function of Galaxies with Theory

NASA Astrophysics Data System (ADS)

Brooks, Alyson M.; Papastergis, Emmanouil; Christensen, Charlotte R.; Governato, Fabio; Stilp, Adrienne; Quinn, Thomas R.; Wadsley, James

2017-11-01

Within a Λ cold dark matter (ΛCDM) scenario, we use high-resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in observed velocity functions (VFs). We measure velocities in as similar a way as possible to observations, including generating mock H I data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an “observed” VF in agreement with observations, reconciling the large number of low-mass halos expected in a ΛCDM cosmological model with the low number of observed dwarfs at a given velocity. We then explore the source of the discrepancy between observations and theory and conclude that the dearth of observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the H I linewidth severely underestimate the maximum halo velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored DM density profiles can contribute to the lower observed velocity of galaxies but only for galaxies in which the velocity is measured interior to the size of the core (˜3 kpc).

POX 4 and Tol 35: Two Peculiar Wolf-Rayet Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Méndez, David I.; Esteban, César

1999-12-01

We present results of narrowband (Hα and adjacent continuum) and broadband (U, B, and V) optical CCD imaging together with high-resolution Hα spectroscopy of the blue compact Wolf-Rayet dwarf galaxies POX 4 and Tol 35. POX 4 has a fainter, irregular, and diffuse companion located 20.5" (4.7 kpc) along the minor axis of the galaxy, which is visible also in the Hα emission. The difference in recession velocity between the galaxy and the companion is about 130 km s-1. The observational results lead us to propose that POX 4 could be interpreted as a low-mass ring galaxy, produced by a head-on intrusion of the fainter companion. Regarding the other object, a spectrum taken along the major axis of Tol 35 shows the coexistence of systems of motion with a velocity difference of about 50 km s-1. Moreover, the deep continuum-subtracted Hα image of the galaxy shows very faint features that resemble the beginning of crossed tidal tails or gaseous filaments powered by the mechanical action of the young stellar population. In this sense, Tol 35 could be interpreted either as an object in an intermediate-stage merging process between two gas-rich dwarf galaxies or as an object suffering the effect of a galactic wind.

The Star-forming Main Sequence of Dwarf Low Surface Brightness Galaxies

NASA Astrophysics Data System (ADS)

McGaugh, Stacy S.; Schombert, James M.; Lelli, Federico

2017-12-01

We explore the star-forming properties of late-type, low surface brightness (LSB) galaxies. The star-forming main sequence ({SFR}-{M}* ) of LSB dwarfs has a steep slope, indistinguishable from unity (1.04 ± 0.06). They form a distinct sequence from more massive spirals, which exhibit a shallower slope. The break occurs around {M}* ≈ {10}10 {M}⊙ , and can also be seen in the gas mass—stellar mass plane. The global Kennicutt-Schmidt law ({SFR}-{M}g) has a slope of 1.47 ± 0.11 without the break seen in the main sequence. There is an ample supply of gas in LSB galaxies, which have gas depletion times well in excess of a Hubble time, and often tens of Hubble times. Only ˜ 3 % of this cold gas needs be in the form of molecular gas to sustain the observed star formation. In analogy with the faint, long-lived stars of the lower stellar main sequence, it may be appropriate to consider the main sequence of star-forming galaxies to be defined by thriving dwarfs (with {M}* < {10}10 {M}⊙ ), while massive spirals (with {M}* > {10}10 {M}⊙ ) are weary giants that constitute more of a turn-off population.

Establishing a Network of faint DA white dwarfs as Spectrophotometric Standards

NASA Astrophysics Data System (ADS)

Saha, Abhijit; Narayan, Gautham; Holberg, Jay; Matheson, Thomas; Olszewski, Edward; Stubbs, Christopher; Bohlin, Ralph; Sabbi, Elena; Deustua, Susana; Rest, Armin; Axelrod, Tim; MacKenty, John W.; Camarota, Larry; Gilliland, Ron

2015-08-01

Systematic uncertainties in photometric calibration are the dominant source of error in current type Ia supernova dark energy studies, as well as other forefront cosmology efforts, e.g. photo-redshift determinations for weak lensing mass tomography. Current and next-generation ground-based all-sky surveys require a network of calibration stars with 1) known SEDs (to properly and unambiguously take into account filter differences), and 2) that are on a common photometric zeropoint scale across the sky to sub-percent accuracy. We are using a combination of HST panchromatic photometry and ground based spectroscopy to establish such an essential network of faint primary photometric standards, exploiting the well-understood spectral energy distributions of DA white dwarf stars that are free from the complications of observing through the Earth's time-variable atmosphere. The Balmer features in the spectra are used to deduce the two parameters (temperature and log(g)) from which we model the spectral energy distribution (SED) from these stars which have pure hydrogen atmospheres. By comparing against panchromatic broadband HST photometry, and allowing for an achromatic zero-point adjustment and mild scaling of the interstellar reddening, we find that model prediction and observation agree to a few milli-mag. By combining the zero-point and reddening adjustments with the modeled SED, for each star we obtain the incident SED above the terrestrial atmosphere, thus establishing these objects as spectrophotometric standards. We are pursuing 23 objects between 16 and 19 mag spread over the sky uniformly around the equator and northern mid-latitudes, with plans to extend this to southern latitudes. This precision photometric heritage from HST will benefit essentially all existing and upcoming survey projects, and in prticular, directly addresses one of the current barriers to understanding the nature of dark energy.

Hubble Sees a “Behemoth” Bleeding Atmosphere Around a Warm Exoplanet

NASA Image and Video Library

2015-06-24

Astronomers using NASA’s Hubble Space Telescope have discovered an immense cloud of hydrogen dubbed “The Behemoth” bleeding from a planet orbiting a nearby star. The enormous, comet-like feature is about 50 times the size of the parent star. The hydrogen is evaporating from a warm, Neptune-sized planet, due to extreme radiation from the star. This phenomenon has never been seen around an exoplanet so small. It may offer clues to how other planets with hydrogen-enveloped atmospheres could have their outer layers evaporated by their parent star, leaving behind solid, rocky cores. Hot, rocky planets such as these that roughly the size of Earth are known as Hot-Super Earths. “This cloud is very spectacular, though the evaporation rate does not threaten the planet right now,” explains the study’s leader, David Ehrenreich of the Observatory of the University of Geneva in Switzerland. “But we know that in the past, the star, which is a faint red dwarf, was more active. This means that the planet evaporated faster during its first billion years of existence because of the strong radiation from the young star. Overall, we estimate that it may have lost up to 10 percent of its atmosphere over the past several billion years.” Caption: This artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. Also depicted is the parent star, which is a faint red dwarf named GJ 436. The hydrogen is evaporating from the planet due to extreme radiation from the star. A phenomenon this large has never before been seen around any exoplanet. Credits: NASA, ESA, and G. Bacon (STScI)

Is This Speck of Light an Exoplanet?

NASA Astrophysics Data System (ADS)

2004-09-01

VLT Images and Spectra of Intriguing Object near Young Brown Dwarf [1] Summary Is this newly discovered feeble point of light the long-sought bona-fide image of an exoplanet? A research paper by an international team of astronomers [2] provides sound arguments in favour, but the definitive answer is now awaiting further observations. On several occasions during the past years, astronomical images revealed faint objects, seen near much brighter stars. Some of these have been thought to be those of orbiting exoplanets, but after further study, none of them could stand up to the real test. Some turned out to be faint stellar companions, others were entirely unrelated background stars. This one may well be different. In April of this year, the team of European and American astronomers detected a faint and very red point of light very near (at 0.8 arcsec angular distance) a brown-dwarf object, designated 2MASSWJ1207334-393254. Also known as "2M1207", this is a "failed star", i.e. a body too small for major nuclear fusion processes to have ignited in its interior and now producing energy by contraction. It is a member of the TW Hydrae stellar association located at a distance of about 230 light-years. The discovery was made with the adaptive-optics supported NACO facility [3] at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory (Chile). The feeble object is more than 100 times fainter than 2M1207 and its near-infrared spectrum was obtained with great efforts in June 2004 by NACO, at the technical limit of the powerful facility. This spectrum shows the signatures of water molecules and confirms that the object must be comparatively small and light. None of the available observations contradict that it may be an exoplanet in orbit around 2M1207. Taking into account the infrared colours and the spectral data, evolutionary model calculations point to a 5 jupiter-mass planet in orbit around 2M1207. Still, they do not yet allow a clear-cut decision about the real nature of this intriguing object. Thus, the astronomers refer to it as a "Giant Planet Candidate Companion (GPCC)" [4]. Observations will now be made to ascertain whether the motion in the sky of GPCC is compatible with that of a planet orbiting 2M1207. This should become evident within 1-2 years at the most. PR Photo 26a/04: NACO image of the brown dwarf object 2M1207 and GPCC PR Photo 26b/04: Near-infrared spectrum of the brown dwarf object 2M1207 and GPCC PR Photo 26c/04: Comparison between the possible 2M1207 system and the solar system Just a speck of light ESO PR Photo 26a/04 ESO PR Photo 26a/04 The Brown Dwarf Object 2M1207 and GPCC [Preview - JPEG: 400 x 471 pix - 65k] [Normal - JPEG: 800 x 942 pix - 158k] Caption: ESO PR Photo 26a/04 is a composite image of the brown dwarf object 2M1207 (centre) and the fainter object seen near it, at an angular distance of 778 milliarcsec. Designated "Giant Planet Candidate Companion" by the discoverers, it may represent the first image of an exoplanet. Further observations, in particular of its motion in the sky relative to 2M1207 are needed to ascertain its true nature. The photo is based on three near-infrared exposures (in the H, K and L' wavebands) with the NACO adaptive-optics facility at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory. Since 1998, a team of European and American astronomers [2] is studying the environment of young, nearby "stellar associations", i.e., large conglomerates of mostly young stars and the dust and gas clouds from which they were recently formed. The stars in these associations are ideal targets for the direct imaging of sub-stellar companions (planets or brown dwarf objects). The leader of the team, ESO astronomer Gael Chauvin notes that "whatever their nature, sub-stellar objects are much hotter and brighter when young - tens of millions of years - and therefore can be more easily detected than older objects of similar mass". The team especially focused on the study of the TW Hydrae Association. It is located in the direction of the constellation Hydra (The Water-Snake) deep down in the southern sky, at a distance of about 230 light-years. For this, they used the NACO facility [3] at the 8.2-m VLT Yepun telescope, one of the four giant telescopes at the ESO Paranal Observatory in northern Chile. The instrument's adaptive optics (AO) overcome the distortion induced by atmospheric turbulence, producing extremely sharp near-infrared images. The infrared wavefront sensor was an essential component of the AO system for the success of these observations. This unique instrument senses the deformation of the near-infrared image, i.e. in a wavelength region where objects like 2M1207 (see below) are much brighter than in the visible range. The TW Hydrae Association contains a star with an orbiting brown dwarf companion, approximately 20 times the mass of Jupiter, and four stars surrounded by dusty proto-planetary disks. Brown dwarf objects are "failed stars", i.e. bodies too small for nuclear processes to have ignited in their interior and now producing energy by contraction. They emit almost no visible light. Like the Sun and the giant planets in the solar system, they are composed mainly of hydrogen gas, perhaps with swirling cloud belts. On a series of exposures made through different optical filters, the astronomers discovered a tiny red speck of light, only 0.8 arcsec from the TW Hydrae Association brown-dwarf object 2MASSWJ1207334-393254, or just "2M1207", cf. PR Photo 26a/04. The feeble image is more than 100 times fainter than that of 2M1207. "If these images had been obtained without adaptive optics, that object would not have been seen," says Gael Chauvin. Christophe Dumas, another member of the team, is enthusiastic: "The thrill of seeing this faint source of light in real-time on the instrument display was unbelievable. Although it is surely much bigger than a terrestrial-size object, it is a strange feeling that it may indeed be the first planetary system beyond our own ever imaged." Exoplanet or Brown Dwarf? ESO PR Photo 26b/04 ESO PR Photo 26b/04 The Brown Dwarf Object 2M1207 and GPCC [Preview - JPEG: 400 x 486 pix - 102k] [Normal - JPEG: 800 x 912 pix - 234k] Caption: ESO PR Photo 26b/04 shows near-infrared H-band spectra of the brown dwarf object 2M1207 and the fainter "GPCC" object seen near it, obtained with the NACO facility at the 8.2-m VLT Yepun telescope. In the upper part, the spectrum of 2M1207 (fully drawn blue curve) is compared with that of another substellar object (T513; dashed line); in the lower, the (somewhat noisy) spectrum of GPCC (fully drawn red curve) is compared with two substellar objects of different types (2M0301 and SDSS0539). The spectrum of GPCC is clearly very similar to these, confirming the substellar nature of this body. The broad dips at the left and the right are clear signatures of water in the (atmospheres of the) objects. What is the nature of this faint object [4]? Could it be an exoplanet in orbit around that young brown dwarf object at a projected distance of about 8,250 million km (about twice the distance between the Sun and Neptune)? "If the candidate companion of 2M1207 is really a planet, this would be the first time that a gravitationally bound exoplanet has been imaged around a star or a brown dwarf" says Benjamin Zuckerman of UCLA, a member of the team and also of NASA's Astrobiology Institute. Using high-angular-resolution spectroscopy with the NACO facility, the team has confirmed the substellar status of this object - now referred to as the "Giant Planet Candidate Companion (GPCC)" - by identifying broad water-band absorptions in its atmosphere, cf. PR Photo 26b/04. The spectrum of a young and hot planet - as the GPCC may well be - will have strong similarities with an older and more massive object such as a brown dwarf. However, when it cools down after a few tens of millions of years, such an object will show the spectral signatures of a giant gaseous planet like those in our own solar system. Although the spectrum of GPCC is quite "noisy" because of its faintness, the team was able to assign to it a spectral characterization that excludes a possible contamination by extra-galactic objects or late-type cool stars with abnormal infrared excess, located beyond the brown dwarf. After a very careful study of all options, the team found that, although this is statistically very improbable, the possibility that this object could be an older and more massive, foreground or background, cool brown dwarf cannot be completely excluded. The related detailed analysis is available in the resulting research paper that has been accepted for publication in the European journal Astronomy & Astrophysics (see below). Implications The brown dwarf 2M1207 has approximately 25 times the mass of Jupiter and is thus about 42 times lighter than the Sun. As a member of the TW Hydrae Association, it is about eight million years old. Because our solar system is 4,600 million years old, there is no way to directly measure how the Earth and other planets formed during the first tens of millions of years following the formation of the Sun. But, if astronomers can study the vicinity of young stars which are now only tens of millions of years old, then by witnessing a variety of planetary systems that are now forming, they will be able to understand much more accurately our own distant origins. Anne-Marie Lagrange, a member of the team from the Grenoble Observatory (France), looks towards the future: "Our discovery represents a first step towards opening a whole new field in astrophysics: the imaging and spectroscopic study of planetary systems. Such studies will enable astronomers to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets." Follow-up observations ESO PR Photo 26c/04 ESO PR Photo 26c/04 Comparison between the solar and 2M1207 systems [Preview - JPEG: 400 x 190 pix - 38k] [Normal - JPEG: 800 x 397 pix - 86k] [HiRes - JPEG: 2000 x 948 pix - 326k] Caption: ESO PR Photo 26c/04 shows for illustration a comparison between the solar system and the brown dwarf object 2M1207 system with its possible planet at 55 AU distance. The sizes of the objects are drawn to the same scale, but the distances have been strongly compressed. Taking into account the infrared colours and the spectral data available for GPCC, evolutionary model calculations point to a 5 jupiter-mass planet, about 55 times more distant from 2M1207 than the Earth is from the Sun (55 AU). The surface temperature appears to be about 10 times hotter than Jupiter, about 1000 °C; this is easily explained by the amount of energy that must be liberated during the current rate of contraction of this young object (indeed, the much older giant planet Jupiter is still producing energy in its interior). The astronomers will now continue their research to confirm or deny whether they have in fact discovered an exoplanet. Over the next few years, they expect to establish beyond doubt whether the object is indeed a planet in orbit around the brown dwarf 2M1207 by watching how the two objects move through space and to learn whether or not they move together. They will also measure the brightness of the GPCC at multiple wavelengths and more spectral observations may be attempted. There is no doubt that future programmes to image exoplanets around nearby stars, either from the ground with extremely large telescopes equipped with specially designed adaptive optics, or from space with special planet-finder telescopes, will greatly profit from current technological achievements. More information The results presented in this ESO Press Release are based on a research paper ("A Giant Planet Candidate near a Young Brown Dwarf" by G. Chauvin et al.) that has been accepted for publication and will appear in the leading research journal "Astronomy and Astrophysics" on September 23, 2004 (Vol. 425, Issue 2, page L29). A preprint is available here and also as astro-ph0409323. Notes [1]: This press release is issued simultaneously by ESO and CNRS (in French). [2]: The team consists of Gael Chauvin and Christophe Dumas (ESO-Chile), Anne-Marie Lagrange and Jean-Luc Beuzit (LAOG, Grenoble, France), Benjamin Zuckerman and Inseok Song (UCLA, Los Angeles, USA), David Mouillet (LAOMP, Tarbes, France) and Patrick Lowrance (IPAC, Pasadena, USA). The American members of the team acknowledge funding in part by NASA's Astrobiology Institute. [3]: The NACO facility (from NAOS/Nasmyth Adaptive Optics System and CONICA/Near-Infrared Imager and Spectrograph) at the 8.2-m VLT Yepun telescope on Paranal offers the capability to produce diffraction-limited near-infrared images of astronomical objects. It senses the radiation in this wavelength region with the N90C10 dichroic; 90 percent of the flux is transmitted to the wavefront sensor and 10 percent to the near-infrared camera CONICA. This mode is particularly useful for sharp imaging of red and very-low-mass stellar or substellar objects. The adaptive optics corrector (NAOS) was built, under an ESO contract, by Office National d'Etudes et de Recherches Aérospatiales (ONERA), Laboratoire d'Astrophysique de Grenoble (LAOG) and the LESIA and GEPI laboratories of the Observatoire de Paris in France, in collaboration with ESO. The CONICA camera was built, under an ESO contract, by the Max-Planck-Institut für Astronomie (MPIA) (Heidelberg) and the Max-Planck Institut für extraterrestrische Physik (MPE) (Garching) in Germany, in collaboration with ESO. [4]: What is the difference between a small brown dwarf and an exoplanet ? The border line between the two is still being investigated but it appears that a brown dwarf object is formed in the same way as stars, i.e. by contraction in an interstellar cloud while planets are formed within stable circumstellar disks via collision/accretion of planetesimals or disk instabilities. This implies that brown dwarfs are formed faster (less than 1 million years) than planets (~10 million years). Another way of separating the two kinds of objects is by mass (as this is also done between brown dwarfs and stars): (giant) planets are lighter than about 13 jupiter-masses (the critical mass needed to ignite deuterium fusion), brown dwarfs are heavier. Unfortunately, the first definition cannot be used in practice, e.g., when detecting a faint companion as in the present case, since the observations do not provide information about the way the object was formed. On the contrary, the above mass criterion is useful in the sense that spectroscopy and astrometry of a faint object, together with the appropriate evolutionary models, may reveal the mass and hence the nature of the object.

Weighing the Smallest Stars

NASA Astrophysics Data System (ADS)

2005-01-01

VLT Finds Young, Very Low Mass Objects Are Twice As Heavy As Predicted Summary Thanks to the powerful new high-contrast camera installed at the Very Large Telescope, photos have been obtained of a low-mass companion very close to a star. This has allowed astronomers to measure directly the mass of a young, very low mass object for the first time. The object, more than 100 times fainter than its host star, is still 93 times as massive as Jupiter. And it appears to be almost twice as heavy as theory predicts it to be. This discovery therefore suggests that, due to errors in the models, astronomers may have overestimated the number of young "brown dwarfs" and "free floating" extrasolar planets. PR Photo 03/05: Near-infrared image of AB Doradus A and its companion (NACO SDI/VLT) A winning combination A star can be characterised by many parameters. But one is of uttermost importance: its mass. It is the mass of a star that will decide its fate. It is thus no surprise that astronomers are keen to obtain a precise measure of this parameter. This is however not an easy task, especially for the least massive ones, those at the border between stars and brown dwarf objects. Brown dwarfs, or "failed stars", are objects which are up to 75 times more massive than Jupiter, too small for major nuclear fusion processes to have ignited in its interior. To determine the mass of a star, astronomers generally look at the motion of stars in a binary system. And then apply the same method that allows determining the mass of the Earth, knowing the distance of the Moon and the time it takes for its satellite to complete one full orbit (the so-called "Kepler's Third Law"). In the same way, they have also measured the mass of the Sun by knowing the Earth-Sun distance and the time - one year - it takes our planet to make a tour around the Sun. The problem with low-mass objects is that they are very faint and will often be hidden in the glare of the brighter star they orbit, also when viewed in large telescopes. Astronomers have however found ways to overcome this difficulty. For this, they rely on a combination of a well-considered observational strategy with state-of-the-art instruments. High contrast camera First, astronomers searching for very low mass objects look at young nearby stars because low-mass companion objects will be brightest while they are young, before they contract and cool off. In this particular case, an international team of astronomers [1] led by Laird Close (Steward Observatory, University of Arizona), studied the star AB Doradus A (AB Dor A). This star is located about 48 light-years away and is "only" 50 million years old. Because the position in the sky of AB Dor A "wobbles", due to the gravitational pull of a star-like object, it was believed since the early 1990s that AB Dor A must have a low-mass companion. To photograph this companion and obtain a comprehensive set of data about it, Close and his colleagues used a novel instrument on the European Southern Observatory's Very Large Telescope. This new high-contrast adaptive optics camera, the NACO Simultaneous Differential Imager, or NACO SDI [2], was specifically developed by Laird Close and Rainer Lenzen (Max-Planck-Institute for Astronomy in Heidelberg, Germany) for hunting extrasolar planets. The SDI camera enhances the ability of the VLT and its adaptive optics system to detect faint companions that would normally be lost in the glare of the primary star. A world premiere ESO PR Photo 03/05 ESO PR Photo 03/05 Infrared image of AB Doradus A and its companion [Preview - JPEG: 400 x 406 pix - 99k] [Normal - JPEG: 800 x 812 pix - 235k] Caption: ESO PR Photo 03/05 is an enhanced, false-colour near-infrared image of AB Dor A and C. The faint companion "AB Dor C" - seen as the pink dot at 8 o'clock - is 120 times fainter than its primary star. The tiny separation between A and C, only 0.156 arcsec, is smaller than a one Euro coin seen at 20 km distance. Nevertheless, the new NACO SDI camera was able to distinguish it as a "redder" dot surrounded by the "bluer" light from AB Dor A. The orbit of AB Dor C around AB Dor A is shown as a yellow ellipse. It takes 11.75 years for the 93 Jupiter-mass companion to complete this orbit. Turning this camera towards AB Dor A in February 2004, they were able for the first time to image a companion so faint - 120 times fainter than its star - and so near its star. Says Markus Hartung (ESO), member of the team: "This world premiere was only possible because of the unique capabilities of the NACO SDI instrument on the VLT. In fact, the Hubble Space Telescope tried but failed to detect the companion, as it was too faint and too close to the glare of the primary star." The tiny distance between the star and the faint companion (0.156 arcsec) is the same as the width of a one Euro coin (2.3 cm) when seen 20 km away. The companion, called AB Dor C, was seen at a distance of 2.3 times the mean distance between the Earth and the Sun. It completes a cycle around its host star in 11.75 years on a rather eccentric orbit. Using the companion's exact location, along with the star's known 'wobble', the astronomers could then accurately determine the companion's mass. The object, more than 100 times fainter than its close primary star, has one tenth of the mass of its host star, i.e., it is 93 times more massive than Jupiter. It is thus slightly above the brown dwarf limit. Using NACO on the VLT, the astronomers further observed AB Dor C at near infrared wavelengths to measure its temperature and luminosity. "We were surprised to find that the companion was 400 degrees (Celsius) cooler and 2.5 times fainter than the most recent models predict for an object of this mass," Close said. "Theory predicts that this low-mass, cool object would be about 50 Jupiter masses. But theory is incorrect: this object is indeed between 88 to 98 Jupiter masses." These new findings therefore challenge current ideas about the brown dwarf population and the possible existence of widely publicized "free-floating" extrasolar planets. Indeed, if young objects hitherto identified as brown dwarfs are twice as massive as was thought, many must rather be low-mass stars. And objects recently identified as "free-floating" planets are in turn likely to be low-mass brown dwarfs. For Close and his colleagues, "this discovery will force astronomers to rethink what masses of the smallest objects produced in nature really are."

1H 1752 + 081: An eclipsing cataclysmic variable with a small accretion disk

NASA Technical Reports Server (NTRS)

Silber, Andrew D.; Remillard, Ronald A.; Horne, Keith; Bradt, Hale V.

1994-01-01

We announce the discovery of an eclipsing nova-like cataclysmic variable (CV) as the optical counterpart to the HEAO 1 X-ray source 1H1752 + 081. This CV has an orbital period of 1.882801 hr, a high equivalent width of H-beta, and an average m(sub v) of 16.4 out of the eclipse. A geometric model is constructed from observations of the eclipse ingress and egress in many optical bandpasses. The broad-band emission originates primarily in two regions; the disk/accretion stream 'hot spot' and a compact central component, which may be a spot on the white dwarf surface, the entire white dwarf surface or the boundary layer between the accretion disk and the white dwarf surface. Based on the durations and offsets of the two eclipses we determined the mass ratio q = 2.5 +/- 0.6 and the angle of inclination i = 77 deg +/- 2 deg. If the central component is the entire white dwarf surface the masses of the stars are M(sub 1) = 0.80 +/- 0.06 solar masses and M(sub 2) = 0.32 +/- 0.06 solar masses. The disk is faint and small (R(sub D) = 0.25 +/- 0.05 r(sub L1), where r(sub L1) is the distance from the primary to the L(sub 1) point), compared to other eclipsing CVs. The small disk may result from the removal of angular momentum from the accretion disk by the magnetic field of the white dwarf; this CV may be a DQ Her type with a slowly rotating white dwarf. The emission-line velocities do not show the 'Z-wave' expected from the eclipse of a Keplerian accretion disk, nor do they have the correct phasing to originate near the white dwarf. The most likely origin of the line emission is the hot spot. The secondary star is visible at wavelengths greater than or equal to 6000 A during eclipse. We estimate a spectral type approximately M6 which, together with the observed m(sub 1) = 16.94 during eclipse, results in a distance estimate of 150 +/- 27 pc.

