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Sample records for galactic stellar halo

  1. Ages, chemistry, and type 1A supernovae: Clues to the formation of the galactic stellar halo

    NASA Technical Reports Server (NTRS)

    Smecker-Hane, Tammy A.; Wyse, Rosemary F. G.

    1993-01-01

    We endeavor to resolve two conflicting constraints on the duration of the formation of the Galactic stellar halo - 2-3 Gyr age differences in halo stars, and the time scale inferred from the observed constant values of chemical element abundance ratios characteristic of enrichment by Type II supernovae - by investigating the time scale for the onset of Type Ia supernovae (SNIa) in the currently favored progenitor model - mergers of carbon and oxygen white dwarfs (CO WDs).

  2. The sagittarius tidal stream and the shape of the galactic stellar halo

    NASA Astrophysics Data System (ADS)

    Newby, Matthew T.

    The stellar halo that surrounds our Galaxy contains clues to understanding galaxy formation, cosmology, stellar evolution, and the nature of dark matter. Gravitationally disrupted dwarf galaxies form tidal streams, which roughly trace orbits through the Galactic halo. The Sagittarius (Sgr) dwarf tidal debris is the most dominant of these streams, and its properties place important constraints on the distribution of mass (including dark matter) in the Galaxy. Stars not associated with substructures form the "smooth" component of the stellar halo, the origin of which is still under investigation. Characterizing halo substructures such as the Sgr stream and the smooth halo provides valuable information on the formation history and evolution of our galaxy, and places constraints on cosmological models. This thesis is primarily concerned with characterizing the 3-dimensional stellar densities of the Sgr tidal debris system and the smooth stellar halo, using data from the Sloan Digital Sky Survey (SDSS). F turnoff stars are used to infer distances, as they are relatively bright, numerous, and distributed about a single intrinsic brightness (magnitude). The inherent spread in brightnesses of these stars is overcome through the use of the recently-developed technique of statistical photometric parallax, in which the bulk properties of a stellar population are used to create a probability distribution for a given star's distance. This was used to build a spatial density model for the smooth stellar halo and tidal streams. The free parameters in this model are then fit to SDSS data with a maximum likelihood technique, and the parameters are optimized by advanced computational methods. Several computing platforms are used in this study, including the RPI SUR Bluegene and the Milkyway home volunteer computing project. Fits to the Sgr stream in 18 SDSS data stripes were performed, and a continuous density profile is found for the major Sgr stream. The stellar halo is found to

  3. Did globular clusters contribute to the stellar population of the Galactic halo?

    NASA Astrophysics Data System (ADS)

    Charbonnel, Corinne; Krause, Martin

    2016-08-01

    The origin of Galactic halo stars and the contribution of globular clusters (GC) to this stellar population have long been (and still are) debated. The discovery of multiple stellar populations with peculiar chemical properties in GCs both in the Milky Way and in Local Group galaxies recently brought a renewal on these questions. Indeed most of the scenarios that compete to reproduce the present-day GC characteristics call for fast expulsion of both gas and low-mass stars from these clusters in their early infancy. In this framework, the initial masses of GCs could have been 8 to 25 times higher than their present-day stellar mass, and they could have contributed to 5 to 20 % of the low-mass stars in the Galactic halo. Here we revisit these conclusions, which are in tension with observations of dwarf galaxies and of young massive star clusters in the Local Group. We come back in particular on the paradigm of gas expulsion from massive star clusters, and propose an alternative interpretation of the GC abundance properties. We conclude by proposing a major revision of the current concepts regarding the role massive star clusters play in the assembly of galactic haloes.

  4. THE STELLAR METALLICITY DISTRIBUTION FUNCTION OF THE GALACTIC HALO FROM SDSS PHOTOMETRY

    SciTech Connect

    An, Deokkeun; Beers, Timothy C.; Johnson, Jennifer A.; Pinsonneault, Marc H.; Lee, Young Sun; Bovy, Jo; Ivezic, Zeljko; Carollo, Daniela; Newby, Matthew

    2013-01-20

    We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5-8 kpc and |b| > 35 Degree-Sign , and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically selected local halo stars from Ryan and Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan and Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] Almost-Equal-To -1.7 and -2.3, the metal-poor component accounts for {approx}20%-35% of the entire halo population in this distance range.

  5. The Stellar Metallicity Distribution Function of the Galactic Halo from SDSS Photometry

    NASA Astrophysics Data System (ADS)

    An, Deokkeun; Beers, Timothy C.; Johnson, Jennifer A.; Pinsonneault, Marc H.; Lee, Young Sun; Bovy, Jo; Ivezić, Željko; Carollo, Daniela; Newby, Matthew

    2013-01-01

    We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5-8 kpc and |b| > 35°, and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically selected local halo stars from Ryan & Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan & Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] ≈ -1.7 and -2.3, the metal-poor component accounts for ~20%-35% of the entire halo population in this distance range.