K2-137 b: an Earth-sized planet in a 4.3-h orbit around an M-dwarf

NASA Astrophysics Data System (ADS)

Smith, A. M. S.; Cabrera, J.; Csizmadia, Sz; Dai, F.; Gandolfi, D.; Hirano, T.; Winn, J. N.; Albrecht, S.; Alonso, R.; Antoniciello, G.; Barragán, O.; Deeg, H.; Eigmüller, Ph; Endl, M.; Erikson, A.; Fridlund, M.; Fukui, A.; Grziwa, S.; Guenther, E. W.; Hatzes, A. P.; Hidalgo, D.; Howard, A. W.; Isaacson, H.; Korth, J.; Kuzuhara, M.; Livingston, J.; Narita, N.; Nespral, D.; Nowak, G.; Palle, E.; Pätzold, M.; Persson, C. M.; Petigura, E.; Prieto-Arranz, J.; Rauer, H.; Ribas, I.; Van Eylen, V.

2018-03-01

We report the discovery in K2's Campaign 10 of a transiting terrestrial planet in an ultra-short-period orbit around an M3-dwarf. K2-137 b completes an orbit in only 4.3 h, the second shortest orbital period of any known planet, just 4 min longer than that of KOI 1843.03, which also orbits an M-dwarf. Using a combination of archival images, adaptive optics imaging, radial velocity measurements, and light-curve modelling, we show that no plausible eclipsing binary scenario can explain the K2 light curve, and thus confirm the planetary nature of the system. The planet, whose radius we determine to be 0.89 ± 0.09 R⊕, and which must have an iron mass fraction greater than 0.45, orbits a star of mass 0.463 ± 0.052 M⊙ and radius 0.442 ± 0.044 R⊙.

POX 186: the ultracompact blue compact dwarf galaxy reveals its nature

NASA Astrophysics Data System (ADS)

Doublier, V.; Kunth, D.; Courbin, F.; Magain, P.

2000-01-01

High resolution, ground based R and I band observations of the ultra compact dwarf galaxy POX 186 are presented. The data, obtained with the ESO New Technology Telescope (NTT), are analyzed using a new deconvolution algorithm which allows one to resolve the innermost regions of this stellar-like object into three Super-Star Clusters (SSC). Upper limits to both masses (M ~ 105 Msun) and physical sizes (<=60pc) of the SSCs are set. In addition, and maybe most importantly, extended light emission underlying the compact star-forming region is clearly detected in both bands. The R-I color rules out nebular Hα contamination and is consistent with an old stellar population. This casts doubt on the hypothesis that Blue Compact Dwarf Galaxies (BCDG) are young galaxies. based on observations carried out at NTT in La Silla, operated by the European Southern Observatory, during Director's Discretionary Time.

The Ursa Major cluster of galaxies - III. Optical observations of dwarf galaxies and the luminosity function down to MR=-11

NASA Astrophysics Data System (ADS)

Trentham, Neil; Tully, R. Brent; Verheijen, Marc A. W.

2001-07-01

Results are presented of a deep optical survey of the Ursa Major cluster, a spiral-rich cluster of galaxies at a distance of 18.6Mpc which contains about 30 per cent of the light but only 5 per cent of the mass of the nearby Virgo cluster. Fields around known cluster members and a pattern of blind fields along the major and minor axes of the cluster were studied with mosaic CCD cameras on the Canada-France-Hawaii Telescope. The dynamical crossing time for the Ursa Major cluster is only slightly less than a Hubble time. Most galaxies in the local Universe exist in similar moderate-density environments. The Ursa Major cluster is therefore a good place to study the statistical properties of dwarf galaxies, since this structure is at an evolutionary stage representative of typical environments, yet has enough galaxies that reasonable counting statistics can be accumulated. The main observational results of our survey are as follows. (i) The galaxy luminosity function is flat, with a logarithmic slope α=-1.1 for -17

A Spectroscopic Search for White Dwarf Companions to 101 Nearby M Dwarfs

NASA Astrophysics Data System (ADS)

Bar, Ira; Vreeswijk, Paul; Gal-Yam, Avishay; Ofek, Eran O.; Nelemans, Gijs

2017-11-01

Recent studies of the stellar population in the solar neighborhood (<20 pc) suggest that there are undetected white dwarfs (WDs) in multiple systems with main-sequence companions. Detecting these hidden stars and obtaining a more complete census of nearby WDs is important for our understanding of stellar and galactic evolution, as well as the study of explosive phenomena. In an attempt to uncover these hidden WDs, we present intermediate resolution spectroscopy over the wavelength range of 3000-25000 Å of 101 nearby M dwarfs (dMs), observed with the Very Large Telescope X-Shooter spectrograph. For each star we search for a hot component superimposed on the dM spectrum. X-Shooter has excellent blue sensitivity and thus can reveal a faint hot WD despite the brightness of its red companion. Visual examination shows no clear evidence of a WD in any of the spectra. We place upper limits on the effective temperatures of WDs that may still be hiding by fitting dM templates to the spectra and modeling the WD spectra. On average our survey is sensitive to WDs hotter than about 5300 K. This suggests that the frequency of WD companions of {T}{eff}≳ 5300 {{K}} with separation of the order of ≲50 au among the local dM population is <3% at the 95% confidence level. The reduced spectra are made available via the WISeREP3 repository. Based on observations collected in service mode using the Very Large Telescope (VLT) under program IDs 095_D-0949(A) and 096_D-0963(A).

High-redshift galaxies and low-mass stars

NASA Astrophysics Data System (ADS)

Wilkins, Stephen M.; Stanway, Elizabeth R.; Bremer, Malcolm N.

2014-03-01

The sensitivity available to near-infrared surveys has recently allowed us to probe the galaxy population at z ≈ 7 and beyond. The existing Hubble Wide Field Camera 3 (WFC3) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond 1 μm - a capability that will be further extended with the launch and commissioning of the James Webb Space Telescope (JWST). As new regions of parameter space in both colour and depth are probed, new challenges for distant galaxy surveys are identified. In this paper, we present an analysis of the colours of L- and T-dwarf stars in widely used photometric systems. We also consider the implications of the newly identified Y-dwarf population - stars that are still cooler and less massive than T-dwarfs for both the photometric selection and spectroscopic follow-up of faint and distant galaxies. We highlight the dangers of working in the low-signal-to-noise regime, and the potential contamination of existing and future samples. We find that Hubble/WFC3 and VISTA/VIRCam Y-drop selections targeting galaxies at z ˜ 7.5 are vulnerable to contamination from T- and Y-class stars. Future observations using JWST, targeting the z ˜ 7 galaxy population, are also likely to prove difficult without deep medium-band observations. We demonstrate that single emission line detections in typical low-signal-to-noise spectroscopic observations may also be suspect, due to the unusual spectral characteristics of the cool dwarf star population.

Spectral calibration in the mid-infrared: Challenges and solutions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sloan, G. C.; Herter, T. L.; Houck, J. R.

2015-01-01

We present spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope of 33 K giants and 20 A dwarfs to assess their suitability as spectrophotometric standard stars. The K giants confirm previous findings that the strength of the SiO absorption band at 8 μm increases for both later optical spectral classes and redder (B–V){sub 0} colors, but with considerable scatter. For K giants, the synthetic spectra underpredict the strengths of the molecular bands from SiO and OH. For these reasons, the assumed true spectra for K giants should be based on the assumption that molecular band strengths inmore » the infrared can be predicted accurately from neither optical spectral class or color nor synthetric spectra. The OH bands in K giants grow stronger with cooler stellar temperatures, and they are stronger than predicted by synthetic spectra. As a group, A dwarfs are better behaved and more predictable than the K giants, but they are more likely to show red excesses from debris disks. No suitable A dwarfs were located in parts of the sky continuously observable from Spitzer, and with previous means of estimating the true spectra of K giants ruled out, it was necessary to use models of A dwarfs to calibrate spectra of K giants from observed spectral ratios of the two groups and then use the calibrated K giants as standards for the full database of infrared spectra from Spitzer. We also describe a lingering artifact that affects the spectra of faint blue sources at 24 μm.« less

Adaptive Optics Observations of Exoplanets, Brown Dwarfs, and Binary Stars

NASA Astrophysics Data System (ADS)

Hinkley, Sasha

2012-04-01

The current direct observations of brown dwarfs and exoplanets have been obtained using instruments not specifically designed for overcoming the large contrast ratio between the host star and any wide-separation faint companions. However, we are about to witness the birth of several new dedicated observing platforms specifically geared towards high contrast imaging of these objects. The Gemini Planet Imager, VLT-SPHERE, Subaru HiCIAO, and Project 1640 at the Palomar 5m telescope will return images of numerous exoplanets and brown dwarfs over hundreds of observing nights in the next five years. Along with diffraction-limited coronagraphs and high-order adaptive optics, these instruments also will return spectral and polarimetric information on any discovered targets, giving clues to their atmospheric compositions and characteristics. Such spectral characterization will be key to forming a detailed theory of comparative exoplanetary science which will be widely applicable to both exoplanets and brown dwarfs. Further, the prevalence of aperture masking interferometry in the field of high contrast imaging is also allowing observers to sense massive, young planets at solar system scales (~3-30 AU)- separations out of reach to conventional direct imaging techniques. Such observations can provide snapshots at the earliest phases of planet formation-information essential for constraining formation mechanisms as well as evolutionary models of planetary mass companions. As a demonstration of the power of this technique, I briefly review recent aperture masking observations of the HR 8799 system. Moreover, all of the aforementioned techniques are already extremely adept at detecting low-mass stellar companions to their target stars, and I present some recent highlights.

Period changes of the long-period cataclysmic binary EX Draconis

NASA Astrophysics Data System (ADS)

Pilarčík, L.; Wolf, M.; Dubovský, P. A.; Hornoch, K.; Kotková, L.

2012-03-01

The cataclysmic variable star EX Dra is a relatively faint but frequently investigated eclipsing dwarf nova. In total 35 new eclipses were measured photometrically as part of our long-term monitoring of interesting eclipsing systems. Using published and new mid-eclipse times obtained between 2004 and 2011 we constructed the observed-minus-calculated diagram. The current data present 21 years of period modulation with a semi-amplitude of 2.5 min. The eclipse timings show significant deviations from the best sinusoidal fit, which indicates that this ephemeris is not a complete description of the data. The fractional period change is roughly ΔP/P = 3 × 10-6.

RS Ophiuchi in Quiescence: Why Is It X-ray Faint?

NASA Technical Reports Server (NTRS)

Mukai, Koji

2007-01-01

The short interval between successive outbursts of RS Oph strongly suggests that it has a high mass white dwarf accreting at a high rate. This, in turn, suggests the possibility of prominent X-ray emission from RS Oph in quiescence. However, archival quiescent X-ray observations of RS Oph show it to be a modest soft X-ray source but not a strong 2-10 keV X-ray source. In this aspect, RS Oph differs markedly from T CrB. We speculate on the possible mechanisms that could significantly suppress the 2-10 keV X-ray emission in RS Oph.

Searching for Faint Companions to Nearby Stars with the Hubble Space Telescope

NASA Technical Reports Server (NTRS)

Schroeder, Daniel J.; Golimowski, David A.

1996-01-01

A search for faint companions (FC's) to selected stars within 5 pc of the Sun using the Hubble Space Telescope's Planetary Camera (PC) has been initiated. To assess the PC's ability to detect FCs, we have constructed both model and laboratory-simulated images and compared them to actual PC images. We find that the PC's point-spread function (PSF) is 3-4 times brighter over the angular range 2-5 sec than the PSF expected for a perfect optical system. Azimuthal variations of the PC's PSF are 10-20 times larger than expected for a perfect PSF. These variations suggest that light is scattered nonuniformly from the surface of the detector. Because the anomalies in the PC's PSF cannot be precisely simulated, subtracting a reference PSF from the PC image is problematic. We have developed a computer algorithm that identifies local brightness anomalies within the PSF as potential FCs. We find that this search algorithm will successfully locate FCs anywhere within the circumstellar field provided that the average pixel signal from the FC is at least 10 sigma above the local background. This detection limit suggests that a comprehensive search for extrasolar Jovian planets with the PC is impractical. However, the PC is useful for detecting other types of substellar objects. With a stellar signal of 10(exp 9) e(-), for example, we may detect brown dwarfs as faint as M(sub I) = 16.7 separated by 1 sec from alpha Cen A.

THE NEXT GENERATION VIRGO CLUSTER SURVEY. VI. THE KINEMATICS OF ULTRA-COMPACT DWARFS AND GLOBULAR CLUSTERS IN M87

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Hong-Xin; Peng, Eric W.; Li, Biao

2015-03-20

The origin of ultra-compact dwarfs (UCDs; r{sub h} ≳ 10 pc)—objects larger and more massive than typical globular clusters (GCs), but more compact than typical dwarf galaxies—has been hotly debated in the 15 years since their discovery. Even whether UCDs should be considered galactic in origin, or simply the most extreme star clusters, is not yet settled. We present the dynamical properties of 97 spectroscopically confirmed UCDs and 911 GCs associated with the central cD galaxy of the Virgo cluster, M87. Our UCDs, of which 89% have M {sub *} ≳ 2× 10{sup 6} M {sub ☉} and 92% are as blue asmore » the classic blue GCs, nearly triple the confirmed sample of Virgo UCDs, providing by far the best opportunity for studying global dynamics of a UCD system. We found that (1) UCDs have a surface number density profile that is shallower than that of blue GCs in the inner ∼70 kpc and as steep as that of red GCs at larger radii; (2) UCDs exhibit a significantly stronger rotation than GCs, and blue GCs seem to have a velocity field that is more consistent with that of the surrounding dwarf ellipticals than with that of UCDs; (3) UCDs have an orbital anisotropy profile that is tangentially biased at radii ≲40 kpc and radially biased farther out, whereas blue GCs become more tangentially biased at larger radii beyond ∼40 kpc; (4) GCs with M {sub *} ≳ 2 × 10{sup 6} M {sub ☉} have rotational properties indistinguishable from the less massive ones, suggesting that it is the size, instead of mass, that differentiates UCDs from GCs as kinematically distinct populations. We conclude that most UCDs in M87 are not consistent with being merely the most luminous and extended examples of otherwise normal GCs. The radially biased orbital structure of UCDs at large radii is in general agreement with the 'tidally threshed dwarf galaxy' scenario.« less

Lyman Break Galaxies in the Hubble Ultra Deep Field through Deep U-Band Imaging

NASA Astrophysics Data System (ADS)

Rafelski, Marc; Wolfe, A. M.; Cooke, J.; Chen, H. W.; Armandroff, T. E.; Wirth, G. D.

2009-12-01

We introduce an extremely deep U-band image taken of the Hubble Ultra Deep Field (HUDF), with a one sigma depth of 30.7 mag arcsec-2 and a detection limiting magnitude of 28 mag arcsec-2. The observations were carried out on the Keck I telescope using the LRIS-B detector. The U-band image substantially improves the accuracy of photometric redshift measurements of faint galaxies in the HUDF at z=[2.5,3.5]. The U-band for these galaxies is attenuated by lyman limit absorption, allowing for more reliable selections of candidate Lyman Break Galaxies (LBGs) than from photometric redshifts without U-band. We present a reliable sample of 300 LBGs at z=[2.5,3.5] in the HUDF. Accurate redshifts of faint galaxies at z=[2.5,3.5] are needed to obtain empirical constraints on the star formation efficiency of neutral gas at high redshift. Wolfe & Chen (2006) showed that the star formation rate (SFR) density in damped Ly-alpha absorption systems (DLAs) at z=[2.5,3.5] is significantly lower than predicted by the Kennicutt-Schmidt law for nearby galaxies. One caveat to this result that we wish to test is whether LBGs are embedded in DLAs. If in-situ star formation is occurring in DLAs, we would see it as extended low surface brightness emission around LBGs. We shall use the more accurate photometric redshifts to create a sample of LBGs around which we will look for extended emission in the more sensitive and higher resolution HUDF images. The absence of extended emission would put limits on the SFR density of DLAs associated with LBGs at high redshift. On the other hand, detection of faint emission on scales large compared to the bright LBG cores would indicate the presence of in situ star formation in those DLAs. Such gas would presumably fuel the higher star formation rates present in the LBG cores.

Extended Schmidt law holds for faint dwarf irregular galaxies

NASA Astrophysics Data System (ADS)

Roychowdhury, Sambit; Chengalur, Jayaram N.; Shi, Yong

2017-12-01

Context. The extended Schmidt law (ESL) is a variant of the Schmidt which relates the surface densities of gas and star formation, with the surface density of stellar mass added as an extra parameter. Although ESL has been shown to be valid for a wide range of galaxy properties, its validity in low-metallicity galaxies has not been comprehensively tested. This is important because metallicity affects the crucial atomic-to-molecular transition step in the process of conversion of gas to stars. Aims: We empirically investigate for the first time whether low metallicity faint dwarf irregular galaxies (dIrrs) from the local universe follow the ESL. Here we consider the "global" law where surface densities are averaged over the galactic discs. dIrrs are unique not only because they are at the lowest end of mass and star formation scales for galaxies, but also because they are metal-poor compared to the general population of galaxies. Methods: Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) which is the largest survey of atomic hydrogen in such galaxies. The gas surface densities are determined using their atomic hydrogen content. The star formation rates are calculated using GALEX far ultraviolet fluxes after correcting for dust extinction, whereas the stellar surface densities are calculated using Spitzer 3.6 μm fluxes. The surface densities are calculated over the stellar discs defined by the 3.6 μm images. Results: We find dIrrs indeed follow the ESL. The mean deviation of the FIGGS galaxies from the relation is 0.01 dex, with a scatter around the relation of less than half that seen in the original relation. In comparison, we also show that the FIGGS galaxies are much more deviant when compared to the "canonical" Kennicutt-Schmidt relation. Conclusions: Our results help strengthen the universality of the ESL, especially for galaxies with low metallicities. We suggest that models of star formation in which feedback from previous generations of stars set the pressure in the interstellar medium and affect ongoing star formation, are promising candidates for explaining the ESL. We also confirm that ESL is an independent relation and not a form of a relation between star formation efficiency and metallicity.

Leo P: A very low-mass, extremely metal-poor, star-forming galaxy

NASA Astrophysics Data System (ADS)

McQuinn, Kristen B.; Leo P Team

2017-01-01

Leo P is a low-luminosity dwarf galaxy just outside the Local Group with properties that make it an ideal probe of galaxy evolution at the faint-end of the luminosity function. Using combined data from 2 Hubble Space Telescope (HST) observing campaigns, the Very Large Array, the Spitzer Space telescope, as well as ground based data, we have constructed a robust evolutionary picture of Leo P. Leo P is one the most metal-poor, gas-rich galaxies ever discovered, has a stellar mass of a 5x105 Msun, comparable gas mass, and a single HII region. The star formation history reconstructed from the resolved stellar populations in Leo P shows it is unquenched, despite its very low mass. Based on the star formation history and metallicity measurements, the galaxy has lost 95% of its oxygen produced via nucleosynthesis, presumably to outflows. The neutral gas in the galaxy shows signs of rotation, although the velocity dispersion is comparable to the rotation velocity. Thus, Leo P bridges the gap between more massive dwarf irregular and less massive dwarf spheroidals on the baryonic Tully-Fisher relation. Furthermore, the galaxy hosts several, extremely dusty AGB candidates which will be probed with new HST and Spitzer observations. If confirmed as AGB stars, these may be our best local proxies for studying chemically unevolved star formation and subsequent dust production in metallicity environments comparable to the early universe.

Digging for substellar objects in the stellar graveyard

NASA Astrophysics Data System (ADS)

Debes, John H., IV

2005-11-01

White dwarfs, the endpoint of stellar evolution for stars with mass < 8 [Special characters omitted.] , possess several attributes favorable for studying planet and brown dwarf formation around stars with primordial masses 1 [Special characters omitted.] . This thesis explores the consequences of post-main-sequence evolution on the dynamics of a planetary system and the observational signatures that arise from such evolution. These signatures are then specifically tested with a direct imaging survey of nearby white dwarfs. Finally, new techniques for high contrast imaging are discussed and placed in the context of further searches for planets and brown dwarfs in the stellar graveyard. While planets closer than ~ 5 AU will most likely not survive the post-main sequence evolution of its parent star, any planet with semimajor axis > 5 AU will survive, and its semimajor axis will increase as the central star loses mass. The stability of adjacent orbits to mutual planet-planet perturbations depends on the ratio of the planet mass to the central star's mass, and I demonstrate that some planets in previously stable orbits around a star undergoing mass loss will become unstable. If pollution of a white dwarf's atmosphere is caused by relic planetary systems, any white dwarf with photospheric absorption due to metals can be searched for substellar companions. Hydrogen white dwarfs with metal absorption, so called DAZ white dwarfs, are hard to explain by simple ISM accretion, and present an opportunity to test the observational signatures of unstable planetary systems. Additionally, field white dwarfs can be searched for substellar companions as well. The search for planetary companions to stars requires further development of high contrast imaging techniques. This thesis studies Gaussian aperture pupil masks (GAPMs) which in theory can achieve the contrast requisite for directly imaging an extrasolar planet around a nearby solar type star. I outline the process of designing, fabricating, and testing a GAPM for use on current telescopes and specifically the Penn State near-IR Imager and Spectrograph (PIRIS) at the Mt. Wilson 100" telescope. I find that observations with a prototype are quite successful, achieving a contrast similar to a traditional Lyot coronagraph without blocking any light from a central object and useful for finding faint companions to nearby young solar analogues. In the lab I can reproduce the expected PSF reasonably well and with a single aperture design which achieves ~ 4 x 10 -5 contrast at 10l/ D . I find that small inaccuracies in the mask fabrication process and insufficient correction of the atmosphere contribute the most degradation to contrast at these levels. (Abstract shortened by UMI.)

NEW BROWN DWARF COMPANIONS TO YOUNG STARS IN SCORPIUS-CENTAURUS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Janson, Markus; Jayawardhana, Ray; Bonavita, Mariangela

2012-10-10

We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of {approx}40-100 M{sub jup} complements previous work in the same region, reporting detections of similarly wide companions with lowermore » masses, in the range of {approx}10-30 M{sub jup}. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass companions to stars.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Makarov, Valeri V.; Goldin, Alexey, E-mail: valeri.makarov@navy.mil, E-mail: alexey.goldin@gmail.com

KIC 7341653 is one of several late-type M dwarfs observed by the main mission of Kepler with peculiar infrared colors placing them in the domain of suspected young stellar objects (YSO). It is likely associated with a powerful X-ray emitter with X-ray flares. Kepler light curves reveal two distinct types of activity: frequent flares lasting from less than 30 minutes to a few hours, and a periodic variability with a period of 0.5463441(7) days. The largest detected flare increased the flux in the Kepler passband by a factor of 2.8 and released an estimated 4 × 10{sup 34} erg ofmore » energy in the Kepler band. Segmented periodogram analysis reveals that the amplitude of the periodic variation was subject to secular changes, dropping from peak values around 20 ppt to below 5 ppt toward the end of the mission, while the phase varied periodically with an amplitude of 0.15 rad and period 362(3) days. Two possible interpretations of the phase periodicity are discussed: a migrating long-lived photospheric spot, and a Doppler frequency shift generated by a solar-mass faint companion, such as a white dwarf.« less

The Universe Going Green: Extraordinarily Strong [OIII]5007 in Typical Dwarf Galaxies at z~3

NASA Astrophysics Data System (ADS)

Malkan, Matthew Arnold; Cohen, Daniel

2017-01-01

We constructed the average SEDs of U-dropout galaxies in the Subaru Deep Field. This sample contains more than 5000 Lyman-break galaxies at z~3. Their average near- and mid-IR colors were obtained by stacking JHK and IRAC imaging, in bins of stellar mass. At the lowest mass bins an increasingly strong excess flux is seen in the K filter. This excess can reach 1 magnitude in the broadband filter, and we attribute it to strong \\OIII $\\lambda{5007}$ line emission. The equivalent width is extraordinarily high, reaching almost 1000\\Ang\\ for the average z=3 galaxy at an i magnitude of 27. Such extreme [OIII] emission is very rare in the current epoch, only seen in a handful of metal-deficient dwarf starbursts sometimes referred to as ''Green Peas". In contrast, extreme [OIII]--strong enough to dominate the entire broad-band SED--was evidently the norm for faint galaxies at high redshift. We present evidence that these small but numerous galaxies were primarily responsible for the reionization of the Universe.

MINERVA-Red: A telescope dedicated to the discovery of planets orbiting the nearest low-mass stars

NASA Astrophysics Data System (ADS)

Sliski, David; Blake, Cullen; Johnson, John A.; Plavchan, Peter; Wittenmyer, Robert A.; Eastman, Jason D.; Barnes, Stuart; Baker, Ashley

2017-01-01

Results from Kepler and ground-based exoplanet surveys suggest that M-dwarfs host numerous small sized planets. Additionally, the discovery of the Earth-sized exoplanets orbiting Proxima Centauri and Trappist 1 demonstrate that these stars can host terrestrial planets in their habitable zones. Since low-mass stars are intrinsically faint at optical wavelengths, obtaining 1 m/s Doppler resolution to detect their planetary companions remains a challenge for instruments designed for sun-like stars. We describe a novel, high-cadence approach aimed at detecting and characterizing planets orbiting the closest low-mass stars to the Sun. MINERVA-Red is an echelle spectrograph optimized for the 'deep red', between 800 nm and 900 nm, where M-dwarfs are brightest. The spectrograph will be temperature controlled at 20C +/- 10mk and in a vacuum chamber which maintains a pressure below 0.01 mbar while using a Fabry-Perot etalon and U/Ne lamp for wavelength calibration. The spectrometer will operate with a robotic, 0.7-meter telescope at Mt. Hopkins, Arizona. We expect first light in 2017.

Coupled modes in magnetized dense plasma with relativistic-degenerate electrons

NASA Astrophysics Data System (ADS)

Khan, S. A.

2012-01-01

Low frequency electrostatic and electromagnetic waves are investigated in ultra-dense quantum magnetoplasma with relativistic-degenerate electron and non-degenerate ion fluids. The dispersion relation is derived for mobile as well as immobile ions by employing hydrodynamic equations for such plasma under the influence of electromagnetic forces and pressure gradient of relativistic-degenerate Fermi gas of electrons. The result shows the coexistence of shear Alfven and ion modes with relativistically modified dispersive properties. The relevance of results to the dense degenerate plasmas of astrophysical origin (for instance, white dwarf stars) is pointed out with brief discussion on ultra-relativistic and non-relativistic limits.

The Extended Halo of Centaurus A: Uncovering Satellites, Streams, and Substructures

NASA Astrophysics Data System (ADS)

Crnojević, D.; Sand, D. J.; Spekkens, K.; Caldwell, N.; Guhathakurta, P.; McLeod, B.; Seth, A.; Simon, J. D.; Strader, J.; Toloba, E.

2016-05-01

We present the widest-field resolved stellar map to date of the closest (D˜ 3.8 Mpc) massive elliptical galaxy NGC 5128 (Centaurus A; Cen A), extending out to a projected galactocentric radius of ˜150 kpc. The data set is part of our ongoing Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) utilizing the Magellan/Megacam imager. We resolve a population of old red giant branch (RGB) stars down to ˜1.5 mag below the tip of the RGB, reaching surface brightness limits as low as {μ }V,0˜ 32 mag arcsec-2. The resulting spatial stellar density map highlights a plethora of previously unknown streams, shells, and satellites, including the first tidally disrupting dwarf around Cen A (CenA-MM-Dw3), which underline its active accretion history. We report 13 previously unknown dwarf satellite candidates, of which 9 are confirmed to be at the distance of Cen A (the remaining 4 are not resolved into stars), with magnitudes in the range {M}V=-7.2 to -13.0, central surface brightness values of {μ }V,0=25.4{--}26.9 mag arcsec-2, and half-light radii of {r}h=0.22{--}2.92 {{kpc}}. These values are in line with Local Group dwarfs but also lie at the faint/diffuse end of their distribution; interestingly, CenA-MM-Dw3 has similar properties to the recently discovered ultradiffuse galaxies in Virgo and Coma. Most of the new dwarfs are fainter than the previously known Cen A satellites. The newly discovered dwarfs and halo substructures are discussed in light of their stellar populations, and they are compared to those discovered by the PAndAS survey of M31. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

M32 analogs? A population of massive ultra-compact dwarf and compact elliptical galaxies in intermediate-redshift clusters

DOE PAGES

Zhang, Yuanyuan; Bell, Eric F.

2017-01-13

Here, we report the discovery of relatively massive, M32-like ultra compact dwarf (UCD) and compact elliptical (CE) galaxy candidates inmore » $$0.2\\lt z\\lt 0.6$$ massive galaxy clusters imaged by the Cluster Lensing And Supernova survey with Hubble (CLASH) survey. Examining the nearly unresolved objects in the survey, we identify a sample of compact objects concentrated around the cluster central galaxies with colors similar to cluster red sequence galaxies. Their colors and magnitudes suggest stellar masses around $${10}^{9}{M}_{\\odot }$$. More than half of these galaxies have half-light radii smaller than 200 pc, falling into the category of massive UCDs and CEs, with properties similar to M32. The properties are consistent with a tidal stripping origin, but we cannot rule out the possibility that they are early-formed compact objects trapped in massive dark matter halos. The 17 CLASH clusters studied in this work on average contain 2.7 of these objects in their central 0.3 Mpc and 0.6 in their central 50 kpc. Our study demonstrates the possibility of statistically characterizing UCDs/CEs with a large set of uniform imaging survey data.« less

M32 analogs? A population of massive ultra-compact dwarf and compact elliptical galaxies in intermediate-redshift clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yuanyuan; Bell, Eric F.

Here, we report the discovery of relatively massive, M32-like ultra compact dwarf (UCD) and compact elliptical (CE) galaxy candidates inmore » $$0.2\\lt z\\lt 0.6$$ massive galaxy clusters imaged by the Cluster Lensing And Supernova survey with Hubble (CLASH) survey. Examining the nearly unresolved objects in the survey, we identify a sample of compact objects concentrated around the cluster central galaxies with colors similar to cluster red sequence galaxies. Their colors and magnitudes suggest stellar masses around $${10}^{9}{M}_{\\odot }$$. More than half of these galaxies have half-light radii smaller than 200 pc, falling into the category of massive UCDs and CEs, with properties similar to M32. The properties are consistent with a tidal stripping origin, but we cannot rule out the possibility that they are early-formed compact objects trapped in massive dark matter halos. The 17 CLASH clusters studied in this work on average contain 2.7 of these objects in their central 0.3 Mpc and 0.6 in their central 50 kpc. Our study demonstrates the possibility of statistically characterizing UCDs/CEs with a large set of uniform imaging survey data.« less

The luminosity function at the end of the main sequence: Results of a deep, large-area, CCD survey for cool dwarfs

NASA Technical Reports Server (NTRS)

Kirkpatrick, J. Davy; Mcgraw, John T.; Hess, Thomas R.; Liebert, James; Mccarthy, Donald W., Jr.

1994-01-01

The luminosity function at the end of the main sequence is determined from V, R, and I data taken by the charge coupled devices (CCD)/Transit Instrument, a dedicated telescope surveying an 8.25 min wide strip of sky centered at delta = +28 deg, thus sampling Galactic latitudes of +90 deg down to -35 deg. A selection of 133 objects chosen via R - I and V - I colors has been observed spectroscopically at the 4.5 m Multiple Mirror Telescope to assess contributions by giants and subdwarfs and to verify that the reddest targets are objects of extremely late spectral class. Eighteen dwarfs of type M6 or later have been discovered, with the latest being of type M8.5. Data used for the determination of the luminosity function cover 27.3 sq. deg down to a completeness limit of R = 19.0. This luminosity function, computed at V, I, and bolometric magnitudes, shows an increase at the lowest luminosities, corresponding to spectral types later than M6- an effect suggested in earlier work by Reid & Gilmore and Legget & Hawkins. When the luminosity function is segregated into north Galactic and south Galactic portions, it is found that the upturn at faint magnitudes exists only in the southern sample. In fact, no dwarfs with M(sub I) is greater than or equal to 12.0 are found within the limiting volume of the 19.4 sq deg northern sample, in stark contrast to the smaller 7.9 sq deg area at southerly latitudes where seven such dwarfs are found. This fact, combined with the fact that the Sun is located approximately 10-40 pc north of the midplane, suggests that the latest dwarfs are part of a young population with a scale height much smaller than the 350 pc value generally adopted for other M dwarfs. These objects comprise a young population either because the lower metallicities prevelant at earlier epochs inhibited the formation of late M dwarfs or because the older counterparts of this population have cooled beyond current detection limits. The latter scenario would hold if these late-type M dwarfs are substellar. The luminosity function data together with an empirical derivation of the mass-luminosity relation (from Henry & McCarthy) are used to compute a mass function independent of theory. This mass function increases toward the end of the main sequence, but the observed density of M dwarfs is still insufficient to account for the missing mass. If the increases seen in the luminosity and mass functions are indicative of a large, unseen, substellar population, brown dwarfs may yet add significantly to the mass of the Galaxy.

Cool Dwarfs 1o-7

NASA Astrophysics Data System (ADS)

Ambruster, Carol W.

Most of the cool dwarfs in the interesting age range 10^7-10^8 yr are too faint for IUE, yet such stars are critically important from the viewpoint of stellar evolution. Among stars of this age are the Pleiades K dwarfs, some of which appear to be on the main sequence, and some of which are still arriving there. Up until last year, only 2 stars in this age range had been observed by IUE, both recently: HD 36705 (AB Dor) and HD 17433. Three more stars were identified by the present investigators and observed with IUE during the past (11th) year: HD 129333, a single, nearby solar-type GOV star; HD 82558, a rapidly rotating, single, K2V star; and Ross 137B, the M dwarf common proper motion companion to AB Dor. We have since identified 5 more stars between 10^7 and 10^8 years old that are bright enough to be observed by IUE. They are physically associated, but distant, companions to main sequence O and B stars, identified in the survey of Lindroos (1986). Their ages are thus determined by the short main sequence lifetimes of the hot primaries. Rotational velocities are not yet known for our 5 proposed targets; we will be obtaining these and other data in the coming year. We therefore request time for basic IUE observations of these stars, an LWP-lo, LWP-hi and SWP-lo, for each star. This will ensure that crucial basic fluxes are in the IUE archives, should the satellite die in the coming year. Furthermore these data are immediately useful in filling the gap in the exhaustive study by Simon, Herbig and Boesgaard (1985) of the evolution of TR and chromospheric activity with age. More in-depth coverage will be proposed next year.

EVIDENCE THAT GAMMA-RAY BURST 130702A EXPLODED IN A DWARF SATELLITE OF A MASSIVE GALAXY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kelly, Patrick L.; Filippenko, Alexei V.; Fox, Ori D.

2013-09-20

GRB 130702A is a nearby long-duration gamma-ray burst (LGRB) discovered by the Fermi satellite whose associated afterglow was detected by the Palomar Transient Factory. Subsequent photometric and spectroscopic monitoring has identified a coincident broad-lined Type Ic supernova (SN), and nebular emission detected near the explosion site is consistent with a redshift of z = 0.145. The SN-GRB exploded at an offset of {approx}7.''6 from the center of an inclined r = 18.1 mag red disk-dominated galaxy, and {approx}0.''6 from the center of a much fainter r = 23 mag object. We obtained Keck-II DEIMOS spectra of the two objects andmore » find a 2{sigma} upper limit on their line-of-sight velocity offset of {approx}<60 km s{sup -1}. If we calculate the inclination angle of the massive red galaxy from its axis ratio and assume that its light is dominated by a very thin disk, the explosion would have a {approx}60 kpc central offset, or {approx}9 times the galaxy's half-light radius. A significant bulge or a thicker disk would imply a higher inclination angle and greater central offset. The substantial offset suggests that the faint source is a separate dwarf galaxy. The star-formation rate of the dwarf galaxy is {approx}0.05 M{sub Sun} yr{sup -1}, and we place an upper limit on its oxygen abundance of 12 + log(O/H) < 8.16 dex. The identification of an LGRB in a dwarf satellite of a massive, metal-rich primary galaxy suggests that recent detections of LGRBs spatially coincident with metal-rich galaxies may be, in some cases, superpositions.« less

The origin of ultra diffuse galaxies: stellar feedback and quenching

NASA Astrophysics Data System (ADS)

Chan, T. K.; Kereš, D.; Wetzel, A.; Hopkins, P. F.; Faucher-Giguère, C.-A.; El-Badry, K.; Garrison-Kimmel, S.; Boylan-Kolchin, M.

2018-05-01

We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies (UDGs). We show that outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching, naturally reproduce the observed population of red UDGs, without the need for high spin halos, or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius and absolute magnitude of the observed red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated dwarf galaxies with M* ˜ 108 M⊙, low metallicity and a broad range of ages; the more massive the UDGs, the older they are. The most massive red UDG in our sample (M* ˜ 3 × 108M⊙) requires quenching at z ˜ 3 when its halo reached Mh ˜ 1011 M⊙. Our simulated UDGs form with normal stellar-to-halo ratios and match the central enclosed masses and the velocity dispersions of the observed UDGs. Enclosed masses remain largely fixed across a broad range of quenching times because the central regions of their dark matter halos complete their growth early. If our simulated dwarfs are not quenched, they evolve into bluer low-surface brightness galaxies with M/L similar to observed field dwarfs. While our simulation sample covers a limited range of formation histories and halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy type around M* ˜ 108 M⊙, both in the field and in clusters.

The origin of ultra diffuse galaxies: stellar feedback and quenching

NASA Astrophysics Data System (ADS)

Chan, T. K.; Kereš, D.; Wetzel, A.; Hopkins, P. F.; Faucher-Giguère, C.-A.; El-Badry, K.; Garrison-Kimmel, S.; Boylan-Kolchin, M.

2018-07-01

We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies (UDGs). We show that outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching, naturally reproduce the observed population of red UDGs, without the need for high spin haloes, or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius, and absolute magnitude of the observed red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated dwarf galaxies with M* ˜ 108 M⊙, low metallicity, and a broad range of ages; the more massive the UDGs, the older they are. The most massive red UDG in our sample (M* ˜ 3 × 108 M⊙) requires quenching at z ˜ 3 when its halo reached Mh ˜ 1011 M⊙. Our simulated UDGs form with normal stellar-to-halo ratios and match the central enclosed masses and the velocity dispersions of the observed UDGs. Enclosed masses remain largely fixed across a broad range of quenching times because the central regions of their dark matter haloes complete their growth early. If our simulated dwarfs are not quenched, they evolve into bluer low surface brightness galaxies with M/L similar to observed field dwarfs. While our simulation sample covers a limited range of formation histories and halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy type around M* ˜ 108 M⊙, both in the field and in clusters.

The globular cluster systems of 54 Coma ultra-diffuse galaxies: statistical constraints from HST data

NASA Astrophysics Data System (ADS)

Amorisco, N. C.; Monachesi, A.; Agnello, A.; White, S. D. M.

2018-04-01

We use data from the HST Coma Cluster Treasury program to assess the richness of the globular cluster systems (GCSs) of 54 Coma ultra-diffuse galaxies (UDGs), 18 of which have a half-light radius exceeding 1.5 kpc. We use a hierarchical Bayesian method tested on a large number of mock data sets to account consistently for the high and spatially varying background counts in Coma. These include both background galaxies and intra-cluster globular clusters (ICGCs), which are disentangled from the population of member globular clusters (GCs) in a probabilistic fashion. We find no candidate for a GCS as rich as that of the Milky Way, our sample has GCSs typical of dwarf galaxies. For the standard relation between GCS richness and halo mass, 33 galaxies have a virial mass Mvir ≤ 1011 M⊙ at 90 per cent probability. Only three have Mvir > 1011 M⊙ with the same confidence. The mean colour and spread in colour of the UDG GCs are indistinguishable from those of the abundant population of ICGCs. The majority of UDGs in our sample are consistent with the relation between stellar mass and GC richness of `normal' dwarf galaxies. Nine systems, however, display GCSs that are richer by a factor of 3 or more (at 90 per cent probability). Six of these have sizes ≲1.4 kpc. Our results imply that the physical mechanisms responsible for the extended size of the UDGs and for the enhanced GC richness of some cluster dwarfs are at most weakly correlated.

Implications of Stellar Feedback for Dynamical Modeling of the Milky Way and Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Wetzel, Andrew

2018-04-01

I will present recent results on dynamical modeling of stellar populations from the FIRE cosmological zoom-in baryonic simulations of Milky Way-like and dwarf galaxies. First, I will discuss the dynamical formation of the Milky Way, including the origin of thin+thick stellar disk morphology. I also will discuss the curious origin of metal-rich stars on halo-like orbits near the Sun, as recently measured by Gaia, with new insights from FIRE simulations on stellar radial migration/heating. Next, I will discuss role of stellar feedback in generating non-equilibrium fluctuations of the gravitational potential in low-mass 'dwarf' galaxies, which can explain the origin of cores in their dark-matter density profiles. In particular, we predict significant observable effects on stellar dynamics, including radial migration, size fluctuations, and population gradients, which can provide observational tests of feedback-driven core formation. Finally, this scenario can explain the formation of newly discovered 'ultra-diffuse' galaxies.

Observing exoplanet populations with high-precision astrometry

NASA Astrophysics Data System (ADS)

Sahlmann, Johannes

2012-06-01

This thesis deals with the application of the astrometry technique, consisting in measuring the position of a star in the plane of the sky, for the discovery and characterisation of extra-solar planets. It is feasible only with a very high measurement precision, which motivates the use of space observatories, the development of new ground-based astronomical instrumentation and of innovative data analysis methods: The study of Sun-like stars with substellar companions using CORALIE radial velocities and HIPPARCOS astrometry leads to the determination of the frequency of close brown dwarf companions and to the discovery of a dividing line between massive planets and brown dwarf companions; An observation campaign employing optical imaging with a very large telescope demonstrates sufficient astrometric precision to detect planets around ultra-cool dwarf stars and the first results of the survey are presented; Finally, the design and initial astrometric performance of PRIMA, ! a new dual-feed near-infrared interferometric observing facility for relative astrometry is presented.

Searching for Planet Nine and Other Nearby Worlds with WISE and NEOWISE

NASA Astrophysics Data System (ADS)

Meisner, Aaron Michael; Bromley, Benjamin; Nugent, Peter; Schlegel, David; Kenyon, Scott; Schlafly, Eddie; Dawson, Kyle; Anderson, Teddy

2018-01-01

Launched nearly eight years ago, NASA's WISE satellite continues to collect millions of infrared images at 3-5 microns as part of its asteroid-hunting NEOWISE mission. We have repurposed these NEOWISE exposures for science beyond the main belt by generating a novel full-sky set of time-resolved coadded images. This reprocessing allows for moving object searches reaching ~1.3 magnitudes fainter than previous studies while also providing a factor of ten increase in time baseline. This creates an exciting opportunity for motion-based discovery of very cold and faint objects which may have thus far eluded detection despite residing close to the Sun. We present the results of our latest WISE/NEOWISE search for 'Planet Nine', a giant planet hypothesized to orbit the Sun at hundreds of astronomical units. Our search incorporates over four years of WISE W1 observations spanning a seven year time period, and covers more than 75% of the sky. Variants of the WISE/NEOWISE Planet Nine search methodology we have developed should enable motion-based discoveries of large numbers of cold, faint brown dwarfs, especially those which are not detectable with Gaia.

Hubble Space Telescope observations of the dwarf Nova Z Chamaeleontis through two eruption cycles

NASA Technical Reports Server (NTRS)

Robinson, E. L.; Wood, Janet H.; Bless, R. C.; Clemens, J. C.; Dolan, J. F.; Elliot, J. L.; Nelson, M. J.; Percival, J. W.; Taylor, M. J.; Van Citters, G. W.

1995-01-01

We have obtained the first high-speed photometry of the eclipsing dwarf nova Z Cha at ultraviolet wavelengths with the Hubble Space Telescope (HST). We observed the eclipse roughly every 4 days over two cycles of the normal eruptions of Z Cha, giving a uniquely complete coverage of its outburst cycle. The accretion disk dominated the ultraviolet light curve of Z Cha at the peak of an eruption; the white dwarf, the bright spot on the edge of the disk, and the boundary layer were all invisible. We were able to obtain an axisymmetric map of the accretion disk at this time only by adopting a flared disk with an opening angle of approximately 8 deg. The run of brightness temperature with radius in the disk at the peak of the eruption was too flat to be consistent with a steady state, optically thick accretion disk. The local rate of mass flow through the disk was approximately 5 x 10(exp -10) solar masses/yr near the center of the disk and approximately 5 x 10(exp -9) solar masses/yr near the outer edge. The white dwarf, the accretion disk, and the boundary layer were all significant contributors to the ultraviolet flux on the descending branches of the eruptions. The temperature of the white dwarf during decline was 18,300 K less than T(sub wd) less than 21,800 K, which is significantly greater than at minimum light. Six days after the maximum of an eruption Z Cha has faded to near minimum light at ultraviolet wavelenghts, but was still approximately 70% brighter at minimum light in the B band. About one-quarter of the excess flux in the B band came from the accretion disk. Thus, the accretion disk faded and became invisible at ultraviolet wavelengths before it faded at optical wavelenghts. The disk did, however, remain optically thick and obscured the lower half of the white dwarf at ultraviolet and possibly at optical wavelenghts for 2 weeks after the eruption ended. By the third week after eruptiuons the eclipse looked like a simple occultation of an unobscured, spherical white dwarf by a dark secondary star. The center of the accretion disk was, therfore, optically thin at ultraviolet wavelenghts and the boundary layer was too faint to be visible.

TRAPPIST-1 Compared to Jovian Moons and Inner Solar System - Updated Feb. 2018

NASA Image and Video Library

2018-02-05

All seven planets discovered in orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. In fact, they would have room to spare. TRAPPIST-1 also is only a fraction of the size of our Sun; it isn't much larger than Jupiter. So, the TRAPPIST-1 system's proportions look more like Jupiter and its moons than those of our solar system. The seven planets of TRAPPIST-1 are all Earth-sized and terrestrial. TRAPPIST-1 is an ultra-cool dwarf star in the constellation Aquarius, and its planets orbit very close to it. https://photojournal.jpl.nasa.gov/catalog/PIA22096

SEGUE 1—A COMPRESSED STAR FORMATION HISTORY BEFORE REIONIZATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webster, David; Bland-Hawthorn, Joss; Frebel, Anna, E-mail: d.webster@physics.usyd.edu.au

Segue 1 is the current best candidate for a “first galaxy,” a system that experienced only a single, short burst of star formation and has since remained unchanged. Here we present possible star formation scenarios that can explain Segue 1’s unique metallicity distribution. While the majority of stars in all other ultra-faint dwarfs are within 0.5 dex of the mean [Fe/H] for the galaxy, five of the seven stars in Segue 1 have a spread of Δ[Fe/H]  > 0.8 dex. We show that this distribution of metallicities cannot be explained by a gradual buildup of stars, but instead requires clustered star formation. Chemicalmore » tagging allows the separate unresolved delta functions in abundance space to be associated with discrete events in space and time. This provides an opportunity to put the enrichment events into a time sequence and unravel the history of the system. We investigate two possible scenarios for the star formation history of Segue 1 using Fyris Alpha simulations of gas in a 10{sup 7} M{sub ⊙} dark matter halo. The lack of stars with intermediate metallicities −3 < [Fe/H] < −2 can be explained either by a pause in star formation caused by supernova feedback or by the spread of metallicities resulting from one or two supernovae in a low-mass dark matter halo. Either possibility can reproduce the metallicity distribution function (MDF) as well as the other observed elemental abundances. The unusual MDF and the low luminosity of Segue 1 can be explained by it being a first galaxy that originated with M{sub vir} ∼ 10{sup 7}M{sub ⊙} at z ∼ 10.« less

A MegaCam Survey of Outer Halo Satellites. III. Photometric and Structural Parameters

NASA Astrophysics Data System (ADS)

Muñoz, Ricardo R.; Côté, Patrick; Santana, Felipe A.; Geha, Marla; Simon, Joshua D.; Oyarzún, Grecco A.; Stetson, Peter B.; Djorgovski, S. G.

2018-06-01

We present structural parameters from a wide-field homogeneous imaging survey of Milky Way satellites carried out with the MegaCam imagers on the 3.6 m Canada–France–Hawaii Telescope and 6.5 m Magellan-Clay telescope. Our survey targets an unbiased sample of “outer halo” satellites (i.e., substructures having galactocentric distances greater than 25 kpc) and includes classical dSph galaxies, ultra-faint dwarfs, and remote globular clusters. We combine deep, panoramic gr imaging for 44 satellites and archival gr imaging for 14 additional objects (primarily obtained with the DECam instrument as part of the Dark Energy Survey) to measure photometric and structural parameters for 58 outer halo satellites. This is the largest and most uniform analysis of Milky Way satellites undertaken to date and represents roughly three-quarters (58/81 ≃ 72%) of all known outer halo satellites. We use a maximum-likelihood method to fit four density laws to each object in our survey: exponential, Plummer, King, and Sérsic models. We systematically examine the isodensity contour maps and color–magnitude diagrams for each of our program objects, present a comparison with previous results, and tabulate our best-fit photometric and structural parameters, including ellipticities, position angles, effective radii, Sérsic indices, absolute magnitudes, and surface brightness measurements. We investigate the distribution of outer halo satellites in the size–magnitude diagram and show that the current sample of outer halo substructures spans a wide range in effective radius, luminosity, and surface brightness, with little evidence for a clean separation into star cluster and galaxy populations at the faintest luminosities and surface brightnesses.

Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina

DOE PAGES

Li, T. S.; Simon, J. D.; Pace, A. B.; ...

2018-04-25

Here, we present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocitymore » $${v}_{\\mathrm{hel}}=477.2\\pm 1.2$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$ and a velocity dispersion of $${\\sigma }_{v}={3.4}_{-0.8}^{+1.2}$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of $${1.0}_{-0.4}^{+0.8}\\times {10}^{6}$$ $${M}_{\\odot }$$, indicating a mass-to-light ratio of $${369}_{-161}^{+309}$$ $${M}_{\\odot }$$/$${L}_{\\odot }$$. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of $${\\rm{[Fe/H]}}=-2.44\\pm 0.09$$ with dispersion $${\\sigma }_{{\\rm{[Fe/H]}}}={0.22}_{-0.07}^{+0.10}$$. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of $${v}_{\\mathrm{hel}}={284.6}_{-3.1}^{+3.4}$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$. The brightest RGB member of Car III has a metallicity of $${\\rm{[Fe/H]}}\\,=-1.97\\pm 0.12$$. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation ($${\\rm{\\Delta }}d\\sim 10\\,\\mathrm{kpc}$$) among Local Group satellites, the large difference in their systemic velocities, $$\\sim 200\\,\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of γ-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.« less

Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, T. S.; Simon, J. D.; Pace, A. B.

Here, we present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocitymore » $${v}_{\\mathrm{hel}}=477.2\\pm 1.2$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$ and a velocity dispersion of $${\\sigma }_{v}={3.4}_{-0.8}^{+1.2}$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of $${1.0}_{-0.4}^{+0.8}\\times {10}^{6}$$ $${M}_{\\odot }$$, indicating a mass-to-light ratio of $${369}_{-161}^{+309}$$ $${M}_{\\odot }$$/$${L}_{\\odot }$$. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of $${\\rm{[Fe/H]}}=-2.44\\pm 0.09$$ with dispersion $${\\sigma }_{{\\rm{[Fe/H]}}}={0.22}_{-0.07}^{+0.10}$$. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of $${v}_{\\mathrm{hel}}={284.6}_{-3.1}^{+3.4}$$ $$\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$. The brightest RGB member of Car III has a metallicity of $${\\rm{[Fe/H]}}\\,=-1.97\\pm 0.12$$. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation ($${\\rm{\\Delta }}d\\sim 10\\,\\mathrm{kpc}$$) among Local Group satellites, the large difference in their systemic velocities, $$\\sim 200\\,\\mathrm{km}\\,{{\\rm{s}}}^{-1}$$, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of γ-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.« less

A Hubble Space Telescope survey for Novae in M87. II. Snuffing out the maximum magnitude–rate of decline relation for novae as a non-standard candle, and a prediction of the existence of ultrafast novae

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shara, Michael M.; Doyle, Trisha; Lauer, Tod R.

The extensive grid of numerical simulations of nova eruptions first predicted that some classical novae might significantly deviate from the Maximum Magnitude–Rate of Decline (MMRD) relation, which purports to characterize novae as standard candles. Kasliwal et al. have announced the observational detection of a new class of faint, fast classical novae in the Andromeda galaxy. These objects deviate strongly from the MMRD relationship, as predicted by Yaron et al. Recently, Shara et al. reported the first detections of faint, fast novae in M87. These previously overlooked objects are as common in the giant elliptical galaxy M87 as they are inmore » the giant spiral M31; they comprise about 40% of all classical nova eruptions and greatly increase the observational scatter in the MMRD relation. We use the extensive grid of the nova simulations of Yaron et al. to identify the underlying causes of the existence of faint, fast novae. These are systems that have accreted, and can thus eject, only very low-mass envelopes, of the order of 10 –7–10 –8 M ⊙, on massive white dwarfs. Such binaries include, but are not limited to, the recurrent novae. As a result, these same models predict the existence of ultrafast novae that display decline times, t 2, to be as short as five hours. We outline a strategy for their future detection.« less

A Hubble Space Telescope Survey for Novae in M87. II. Snuffing out the Maximum Magnitude–Rate of Decline Relation for Novae as a Non-standard Candle, and a Prediction of the Existence of Ultrafast Novae

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shara, Michael M.; Doyle, Trisha; Zurek, David

The extensive grid of numerical simulations of nova eruptions from the work of Yaron et al. first predicted that some classical novae might significantly deviate from the Maximum Magnitude–Rate of Decline (MMRD) relation, which purports to characterize novae as standard candles. Kasliwal et al. have announced the observational detection of a new class of faint, fast classical novae in the Andromeda galaxy. These objects deviate strongly from the MMRD relationship, as predicted by Yaron et al. Recently, Shara et al. reported the first detections of faint, fast novae in M87. These previously overlooked objects are as common in the giantmore » elliptical galaxy M87 as they are in the giant spiral M31; they comprise about 40% of all classical nova eruptions and greatly increase the observational scatter in the MMRD relation. We use the extensive grid of the nova simulations of Yaron et al. to identify the underlying causes of the existence of faint, fast novae. These are systems that have accreted, and can thus eject, only very low-mass envelopes, of the order of 10{sup −7}–10{sup −8} M {sub ⊙}, on massive white dwarfs. Such binaries include, but are not limited to, the recurrent novae. These same models predict the existence of ultrafast novae that display decline times, t {sub 2,} to be as short as five hours. We outline a strategy for their future detection.« less

A Hubble Space Telescope survey for Novae in M87. II. Snuffing out the maximum magnitude–rate of decline relation for novae as a non-standard candle, and a prediction of the existence of ultrafast novae

DOE PAGES

Shara, Michael M.; Doyle, Trisha; Lauer, Tod R.; ...