  6. Beryllium in the Galactic halo - Surface abundances from standard, diffusive, and rotational stellar evolution, and implications

    NASA Technical Reports Server (NTRS)

    Deliyannis, Constantine P.; Pinsonneault, Marc H.

    1990-01-01

    The recently observed upper limits to the beryllium abundances in population II stars are much lower than population I detections. This difference reflects an intrinsic difference in the initial abundances and is not caused by different degrees of depletion driven by stellar evolution processes from similar initial abundances. Evolutionary sequences of models from the early premain sequence to beyond the turnoff that correspond to halo dwarfs with Fe/H abundances of -1.3, -2.3, and -3.3 are constructed, and standard, diffusive, and rotational mechanisms are used to estimate a maximal possible beryllium depletion. Halo star models in the T(eff) range 6000 to 5000 K might be rotationally depleted by a factor of 1.5-2, and the total depletion should be no more than (conservatively) a factor of 3. Implications for cosmology, cosmic-ray theory, and Galactic chemical evolution are discussed.

  7. Hot subdwarf stars in the Galactic halo Tracers of prominent events in late stellar evolution

    NASA Astrophysics Data System (ADS)

    Geier, Stephan; Kupfer, Thomas; Schaffenroth, Veronika; Heber, Ulrich

    2016-08-01

    Hot subdwarf stars (sdO/Bs) are the stripped cores of red giants located at the bluest extension of the horizontal branch. They constitute the dominant population of UV-bright stars in old stellar environments and are most likely formed by binary interactions. We perform the first systematic, spectroscopic analysis of a sample of those stars in the Galactic halo based on data from SDSS. In the course of this project we discovered 177 close binary candidates. A significant fraction of the sdB binaries turned out to have close substellar companions, which shows that brown dwarfs and planets can significantly influence late stellar evolution. Close hot subdwarf binaries with massive white dwarf companions on the other hand are good candidates for the progenitors of type Ia supernovae. We discovered a hypervelocity star, which not only turned out to be the fastest unbound star known in our Galaxy, but also the surviving companion of such a supernova explosion.

  8. ASSEMBLY OF THE OUTER GALACTIC STELLAR HALO IN THE HIERARCHICAL MODEL

    SciTech Connect

    Murante, Giuseppe; Curir, Anna; Poglio, Eva; Villalobos, Alvaro E-mail: curir@oato.inaf.i E-mail: villalobos@oats.inaf.i

    2010-06-20

    We provide a set of numerical N-body simulations for studying the formation of the outer Milky Ways' stellar halo through accretion events. After simulating minor mergers of prograde and retrograde orbiting satellite halos with a dark matter main halo, we analyze the signal left by satellite stars in the rotation velocity distribution. The aim is to explore the orbital conditions where a retrograde signal in the outer part of the halo can be obtained, in order to give a possible explanation of the observed rotational properties of the Milky Way stellar halo. Our results show that, for satellites more massive than {approx}1/40 of the main halo, the dynamical friction has a fundamental role in assembling the final velocity distributions resulting from different orbits and that retrograde satellites moving on low-inclination orbits deposit more stars in the outer halo regions and therefore can produce the counter-rotating behavior observed in the outer Milky Way halo.

  9. DETECTING TRIAXIALITY IN THE GALACTIC DARK MATTER HALO THROUGH STELLAR KINEMATICS

    SciTech Connect

    Rojas-Nino, Armando; Valenzuela, Octavio; Pichardo, Barbara; Aguilar, Luis A. E-mail: barbara@astro.unam.mx

    2012-10-01

    Assuming the dark matter halo of the Milky Way to be a non-spherical potential (i.e., triaxial, prolate, oblate), we show how the assembling process of the Milky Way halo may have left long-lasting stellar halo kinematic fossils due to the shape of the dark matter halo. In contrast with tidal streams, which are associated with recent satellite accretion events, these stellar kinematic groups will typically show inhomogeneous chemical and stellar population properties. However, they may be dominated by a single accretion event for certain mass assembling histories. If the detection of these peculiar kinematic stellar groups were confirmed, they would be the smoking gun for the predicted triaxiality of dark halos in cosmological galaxy formation scenarios.

  10. 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.

  11. 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.

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

    SciTech Connect

    Dall'Ora, M.; Ripepi, Vincenzo; Marconi, Marcella; Musella, Ilaria E-mail: ripepi@na.astro.it E-mail: ilaria@na.astro.it; and others

    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} = 17.70 {+-} 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.

  13. Detecting Stellar Streams in the Galactic Halo with a Modified GC3 Method

    NASA Astrophysics Data System (ADS)

    Mateu, C.; Bruzual, G.; Aguilar, L.; Brown, A.; Valenzuela, O.; Carigi, L.; Hernández, F.; Velázquez, H.

    2011-10-01

    In the following contribution we present results on the applicability of a modified Great Circle Cell Counts (GC3) method to detect stellar streams from satellites with different orbits, dynamical ages, total luminosities and star formation histories.

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

    SciTech Connect

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

    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. 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.