2017-04-20

The extensive grid of numerical simulations of nova eruptions first predicted that some classical novae might significantly deviate from the Maximum Magnitude–Rate of Decline (MMRD) relation, which purports to characterize novae as standard candles. Kasliwal et al. have announced the observational detection of a new class of faint, fast classical novae in the Andromeda galaxy. These objects deviate strongly from the MMRD relationship, as predicted by Yaron et al. Recently, Shara et al. reported the first detections of faint, fast novae in M87. These previously overlooked objects are as common in the giant elliptical galaxy M87 as they are inmore » the giant spiral M31; they comprise about 40% of all classical nova eruptions and greatly increase the observational scatter in the MMRD relation. We use the extensive grid of the nova simulations of Yaron et al. to identify the underlying causes of the existence of faint, fast novae. These are systems that have accreted, and can thus eject, only very low-mass envelopes, of the order of 10 –7–10 –8 M ⊙, on massive white dwarfs. Such binaries include, but are not limited to, the recurrent novae. As a result, these same models predict the existence of ultrafast novae that display decline times, t 2, to be as short as five hours. We outline a strategy for their future detection.« less

A surprise at the bottom of the main sequence: Rapid rotation and NO H-alpha emission

NASA Technical Reports Server (NTRS)

Basri, Gibor; Marcy, Geoffrey W.

1995-01-01

We report Kech Observatory high-resolution echelle spectra from 640-850 nm for eight stars near the faint end of the main sequence. These spectra are the highest resolution spectra of such late-type stars, and clearly resolve the TiO, VO, and atomic lines. The sample includes the field brown-dwarf candidate, BRI 0021-0214 (M9.5+). Very unexpectedly, it shows the most rapid rotation in the entire samples, v sin i approximately 40 km/s, which is 20x faster than typical field nonemission M stars. Equally surprising is that BRI 0021 exhibits no emission or absorptionat H-alpha. We argue that this absence is not simply due to its cool photosphere, but that stellar activity declines in a fundamental way at the end of the main sequence. As it is the first very late M dwarf observed at high spectral resolution, BRI 0021 may be signaling a qualitative change in the angular momentum loss rate among the lowest mass stars. Conventionally, its rapid rotation would have marked BRI 0021 as very young, consistent with the selection effect which arises if the latest-type dwarfs are really brown dwarfs on cooling curves. In any case, it is unprecedented to find no sign of stellar activity in such a rapidly rotating convective star. We also discuss the possible conflict between this observation and the extremely strong H-alpha seen in another very cool star, PC 0025+0447. Extrapolation of M-L relations for BRI 0021 yields M approximately 0.065 solar mass, and the other sample objects have expected masses near the H-burning limit. These include two Pleiades brown-dwarf candidates, four field M6 dwarfs and one late-type T Tauri star. The two Pleiades M6 dwarfs have v sin i of 26 and 37 km/s, H-alpha in emission, and radial velocities consistent with Pleiades M6 dwarfs have v sin i of 26 and 37 km/s, H-alpha in emission, and radial velocities consistent with Pleiades membership. Similarly, the late-type T Tauri star has v sin i approximately 30 km/s and H alpha emission indicate of its youth. Two of the four late-type field dMe star also exhibit rotation above 5 km/s, consistent with expectations. BRI 0021 has no measurable absoprtion due to lithium, indicating that it is likely to be more massive than 0.065 solar mass.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tauris, T. M.; Langer, N.; Moriya, T. J.

Recent discoveries of weak and fast optical transients raise the question of their origin. We investigate the minimum ejecta mass associated with core-collapse supernovae (SNe) of Type Ic. We show that mass transfer from a helium star to a compact companion can produce an ultra-stripped core which undergoes iron core collapse and leads to an extremely fast and faint SN Ic. In this Letter, a detailed example is presented in which the pre-SN stellar mass is barely above the Chandrasekhar limit, resulting in the ejection of only ∼0.05-0.20 M {sub ☉} of material and the formation of a low-mass neutron star (NS).more » We compute synthetic light curves of this case and demonstrate that SN 2005ek could be explained by our model. We estimate that the fraction of such ultra-stripped to all SNe could be as high as 10{sup –3}-10{sup –2}. Finally, we argue that the second explosion in some double NS systems (for example, the double pulsar PSR J0737–3039B) was likely associated with an ultra-stripped SN Ic.« less

Spitzer Photometry of WISE-Selected Brown Dwarf and Hyper-Lumninous Infrared Galaxy Candidates

NASA Technical Reports Server (NTRS)

Griffith, Roger L.; Kirkpatrick, J. Davy; Eisenhardt, Peter R. M.; Gelino, Christopher R.; Cushing, Michael C.; Benford, Dominic; Blain, Andrew; Bridge, Carrie R.; Cohen, Martin; Cutri, Roc M.;

Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk.

PubMed

Bergemann, Maria; Sesar, Branimir; Cohen, Judith G; Serenelli, Aldo M; Sheffield, Allyson; Li, Ting S; Casagrande, Luca; Johnston, Kathryn V; Laporte, Chervin F P; Price-Whelan, Adrian M; Schönrich, Ralph; Gould, Andrew

2018-03-15

Our Galaxy is thought to have an active evolutionary history, dominated over the past ten billion years or so by star formation, the accretion of cold gas and, in particular, the merging of clumps of baryonic and dark matter. The stellar halo-the faint, roughly spherical component of the Galaxy-reveals rich 'fossil' evidence of these interactions, in the form of stellar streams, substructures and chemically distinct stellar components. The effects of interactions with dwarf galaxies on the content and morphology of the Galactic disk are still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups of stars in our Galaxy, which may have extragalactic origins. There is also mounting evidence that stellar overdensities (regions with greater-than-average stellar density) at the interface between the outer disk and the halo could have been caused by the interaction of a dwarf galaxy with the disk. Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane-locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies.

Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk

NASA Astrophysics Data System (ADS)

Bergemann, Maria; Sesar, Branimir; Cohen, Judith G.; Serenelli, Aldo M.; Sheffield, Allyson; Li, Ting S.; Casagrande, Luca; Johnston, Kathryn V.; Laporte, Chervin F. P.; Price-Whelan, Adrian M.; Schönrich, Ralph; Gould, Andrew

2018-03-01

Our Galaxy is thought to have an active evolutionary history, dominated over the past ten billion years or so by star formation, the accretion of cold gas and, in particular, the merging of clumps of baryonic and dark matter. The stellar halo—the faint, roughly spherical component of the Galaxy—reveals rich ‘fossil’ evidence of these interactions, in the form of stellar streams, substructures and chemically distinct stellar components. The effects of interactions with dwarf galaxies on the content and morphology of the Galactic disk are still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups of stars in our Galaxy, which may have extragalactic origins. There is also mounting evidence that stellar overdensities (regions with greater-than-average stellar density) at the interface between the outer disk and the halo could have been caused by the interaction of a dwarf galaxy with the disk. Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane—locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies.

WISE J061213.85-303612.5: a new T-dwarf binary candidate

NASA Astrophysics Data System (ADS)

Huélamo, N.; Ivanov, V. D.; Kurtev, R.; Girard, J. H.; Borissova, J.; Mawet, D.; Mužić, K.; Cáceres, C.; Melo, C. H. F.; Sterzik, M. F.; Minniti, D.

2015-06-01

Context. T and Y dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Up to now, there are more than 350 T dwarfs that have been identified thanks to large imaging surveys in the infrared, and their multiplicity properties can shed light on the formation process. Aims: The aim of this work is to look for companions around a sample of seven ultracoool objects. Most of them have been discovered by the WISE observatory and have not been studied before for multiplicity. Methods: We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope. We observed all the objects in one or more near-IR filters (JHKs). Results: From the seven observed objects, we have identified a subarcsecond binary system, WISE J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of ρ = 350 ± 5 mas and a position angle of PA = 235 ± 1°. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS JHKs bands, we estimate a distance of d = 31 ± 6 pc and derive a projected separation of ρ ~ 11 ± 2 au. Another target, WISE J2255-3118, shows a very faint object at 1.̋3 in the Ks image. The object is marginally detected in H, and we derive a near infrared color of H - Ks> 0.1 mag. HST/WFC3 public archival data reveals that the companion candidate is an extended source. Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-σ sensitivity limits between H = 19-21 for companions at separations ≥0.̋5. Conclusions: WISE 0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, ~11 au, is larger than the average (~5 au). Additional observations are needed to confirm that the system is bound. This work used data collected at the VLT under runs 89.C-0494(A, B, C) and 91.C-0501(A, B).Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA), and the Canadian Astronomy Data Centre (CADC/NRC/CSA).

Contribution of HI-bearing ultra-diffuse galaxies to the cosmic number density of galaxies

NASA Astrophysics Data System (ADS)

Jones, M. G.; Papastergis, E.; Pandya, V.; Leisman, L.; Romanowsky, A. J.; Yung, L. Y. A.; Somerville, R. S.; Adams, E. A. K.

2018-06-01

We estimate the cosmic number density of the recently identified class of HI-bearing ultra-diffuse sources (HUDs) based on the completeness limits of the ALFALFA survey. These objects have HI masses approximately in the range 8.5 < logMHI/M⊙ < 9.5, average r-band surface brightnesses fainter than 24 mag arcsec-2, half-light radii greater than 1.5 kpc, and are separated from neighbours by at least 350 kpc. In this work we demonstrate that they contribute at most 6% of the population of HI-bearing dwarfs detected by ALFALFA (with similar HI masses), have a total cosmic number density of (1.5 ± 0.6) × 10-3 Mpc-3, and an HI mass density of (6.0 ± 0.8) × 105 M⊙ Mpc-3. We estimate that this is similar to the total cosmic number density of ultra-diffuse galaxies (UDGs) in groups and clusters, and conclude that the relation between the number of UDGs hosted in a halo and the halo mass must have a break below M200 1012 M⊙ in order to account for the abundance of HUDs in the field. The distribution of the velocity widths of HUDs rises steeply towards low values, indicating a preference for slow rotation rates compared to the global HI-rich dwarf population. These objects were already included in previous measurements of the HI mass function, but have been absent from measurements of the galaxy stellar mass function owing to their low surface brightness. However, we estimate that due to their low number density the inclusion of HUDs would constitute a correction of less than 1%. Comparison with the Santa Cruz semi-analytic model shows that it produces HI-rich central UDGs that have similar colours to HUDs, but that these UDGs are currently produced in a much greater number. While previous results from this sample have favoured formation scenarios where HUDs form in high spin-parameter halos, comparisons with recent results which invoke that formation mechanism reveal that this model produces an order of magnitude more field UDGs than we observe in the HUD population, and these have an occurrence rate (relative to other dwarfs) that is approximately double what we observe. In addition, the colours of HUDs are bluer than predicted, although we suspect this is due to a systematic problem in reproducing the star formation histories of low-mass galaxies rather than being specific to the ultra-diffuse nature of these sources.

Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B

NASA Technical Reports Server (NTRS)

Zhou, Yifan; Apai, Daniel; Metchev, Stanimir; Lew, Ben W. P.; Schneider, Glenn; Marley, Mark S.; Karalidi, Theodora; Manjavacas, Elena; Bedin, Luigi R.; Cowan, Nicolas B.;

Soft gamma-ray repeaters and anomalous X-ray pulsars as highly magnetized white dwarfs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mukhopadhyay, Banibrata; Rao, A.R., E-mail: bm@physics.iisc.ernet.in, E-mail: arrao@tifr.res.in

We explore the possibility that soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are powered by highly magnetized white dwarfs (B-WDs). We take a sample of SGRs and AXPs and provide the possible parameter space in mass, radius, and surface magnetic field based on their observed properties (period and its derivative) and the assumption that these sources obey the mass-radius relation derived for the B-WDs. The radius and magnetic field of B-WDs are adequate to explain energies in SGRs/AXPs as the rotationally powered energy. In addition, B-WDs also adequately explain the perplexing radio transient GCRT J1745-3009 as a whitemore » dwarf pulsar. Note that the radius and magnetic fields of B-WDs are neither extreme (unlike of highly magnetized neutron stars) nor ordinary (unlike of magnetized white dwarfs, yet following the Chandrasekhar's mass-radius relation (C-WDs)). In order to explain SGRs/AXPs, while the highly magnetized neutron stars require an extra, observationally not well established yet, source of energy, the C-WDs predict large ultra-violet luminosity which is observationally constrained from a strict upper limit. Finally, we provide a set of basic differences between the magnetar and B-WD hypotheses for SGRs/AXPs.« less

Andromeda IV: A new local volume very metal-poor galaxy

NASA Astrophysics Data System (ADS)

Pustilnik, S. A.; Tepliakova, A. L.; Kniazev, A. Y.; Burenkov, A. N.

2008-06-01

And IV is a low surface brightness (LSB) dwarf galaxy at a distance of 6.1 Mpc, projecting close to M 31. In this paper the results of spectroscopy of the And IV two brightest HII regions with the SAO 6-m telescope (BTA) are presented. In spectra of both of them the faint line [OIII] λ4363 Å was detected and this allowed us to determine their O/H by the classical Te method. Their values for 12+log(O/H) are equal to 7.49±0.06 and 7.55±0.23, respectively. The comparison of the direct O/H calculations with the two most reliable semi-empirical and empirical methods shows the good consistency between these methods. For And IV absolute blue magnitude, MB = -12.6, our value for O/H corresponds to the ‘standard’ relation between O/H and LB for dwarf irregular galaxies (DIGs). And IV appears to be a new representative of the extremely metal-deficient gas-rich galaxies in the Local Volume. The very large range of M(HI) for LSB galaxies with close metallicities and luminosities indicates that simple models of LSBG chemical evolution are too limited to predict such striking diversity.

Ensemble Atmospheric Properties of Small Planets around M Dwarfs

NASA Astrophysics Data System (ADS)

Guo, Xueying; Ballard, Sarah; Dragomir, Diana

2018-01-01

With the growing number of planets discovered by the Kepler mission and ground-base surveys, people start to try to understand the atmospheric features of those uncovered new worlds. While it has been found that hot Jupiters exhibit diverse atmosphere composition with both clear and cloudy/hazy atmosphere possible, similar studies on ensembles of smaller planets (Earth analogs) have been held up due to the faintness of most of their host stars. In this work, a sample of 20 Earth analogs of similar periods around M dwarfs with existing Kepler transit information and Spitzer observations is composed, complemented with previously studies GJ1214b and GJ1132b, as well as the recently announced 7 small planets in the TRAPPIST-1 system. We evaluate their transit depths with uncertainties on the Spitzer 4.5 micron band using the “pixel-level decorrelation” method, and together with their well analyzed Kepler data and Hubble data, we put constraints on their atmosphere haze slopes and cloud levels. Aside from improving the understanding of ensemble properties of small planets, this study will also provide clues of potential targets for detailed atmospheric studies using the upcoming James Webb Telescope.

HPF: The Habitable Zone Planet Finder at the Hobby-Eberly Telescope

NASA Astrophysics Data System (ADS)

Wright, Jason T.; Mahadevan, Suvrath; Hearty, Fred; Monson, Andy; Stefansson, Gudmundur; Ramsey, Larry; Ninan, Joe; Bender, Chad; Kaplan, Kyle; Roy, Arpita; Terrien, Ryan; Robertson, Paul; Halverson, Sam; Schwab, Christian; Kanodia, Shubham

2018-01-01

The Habitable Zone Planet Finder (HPF) is an ultra-stable NIR (ZYJ) high resolution echelle spectrograph on the 10-m Hobby-Eberly Telescope capable of 1-3 m/s Doppler velocimetry on nearby late M dwarfs (M4-M9). This precision is sufficient to detect terrestrial planets in the Habitable Zones of these relatively unexplored stars. Here we present its capabilities and early commissioning results.

(Almost) Dark Galaxies in the ALFALFA Survey: HI-bearing Ultra-Diffuse Galaxies, and Beyond

NASA Astrophysics Data System (ADS)

Leisman, Luke; Haynes, Martha P.; Giovanelli, Riccardo; ALFALFA Almost Darks Team

2017-01-01

Scaling relations between HI and stars in galaxies suggest strong ties between their atomic gas content and star formation laws. The Arecibo Legacy Fast ALFA (ALFALFA) blind extragalactic HI survey is well positioned to locate very low surface brightness sources that lie off these relations, the most extreme of which may fall below optical detection limits. Thus, the ALFALFA (Almost) Darks Project has been investigating extreme outliers from these relations by studying the ~1% of ALFALFA sources without apparent stellar counterparts in major optical surveys. We have obtained deep HI and optical imaging of 25 of these candidate "dark" sources. We find that most "dark" sources are not extreme "(almost) dark" galaxies. A few are rare OH Megamasers, redshifted into the ALFALFA bandpass, and many are part of large galactic plumes, stretching as far as 600 kpc from their host galaxy. However, a small handful of sources appear to be galaxies with extreme stellar systems. We find multiple systems with HI mass to stellar mass ratios an order of magnitude larger than typical gas rich dwarfs. Further, we find an isolated population of HI-bearing "ultra diffuse" galaxies (UDGs), with stellar masses of dwarfs, but HI and optical radii of L* galaxies. We suggest that these sources may be related to recently reported gas poor, quiescent UDGs.

WiseView: Visualizing motion and variability of faint WISE sources

NASA Astrophysics Data System (ADS)

Caselden, Dan; Westin, Paul, III; Meisner, Aaron; Kuchner, Marc; Colin, Guillaume

2018-06-01

WiseView renders image blinks of Wide-field Infrared Survey Explorer (WISE) coadds spanning a multi-year time baseline in a browser. The software allows for easy visual identification of motion and variability for sources far beyond the single-frame detection limit, a key threshold not surmounted by many studies. WiseView transparently gathers small image cutouts drawn from many terabytes of unWISE coadds, facilitating access to this large and unique dataset. Users need only input the coordinates of interest and can interactively tune parameters including the image stretch, colormap and blink rate. WiseView was developed in the context of the Backyard Worlds: Planet 9 citizen science project, and has enabled hundreds of brown dwarf candidate discoveries by citizen scientists and professional astronomers.

VizieR Online Data Catalog: BVI photometry of two Orion galaxies (Karachentsev+ 1996)

NASA Astrophysics Data System (ADS)

Karachentsev, I.; Musella, I.

1996-05-01

Two relatively faint (B=15.7 and B=18.4mag, respectively), low velocity (+276 and +322km/s) galaxies were imaged with a CCD in the B, V, I bands at the prime focus of the 6m telescope on 16 October 1993. By means of the brightest stars we estimated their distances to be 6.4 and 5.5(+/-2)Mpc, assuming a galactic extinction of 2.7 and 2.9mag, respectively. We note that these isolated irregular dwarfs are located to a high Supergalactic latitude, -63deg, and their low radial velocities may be the result of a retarded expansion along the polar axis of the Local cloud of galaxies. (2 data files).

Ultra-precise Masses and Magnitudes for the Gliese 268 M-dwarf Binary

NASA Astrophysics Data System (ADS)

Barry, R. K.; Demory, B. O.; Ségransan, D.; Forveille, T.; Danchi, W. C.; di Folco, E.; Queloz, D.; Torres, G.; Traub, W. A.; Delfosse, X.; Mayor, M.; Perrier, C.; Udry, S.

2009-02-01

Recent advances in astrometry using interferometry and precision radial velocity techniques combined allow for a significant improvement in the precision of masses of M-dwarf stars in visual systems. We report recent astrometric observations of Gliese 268, an M-dwarf binary with a 10.4 day orbital period, with the IOTA interferometer and radial velocity observations with the ELODIE instrument. Combining these measurements leads to preliminary masses of the constituent stars with uncertainties of 0.4%. The masses of the components are 0.22596+/-0.00084 Msolar for the primary and 0.19230+/-0.00071 Msolar for the secondary. The system parallax is determined by these observations to be 0.1560+/-.0030 arcsec (2.0% uncertainty) and is within Hipparcos error bars (0.1572+/-.0033). We tested these physical parameters, along with the near-infrared luminosities of the stars, against stellar evolution models for low-mass stars. Discrepancies between the measured and theoretical values point toward a low-level departure from the predictions. These results are among the most precise masses measured for visual binaries.

The mass spectrum of the white dwarfs in cataclysmic binaries - Supplementary computations

NASA Astrophysics Data System (ADS)

Ritter, H.; Ozkan, M. T.

1986-10-01

Numerical computations supplementing those of Ritter and Burkert (1986) for the study of observational selection in favor of massive white dwarfs among cataclysmic binaries (CBs) are presented. In particular the contribution of CBs beyond the period minimum and the influence of the inclination and of limb-darkening of the accretion disk are investigated. It is found that: (1) the main conclusions of Ritter and Burkert remain unchanged; (2) neither the inclusion of CBs containing a black-dwarf secondary nor the variation of the inclination or limb-darkening change the selection significantly; (3) in a magnitude-limited sample, about 22 percent of the ultra-short-period CBs contain a degenerate secondary; (4) the mean inclination of the accretion disk in a magnitude-limited sample is always close to 60 deg; (5) the fraction of eclipsing systems is of order 10-15 percent at m(v) = 10.0; (6) the mean inclination and the fraction of eclipsing systems increases with the limiting magnitude; and (7) the intrinsic space density of CBs is n(CB) approximately .0001-.0002/cu pc.

Planets around Low-mass Stars (PALMS). VI. Discovery of a Remarkably Red Planetary-mass Companion to the AB Dor Moving Group Candidate 2MASS J22362452+4751425*

NASA Astrophysics Data System (ADS)

Bowler, Brendan P.; Liu, Michael C.; Mawet, Dimitri; Ngo, Henry; Malo, Lison; Mace, Gregory N.; McLane, Jacob N.; Lu, Jessica R.; Tristan, Isaiah I.; Hinkley, Sasha; Hillenbrand, Lynne A.; Shkolnik, Evgenya L.; Benneke, Björn; Best, William M. J.

2017-01-01

We report the discovery of an extremely red planetary-mass companion to 2MASS J22362452+4751425, a ≈0.6 M⊙ late-K dwarf likely belonging to the ˜120 Myr AB Doradus moving group. 2M2236+4751 b was identified in multi-epoch NIRC2 adaptive optics imaging at Keck Observatory at a separation of 3\\buildrel{\\prime\\prime}\\over{.} 7, or 230 ± 20 AU in projection at the kinematic distance of 63 ± 5 pc to its host star. Assuming membership in the AB Dor group, as suggested from its kinematics, the inferred mass of 2M2236+4751 b is 11-14 MJup. Follow-up Keck/OSIRIS K-band spectroscopy of the companion reveals strong CO absorption similar to other faint red L dwarfs and lacks signs of methane absorption, despite having an effective temperature of ≈900-1200 K. With a (J-K)MKO color of 2.69 ± 0.12 mag, the near-infrared slope of 2M2236+4751 b is redder than all of the HR 8799 planets and instead resembles the ≈23 Myr isolated planetary-mass object PSO J318.5-22, implying that similarly thick photospheric clouds can persist in the atmospheres of giant planets at ages beyond 100 Myr. In near-infrared color-magnitude diagrams, 2M2236+4751 b is located at the tip of the red L dwarf sequence and appears to define the “elbow” of the AB Dor substellar isochrone separating low-gravity L dwarfs from the cooler young T dwarf track. 2M2236+4751 b is the reddest substellar companion to a star and will be a valuable benchmark to study the shared atmospheric properties of young low-mass brown dwarfs and extrasolar giant planets. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

Discovery of bright z ≃ 7 galaxies in the UltraVISTA survey

NASA Astrophysics Data System (ADS)

Bowler, R. A. A.; Dunlop, J. S.; McLure, R. J.; McCracken, H. J.; Milvang-Jensen, B.; Furusawa, H.; Fynbo, J. P. U.; Le Fèvre, O.; Holt, J.; Ideue, Y.; Ihara, Y.; Rogers, A. B.; Taniguchi, Y.

2012-11-01

We have exploited the new, deep, near-infrared UltraVISTA imaging of the Cosmological Evolution Survey (COSMOS) field, in tandem with deep optical and mid-infrared imaging, to conduct a new search for luminous galaxies at redshifts z ≃ 7. The year-one UltraVISTA data provide contiguous Y, J, H, Ks imaging over 1.5 deg2, reaching a 5σ detection limit of Y + J ≃ 25 (AB mag, 2-arcsec-diameter aperture). The central ≃1 deg2 of this imaging coincides with the final deep optical (u*, g, r, i) data provided by the Canada-France-Hawaii Telescope (CFHT) Legacy Survey and new deep Subaru/Suprime-Cam z'-band imaging obtained specifically to enable full exploitation of UltraVISTA. It also lies within the Hubble Space Telescope (HST) I814 band and Spitzer/Infrared Array Camera imaging obtained as part of the COSMOS survey. We have utilized this unique multiwavelength dataset to select galaxy candidates at redshifts z > 6.5 by searching first for Y + J-detected objects which are undetected in the CFHT and HST optical data. This sample was then refined using a photometric redshift fitting code, enabling the rejection of lower redshift galaxy contaminants and cool galactic M, L, T dwarf stars. The final result of this process is a small sample of (at most) 10 credible galaxy candidates at z > 6.5 (from over 200 000 galaxies detected in the year-one UltraVISTA data) which we present in this paper. The first four of these appear to be robust galaxies at z > 6.5, and fitting to their stacked spectral energy distribution yields zphot = 6.98 ± 0.05 with a stellar mass M* ≃ 5 × 109 M⊙ and rest-frame ultraviolet (UV) spectral slope β ≃ -2.0 ± 0.2 (where fλ ∝ λβ). The next three are also good candidates for z > 6.5 galaxies, but the possibility that they are dwarf stars cannot be completely excluded. Our final subset of three additional candidates is afflicted not only by potential dwarf star contamination, but also contains objects likely to lie at redshifts just below z = 6.5. We show that the three even-brighter z ≳ 7 galaxy candidates reported in the COSMOS field by Capak et al. are in fact all lower redshift galaxies at z ≃ 1.5-3.5. Consequently the new z ≃ 7 galaxies reported here are the first credible z ≃ 7 Lyman-break galaxies discovered in the COSMOS field and, as the most UV luminous discovered to date at these redshifts, are prime targets for deep follow-up spectroscopy. We explore their physical properties, and briefly consider the implications of their inferred number density for the form of the galaxy luminosity function at z ≃ 7.

The faint X-ray sources in and out of omega Centauri: X-ray observations and optical identifications

NASA Technical Reports Server (NTRS)

Cool, Adrienne M.; Grindlay, Jonathan E.; Bailyn, Charles D.; Callanan, Paul J.; Hertz, Paul

1995-01-01

We present the results of an observation of the globular cluster omega Cen (NGC 5139) with the Einstein high-resolution imager (HRI). Of the five low-luminosity X-ray sources toward omega Cen which were first identified with the Einstein imaging proportional counter (IPC) (Hertz and Grindlay 1983a, b), two are detected in the Einstein HRI observation: IPC sources A and D. These detections provide source positions accurate to 3 sec-4 sec; the positions are confirmed in a ROSAT HRI observation reported here. Using CCD photometry and spectroscopy, we have identified both sources as foreground dwarf M stars with emission lines (dMe). The chance projection of two Mde stars within approximately 13 min of the center of omega Cen is not extraordinary, given the space density of these stellar coronal X-ray sources. We discuss the possible nature of the three as yet unidentified IPC sources toward omega Cen, and consider the constraints that the Einstein observations place on the total population of X-ray sources in this cluster. The integrated luminosity from faint X-ray sources in omega Cen appears to be low relative to both the old open cluster M67 and the post-core-collapse globular, NGC 6397.

Seeing Baby Dwarf Galaxies

NASA Technical Reports Server (NTRS)

2009-01-01

[figure removed for brevity, see original site] [figure removed for brevity, see original site] [figure removed for brevity, see original site] Visible/DSS Click on image for larger version Ultraviolet/GALEX Click on image for larger version Poster Version Click on image for larger version

The unique ultraviolet vision of NASA's Galaxy Evolution Explorer reveals, for the first time, dwarf galaxies forming out of nothing more than pristine gas likely leftover from the early universe. Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way.

The forming dwarf galaxies shine in the far ultraviolet spectrum, rendered as blue in the call-out on the right hand side of this image. Near ultraviolet light, also obtained by the Galaxy Evolution Explorer, is displayed in green, and visible light from the blue part of the spectrum here is represented by red. The clumps (in circles) are distinctively blue, indicating they are primarily detected in far ultraviolet light.

The faint blue overlay traces the outline of the Leo Ring, a huge cloud of hydrogen and helium that orbits around two massive galaxies in the constellation Leo (left panel). The cloud is thought likely to be a primordial object, an ancient remnant of material that has remained relatively unchanged since the very earliest days of the universe. Identified about 25 years ago by radio waves, the ring cannot be seen in visible light.

Only a portion of the Leo Ring has been imaged in the ultraviolet, but this section contains the telltale ultraviolet signature of recent massive star formation within this ring of pristine gas. Astronomers have previously only seen dwarf galaxies form out of gas that has already been cycled through a galaxy and enriched with metals elements heavier than helium produced as stars evolve.

The visible data come from the Digitized Sky Survey of the Space Telescope Science Institute in Baltimore, Md. The Leo Ring visible image (left) represents the survey's blue, red, and infrared bands with the colors blue, green, and red. The overlay indicating the location of hydrogen gas in the Leo Ring is based on observations made at the Arecibo Observatory in Puerto Rico.

The optical + infrared L dwarf spectral sequence of young planetary-mass objects in the Upper Scorpius association

NASA Astrophysics Data System (ADS)

Lodieu, N.; Zapatero Osorio, M. R.; Béjar, V. J. S.; Peña Ramírez, K.

2018-01-01

We present the results of photometric and spectroscopic follow-ups of the lowest mass member candidates in the nearest OB association, Upper Scorpius (∼5-10 Myr; 145 ± 17 pc), with the Gran Telescopio de Canarias (GTC) and European Southern Observatory (ESO) Very Large Telescope (VLT). We confirm the membership of the large majority (>80 per cent) of candidates originally selected photometrically and astrometrically based on their spectroscopic features, weak equivalent widths of gravity-sensitive doublets and radial velocities. Confirmed members follow a sequence over a wide magnitude range (J = 17.0-19.3 mag) in several colour-magnitude diagrams with optical, near- and mid-infrared photometry and have near-infrared spectral types in the L1-L7 interval with likely masses below 15 Jupiter masses. We find that optical spectral types tend to be earlier than near-infrared spectral types by a few subclasses for spectral types later than M9. We investigate the behaviour of spectral indices, defined in the literature as a function of spectral type and gravity, by comparison with values reported in the literature for young and old dwarfs. We also derive effective temperatures in the 1900-1600 K range from fits of synthetic model-atmosphere spectra to the observed photometry, but we caution that the procedure carries large uncertainties. We determine bolometric corrections for young L dwarfs with ages of ∼5-10 Myr (Upper Sco association) and find them to be similar in the J band but larger by 0.1-0.4 mag in the K band with respect to field L dwarfs. Finally, we discover two faint young L dwarfs, Visible and Infrared Survey Telescope for Astronomy (VISTA) J1607-2146 (L4.5) and VISTA J1611-2215 (L5), that have Hα emission and possible flux excesses at 4.5 μm, pointing to the presence of accretion from a disc on to the central objects of mass below ∼15MJup at an age of 5-10 Myr.

The Breakthrough Listen Search for Intelligent Life: Target Selection of Nearby Stars and Galaxies

NASA Astrophysics Data System (ADS)

Isaacson, Howard; Siemion, Andrew P. V.; Marcy, Geoffrey W.; Lebofsky, Matt; Price, Danny C.; MacMahon, David; Croft, Steve; DeBoer, David; Hickish, Jack; Werthimer, Dan; Sheikh, Sofia; Hellbourg, Greg; Enriquez, J. Emilio

2017-05-01

We present the target selection for the Breakthrough Listen search for extraterrestrial intelligence during the first year of observations at the Green Bank Telescope, Parkes Telescope, and Automated Planet Finder. On the way to observing 1,000,000 nearby stars in search of technological signals, we present three main sets of objects we plan to observe in addition to a smaller sample of exotica. We chose the 60 nearest stars, all within 5.1 pc from the Sun. Such nearby stars offer the potential to observe faint radio signals from transmitters that have a power similar to those on Earth. We add a list of 1649 stars drawn from the Hipparcos catalog that span the Hertzprung-Russell diagram, including all spectral types along the main sequence, subgiants, and giant stars. This sample offers diversity and inclusion of all stellar types, but with thoughtful limits and due attention to main sequence stars. Our targets also include 123 nearby galaxies composed of a “morphological-type-complete” sample of the nearest spirals, ellipticals, dwarf spherioidals, and irregulars. While their great distances hamper the detection of technological electromagnetic radiation, galaxies offer the opportunity to observe billions of stars simultaneously and to sample the bright end of the technological luminosity function. We will also use the Green Bank and Parkes telescopes to survey the plane and central bulge of the Milky Way. Finally, the complete target list includes several classes of exotica, including white dwarfs, brown dwarfs, black holes, neutron stars, and asteroids in our solar system.

A very dark stellar system lost in Virgo: kinematics and metallicity of SECCO 1 with MUSE

NASA Astrophysics Data System (ADS)

Beccari, G.; Bellazzini, M.; Magrini, L.; Coccato, L.; Cresci, G.; Fraternali, F.; de Zeeuw, P. T.; Husemann, B.; Ibata, R.; Battaglia, G.; Martin, N.; Testa, V.; Perina, S.; Correnti, M.

2017-02-01

We present the results of VLT-MUSE (Very Large Telescope-Multi Unit Spectroscopic Explorer) integral field spectroscopy of SECCO 1, a faint, star-forming stellar system recently discovered as the stellar counterpart of an ultracompact high-velocity cloud (HVC 274.68+74.0), very likely residing within a substructure of the Virgo cluster of galaxies. We have obtained the radial velocity of a total of 38 individual compact sources identified as H II regions in the main and secondary bodies of the system, and derived the metallicity for 18 of them. We provide the first direct demonstration that the two stellar bodies of SECCO 1 are physically associated and that their velocities match the H I velocities. The metallicity is quite uniform over the whole system, with a dispersion lower than the uncertainty on individual metallicity estimates. The mean abundance, <12 + log(O/H)> = 8.44, is much higher than the typical values for local dwarf galaxies of similar stellar mass. This strongly suggests that the SECCO 1 stars were born from a pre-enriched gas cloud, possibly stripped from a larger galaxy. Using archival Hubble Space Telescope (HST) images, we derive a total stellar mass of ≃1.6 × 105 M⊙ for SECCO 1, confirming that it has a very high H I-to-stellar mass ratio for a dwarf galaxy, M_{H I}/M* ˜ 100. The star formation rate, derived from the Hα flux, is a factor of more than 10 higher than in typical dwarf galaxies of similar luminosity.

Optical linear polarization of 74 white dwarfs with the RoboPol polarimeter

NASA Astrophysics Data System (ADS)

Żejmo, Michał; Słowikowska, Aga; Krzeszowski, Krzysztof; Reig, Pablo; Blinov, Dmitry

2017-01-01

We present the first linear polarimetric survey of white dwarfs (WDs). Our sample consists of WDs of DA and DC spectral types in the SDSS r magnitude range from 13 to 17. We performed polarimetric observations using the RoboPol polarimeter attached to the 1.3-m telescope at Skinakas Observatory. We have 74 WDs in our sample, of which almost all are low-polarized WDs with a polarization degree (PD) lower than 1 per cent; only two have a PD higher than 1 per cent. There is evidence that on average isolated DC-type WDs have a higher PD (with a median PD of 0.78 per cent) than isolated DA-type WDs (with a median PD of 0.36 per cent). On the other hand, the median PD of isolated DA-type WDs is almost the same (I.e. 0.36 per cent) as the median PD of DA-type WDs in binary systems with red dwarfs (dM type; I.e. 0.33 per cent). This shows, as expected, that there is no contribution to the PD from the companion if the WD companion is a red dwarf, which is the most common situation for WD binary systems. We do not find differences in the PD between magnetic and non-magnetic WDs. Because 97 per cent of WDs in our sample have a PD lower than 1 per cent, they can be used as faint zero-polarized standard stars in the magnitude range from 13 to 17 of the SDSS r filter. They cover the Northern sky between 13h and 23h in right ascension and between -11° and 78° in declination. In addition, we found that for low extinction values (<0.04), the best model that describes the dependence of the PD on E(B - V) is given by the equation PDmax, ISM[per cent] = 0.65 E(B - V)0.12.

The Fornax Deep Survey with VST. III. Low surface brightness dwarfs and ultra diffuse galaxies in the center of the Fornax cluster

NASA Astrophysics Data System (ADS)

Venhola, Aku; Peletier, Reynier; Laurikainen, Eija; Salo, Heikki; Lisker, Thorsten; Iodice, Enrichetta; Capaccioli, Massimo; Kleijn, Gijs Verdoes; Valentijn, Edwin; Mieske, Steffen; Hilker, Michael; Wittmann, Carolin; van de Ven, Glenn; Grado, Aniello; Spavone, Marilena; Cantiello, Michele; Napolitano, Nicola; Paolillo, Maurizio; Falcón-Barroso, Jesús

2017-12-01

Context. Studies of low surface brightness (LSB) galaxies in nearby clusters have revealed a sub-population of extremely diffuse galaxies with central surface brightness of μ0,g' > 24 mag arcsec-2, total luminosity Mg' fainter than -16 mag and effective radius between 1.5 kpc 23 mag arcsec-2. We classified the objects based on their appearance into galaxies and tidal structures, and perform 2D Sérsic model fitting with GALFIT to measure the properties of those classified as galaxies. We analyzed their radial distribution and orientations with respect of the cluster center, and with respect to the other galaxies in our sample. We also studied their colors and compare the LSB galaxies in Fornax with those in other environments. Results: Our final sample complete in the parameter space of the previously known UDGs, consists of 205 galaxies of which 196 are LSB dwarfs (with Re < 1.5 kpc) and nine are UDGs (Re > 1.5 kpc). We show that the UDGs have (1) g'-r' colors similar to those of LSB dwarfs of the same luminosity; (2) the largest UDGs (Re > 3 kpc) in our sample appear different from the other LSB galaxies, in that they are significantly more elongated and extended; whereas (3) the smaller UDGs differ from the LSB dwarfs only by having slightly larger effective radii; (4) we do not find clear differences between the structural parameters of the UDGs in our sample and those of UDGs in other galaxy environments; (5) we find that the dwarf LSB galaxies in our sample are less concentrated in the cluster center than the galaxies with higher surface brightness, and that their number density drops within 180 kpc from the cluster center. We also compare the LSB dwarfs in Fornax with the LSB dwarfs in the Centaurus group, where data of similar quality to ours is available. (6) We find the smallest LSB dwarfs to have similar colors, sizes and Sérsic profiles regardless of their environment. However, in the Centaurus group the colors become bluer with increasing galaxy magnitudes, an effect which is probably due to smaller mass and hence weaker environmental influence of the Centaurus group. Conclusions: Our findings are consistent with the small UDGs forming the tail of a continuous distribution of less extended LSB galaxies. However, the elongated and distorted shapes of the large UDGs could imply that they are tidally disturbed galaxies. Due to limitations of the automatic detection methods and uncertainty in the classification the objects, it is yet unclear what is the total contribution of the tidally disrupted galaxies in the UDG population.

ARE ULTRA-LONG GAMMA-RAY BURSTS CAUSED BY BLUE SUPERGIANT COLLAPSARS, NEWBORN MAGNETARS, OR WHITE DWARF TIDAL DISRUPTION EVENTS?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ioka, Kunihito; Hotokezaka, Kenta; Piran, Tsvi, E-mail: kunihito.ioka@yukawa.kyoto-u.ac.jp

Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with extreme durations of ∼10{sup 4} s. Leading candidates for their origin are blue supergiant collapsars, magnetars, and white dwarf tidal disruption events (WD-TDEs) caused by massive black holes (BHs). Recent observations of supernova-like (SN-like) bumps associated with ulGRBs challenged both the WD-TDE and the blue supergiant models because of the detection of SNe and the absence of hydrogen lines, respectively. We propose that WD-TDEs can accommodate the observed SN-like bumps if the fallback WD matter releases energy into the unbound WD ejecta. The observed ejecta energy, luminosity, and velocitymore » are explained by the gravitational energy, Eddington luminosity, and escape velocity of the formed accretion disk, respectively. We also show that the observed X-rays can ionize the ejecta, eliminating lines. The SN-like light curves (SN 2011kl) for the ulGRB 111209A are consistent with all three models, although a magnetar model is unnatural because the spin-down time required to power the SN-like bump is a hundred times longer than the GRB. Our results imply that TDEs are a possible energy source for SN-like events in general and for ulGRBs in particular.« less

Spectroscopic characterisation of the stellar content of ultra diffuse galaxies

NASA Astrophysics Data System (ADS)

Ruiz-Lara, T.; Beasley, M. A.; Falcón-Barroso, J.; Román, J.; Pinna, F.; Brook, C.; Di Cintio, A.; Martín-Navarro, I.; Trujillo, I.; Vazdekis, A.

2018-05-01

Understanding the peculiar properties of Ultra Diffuse Galaxies (UDGs) via spectroscopic analysis is a challenging task requiring very deep observations and exquisite data reduction. In this work we perform one of the most complete characterisations of the stellar component of UDGs to date using deep optical spectroscopic data from OSIRIS at GTC. We measure radial and rotation velocities, star formation histories (SFH) and mean population parameters, such as ages and metallicities, for a sample of five UDG candidates in the Coma cluster. From the radial velocities, we confirm the Coma membership of these galaxies. We find that their rotation properties, if detected at all, are compatible with dwarf-like galaxies. The SFHs of the UDG are dominated by old (˜ 7 Gyr), metal-poor ([M/H] ˜ -1.1) and α-enhanced ([Mg/Fe] ˜ 0.4) populations followed by a smooth or episodic decline which halted ˜ 2 Gyr ago, possibly a sign of cluster-induced quenching. We find no obvious correlation between individual SFH shapes and any UDG morphological properties. The recovered stellar properties for UDGs are similar to those found for DDO 44, a local UDG analogue resolved into stars. We conclude that the UDGs in our sample are extended dwarfs whose properties are likely the outcome of both internal processes, such as bursty SFHs and/or high-spin haloes, as well as environmental effects within the Coma cluster.

ULTRA-COMPACT DWARFS IN THE COMA CLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chiboucas, Kristin; Tully, R. Brent; Marzke, R. O.

2011-08-20

We have undertaken a spectroscopic search for ultra-compact dwarf galaxies (UCDs) in the dense core of the dynamically evolved, massive Coma cluster as part of the Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) Coma Cluster Treasury Survey. UCD candidates were initially chosen based on color, magnitude, degree of resolution within the ACS images, and the known properties of Fornax and Virgo UCDs. Follow-up spectroscopy with Keck/Low-Resolution Imaging Spectrometer confirmed 27 candidates as members of the Coma cluster, a success rate >60% for targeted objects brighter than M{sub R} = -12. Another 14 candidates may also prove to be Coma members,more » but low signal-to-noise spectra prevent definitive conclusions. An investigation of the properties and distribution of the Coma UCDs finds these objects to be very similar to UCDs discovered in other environments. The Coma UCDs tend to be clustered around giant galaxies in the cluster core and have colors/metallicity that correlate with the host galaxy. With properties and a distribution similar to that of the Coma cluster globular cluster population, we find strong support for a star cluster origin for the majority of the Coma UCDs. However, a few UCDs appear to have stellar population or structural properties which differentiate them from the old star cluster populations found in the Coma cluster, perhaps indicating that UCDs may form through multiple formation channels.« less

The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey

NASA Astrophysics Data System (ADS)

Reylé, C.; Delorme, P.; Willott, C. J.; Albert, L.; Delfosse, X.; Forveille, T.; Artigau, E.; Malo, L.; Hill, G. J.; Doyon, R.

2010-11-01

Context. Thanks to recent and ongoing large scale surveys, hundreds of brown dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam camera on the Canada-France-Hawaii Telescope. Aims: Our objectives are to find ultracool brown dwarfs and to constrain the field brown-dwarf luminosity function and the mass function from a large and homogeneous sample of L and T dwarfs. Methods: We identify candidates in CFHT/MegaCam i' and z' images and follow them up with pointed near infrared (NIR) imaging on several telescopes. Halfway through our survey we found ~50 T dwarfs and ~170 L or ultra cool M dwarfs drawn from a larger sample of 1400 candidates with typical ultracool dwarfs i'-z' colours, found in 780 square degrees. Results: We have currently completed the NIR follow-up on a large part of the survey for all candidates from mid-L dwarfs down to the latest T dwarfs known with utracool dwarfs' colours. This allows us to draw on a complete and well defined sample of 102 ultracool dwarfs to investigate the luminosity function and space density of field dwarfs. Conclusions: We found the density of late L5 to T0 dwarfs to be 2.0+0.8-0.7 × 10-3 objects pc-3, the density of T0.5 to T5.5 dwarfs to be 1.4+0.3-0.2 × 10-3 objects pc-3, and the density of T6 to T8 dwarfs to be 5.3+3.1-2.2 × 10-3 objects pc-3. We found that these results agree better with a flat substellar mass function. Three latest dwarfs at the boundary between T and Y dwarfs give the high density 8.3+9.0-5.1 × 10-3 objects pc-3. Although the uncertainties are very large this suggests that many brown dwarfs should be found in this late spectral type range, as expected from the cooling of brown dwarfs, whatever their mass, down to very low temperature. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS. Based on observations made with the ESO New Technology Telescope at the La Silla Observatory. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina). Based on observations with the Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on observations made at The McDonald Observatory of the University of Texas at Austin.Tables 3, 5 and 8 are only available in electronic form at http://www.aanda.org

A calibration of the stellar mass fundamental plane at z ∼ 0.5 using the micro-lensing-induced flux ratio anomalies of macro-lensed quasars {sup ,} {sup ,}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schechter, Paul L.; Pooley, David; Blackburne, Jeffrey A.

2014-10-01

We measure the stellar mass surface densities of early-type galaxies by observing the micro-lensing of macro-lensed quasars caused by individual stars, including stellar remnants, brown dwarfs, and red dwarfs too faint to produce photometric or spectroscopic signatures. Instead of observing multiple micro-lensing events in a single system, we combine single-epoch X-ray snapshots of 10 quadruple systems, and compare the measured relative magnifications for the images with those computed from macro-models. We use these to normalize a stellar mass fundamental plane constructed using a Salpeter initial mass function with a low-mass cutoff of 0.1 M {sub ☉} and treat the zeropointmore » of the surface mass density as a free parameter. Our method measures the graininess of the gravitational potential produced by individual stars, in contrast to methods that decompose a smooth total gravitational potential into two smooth components, one stellar and one dark. We find the median likelihood value for the normalization factor F by which the Salpeter stellar masses must be multiplied is 1.23, with a one sigma confidence range, dominated by small number statistics, of 0.77« less

Tidal interaction, star formation and chemical evolution in blue compact dwarf galaxy Mrk 22

NASA Astrophysics Data System (ADS)

Paswan, A.; Omar, A.; Jaiswal, S.

2018-02-01

The optical spectroscopic and radio interferometric H I 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission-line features corresponding to high ionization lines of He II λ4686 and C IV λ5808 from young massive stars are detected. The ages of two prominent star-forming regions in the galaxy are estimated as ∼10 and ∼ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young starburst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission-line features. A significant N/O enrichment is seen in the fainter star-forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98±0.07 and 7.46±0.09, respectively. The galaxy has a large diffuse H I envelop. The H I images reveal disturbed gas kinematics and H I clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.

Pox 186: A Nearby Protogalaxy?

NASA Astrophysics Data System (ADS)

Corbin, Michael

1999-07-01

Blue Compact Dwarf Galaxies {BCDGs} typically consist of clusters of early-type stars embedded in older, evolved stellar populations similar in size and shape to normal dwarf ellipticals. However, deep ground-based CCD images of one faint BCDG, Pox 186, reveal a very compact { 5" diameter} structure with no evidence of an underlying older population. Optical spectroscopy of this object also indicates that a large number of Wolf-Rayet stars are present, which implies that a burst of star formation must have occurred very recently {<=sssim 10^7 years ago}. It has thus been suggested that Pox 186 is a protogalaxy, forming its very first generation of stars. Further investigation of this possibility requires the high angular resolution and ultraviolet spectral coverage that only HST can provide. Using WFPC2, we propose to image the galaxy in the U, V, and I bands, in order to better test for the presence of an underlying evolved population and to reveal any substructure in its star-forming regions. Using STIS, we will obtain low-resolution ultraviolet spectra of the galaxy for combination with ground-based spectra covering the optical through near infrared. This will allow us to determine its spectral energy distribution, metallicity, and dust content, which will in turn constrain its age and star formation history.

Featured Image: Identifying a Glowing Shell

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2018-05-01

New nebulae are being discovered and classified every day and this false-color image reveals one of the more recent objects of interest. This nebula, IPHASX J210204.7+471015, was recently imaged by the Andalucia Faint Object Spectrograph and Camera mounted on the 2.5-m Nordic Optical Telescope in La Palma, Spain. J210204 was initially identified as a possible planetary nebula a remnant left behind at the end of a red giants lifetime. Based on the above imaging, however, a team of authors led by Martn Guerrero (Institute of Astrophysics of Andalusia, Spain) is arguing that this shell of glowing gas was instead expelled around a classical nova. In a classical nova eruption, a white dwarf and its binary companion come very close together, and mass transfers to form a thin atmosphere of hydrogen around the white dwarf. When this hydrogen suddenly ignites in runaway fusion, this outer atmosphere can be expelled, forming a short-lived nova remnant which is what Guerrero and collaborators think were seeing with J210204. If so, this nebula can reveal information about the novathat caused it. To find out more about what the authors learned from this nebula, check out the paper below.CitationMartn A. Guerrero et al 2018 ApJ 857 80. doi:10.3847/1538-4357/aab669

SEARCH FOR RED DWARF STARS IN GLOBULAR CLUSTER NGC 6397

NASA Technical Reports Server (NTRS)

2002-01-01

Left A NASA Hubble Space Telescope image of a small region (1.4 light-years across) in the globular star cluster NGC 6397. Simulated stars (diamonds) have been added to this view of the same region of the cluster to illustrate what astronomers would have expected to see if faint red dwarf stars were abundant in the Milky Way Galaxy. The field would then contain 500 stars, according to theoretical calculations. Right The unmodified HST image shows far fewer stars than would be expected, according to popular theories of star formation. HST resolves about 200 stars. The stellar density is so low that HST can literally see right through the cluster and resolve far more distant background galaxies. From this observation, scientists have identified the surprising cutoff point below which nature apparently doesn't make many stars smaller that 1/5 the mass of our Sun. These HST findings provide new insights into star formation in our Galaxy. Technical detail:The globular cluster NGC 6397, one of the nearest and densest agglomerations of stars, is located 7,200 light-years away in the southern constellation Ara. This visible-light picture was taken on March 3, 1994 with the Wide Field Planetary Camera 2, as part the HST parallel observing program. Credit: F. Paresce, ST ScI and ESA and NASA

A photometric study of the dwarf spheroidal galaxies Leo IV and Bootes II

NASA Astrophysics Data System (ADS)

Cheng, Haw

A photometric study of the ultra-faint dwarf (UFD) galaxies Leo IV and Bootes II in the V and IC filters is here presented. The age of Leo IV relative to M92 were derived by fitting of Dartmouth isochrones, by a "standard" VHBTO method, and by the V HBTO method of VandenBerg et al. 2013. The age of Bootes II relative to M92 was derived by fitting of Dartmouth isochrones. Leo IV is found to be between 2 to 2.5 Gyr younger than M92 by these three methods. It is found to be predominantly old and metal poor and is well fit by isochrones of [Fe/H] = ---2.46 and [alpha/Fe] = 0.2 and 0.4. An age spread with a plausible value of ˜ 2 Gyr cannot be ruled out. A 10 Gyr old synthetic horizontal branch with [Fe/H] = ---1.70 and [alpha/Fe] of 0.2 is fit to Leo IV's red horizontal branch (RHB). The good fit of this model and its matching isochrone to Leo IV's CMD suggests that the RHB is real and not an observational artifact as proposed by Okamoto et al. 2012. Two RRab Lyraes previously observed by Moretti et al. 2009 were observed in Leo IV. One of the stars, V1, is observed to exhibit the Blazhko effect. No further RR Lyraes were uncovered in Leo IV. Comparison of the horizontal branch's observed V magnitude to the absolute magnitudes of the RR Lyraes yields a distance modulus of (m---M)0 = 21.01 +/- 0.07, in good agreement with previous studies. Leo IV's possible population of blue stragglers is found to show no signs of central concentration, though this study's sample and spatial coverage are too small for any detailed spatial distribution study. Bootes II's CMD is found to be consistent with that of a single age, mono-metallicity system. It is well fit by isochrones of [Fe/H] = ---1.79 and [alpha/Fe] = 0.2 and 0.4. Bootes II is found to be between 0.5 to 1.5 Gyr younger than M92. Distance was left as a free parameter in the fits. Bootes II is found to have distance modulus (m---M)0 lying between 18.02 to 18.15, in good agreement with previous studies. A single RRab Lyrae is discovered in Bootes II.

In situ study of maize starch gelatinization under ultra-high hydrostatic pressure using X-ray diffraction.

PubMed

Yang, Zhi; Gu, Qinfen; Hemar, Yacine

2013-08-14

The gelatinization of waxy (very low amylose) and high-amylose maize starches by ultra-high hydrostatic pressure (up to 6 GPa) was investigated in situ using synchrotron X-ray powder diffraction on samples held in a diamond anvil cell (DAC). The starch pastes, made by mixing starch and water in a 1:1 ratio, were pressurized and measured at room temperature. X-ray diffraction pattern showed that at 2.7 GPa waxy starch, which displayed A-type XRD pattern at atmospheric pressure, exhibited a faint B-type-like pattern. The B-type crystalline structures of high-amylose starch were not affected even when 1.5 GPa pressure was applied. However, both waxy and high-amylose maize starches can be fully gelatinized at 5.9 GPa and 5.1 GPa, respectively. In the case of waxy maize starch, upon release of pressure (to atmospheric pressure) crystalline structure appeared as a result of amylopectin aggregation. Copyright © 2013 Elsevier Ltd. All rights reserved.

Hubble Sees a “Behemoth” Bleeding Atmosphere Around a Warm Exoplanet

NASA Image and Video Library

2015-06-24

Astronomers using NASA’s Hubble Space Telescope have discovered an immense cloud of hydrogen dubbed “The Behemoth” bleeding from a planet orbiting a nearby star. The enormous, comet-like feature is about 50 times the size of the parent star. The hydrogen is evaporating from a warm, Neptune-sized planet, due to extreme radiation from the star. This phenomenon has never been seen around an exoplanet so small. It may offer clues to how other planets with hydrogen-enveloped atmospheres could have their outer layers evaporated by their parent star, leaving behind solid, rocky cores. Hot, rocky planets such as these that roughly the size of Earth are known as Hot-Super Earths. “This cloud is very spectacular, though the evaporation rate does not threaten the planet right now,” explains the study’s leader, David Ehrenreich of the Observatory of the University of Geneva in Switzerland. “But we know that in the past, the star, which is a faint red dwarf, was more active. This means that the planet evaporated faster during its first billion years of existence because of the strong radiation from the young star. Overall, we estimate that it may have lost up to 10 percent of its atmosphere over the past several billion years.” Read more: www.nasa.gov/feature/goddard/hubble-sees-a-behemoth-bleed... Caption: This artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. Also depicted is the parent star, which is a faint red dwarf named GJ 436. The hydrogen is evaporating from the planet due to extreme radiation from the star. A phenomenon this large has never before been seen around any exoplanet. Credits: NASA, ESA, and G. Bacon (STScI) NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

Abundance analysis of a CEMP-no star in the Carina dwarf spheroidal galaxy

NASA Astrophysics Data System (ADS)

Susmitha, A.; Koch, A.; Sivarani, T.

2017-10-01

Carbon-enhanced metal-poor (CEMP) stars bear important imprints of the early chemical enrichment of any stellar system. While these stars are known to exist in copious amounts in the Milky Way halo, detailed chemical abundance data from the faint dwarf spheroidal (dSph) satellites are still sparse, although the relative fraction of these stars increases with decreasing metallicity. Here, we report the abundance analysis of a metal-poor ([ Fe / H ] = - 2.5 dex), carbon-rich ([C/Fe] = 1.4 dex) star, ALW-8, in the Carina dSph using high-resolution spectroscopy obtained with the ESO/UVES instrument. Its spectrum does not indicate any over-enhancements of neutron capture elements. Thus classified as a CEMP-no star, this is the first detection of this kind of star in Carina. Another of our sample stars, ALW-1, is shown to be a CEMP-s star, but its immediate binarity prompted us to discard it from a detailed analysis. The majority of the 18 chemical elements we measured are typical of Carina's field star population and also agree with CEMP stars in other dSph galaxies. Similar to the only known CEMP-no star in the Sculptor dSph and the weak-r-process star HD 122563, the lack of any strong barium-enhancement is accompanied by a moderate overabundance in yttrium, indicating a weak r-process activity. The overall abundance pattern confirms that, also in Carina, the formation site for CEMP-no stars has been affected by both faint supernovae and by standard core collapse supernovae. Whichever process was responsible for the heavy element production in ALW-8 must be a ubiquitous source to pollute the CEMP-no stars, acting independently of the environment such as in the Galactic halo or in dSphs. Based on observations collected at the European Southern Observatory at Paranal, Chile; Large Programme proposal 171.B- 0520.Table A.1 is also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/606/A112

Ionization processes in a local analogue of distant clumpy galaxies: VLT MUSE IFU spectroscopy and FORS deep images of the TDG NGC 5291N

NASA Astrophysics Data System (ADS)

Fensch, J.; Duc, P.-A.; Weilbacher, P. M.; Boquien, M.; Zackrisson, E.

2016-01-01

Context. We present Integral Field Unit (IFU) observations with MUSE and deep imaging with FORS of a dwarf galaxy recently formed within the giant collisional HI ring surrounding NGC 5291. This Tidal Dwarf Galaxy (TDG) -like object has the characteristics of typical z = 1-2 gas-rich spiral galaxies: a high gas fraction, a rather turbulent clumpy interstellar medium, the absence of an old stellar population, and a moderate metallicity and star formation efficiency. Aims: The MUSE spectra allow us to determine the physical conditions within the various complex substructures revealed by the deep optical images and to scrutinize the ionization processes at play in this specific medium at unprecedented spatial resolution. Methods: Starburst age, extinction, and metallicity maps of the TDG and the surrounding regions were determined using the strong emission lines Hβ, [OIII], [OI], [NII], Hα, and [SII] combined with empirical diagnostics. Different ionization mechanisms were distinguished using BPT-like diagrams and shock plus photoionization models. Results: In general, the physical conditions within the star-forming regions are homogeneous, in particular with a uniform half-solar oxygen abundance. On small scales, the derived extinction map shows narrow dust lanes. Regions with atypically strong [OI] emission line immediately surround the TDG. The [OI]/ Hα ratio cannot be easily accounted for by the photoionization by young stars or shock models. At greater distances from the main star-foming clumps, a faint diffuse blue continuum emission is observed, both with the deep FORS images and the MUSE data. It does not have a clear counterpart in the UV regime probed by GALEX. A stacked spectrum towards this region does not exhibit any emission line, excluding faint levels of star formation, or stellar absorption lines that might have revealed the presence of old stars. Several hypotheses are discussed for the origin of these intriguing features. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile: ESO MUSE programme 60.A-9320(A) and FORS programme 382.B-0213(A).

Upper Limits on the Presence of Central Massive Black Holes in Two Ultra-compact Dwarf Galaxies in Centaurus A

NASA Astrophysics Data System (ADS)

Voggel, Karina T.; Seth, Anil C.; Neumayer, Nadine; Mieske, Steffen; Chilingarian, Igor; Ahn, Christopher; Baumgardt, Holger; Hilker, Michael; Nguyen, Dieu D.; Romanowsky, Aaron J.; Walsh, Jonelle L.; den Brok, Mark; Strader, Jay

2018-05-01

The recent discovery of massive black holes (BHs) in the centers of high-mass ultra-compact dwarf galaxies (UCDs) suggests that at least some are the stripped nuclear star clusters of dwarf galaxies. We present the first study that investigates whether such massive BHs, and therefore stripped nuclei, also exist in low-mass (M < 107 M ⊙) UCDs. We constrain the BH masses of two UCDs located in Centaurus A (UCD 320 and UCD 330) using Jeans modeling of the resolved stellar kinematics from adaptive optics data obtained with the SINFONI integral field spectrograph at the Very Large Telescope (VLT/SINFONI). No massive BHs are found in either UCD. We find a 3σ upper limit on the central BH mass in UCD 330 of M • < 1.0 × 105 M ⊙, which corresponds to 1.7% of the total mass. This excludes a high-mass fraction BH and would only allow low-mass BHs similar to those claimed to be detected in Local Group globular clusters. For UCD 320, poorer data quality results in a less constraining 3σ upper limit of M • < 1 × 106 M ⊙, which is equal to 37.7% of the total mass. The dynamical mass-to-light ratios of UCD 320 and UCD 330 are not inflated compared to predictions from stellar population models. The non-detection of BHs in these low-mass UCDs is consistent with the idea that elevated dynamical mass-to-light ratios do indicate the presence of a substantial BH. Although no massive BHs are detected, these systems could still be stripped nuclei. The strong rotation (v/σ of 0.3–0.4) in both UCDs and the two-component light profile in UCD 330 support the idea that these UCDs may be stripped nuclei of low-mass galaxies whose BH occupation fraction is not yet known.

The MUSE Hubble Ultra Deep Field Survey. VI. The faint-end of the Lyα luminosity function at 2.91 < z < 6.64 and implications for reionisation

NASA Astrophysics Data System (ADS)

Drake, A. B.; Garel, T.; Wisotzki, L.; Leclercq, F.; Hashimoto, T.; Richard, J.; Bacon, R.; Blaizot, J.; Caruana, J.; Conseil, S.; Contini, T.; Guiderdoni, B.; Herenz, E. C.; Inami, H.; Lewis, J.; Mahler, G.; Marino, R. A.; Pello, R.; Schaye, J.; Verhamme, A.; Ventou, E.; Weilbacher, P. M.

2017-11-01

We present the deepest study to date of the Lyα luminosity function in a blank field using blind integral field spectroscopy from MUSE. We constructed a sample of 604 Lyα emitters (LAEs) across the redshift range 2.91 < z < 6.64 using automatic detection software in the Hubble Ultra Deep Field. The deep data cubes allowed us to calculate accurate total Lyα fluxes capturing low surface-brightness extended Lyα emission now known to be a generic property of high-redshift star-forming galaxies. We simulated realistic extended LAEs to fully characterise the selection function of our samples, and performed flux-recovery experiments to test and correct for bias in our determination of total Lyα fluxes. We find that an accurate completeness correction accounting for extended emission reveals a very steep faint-end slope of the luminosity function, α, down to luminosities of log10L erg s-1< 41.5, applying both the 1 /Vmax and maximum likelihood estimators. Splitting the sample into three broad redshift bins, we see the faint-end slope increasing from -2.03-0.07+ 1.42 at z ≈ 3.44 to -2.86-∞+0.76 at z ≈ 5.48, however no strong evolution is seen between the 68% confidence regions in L∗-α parameter space. Using the Lyα line flux as a proxy for star formation activity, and integrating the observed luminosity functions, we find that LAEs' contribution to the cosmic star formation rate density rises with redshift until it is comparable to that from continuum-selected samples by z ≈ 6. This implies that LAEs may contribute more to the star-formation activity of the early Universe than previously thought, as any additional intergalactic medium (IGM) correction would act to further boost the Lyα luminosities. Finally, assuming fiducial values for the escape of Lyα and LyC radiation, and the clumpiness of the IGM, we integrated the maximum likelihood luminosity function at 5.00

A Study of Planetary Nebulae using the Faint Object Infrared Camera for the SOFIA Telescope

NASA Technical Reports Server (NTRS)

Davis, Jessica

2012-01-01

A planetary nebula is formed following an intermediate-mass (1-8 solar M) star's evolution off of the main sequence; it undergoes a phase of mass loss whereby the stellar envelope is ejected and the core is converted into a white dwarf. Planetary nebulae often display complex morphologies such as waists or torii, rings, collimated jet-like outflows, and bipolar symmetry, but exactly how these features form is unclear. To study how the distribution of dust in the interstellar medium affects their morphology, we utilize the Faint Object InfraRed CAmera for the SOFIA Telescope (FORCAST) to obtain well-resolved images of four planetary nebulae--NGC 7027, NGC 6543, M2-9, and the Frosty Leo Nebula--at wavelengths where they radiate most of their energy. We retrieve mid infrared images at wavelengths ranging from 6.3 to 37.1 micron for each of our targets. IDL (Interactive Data Language) is used to perform basic analysis. We select M2-9 to investigate further; analyzing cross sections of the southern lobe reveals a slight limb brightening effect. Modeling the dust distribution within the lobes reveals that the thickness of the lobe walls is higher than anticipated, or rather than surrounding a vacuum surrounds a low density region of tenuous dust. Further analysis of this and other planetary nebulae is needed before drawing more specific conclusions.

The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey. A ground-based follow-up survey of CO(1-0), CO(2-1), and CO(3-2)

NASA Astrophysics Data System (ADS)

Cormier, D.; Madden, S. C.; Lebouteiller, V.; Hony, S.; Aalto, S.; Costagliola, F.; Hughes, A.; Rémy-Ruyer, A.; Abel, N.; Bayet, E.; Bigiel, F.; Cannon, J. M.; Cumming, R. J.; Galametz, M.; Galliano, F.; Viti, S.; Wu, R.

2014-04-01

Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments. Aims: We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity. Methods: We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [C ii] 157μm and [O i] 63μm observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC 4861, NGC 625, and UM 311. We derived their molecular gas masses from several methods, including using the CO-to-H2 conversion factor XCO (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Results: We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z ~ 0.4 Z⊙), Mrk 930 (0.2 Z⊙), and UM 311 (0.5 Z⊙), but CO remains undetected in NGC 4861 (0.2 Z⊙). The CO luminosities are low, while [C ii] is bright in these galaxies, resulting in [C ii]/CO(1-0) ≥ 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled XCO factors. Those galaxies are dominated by their H i gas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro 11 using Cloudy and estimate an equivalent XCO factor that is 10 times higher than the Galactic value. Overall, our results confirm the emerging picture that CO suffers from significant selective photodissociation in low-metallicity dwarf galaxies.

Stellar Population Properties of Ultracompact Dwarfs in M87: A Mass–Metallicity Correlation Connecting Low-metallicity Globular Clusters and Compact Ellipticals

NASA Astrophysics Data System (ADS)

Zhang, Hong-Xin; Puzia, Thomas H.; Peng, Eric W.; Liu, Chengze; Côté, Patrick; Ferrarese, Laura; Duc, Pierre-Alain; Eigenthaler, Paul; Lim, Sungsoon; Lançon, Ariane; Muñoz, Roberto P.; Roediger, Joel; Sánchez-Janssen, Ruben; Taylor, Matthew A.; Yu, Jincheng

2018-05-01

We derive stellar population parameters for a representative sample of ultracompact dwarfs (UCDs) and a large sample of massive globular clusters (GCs) with stellar masses ≳ 106 M ⊙ in the central galaxy M87 of the Virgo galaxy cluster, based on model fitting to the Lick-index measurements from both the literature and new observations. After necessary spectral stacking of the relatively faint objects in our initial sample of 40 UCDs and 118 GCs, we obtain 30 sets of Lick-index measurements for UCDs and 80 for GCs. The M87 UCDs have ages ≳ 8 Gyr and [α/Fe] ≃ 0.4 dex, in agreement with previous studies based on smaller samples. The literature UCDs, located in lower-density environments than M87, extend to younger ages and smaller [α/Fe] (at given metallicities) than M87 UCDs, resembling the environmental dependence of the stellar nuclei of dwarf elliptical galaxies (dEs) in the Virgo cluster. The UCDs exhibit a positive mass–metallicity relation (MZR), which flattens and connects compact ellipticals at stellar masses ≳ 108 M ⊙. The Virgo dE nuclei largely follow the average MZR of UCDs, whereas most of the M87 GCs are offset toward higher metallicities for given stellar masses. The difference between the mass–metallicity distributions of UCDs and GCs may be qualitatively understood as a result of their different physical sizes at birth in a self-enrichment scenario or of galactic nuclear cluster star formation efficiency being relatively low in a tidal stripping scenario for UCD formation. The existing observations provide the necessary but not sufficient evidence for tidally stripped dE nuclei being the dominant contributors to the M87 UCDs.

STELLAR POPULATIONS IN COMPACT GALAXY GROUPS: A MULTI-WAVELENGTH STUDY OF HCGs 16, 22, AND 42, THEIR STAR CLUSTERS, AND DWARF GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Konstantopoulos, I. S.; Maybhate, A.; Charlton, J. C.

2013-06-20

We present a multi-wavelength analysis of three compact galaxy groups, Hickson compact groups (HCGs) 16, 22, and 42, which describe a sequence in terms of gas richness, from space- (Swift, Hubble Space Telescope (HST), and Spitzer) and ground-based (Las Campanas Observatory and Cerro Tololo Inter-American Observatory) imaging and spectroscopy. We study various signs of past interactions including a faint, dusty tidal feature about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the possible detection of a tidal feature at the end of its phase of optical observability. Our HST images also resolve what were thought to bemore » double nuclei in HCG 16C and D into multiple, distinct sources, likely to be star clusters. Beyond our phenomenological treatment, we focus primarily on contrasting the stellar populations across these three groups. The star clusters show a remarkable intermediate-age population in HCG 22, and identify the time at which star formation was quenched in HCG 42. We also search for dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27 ''associates'' (possible members) radically changes group dynamical masses, which in turn may affect previous evolutionary classifications. The extended membership paints a picture of relative isolation in HCGs 16 and 22, but shows HCG 42 to be part of a larger structure, following a dichotomy expected from recent studies. We conclude that (1) star cluster populations provide an excellent metric of evolutionary state, as they can age-date the past epochs of star formation; and (2) the extended dwarf galaxy population must be considered in assessing the dynamical state of a compact group.« less

THE CIRCUMSTELLAR ENVIRONMENT OF R CORONAE BOREALIS: WHITE DWARF MERGER OR FINAL-HELIUM-SHELL FLASH?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clayton, Geoffrey C.; Andrews, J. E.; Sugerman, Ben E. K.

2011-12-10

In 2007, R Coronae Borealis (R CrB) went into a historically deep and long decline. In this state, the dust acts like a natural coronagraph at visible wavelengths, allowing faint nebulosity around the star to be seen. Imaging has been obtained from 0.5 to 500 {mu}m with Gemini/GMOS, Hubble Space Telescope/WFPC2, Spitzer/MIPS, and Herschel/SPIRE. Several of the structures around R CrB are cometary globules caused by wind from the star streaming past dense blobs. The estimated dust mass of the knots is consistent with their being responsible for the R CrB declines if they form along the line of sightmore » to the star. In addition, there is a large diffuse shell extending up to 4 pc away from the star containing cool 25 K dust that is detected all the way out to 500 {mu}m. The spectral energy distribution of R CrB can be well fitted by a 150 AU disk surrounded by a very large diffuse envelope which corresponds to the size of the observed nebulosity. The total masses of the disk and envelope are 10{sup -4} and 2 M{sub Sun }, respectively, assuming a gas-to-dust ratio of 100. The evidence pointing toward a white dwarf merger or a final-helium-shell flash origin for R CrB is contradictory. The shell and the cometary knots are consistent with a fossil planetary nebula. Along with the fact that R CrB shows significant lithium in its atmosphere, this supports the final-helium-shell flash. However, the relatively high inferred mass of R CrB and its high fluorine abundance support a white dwarf merger.« less

An L Band Spectrum of the Coldest Brown Dwarf

NASA Astrophysics Data System (ADS)

Morley, Caroline V.; Skemer, Andrew J.; Allers, Katelyn N.; Marley, Mark. S.; Faherty, Jacqueline K.; Visscher, Channon; Beiler, Samuel A.; Miles, Brittany E.; Lupu, Roxana; Freedman, Richard S.; Fortney, Jonathan J.; Geballe, Thomas R.; Bjoraker, Gordon L.

2018-05-01

The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. presented a spectrum of WISE 0855 from 4.5–5.1 μm (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in the L band, from 3.4–4.14 μm. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. The James Webb Space Telescope will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes.

The fight for accretion: discovery of intermittent mass transfer in BB Doradus in the low state

NASA Astrophysics Data System (ADS)

Rodríguez-Gil, P.; Schmidtobreick, L.; Long, K. S.; Gänsicke, B. T.; Torres, M. A. P.; Rubio-Díez, M. M.; Santander-García, M.

2012-05-01

Our long-term photometric monitoring of southern nova-like cataclysmic variables with the 1.3-m Small and Moderate Aperture Research Telescope System (SMARTS) telescope found BB Doradus fading from V˜ 14.3 towards a deep low state at V˜ 19.3 in 2008 April. Here we present time-resolved optical spectroscopy of BB Dor in this faint state in 2009. The optical spectrum in quiescence is a composite of a hot white dwarf with Teff= 30 000 ± 5000 K and a M3-M4 secondary star with narrow emission lines (mainly of the Balmer series and He I) superposed. We associate these narrow profiles with an origin on the donor star. An analysis of the radial velocity curve of the Hα emission from the donor star allowed the measurement of an orbital period of 0.154 095 ± 0.000 003 d (3.698 28 ± 0.000 07 h), different from all previous estimates. We detected episodic accretion events which veiled the spectra of both stars and radically changed the line profiles within a time-scale of tens of minutes. This shows that accretion is not completely quenched in the low state. During these accretion episodes the line wings are stronger and their radial velocity curve is delayed by ˜0.2 cycle, similar to that observed in SW Sex and AM Her stars in the high state, with respect to the motion of the white dwarf. Two scenarios are proposed to explain the extra emission: impact of the material on the outer edge of a cold, remnant accretion disc, or the combined action of a moderately magnetic white dwarf (B1≲ 5 MG) and the magnetic activity of the donor star.

The Origin of Dwarf Early-Type Galaxies

NASA Astrophysics Data System (ADS)

Toloba, Elisa

2012-10-01

Abridge. We have conducted a spectrophotometric study of dwarf early-type galaxies (dEs) in the Virgo cluster and in regions of lower density. We have found that these galaxies show many properties in common with late-type galaxies but not with more massive early-types (E/S0). The properties of the dEs in Virgo show gradients within the cluster. dEs in the outer parts of the Virgo cluster are kinematically supported by rotation, while those in the center are supported by the random motions of their stars (i.e. pressure supported). The rotationally supported dEs have disky isophotes and faint underlying spiral/irregular substructures, they also show younger ages than those pressure supported, which have boxy isophotes and are smooth and regular, without any substructure. We compare the position of these dEs with massive early-type galaxies in the Faber-Jackson and Fundamental Plane relations, and we find that, although there is no difference between the position of rotationally and pressure supported dEs, both deviate from the relations of massive early-type galaxies in the direction of dwarf spheroidal systems (dSphs). We have used their offset with respect to the Fundamental Plane of E/S0 galaxies to estimate their dark matter fraction. All the properties studied in this work agree with a ram pressure stripping scenario, where late-type galaxies infall into the cluster, their interaction with the intergalactic medium blows away their gas and, as a result, they are quenched in a small amount of time. However, those dEs in the center of the cluster seem to have been fully transformed leaving no trace of their possible spiral origin, thus, if that is the case, they must have experienced a more violent mechanism in combination with ram pressure stripping.

Stellar Populations in Compact Galaxy Groups: a Multi-wavelength Study of HCGs 16, 22, and 42, Their Star Clusters, and Dwarf Galaxies

NASA Technical Reports Server (NTRS)

Konstantopoulos, I. S.; Maybhate, A.; Charlton, J. C.; Fedotov, K.; Durrell, P. R.; Mulchaey, J. S.; English, J.; Desjardins, T. D.; Gallagher, S. C.; Walker, L. M.;

CHARACTERIZING THE STAR FORMATION OF THE LOW-MASS SHIELD GALAXIES FROM HUBBLE SPACE TELESCOPE IMAGING

DOE Office of Scientific and Technical Information (OSTI.GOV)

McQuinn, Kristen B. W.; Skillman, Evan D.; Simones, Jacob E.

The Survey of Hi in Extremely Low-mass Dwarfs is an on-going multi-wavelength program to characterize the gas, star formation, and evolution in gas-rich, very low-mass galaxies that populate the faint end of the galaxy luminosity function. The galaxies were selected from the first ∼10% of the Hi Arecibo Legacy Fast ALFA survey based on their low Hi mass and low baryonic mass. Here, we measure the star formation properties from optically resolved stellar populations for 12 galaxies using a color–magnitude diagram fitting technique. We derive lifetime average star formation rates (SFRs), recent SFRs, stellar masses, and gas fractions. Overall, themore » recent SFRs are comparable to the lifetime SFRs with mean birthrate parameter of 1.4, with a surprisingly narrow standard deviation of 0.7. Two galaxies are classified as dwarf transition galaxies (dTrans). These dTrans systems have star formation and gas properties consistent with the rest of the sample, in agreement with previous results that some dTrans galaxies may simply be low-luminosity dwarf irregulars. We do not find a correlation between the recent star formation activity and the distance to the nearest neighboring galaxy, suggesting that the star formation process is not driven by gravitational interactions, but regulated internally. Further, we find a broadening in the star formation and gas properties (i.e., specific SFRs, stellar masses, and gas fractions) compared to the generally tight correlation found in more massive galaxies. Overall, the star formation and gas properties indicate these very low-mass galaxies host a fluctuating, non-deterministic, and inefficient star formation process.« less

A strange dwarf scenario for the formation of the peculiar double white dwarf binary SDSS J125733.63+542850.5

NASA Astrophysics Data System (ADS)

Jiang, Long; Chen, Wen-Cong; Li, Xiang-Dong

2018-05-01

The Hubble Space Telescope observation of the double white dwarf (WD) binary SDSS J125733.63+542850.5 reveals that the massive WD has a surface gravity log g1 ˜ 8.7 (which implies a mass of M1 ˜ 1.06 M⊙) and an effective temperature T1 ˜ 13 000 K, while the effective temperature of the low-mass WD (M2 < 0.24 M⊙) is T2 ˜ 6400K. Therefore, the massive and the low-mass WDs have a cooling age τ1 ˜ 1 Gyr and τ2 ≥ 5 Gyr, respectively. This is in contradiction with traditional binary evolution theory. In this paper, we propose a strange dwarf (SD) scenario to explain the formation of this double WD binary. We assume that the massive WD is a SD originating from a phase transition (PT) in a ˜1.1 M⊙ WD, which has experienced accretion and spin-down processes. Its high effective temperature could arise from the heating process during the PT. Our simulations suggest that the progenitor of SDSS J125733.63+542850.5 can be a binary system consisting of a 0.65 M⊙ WD and a 1.5 M⊙ main-sequence star in a 1.492 d orbit. Especially, the secondary star (i.e. the progenitor of the low-mass WD) is likely to have an ultra-low metallicity of Z = 0.0001.

Towards Understanding The Origin And Evolution Of Ultra-Diffuse Galaxies

NASA Astrophysics Data System (ADS)

van der Burg, Remco F. J.; Sifón, Cristóbal; Muzzin, Adam; Hoekstra, Henk; KiDS Collaboration; GAMA Collaboration

2017-06-01

Recent observations have shown that Ultra-Diffuse Galaxies (UDGs, which have the luminosities of dwarfs but sizes of giant galaxies) are surprisingly abundant in clusters of galaxies. The origin of these galaxies remains unclear, since one would naively expect them to be easily disrupted by tidal interactions in the cluster environment. Several formation scenarios have been proposed for UDGs, but these make a wide range of different testable observational predictions. I'll summarise recent results on two key observables that have the potential to differentiate between the proposed models, namely 1) a measurement of their (sub)halo masses using weak gravitational lensing, and 2) their abundance in lower-mass haloes using data from the GAMA and KiDS surveys. I'll discuss implications and future prospects to learn more about the properties and formation histories of these elusive galaxies.

From Actinides to Zinc: Using the Full Abundance Pattern of the Brightest Star in Reticulum II to Distinguish between Different r-process Sites

NASA Astrophysics Data System (ADS)

Ji, Alexander P.; Frebel, Anna

2018-04-01

The ultra-faint dwarf galaxy Reticulum II was enriched by a rare and prolific r-process event, such as a neutron star merger (NSM). To investigate the nature of this event, we present high-resolution Magellan/MIKE spectroscopy of the brightest star in this galaxy. The high signal-to-noise allows us to determine the abundances of 41 elements, including the radioactive actinide element Th and first ever detections of third r-process peak elements (Os and Ir) in a star outside the Milky Way. The observed neutron-capture element abundances closely match the solar r-process component, except for the first r-process peak, which is significantly lower than solar but matches other r-process enhanced stars. The ratio of the first peak to heavier r-process elements implies that the r-process site produces roughly equal masses of high and low electron fraction ejecta, within a factor of 2. We compare the detailed abundance pattern to predictions from nucleosynthesis calculations of NSMs and magnetorotationally driven jet supernovae, finding that nuclear physics uncertainties dominate over astrophysical uncertainties. We measure {log}{{Th/Eu}}=-0.84+/- 0.06 ({stat})+/- 0.22 ({sys}), somewhat lower than all previous Th/Eu observations. The youngest age we derive from this ratio is 21.7 ± 2.8 (stat) ± 10.3 (sys) Gyr, indicating that current initial production ratios do not describe the r-process event in Reticulum II. The abundances of light elements up to Zn are consistent with extremely metal-poor Milky Way halo stars. They may eventually provide a way to distinguish between NSMs and magnetorotationally driven jet supernovae, but this would require more detailed knowledge of the chemical evolution of Reticulum II. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Surprising Rapid Collapse of Sirius B from Red Giant to White Dwarf Through Mass Transfer to Sirius a

NASA Astrophysics Data System (ADS)

Yousef, Shahinaz; Ali, Ola

2013-03-01

Sirius was observed in antiquity as a red star. In his famous astronomy textbook the Almagest written 140 AD, Ptolemy described the star Sirius as fiery red. He curiously depicted it as one of six red-colored stars. The other five are class M and K stars, such as Arcturus and Betelgeuse. Apparent confirmation in ancient Greek and Roman sources are found and Sirius was also reported red in Europe about 1400 years ago. Sirius must have changed to a white dwarf in the night of Ascension. The star chapter in the Quran started with "by the star as it collapsed (1) your companion have not gone astray nor being misled (2), and in verse 49 which is the rotation period of the companion Sirius B around Sirius A, it is said" He is the Lord of Sirius (49). If Sirius actually was red what could have caused it to change into the brilliant bluish-white star we see today? What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The red color indicates that the star seen then was a red giant. It looks that what they have seen in antiquity was Sirius B which was then a red giant and it collapsed to form a white dwarf. Since there is no evidence of a planetary nebula, then the red Sirius paradox can be solved in terms of stellar evolution with mass transfer. Sirius B was the most massive star which evolved to a red giant and filled the Roche lobe. Mass transfer to Sirius A occurred through the Lagrangian point. Sirius A then became more massive while Sirius B lost mass and shrank. Sirius B then collapsed abruptly into a white dwarf. In the case of Algol, Ptolmy observed it as white star but it was red at the time of El sufi. At present it is white. The rate of mass transfer from Sirius B to Sirius A, and from Algol B to A is estimated from observational data of colour change from red to bullish white to be 0.0021 and 0.0024 M⊙/yr respectively.

Fossil group origins. VIII. RX J075243.6+455653 a transitionary fossil group

NASA Astrophysics Data System (ADS)

Aguerri, J. A. L.; Longobardi, A.; Zarattini, S.; Kundert, A.; D'Onghia, E.; Domínguez-Palmero, L.

2018-01-01

Context. It is thought that fossil systems are relics of structure formation in the primitive Universe. They are galaxy aggregations that have assembled their mass at high redshift with few or no subsequent accretion. Observationally these systems are selected by large magnitude gaps between their 1st and 2nd ranked galaxies (Δm12). Nevertheless, there is still debate over whether or not this observational criterium selects dynamically evolved ancient systems. Aims: We have studied the properties of the nearby fossil group RX J075243.6+455653 in order to understand the mass assembly of this system. Methods: Deep spectroscopic observations allow us to construct the galaxy luminosity function (LF) of RX J075243.6+455653 down to Mr*+6. The analysis of the faint-end of the LF in groups and clusters provides valuable information about the mass assembly of the system. In addition, we have analyzed the nearby large-scale structure around this group. Results: We identified 26 group members within r200 0.96 Mpc. These galaxies are located at Vc = 15551 ± 65 km s-1 and have a velocity dispersion of σc = 333 ± 46 km s-1. The X-ray luminosity of the group is LX = 2.2 × 1043 h70-2 erg s-1, resulting in a mass of M = 4.2 × 1013 h70-1 within 0.5r200. The group has Δm12 = 2.1 within 0.5r200, confirming the fossil nature of this system. RX J075243.6+455653 has a central brightest group galaxy (BGG) with Mr = -22.67, one of the faintest BGGs observed in fossil systems. The LF of the group shows a flat faint-end slope (α = -1.08 ± 0.33). This low density of dwarf galaxies is confirmed by the low value of the dwarf-to-giant ratio (DGR = 0.99 ± 0.49) for this system. Both the lack of dwarf galaxies and the low luminosity of the BGG suggests that RX J075243.6+455653 still has to accrete mass from its nearby environment. This mass accretion will be achieved because it is the dominant structure of a rich environment formed by several groups of galaxies (15) within 7 Mpc from the group center and with ± 1000 km s-1. Conclusions: RX J075243.6+455653 is a group of galaxies that has not yet completed the process of its mass assembly. This new mass accretion will change the fossil state of the group. This group is an example of a galaxy aggregation selected by a large magnitude gap but still in the process of the accretion of its mass.

(Almost) Dark Galaxies in the ALFALFA Survey: Isolated H I-bearing Ultra-diffuse Galaxies

NASA Astrophysics Data System (ADS)

Leisman, Lukas; Haynes, Martha P.; Janowiecki, Steven; Hallenbeck, Gregory; Józsa, Gyula; Giovanelli, Riccardo; Adams, Elizabeth A. K.; Bernal Neira, David; Cannon, John M.; Janesh, William F.; Rhode, Katherine L.; Salzer, John J.

2017-06-01

We present a sample of 115 very low optical surface brightness, highly extended, H I-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface brightnesses, and radii to recently discovered “ultra-diffuse” galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of H I. We find that while these sources have normal star formation rates for H I-selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and H I-synthesis follow-up imaging of three of these H I-bearing ultra-diffuse sources. We measure H I diameters extending to ˜40 kpc, but note that while all three sources have large H I diameters for their stellar mass, they are consistent with the H I mass-H I radius relation. We further analyze the H I velocity widths and rotation velocities for the unresolved and resolved sources, respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential H I-rich progenitors to the ultra-diffuse galaxies observed in cluster environments.

An Unusual Stellar Death on Christmas Day

NASA Technical Reports Server (NTRS)

Thone, C. C.; de Ugarte Postigo, A.; Fryer, C. L.; Page, K. L.; Gorosabel, J.; Aloy, M. A.; Perley, D. A.; Kouveliotou, C.; Janka, H. T.; Mimica, P.;

Discovery of Rotational Modulations in the Planetary-mass Companion 2M1207b: Intermediate Rotation Period and Heterogeneous Clouds in a Low Gravity Atmosphere

NASA Astrophysics Data System (ADS)

Zhou, Yifan; Apai, Dániel; Schneider, Glenn H.; Marley, Mark S.; Showman, Adam P.

2016-02-01

Rotational modulations of brown dwarfs have recently provided powerful constraints on the properties of ultra-cool atmospheres, including longitudinal and vertical cloud structures and cloud evolution. Furthermore, periodic light curves directly probe the rotational periods of ultra-cool objects. We present here, for the first time, time-resolved high-precision photometric measurements of a planetary-mass companion, 2M1207b. We observed the binary system with Hubble Space Telescope/Wide Field Camera 3 in two bands and with two spacecraft roll angles. Using point-spread function-based photometry, we reach a nearly photon-noise limited accuracy for both the primary and the secondary. While the primary is consistent with a flat light curve, the secondary shows modulations that are clearly detected in the combined light curve as well as in different subsets of the data. The amplitudes are 1.36% in the F125W and 0.78% in the F160W filters, respectively. By fitting sine waves to the light curves, we find a consistent period of {10.7}-0.6+1.2 hr and similar phases in both bands. The J- and H-band amplitude ratio of 2M1207b is very similar to a field brown dwarf that has identical spectral type but different J-H color. Importantly, our study also measures, for the first time, the rotation period for a directly imaged extra-solar planetary-mass companion.

Minor Contribution of Quasars to Ionizing Photon Budget at z ˜ 6: Update on Quasar Luminosity Function at the Faint End with Subaru/Suprime-Cam

NASA Astrophysics Data System (ADS)

Onoue, Masafusa; Kashikawa, Nobunari; Willott, Chris J.; Hibon, Pascale; Im, Myungshin; Furusawa, Hisanori; Harikane, Yuichi; Imanishi, Masatoshi; Ishikawa, Shogo; Kikuta, Satoshi; Matsuoka, Yoshiki; Nagao, Tohru; Niino, Yuu; Ono, Yoshiaki; Ouchi, Masami; Tanaka, Masayuki; Tang, Ji-Jia; Toshikawa, Jun; Uchiyama, Hisakazu

2017-10-01

We constrain the quasar contribution to the cosmic reionization based on our deep optical survey of z ˜ 6 quasars down to z R = 24.15 using Subaru/Suprime-Cam in three UKIDSS-DXS fields covering 6.5 deg2. In Kashikawa et al. (2015), we select 17 quasar candidates and report our initial discovery of two low-luminosity quasars ({M}1450˜ -23) from seven targets, one of which might be a Lyα-emitting galaxy. From an additional optical spectroscopy, none of the four candidates out of the remaining 10 turn out to be genuine quasars. Moreover, the deeper optical photometry provided by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) shows that, unlike the two already-known quasars, the I - z and z - y colors of the last six candidates are consistent with M- or L-type brown dwarfs. Therefore, the quasar luminosity function (QLF) measurement in the previous paper is confirmed. Compiling the QLF measurements from the literature over a wide magnitude range, including an extremely faint AGN candidate from Parsa et al. (2017), to fit them with a double power law, we find that the best-fit faint-end slope is α =-{2.04}-0.18+0.33 (-{1.98}-0.21+0.48) and characteristic magnitude is {M}1450* =-{25.8}-1.9+1.1 (-{25.7}-1.8+1.0) in the case of two (one) quasar detection. Our result suggests that, if the QLF is integrated down to {M}1450=-18, quasars produce ˜1%-12% of the ionizing photons required to fully ionize the universe at z ˜ 6 with a 2σ confidence level, assuming that the escape fraction is {f}{esc}=1 and the intergalactic medium clumpy factor is C = 3. Even when the systematic uncertainties are taken into account, our result supports the scenario that quasars are the minor contributors of the reionization.

The Evolution of the Faint End of the UV Luminosity Function during the Peak Epoch of Star Formation (1 < z < 3)

NASA Astrophysics Data System (ADS)

Alavi, Anahita; Siana, Brian; Richard, Johan; Rafelski, Marc; Jauzac, Mathilde; Limousin, Marceau; Freeman, William R.; Scarlata, Claudia; Robertson, Brant; Stark, Daniel P.; Teplitz, Harry I.; Desai, Vandana

2016-11-01

We present a robust measurement of the rest-frame UV luminosity function (LF) and its evolution during the peak epoch of cosmic star formation at 1\\lt z\\lt 3. We use our deep near-ultraviolet imaging from WFC3/UVIS on the Hubble Space Telescope and existing Advanced Camera for Surveys (ACS)/WFC and WFC3/IR imaging of three lensing galaxy clusters, Abell 2744 and MACS J0717 from the Hubble Frontier Field survey and Abell 1689. Combining deep UV imaging and high magnification from strong gravitational lensing, we use photometric redshifts to identify 780 ultra-faint galaxies with {M}{UV}\\lt -12.5 AB mag at 1\\lt z\\lt 3. From these samples, we identified five new, faint, multiply imaged systems in A1689. We run a Monte Carlo simulation to estimate the completeness correction and effective volume for each cluster using the latest published lensing models. We compute the rest-frame UV LF and find the best-fit faint-end slopes of α =-1.56+/- 0.04, α =-1.72+/- 0.04, and α =-1.94+/- 0.06 at 1.0\\lt z\\lt 1.6, 1.6\\lt z\\lt 2.2, and 2.2\\lt z\\lt 3.0, respectively. Our results demonstrate that the UV LF becomes steeper from z˜ 1.3 to z˜ 2.6 with no sign of a turnover down to {M}{UV}=-14 AB mag. We further derive the UV LFs using the Lyman break “dropout” selection and confirm the robustness of our conclusions against different selection methodologies. Because the sample sizes are so large and extend to such faint luminosities, the statistical uncertainties are quite small, and systematic uncertainties (due to the assumed size distribution, for example) likely dominate. If we restrict our analysis to galaxies and volumes above \\gt 50 % completeness in order to minimize these systematics, we still find that the faint-end slope is steep and getting steeper with redshift, though with slightly shallower (less negative) values (α =-1.55+/- 0.06, -1.69 ± 0.07, and -1.79 ± 0.08 for z˜ 1.3, 1.9, and 2.6, respectively). Finally, we conclude that the faint star-forming galaxies with UV magnitudes of -18.5\\lt {M}{UV}\\lt -12.5 covered in this study produce the majority (55%-60%) of the unobscured UV luminosity density at 1\\lt z\\lt 3. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B

NASA Astrophysics Data System (ADS)

Zhou, Yifan; Apai, Dániel; Metchev, Stanimir; Lew, Ben W. P.; Schneider, Glenn; Marley, Mark S.; Karalidi, Theodora; Manjavacas, Elena; Bedin, Luigi R.; Cowan, Nicolas B.; Miles-Páez, Paulo A.; Lowrance, Patrick J.; Radigan, Jacqueline; Burgasser, Adam J.

2018-03-01

Time-resolved observations of brown dwarfs’ rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres. Multi-wavelength light curves reveal cloud vertical structures, condensate particle sizes, and cloud morphology, which directly constrain condensate cloud and atmospheric circulation models. We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared G141 taken in six consecutive orbits observations of HN Peg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the 1.1–1.7 μm broadband light curve has an amplitude of 1.206% ± 0.025% and period of 15.4 ± 0.5 hr. The modulation amplitude has no detectable wavelength dependence except in the 1.4 μm water absorption band, indicating that the characteristic condensate particle sizes are large (>1 μm). We detect significantly (4.4σ) lower modulation amplitude in the 1.4 μm water absorption band and find that HN Peg B’s spectral modulation resembles those of early T type brown dwarfs. We also describe a new empirical interpolation method to remove spectral contamination from the bright host star. This method may be applied in other high-contrast time-resolved observations with WFC3.

Searching Ultra-compact Pulsar Binaries with Abnormal Timing Behavior

NASA Astrophysics Data System (ADS)

Gong, B. P.; Li, Y. P.; Yuan, J. P.; Tian, J.; Zhang, Y. Y.; Li, D.; Jiang, B.; Li, X. D.; Wang, H. G.; Zou, Y. C.; Shao, L. J.

2018-03-01

Ultra-compact pulsar binaries are both ideal sources of gravitational radiation for gravitational wave detectors and laboratories for fundamental physics. However, the shortest orbital period of all radio pulsar binaries is currently 1.6 hr. The absence of pulsar binaries with a shorter orbital period is most likely due to technique limit. This paper points out that a tidal effect occurring on pulsar binaries with a short orbital period can perturb the orbital elements and result in a significant change in orbital modulation, which dramatically reduces the sensitivity of the acceleration searching that is widely used. Here a new search is proposed. The abnormal timing residual exhibited in a single pulse observation is simulated by a tidal effect occurring on an ultra-compact binary. The reproduction of the main features represented by the sharp peaks displayed in the abnormal timing behavior suggests that pulsars like PSR B0919+06 could be a candidate for an ultra-compact binary of an orbital period of ∼10 minutes and a companion star of a white dwarf star. The binary nature of such a candidate is further tested by (1) comparing the predicted long-term binary effect with decades of timing noise observed and (2) observing the optical counterpart of the expected companion star. Test (1) likely supports our model, while more observations are needed in test (2). Some interesting ultra-compact binaries could be found in the near future by applying such a new approach to other binary candidates.

Faintest Methane Brown Dwarf Discovered with the NTT and VLT

NASA Astrophysics Data System (ADS)

1999-08-01

A team of European astronomers [1] has found a cold and extremely faint object in interstellar space, high above the galactic plane. It is a Methane Brown Dwarf of which only a few are known. This is by far the most distant one identified to date. Brown Dwarfs are star-like objects which are heavier than planets but not massive enough to trigger the nuclear burning of hydrogen and other elements which powers normal stars. They are, nevertheless, heated during their formation by gravitational contraction but then continuously cool as this energy is radiated away. The so-called Methane Brown Dwarfs are the coolest members of the class detected so far, with temperatures around 700 °C, i.e. around 1000 degrees cooler than the coldest stars. The new object, provisionally known as NTTDF J1205-0744 , was found during a deep survey of a small sky region in the constellation Virgo (The Virgin), just south of the celestial equator. The chances of identifying a rare object like this in such a restricted area are very small and the astronomers readily admit that they must have been very lucky. This is the story of an (unexpected) astronomical discovery that may prove to be very important for galactic studies. It also demonstrates the power of modern observational techniques. The NTT Deep Field A long series of exposures of a small sky field in Virgo were made in 1997 and 1998 with the ESO 3.58-m New Technology Telescope (NTT) at La Silla. They were carried out with the aim of measuring and demonstrating the limiting performance of two astronomical instruments at this telescope, the SUperb-Seeing Imager (SUSI) in the visible part of the spectrum (0.35 - 1.00 µm), and the multi-mode Son of ISAAC (SOFI) in the near-infrared region (1.0 - 2.5 µm). The observed sky area measures only 2.3 x 2.3 arcmin 2 and is referred to as the NTT Deep Field. It has been studied in great detail, in particular to identify very distant galaxies for spectroscopic follow-up observations with the FORS1 and ISAAC instruments at the VLT 8.2-m ANTU telescope during the first period of VLT observations. Such distant objects are quite red (due to their high redshift) and are best detected by a combination of visible and infrared exposures. Discovery of an extremely infrared object ESO PR Photo 35a/99 ESO PR Photo 35a/99 [Preview - JPEG: 400 x 251 pix - 72k] [Normal - JPEG: 800 x 502 pix - 224k] [High-Res - JPEG: 3000 x 1881 pix - 1.7M] Caption to ESO PR Photo 35a/99 : Part of the NTT Deep Field , with the new Methane Brown Dwarf NTTDF J1205-0744 at the centre. The field measures 1.3 x 1.3 arcmin 2. The object is well visible in the SOFI infrared exposure (left) in the J-band at wavelength 1.25 µm, but not in the SUSI one at a shorter wavelength (right) in the i-band at 0.8 µm. North is up and East is left. The astronomers noted a star-like object of extreme colour in this field. While it was well visible and similarly bright in both SOFI infrared images (J = 20.2 and K = 20.3), it could not be seen at all on the SUSI images in the visible spectral region, even at the longest wavelength (i-band) observed with that instrument (i-J > 6 mag), cf. PR Photo 35a/99 . No "normal" object is known to have such extreme colours. The new object now received the designation NTTDF J1205-0744 , indicating that it was discovered in the NTT Deep Field at the given position on the sky. It seemed that there were only two possibilities. Either it was an extremely distant quasar (redshift about 8) at the edge of the observable universe, or it must be a very cold object in the Milky Way Galaxy. Whatever its nature, this was obviously a most interesting object. Spectroscopic observations of NTTDF J1205-0744 ESO PR Photo 35b/99 ESO PR Photo 35b/99 [Preview - JPEG: 400 x 337 pix - 56k] [Normal - JPEG: 800 x 674 pix - 124k] Caption to ESO PR Photo 35b/99 : The infrared spectrum of NTTDF J1205-0744 , as obtained with SOFI at the NTT and ISAAC at VLT ANTU, and compared to the spectrum of the much closer and brighter Methane Brown Dwarf Gliese 229B . This issue was resolved by obtaining infrared spectra of NTTDF J1205-0744 . Despite its faintness, initial observations with SOFI at the NTT covering the infrared J and H-bands already revealed some of the molecular absorptions characteristic of methane brown dwarfs. More recently, complementary longer wavelength observations with ISAAC at the first VLT 8.2-m Unit Telescope (ANTU) at Paranal have now confirmed the nature of this object. The combined SOFI/ISAAC infrared spectrum shown in PR Photo 35b/99 is clearly extremely similar to that of Gliese 229B , the first Methane Brown Dwarf discovered a few years ago and which is a member of a binary system at a distance of about 19 light-years. The features in the spectra result from strong absorption by methane (CH 4 ) and water (H 2 O). There is thus no doubt that NTTDF J1205-0744 is of the same type (stellar class T). Unlike Gliese 229B , however, it does not appear to be a member of a binary system. It is also 5-6 magnitudes (i.e., a factor of about 250) fainter than this and a few similar objects discovered recently in large-area sky surveys, implying that it is considerably more distant. Properties of NTTDF J1205-0744 NTTDF J1205-0744 is located at a distance of about 300 light-years (90 pc) and some 240 light-years (75 pc) above the plane of our Milky Way galaxy. Its mass is probably about 20-50 times that of Jupiter, or less than 2% of that of the Sun. Its temperature is around 700 °C (1000 K), suggesting an age of 500 to 1,000 million years. Lacking a stable source of energy at its centre, it is becoming continuously fainter and cooler and will continue to do so for tens of thousands of millions of years. NTTDF J1205-0744 is a very faint and small object indeed, on the still not well understood border zone between stars and planets [2]. How many Brown Dwarfs? How many T-class objects are there in the Milky Way? What is the space density of these extreme objects? Since only a few have been identified so far, any statistics must be quite uncertain. Until now, the best estimates have been of the order of 1 per 3,500 cubic light-years (0.01/pc 3 ). A surprising aspect of this discovery is that NTTDF J1205-0744 was found within a sky area of only 2.3 x 2.3 arcmin 2 , specially selected to be as "empty" as possible in order to facilitate studies of distant galaxies. Based on the above density estimate, the chance of finding such an object should only have been about 1%. Based on model predictions, the chance would have been even smaller than this. Searches like the one described here, based on the combination of optical and infrared data, therefore appear particularly effective at detecting such objects. It is now of high interest to test if this first discovery was just extremely lucky, or if the space density of these extreme objects is in fact much higher than expected. More information A research article about these new results ( Discovery of a faint Field Methane Brown Dwarf from ES0 NTT and VLT observations), will appear in the European journal Astronomy & Astrophysics . Note [1] The team consists of Jean Gabriel Cuby, Alan Moorwood, Sandro D'Odorico, Chris Lidman, Fernando Comeron, Jason Spyromilio (ESO) and Paolo Saracco (Osservatorio Astronomico di Brera, Merate, Milan, Italy). [2] A more nearby, hotter brown dwarf, KELU-1 , was found at La Silla in 1997 at a distance of 33 light-years, cf. ESO Press Release 07/97. How to obtain ESO Press Information ESO Press Information is made available on the World-Wide Web (URL: http://www.eso.org../ ). ESO Press Photos may be reproduced, if credit is given to the European Southern Observatory.

Searching for Ultra-cool Objects at the Limits of Large-scale Surveys

NASA Astrophysics Data System (ADS)

Pinfield, D. J.; Patel, K.; Zhang, Z.; Gomes, J.; Burningham, B.; Day-Jones, A. C.; Jenkins, J.

2011-12-01

We have made a search (to Y=19.6) of the UKIDSS Large Area Survey (LAS DR7) for objects detected only in the Y-band. We have identified and removed contamination due to solar system objects, dust specs in the WFCAM optical path, persistence in the WFCAM detectors, and other sources of spurious single source Y-detections in the UKIDSS LAS data-base. In addition to our automated selection procedure we have visually inspected the ˜600 automatically selected candidates to provide an additional level of quality filtering. This has resulted in 55 good candidates that await follow-up observations to confirm their nature. Ultra-cool LAS Y-only objects would have blue Y-J colours combined with very red optical-NIR SEDs - characteristics shared by Jupiter, and suggested by an extrapolation of the Y-J colour trend seen for the latest T dwarfs currently known.

Ultra high energy gamma rays, cosmic rays and neutrinos from accreting degenerate stars

NASA Technical Reports Server (NTRS)

Brecher, K.; Chanmugam, G.

1985-01-01

Super-Eddington accretion for a recently proposed unipolar induction model of cosmic ray acceleration in accreting binary star systems containing magnetic white dwarfs or neutron stars is considered. For sufficiently high accretion rates and low magnetic fields, the model can account for: (1) acceleration of cosmic ray nuclei up to energies of 10 to the 19th power eV; (2) production of more or less normal solar cosmic ray composition; (3) the bulk of cosmic rays observed with energies above 1 TeV, and probably even down to somewhat lower energies as well; and (4) possibly the observed antiproton cosmic ray flux. It can also account for the high ultra high energy (UHE) gamma ray flux observed from several accreting binary systems (including Cygnus X-3), while allowing the possibility of an even higher neutrino flux from these sources, with L sub nu/L sub gamma is approximately 100.

OGLE-2016-BLG-0168 Binary Microlensing Event: Prediction and Confirmation of the Microlens Parallax Effect from Space-based Observations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shin, I.-G.; Yee, J. C.; Jung, Y. K.

The microlens parallax is a crucial observable for conclusively identifying the nature of lens systems in microlensing events containing or composed of faint (even dark) astronomical objects such as planets, neutron stars, brown dwarfs, and black holes. With the commencement of a new era of microlensing in collaboration with space-based observations, the microlens parallax can be routinely measured. In addition, space-based observations can provide opportunities to verify the microlens parallax measured from ground-only observations and to find a unique solution to the lensing light-curve analysis. Furthermore, since most space-based observations cannot cover the full light curves of lensing events, itmore » is also necessary to verify the reliability of the information extracted from fragmentary space-based light curves. We conduct a test based on the microlensing event OGLE-2016-BLG-0168, created by a binary lens system consisting of almost equal mass M-dwarf stars, to demonstrate that it is possible to verify the microlens parallax and to resolve degeneracies using the space-based light curve even though the observations are fragmentary. Since space-based observatories will frequently produce fragmentary light curves due to their short observing windows, the methodology of this test will be useful for next-generation microlensing experiments that combine space-based and ground-based collaboration.« less

EVERY INTERACTING DOUBLE WHITE DWARF BINARY MAY MERGE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shen, Ken J.

2015-05-20

Interacting double white dwarf (WD) binaries can give rise to a wide variety of astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae to the formation of neutron stars and stably accreting AM Canum Venaticorum systems. One key factor affecting the final outcome is whether mass transfer remains dynamically stable or instead diverges, leading to the tidal disruption of the donor and the merger of the binary. It is typically thought that for low ratios of the donor mass to the accretor mass, mass transfer remains stable, especially if accretion occurs via a disk. In this Letter, we examinemore » low mass ratio double WD binaries and find that the initial phase of hydrogen-rich mass transfer leads to a classical nova-like outburst on the accretor. Dynamical friction within the expanding nova shell shrinks the orbit and causes the mass transfer rate to increase dramatically above the accretor's Eddington limit, possibly resulting in a binary merger. If the binary survives the first hydrogen-rich nova outbursts, dynamical friction within the subsequent helium-powered nova shells pushes the system even more strongly toward merger. While further calculations are necessary to confirm this outcome for the entire range of binaries previously thought to be dynamically stable, it appears likely that most, if not all, interacting double WD binaries will merge during the course of their evolution.« less

NGST: Exploring the Fossil Record of Galaxy Formation

NASA Technical Reports Server (NTRS)

Rich, R. Michael

1998-01-01

During this grant period the investigator has accomplished the following: developed and researched the science case for high resolution optical imaging with NGST (Next Generation Space Telescope); presented to the ASWG (Ad-Hoc Science Working Group) on the use of NGST to image and measure the proper motions of white dwarf stars that could account for the MACHO events toward the LMC (Large Magellanic Cloud); wrote proposals for the Design Reference Mission. Three proposals have been written on the following topics: Measuring the faint end of the white dwarf luminosity function to get an independent measure of the age of the oldest stars, measurement of the stellar luminosity function over the full range of age, abundance, and population type, and deep imaging of Local Group halos to measure the relative ages of the oldest stars in the Milky Way and other galaxies; he has introduced simulations of crowded field stellar photometry on a hypothetical population in the M31 halo. Using a physically correct luminosity function and a surface brightness of 24.5 mag/sq. arcsec, NGST imaging in 10 hours easily measures the turnoff and reaches 3 mags down the main sequence in the halo of M31; and has delivered talks at the NGST workshop in Liege, and at the meeting in Paris entitled "Connecting the Distant Universe with the Local Fossil Record".

THE NEW ECLIPSING CV MASTER OTJ192328.22+612413.5—A POSSIBLE SW SEXTANTIS STAR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kennedy, M. R.; Callanan, P.; Bouanane, S.

2016-07-01

We present optical photometry and spectroscopy of the new eclipsing cataclysmic variable MASTER OTJ192328.22+612413.5, discovered by the MASTER team. We find the orbital period to be P = 0.16764612(5) day/4.023507(1) hr. The depth of the eclipse (2.9 ± 0.1 mag) suggests that the system is nearly edge on, and modeling of the system confirms the inclination to be between 81.°3 and 83.°6. The brightness outside the eclipse varies between observations, with a change of 1.6 ± 0.1 mag. Spectroscopy reveals double-peaked Balmer emission lines. By using spectral features matching a late M-type companion, we bound the distance to be 750more » ± 250 pc, depending on the companion’s spectral type. The source displays 2 mag brightness changes on timescales of days. The amplitude of these changes, along with the spectrum at the faint state, suggest that the system is possibly a dwarf nova. The lack of any high-excitation He ii lines suggests that this system is not magnetically dominated. The light curve in both quiescence and outburst resembles that of Lanning 386, implying MASTER OTJ192328.22+612413.5 is a possible cross between a dwarf nova and a SW Sextantis star.« less

OGLE-2016-BLG-0168 Binary Microlensing Event: Prediction and Confirmation of the Microlens Parallax Effect from Space-based Observations

NASA Astrophysics Data System (ADS)

Shin, I.-G.; Udalski, A.; Yee, J. C.; Calchi Novati, S.; Han, C.; Skowron, J.; Mróz, P.; Soszyński, I.; Poleski, R.; Szymański, M. K.; Kozłowski, S.; Pietrukowicz, P.; Ulaczyk, K.; Pawlak, M.; OGLE Collaboration; Albrow, M. D.; Gould, A.; Chung, S.-J.; Hwang, K.-H.; Jung, Y. K.; Ryu, Y.-H.; Zhu, W.; Cha, S.-M.; Kim, D.-J.; Kim, H.-W.; Kim, S.-L.; Lee, C.-U.; Lee, Y.; Park, B.-G.; Pogge, R. W.; KMTNet Group; Beichman, C.; Bryden, G.; Carey, S.; Gaudi, B. S.; Henderson, C. B.; Shvartzvald, Y.; Spitzer Team

2017-11-01

The microlens parallax is a crucial observable for conclusively identifying the nature of lens systems in microlensing events containing or composed of faint (even dark) astronomical objects such as planets, neutron stars, brown dwarfs, and black holes. With the commencement of a new era of microlensing in collaboration with space-based observations, the microlens parallax can be routinely measured. In addition, space-based observations can provide opportunities to verify the microlens parallax measured from ground-only observations and to find a unique solution to the lensing light-curve analysis. Furthermore, since most space-based observations cannot cover the full light curves of lensing events, it is also necessary to verify the reliability of the information extracted from fragmentary space-based light curves. We conduct a test based on the microlensing event OGLE-2016-BLG-0168, created by a binary lens system consisting of almost equal mass M-dwarf stars, to demonstrate that it is possible to verify the microlens parallax and to resolve degeneracies using the space-based light curve even though the observations are fragmentary. Since space-based observatories will frequently produce fragmentary light curves due to their short observing windows, the methodology of this test will be useful for next-generation microlensing experiments that combine space-based and ground-based collaboration.

NEAR-INFRARED THERMAL EMISSION DETECTIONS OF A NUMBER OF HOT JUPITERS AND THE SYSTEMATICS OF GROUND-BASED NEAR-INFRARED PHOTOMETRY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Croll, Bryce; Albert, Loic; Lafreniere, David

We present detections of the near-infrared thermal emission of three hot Jupiters and one brown dwarf using the Wide-field Infrared Camera (WIRCam) on the Canada-France-Hawaii Telescope (CFHT). These include Ks-band secondary eclipse detections of the hot Jupiters WASP-3b and Qatar-1b and the brown dwarf KELT-1b. We also report Y-band, K {sub CONT}-band, and two new and one reanalyzed Ks-band detections of the thermal emission of the hot Jupiter WASP-12b. We present a new reduction pipeline for CFHT/WIRCam data, which is optimized for high precision photometry. We also describe novel techniques for constraining systematic errors in ground-based near-infrared photometry, so asmore » to return reliable secondary eclipse depths and uncertainties. We discuss the noise properties of our ground-based photometry for wavelengths spanning the near-infrared (the YJHK bands), for faint and bright stars, and for the same object on several occasions. For the hot Jupiters WASP-3b and WASP-12b we demonstrate the repeatability of our eclipse depth measurements in the Ks band; we therefore place stringent limits on the systematics of ground-based, near-infrared photometry, and also rule out violent weather changes in the deep, high pressure atmospheres of these two hot Jupiters at the epochs of our observations.« less

Searching for faint comoving companions to the α Centauri system in the VVV survey infrared images

NASA Astrophysics Data System (ADS)

Beamín, J. C.; Minniti, D.; Pullen, J. B.; Ivanov, V. D.; Bendek, E.; Bayo, A.; Gromadzki, M.; Kurtev, R.; Lucas, P. W.; Butler, R. P.

2017-12-01

The VVV survey has observed the southern disc of the Milky Way in the near-infrared, covering 240 deg2 in the ZYJHKs filters. We search the VVV survey images in a ∼19 deg2 field around α Centauri, the nearest stellar system to the Sun, to look for possible overlooked companions that the baseline in time of VVV would be able to uncover. The photometric depth of our search reaches Y ∼ 19.3 mag, J ∼ 19 mag, and Ks ∼ 17 mag. This search has yielded no new companions in α Centauri system, setting an upper mass limit for any unseen companion well into the brown dwarf/planetary mass regime. The apparent magnitude limits were turned into effective temperature limits, and the presence of companion objects with effective temperatures warmer than 325 K can be ruled out using different state-of-the-art atmospheric models. These limits were transformed into mass limits using evolutionary models, companions with masses above 11MJup were discarded, extending the constraints recently provided in the literature up to projected distances of d < 7000 au from α Cen AB and ∼1 200 au from Proxima. In the next few years, the VVV extended survey (VVVX) will allow us to extend the search and place similar limits on brown dwarfs/planetary companions to α Cen AB for separations up to 20 000 au.

A DWARF NOVA IN THE GLOBULAR CLUSTER M13

DOE Office of Scientific and Technical Information (OSTI.GOV)

Servillat, M.; Van den Berg, M.; Grindlay, J.

Dwarf novae (DNe) in globular clusters (GCs) seem to be rare with only 13 detections in the 157 known Galactic GCs. We report the identification of a new DN in M13, the 14th DN identified in a GC to date. Using the 2 m Faulkes Telescope North, we conducted a search for stars in M13 that show variability over a year (2005-2006) on timescales of days and months. This led to the detection of one DN showing several outbursts. A Chandra X-ray source is coincident with this DN and shows both a spectrum and variability consistent with that expected frommore » a DN, thus supporting the identification. We searched for a counterpart in Hubble Space Telescope Advanced Camera for Surveys/Wide Field Camera archived images and found at least 11 candidates, of which we could characterize only the 7 brightest, including one with a 3{sigma} H{alpha} excess and a faint blue star. The detection of one DN when more could have been expected likely indicates that our knowledge of the global Galactic population of cataclysmic variables is too limited. The proportion of DNe may be lower than found in catalogs, or they may have a much smaller mean duty cycle ({approx}1%) as proposed by some population synthesis models and recent observations in the field.« less

A Universe of ultradiffuse galaxies: theoretical predictions from ΛCDM simulations

NASA Astrophysics Data System (ADS)

Rong, Yu; Guo, Qi; Gao, Liang; Liao, Shihong; Xie, Lizhi; Puzia, Thomas H.; Sun, Shuangpeng; Pan, Jun

2017-10-01

A particular population of galaxies have drawn much interest recently, which are as faint as typical dwarf galaxies but have the sizes as large as L* galaxies, the so called ultradiffuse galaxies (UDGs). The lack of tidal features of UDGs in dense environments suggests that their host haloes are perhaps as massive as that of the Milky Way. On the other hand, galaxy formation efficiency should be much higher in the haloes of such masses. Here, we use the model galaxy catalogue generated by populating two large simulations: the Millennium-II cosmological simulation and Phoenix simulations of nine big clusters with the semi-analytic galaxy formation model. This model reproduces remarkably well the observed properties of UDGs in the nearby clusters, including the abundance, profile, colour and morphology, etc. We search for UDG candidates using the public data and find two UDG candidates in our Local Group and 23 in our Local Volume, in excellent agreement with the model predictions. We demonstrate that UDGs are genuine dwarf galaxies, formed in the haloes of ˜1010 M⊙. It is the combination of the late formation time and high spins of the host haloes that results in the spatially extended feature of this particular population. The lack of tidal disruption features of UDGs in clusters can also be explained by their late infall-time.

SPECTROSCOPIC CONFIRMATION OF THE DWARF SPHEROIDAL GALAXY d0994+71 AS A MEMBER OF THE M81 GROUP OF GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Toloba, Elisa; Sand, David; Crnojević, Denija

2016-10-10

We use Keck/DEIMOS spectroscopy to measure the first velocity and metallicity of a dwarf spheroidal (dSph) galaxy beyond the Local Group using resolved stars. Our target, d0944+71, is a faint dSph found in the halo of the massive spiral galaxy M81 by Chiboucas et al. We coadd the spectra of 27 individual stars and measure a heliocentric radial velocity of −38 ± 10 km s{sup −1}. This velocity is consistent with d0944+71 being gravitationally bound to M81. We coadd the spectra of the 23 stars that are consistent with being red giant branch stars and measure an overall metallicity ofmore » [Fe/H] = −1.3 ± 0.3 based on the calcium triplet lines. This metallicity is consistent with d0944+71 following the metallicity−luminosity relation for Local Group dSphs. We investigate several potential sources of observational bias but find that our sample of targeted stars is representative of the metallicity distribution function of d0944+71 and any stellar contamination due to seeing effects is negligible. The low ellipticity of the galaxy and its position in the metallicity−luminosity relation suggest that d0944+71 has not been affected by strong tidal stripping.« less

New spectroscopic tools and techniques for characterizing M dwarfs and discovering their planets in the near-infrared

NASA Astrophysics Data System (ADS)

Terrien, Ryan C.

M dwarfs are the least massive and most common stars in the Galaxy. Due to their prevalence and long lifetimes, these diminutive stars play an outsize role in several fields of astronomical study. In particular, it is now known that they commonly host planetary systems, and may be the most common hosts of Earth-size, rocky planets in the habitable zone. A comprehensive understanding of M dwarfs is crucial for understanding the origins and conditions of their planetary systems, including their potential habitability. Such an understanding depends on methods for precisely and accurately measuring their properties. These tools have broader applicability as well, underlying the use of M dwarfs as fossils of Galactic evolution, and helping to constrain the structures and interiors of these stars. The measurement of the fundamental parameters of M dwarfs is encumbered by their spectral complexity. Unlike stars of spectral type F, G, or K that are similar to our G type Sun, whose spectra are dominated by continuum emission and atomic features, the cool atmospheres of M dwarfs are dominated by complex molecular absorption. Another challenge for studies of M dwarfs is that these stars are optically faint, emitting much of their radiation in the near-infrared (NIR). The availability and performance of NIR spectrographs have lagged behind those of optical spectrographs due to the challenges of producing low-noise, high-sensitivity NIR detector arrays, which have only recently become available. This thesis discusses two related lines of work that address these challenges, motivated by the development of the Habitable Zone Planet Finder (HPF), a NIR radial velocity (RV) spectrograph under development at Penn State that will search for and confirm planets around nearby M dwarfs. This work includes the development and application of new NIR spectroscopic techniques for characterizing M dwarfs, and the development and optimization of new NIR instrumentation for HPF. The first line of work is centered on a large NIR spectroscopic survey of nearby M dwarfs, undertaken to characterize potential targets for HPF. This survey, and new techniques for measuring M dwarf metallicity, are the subject of Chapter 2. These data will provide crucial information to assess planetary composition, and the stellar metallicities will help us understand the process of planet formation around M dwarfs. These techniques have also enabled strong tests of low-mass stellar models in the benchmark eclipsing binary system CM Draconis, and have helped identify potential directions for improvement in the models, as presented in Chapter 3. The development of new spectroscopic indices for measuring M dwarf luminosity, radius, and potentially alpha-element abundance is discussed in Chapter 4. Finally, Chapter 5 presents a synthesis of these M dwarf characterization techniques and radial velocity (RV) measurements from the SDSS-III APOGEE spectrograph, which we applied to confirm and characterize the first M dwarfs in the nearby Coma Berenices cluster. The second line of work relates to the optimization of HPF. By targeting M dwarfs, HPF will take advantage of the large signal induced by an Earth-mass planet orbiting an M dwarf compared to the same planet orbiting an FGK star. Chapter 6 discusses a number of design trades and parameter optimizations undertaken in order to ensure the best sensitivity to Earth-mass planets. These subtopics include the optimization of the HPF resolution, bandpass, operating temperature, and vacuum phase holographic cross-disperser, as well as prediction of anticipated HPF performance, and the development of an HPF software simulator tool. In carrying out NIR detector tests for HPF, we have also tested an optical filter that selectively blocks long-wavelength thermal background radiation. This type of contamination is a perennial source of noise for NIR instruments, and typically forces these instruments to operate fully cryogenically. The complexity and cost of this approach may be avoided: for instruments operating in the H-band or bluer, the thermal background can be optically filtered, freeing the instrument to operate at warmer temperatures. Chapter 7 details our characterization and application of an interference filter that effectively blocks thermal background when used with a 1.7mum-cutoff HAWAII-2RG NIR detector array. By effectively filtering the thermal background with a single coated optic, this filter offers the potential for simple, cost-effective, warm-pupil NIR astronomical instruments, which can take advantage of the increasing availability of low-noise, high-efficiency NIR detectors.

X-ray and optical observations of four polars

NASA Astrophysics Data System (ADS)

Worpel, H.; Schwope, A. D.; Granzer, T.; Reinsch, K.; Schwarz, R.; Traulsen, I.

2016-08-01

Aims: We investigate the temporal and spectral behaviour of four polar cataclysmic variables from the infrared to X-ray regimes, refine our knowledge of the physical parameters of these systems at different accretion rates, and search for a possible excess of soft X-ray photons. Methods: We obtained and analysed four XMM-Newton X-ray observations of three of the sources, two of them discovered with the SDSS and one in the RASS. The X-ray data were complemented by optical photometric and spectroscopic observations and, for two sources, archival Swift observations. Results: SDSSJ032855.00+052254.2 was X-ray bright in two XMM-Newton and two Swift observations, and shows transitions from high and low accretion states on a timescale of a few months. The source shows no significant soft excess. We measured the magnetic field strength at the main accreting pole to be 39 MG and the inclination to be 45° ≤ I ≤ 77°, and we refined the long-term ephemeris. SDSSJ133309.20+143706.9 was X-ray faint. We measured a faint phase X-ray flux and plasma temperature for this source, which seems to spend almost all of its time accreting at a low level. Its inclination is less than about 76°. 1RXSJ173006.4+033813 was X-ray bright in the XMM-Newton observation. Its spectrum contained a modest soft blackbody component, not luminous enough to be considered a significant soft excess. We inferred a magnetic field strength at the main accreting pole of 20 to 25 MG, and that the inclination is less than 77° and probably less than 63°. V808 Aur, also known as CSS081231:J071126+440405, was X-ray faint in the Swift observation, but there is nonetheless strong evidence for bright and faint phases in X-rays and perhaps in UV. Residual X-ray flux from the faint phase is difficult to explain by thermal emission from the white dwarf surface, or by accretion onto the second pole. We present a revised distance estimate of 250 pc. Conclusions: The three systems we were able to study in detail appear to be normal polars with luminosities and magnetic field strengths typical for this class of accreting binary. None of the four systems studied shows the strong soft excess thought commonplace in polars prior to the XMM-Newton era. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).

Near-InfraRed Planet Searcher to Join HARPS on the ESO 3.6-metre Telescope

NASA Astrophysics Data System (ADS)

Bouchy, F.; Doyon, R.; Artigau, É.; Melo, C.; Hernandez, O.; Wildi, F.; Delfosse, X.; Lovis, C.; Figueira, P.; Canto Martins, B. L..; González Hernández, J. I..; Thibault, S.; Reshetov, V.; Pepe, F.; Santos, N. C.; de Medeiros, J. R..; Rebolo, R.; Abreu, M.; Adibekyan, V. Z.; Bandy, T.; Benz, W.; Blind, N.; Bohlender, D.; Boisse, I.; Bovay, S.; Broeg, C.; Brousseau, D.; Cabral, A.; Chazelas, B.; Cloutier, R.; Coelho, J.; Conod, U.; Cumming, A.; Delabre, B.; Genolet, L.; Hagelberg, J.; Jayawardhana, R.; Käufl, H.-U.; Lafrenière, D.; de Castro Leão, I..; Malo, L.; de Medeiros Martins, A..; Matthews, J. M.; Metchev, S.; Oshagh, M.; Ouellet, M.; Parro, V. C.; Rasilla Piñeiro, J. L..; Santos, P.; Sarajlic, M.; Segovia, A.; Sordet, M.; Udry, S.; Valencia, D.; Vallée, P.; Venn, K.; Wade, G. A.; Saddlemyer, L.

2017-09-01

The Near-InfraRed Planet Searcher (NIRPS) is a new ultra-stable infrared (YJH) spectrograph that will be installed on ESO's 3.6-metre Telescope in La Silla, Chile. Aiming to achieve a precision of 1 m s-1, NIRPS is designed to find rocky planets orbiting M dwarfs, and will operate together with the High Accuracy Radial velocity Planet Searcher (HARPS), also on the 3.6-metre Telescope. In this article we describe the NIRPS science cases and present its main technical characteristics.

TRAPPIST-1 Comparison to Solar System and Jovian Moons

NASA Image and Video Library

2017-02-22

All seven planets discovered in orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. In fact, they would have room to spare. TRAPPIST-1 also is only a fraction of the size of our sun; it isn't much larger than Jupiter. So the TRAPPIST-1 system's proportions look more like Jupiter and its moons than those of our solar system. The seven planets of TRAPPIST-1 are all Earth-sized and terrestrial, according to research published in 2017 in the journal Nature. TRAPPIST-1 is an ultra-cool dwarf star in the constellation Aquarius, and its planets orbit very close to it. The system has been revealed through observations from NASA's Spitzer Space Telescope and the ground-based TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescope, as well as other ground-based observatories. The system was named for the TRAPPIST telescope. http://photojournal.jpl.nasa.gov/catalog/PIA21428

A Neutron Star-White Dwarf Binary Model for Repeating Fast Radio Burst 121102

NASA Astrophysics Data System (ADS)

Gu, Wei-Min; Dong, Yi-Ze; Liu, Tong; Ma, Renyi; Wang, Junfeng

2016-06-01

We propose a compact binary model for the fast radio burst (FRB) repeaters, where the system consists of a magnetic white dwarf (WD) and a neutron star (NS) with strong bipolar magnetic fields. When the WD fills its Roche lobe, mass transfer will occur from the WD to the NS through the inner Lagrange point. The accreted magnetized materials may trigger magnetic reconnection when they approach the NS surface, and therefore the electrons can be accelerated to an ultra-relativistic speed. In this scenario, the curvature radiation of the electrons moving along the NS magnetic field lines can account for the characteristic frequency and the timescale of an FRB. Owing to the conservation of angular momentum, the WD may be kicked away after a burst, and the next burst may appear when the system becomes semi-detached again through the gravitational radiation. By comparing our analyses with the observations, we show that such an intermittent Roche-lobe overflow mechanism can be responsible for the observed repeating behavior of FRB 121102.

Dynamics of exoplanetary systems, links to their habitability

NASA Astrophysics Data System (ADS)

Bolmont, E.; Raymond, S. N.; Selsis, F.

2014-12-01

Our knowledge of planets' orbital dynamics, which was based on Solar System studies, has been challenged by the diversity of exoplanetary systems. Around cool and ultra cool dwarfs, the influence of tides on the orbital and spin evolution of planets can strongly affect their climate and their capacity to host surface liquid water. We illustrate the role of tides and dynamics with the extreme case of planets orbiting around brown dwarfs. In multiple planet systems, the eccentricity is excited by planet-planet interactions. Planets are therefore heated up from the inside by the tidally-induced friction. This process can heat a habitable zone planet to such a level that surface liquid water cannot exist. We also talk about the newly discovered potentially habitable Earth-sized planet Kepler-186f. Given the poorly estimated age of the system, the planet could still be evolving towards synchronization and have a high obliquity or be pseudo-synchronized with a zero obliquity. These two configurations would have a different effect on the climate of this planet.

Faint CO Line Wings in Four Star-forming (Ultra)luminous Infrared Galaxies

NASA Astrophysics Data System (ADS)

Leroy, Adam K.; Walter, Fabian; Decarli, Roberto; Bolatto, Alberto; Zschaechner, Laura; Weiss, Axel

2015-09-01

We report the results of a search for large velocity width, low-intensity line wings—a commonly used signature of molecular outflows—in four low redshift (ultra)luminous infrared galaxies that appear to be dominated by star formation. The targets were drawn from a sample of fourteen targets presented in Chung et al., who showed the stacked CO spectrum of the sample to exhibit 1000 km s-1-wide line wings. We obtained sensitive, wide bandwidth imaging of our targets using the IRAM Plateau de Bure Interferometer. We detect each target at very high significance but do not find the claimed line wings in these four targets. Instead, we constrain the flux in the line wings to be only a few percent. Casting our results as mass outflow rates following Cicone et al. we show them to be consistent with a picture in which very high mass loading factors preferentially occur in systems with high active galactic nucleus contributions to their bolometric luminosity. We identify one of our targets, IRAS 05083 (VII Zw 31), as a candidate molecular outflow.

The eclipsing AM Herculis variable H1907 + 690

NASA Technical Reports Server (NTRS)

Remillard, R. A.; Silber, A.; Stroozas, B. A.; Tapia, S.

1991-01-01

The discovery is reported of an eclipsing cataclysmic variable that exhibits up to 10 percent circular polarization at optical wavelengths, securing its classification as an AM Herculis type binary. The object, H1907 + 609, was located with the guidance of X-ray positions from the HEAO 1 survey. Optical CCD photometry exhibits deep eclipses, from which is derived a precise orbital period of 1.743750 hr. The eclipse duration suggests an inclination angle about 80 deg for a main-sequence secondary star. The optical flux has been persistently faint during observations spanning 1987-1990, while the X-ray measurements suggest long-term X-ray variability. The polarization and photometric light curves can be interpreted with a geometric model in which most of the accretion is directed toward a single magnetic pole, with an accretion spot displaced about 17 deg in longitude from the projection of the secondary star on the white dwarf surface.

CCD time-resolved photometry of faint cataclysmic variables. I

NASA Technical Reports Server (NTRS)

Howell, Steve; Szkody, Paula

1988-01-01

CCD time-resolved V and B differential light curves are presented for the dwarf novae AR And, FS Aur, TT Boo, UZ Boo, AF Cam, AL Com, AW Gem, X Leo, RZ Leo, CW Mon, SW UMa, and TW Vir. The time-series observations ranged from 2 to 6 hours and have accuracies of 0.025 mag or better for the majority of the runs. Except for AR And, X Leo, CW Mon, and TW Vir, the periods are below the cataclysmic-variable period gap (about 2 hours), and the systems are potential SU UMa stars. Photometric periods for five of the stars are the first such determinations, while those for the other seven generally confirm previous spectroscopic or photometric observations. In several cases, the photometric modulations are large amplitude (up to 0.5 mag). The results on AL Com and SW UMa indicate they may be magnetic variables.