  15. Stellar Archeology in the Galactic Halo with Ultra-faint Dwarfs. VII. Hercules

    NASA Astrophysics Data System (ADS)

    Musella, Ilaria; Ripepi, Vincenzo; Marconi, Marcella; Clementini, Gisella; Dall'Ora, Massimo; Scowcroft, Victoria; Moretti, Maria Ida; Di Fabrizio, Luca; Greco, Claudia; Coppola, Giuseppina; Bersier, David; Catelan, Márcio; Grado, Aniello; Limatola, Luca; Smith, Horace A.; Kinemuchi, Karen

    2012-09-01

    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, langP abrang = 0.68 days (σ = 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. 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)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 ~ 25 mag and extends beyond the galaxy's half-light radius over a total area of 40' × 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. Based on data collected at the 2.5 m Isaac Newton Telescope, La Palma, Canary Islands, Spain, at the 4.2 m William Herschel Telescope, Roche de los Muchachos, Canary Islands, Spain, at the 2.2 m ESO/MPI telescope, La Silla, Chile, Proposal 079.D-0587, at the 2 m Liverpool Telescope, Roche de los Muchachos, Canary Islands, Spain, and at the 2 m Faulkes Telescope North, Haleakala Observatory, Hawaii, USA.

  16. "Invisible" Galactic Halos.

    ERIC Educational Resources Information Center

    Lugt, Karel Vander

    1993-01-01

    Develops a simple core-halo model of a galaxy that exhibits the main features of observed rotation curves and quantitatively illustrates the need to postulate halos of dark matter. Uses only elementary mechanics. (Author/MVL)

  17. Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey

    SciTech Connect

    Miceli, A; Rest, A; Stubbs, C W; Hawley, S L; Cook, K H; Magnier, E A; Krisciunas, K; Bowell, E; Koehn, B

    2007-02-23

    We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg{sup 2} and span distances ranging from 3-30kpc from the Galactic Center. Object selection is based on phased, photometric data with 28-50 epochs. We use this large sample to explore the bulk properties of the stellar halo, including the spatial distribution. The period-amplitude distribution of this sample shows that the majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a significant fraction (26%) which have longer periods and appear to be Oosterhoff type II. We find that the radial distributions of these two populations have significantly different profiles ({rho}{sub OoI} {approx} R{sup -2.26{+-}0.07} and {rho}{sub OoII} {approx} R{sup -2.88{+-}0.11}). This suggests that the stellar halo was formed by at least two distinct accretion processes and supports dual-halo models.

  18. GCN: a gaseous Galactic halo stream?

    NASA Astrophysics Data System (ADS)

    Jin, Shoko

    2010-10-01

    We show that a string of HI clouds that form part of the high-velocity cloud complex known as GCN is a probable gaseous stream extending over more than 50° in the Galactic halo. The radial velocity gradient along the stream is used to deduce transverse velocities as a function of distance, enabling a family of orbits to be computed. We find that a direction of motion towards the Galactic disc coupled with a mid-stream distance of ~20kpc provides a good match to the observed sky positions and radial velocities of the HI clouds comprising the stream. With an estimated mass of 105Msolar, its progenitor is likely to be a dwarf galaxy. However, no stellar counterpart has been found amongst the currently known Galactic dwarf spheroidal galaxies or stellar streams and the exact origin of the stream is therefore currently unknown.

  19. Stellar halos: a rosetta stone for galaxy formation and cosmology

    NASA Astrophysics Data System (ADS)

    Inglis Read, Justin

    2015-08-01

    Stellar halos make up about a percent of the total stellar mass in galaxies. Yet their old age and long phase mixing times make them living fossil records of galactic history. In this talk, I review the latest simulations of structure formation in our standard Lambda Cold Dark Matter cosmology. I discuss the latest predictions for stellar halos and the relationship between the stellar halo light and the underlying dark matter. Finally, I discuss how these simulations compare to observations of the Milky Way and Andromeda and, ultimately, what this means for our cosmological model and the formation history of the Galaxy.

  20. Stellar populations of stellar halos: Results from the Illustris simulation

    NASA Astrophysics Data System (ADS)

    Cook, B. A.; Conroy, C.; Pillepich, A.; Hernquist, L.

    2016-08-01

    The influence of both major and minor mergers is expected to significantly affect gradients of stellar ages and metallicities in the outskirts of galaxies. Measurements of observed gradients are beginning to reach large radii in galaxies, but a theoretical framework for connecting the findings to a picture of galactic build-up is still in its infancy. We analyze stellar populations of a statistically representative sample of quiescent galaxies over a wide mass range from the Illustris simulation. We measure metallicity and age profiles in the stellar halos of quiescent Illustris galaxies ranging in stellar mass from 1010 to 1012 M ⊙, accounting for observational projection and luminosity-weighting effects. We find wide variance in stellar population gradients between galaxies of similar mass, with typical gradients agreeing with observed galaxies. We show that, at fixed mass, the fraction of stars born in-situ within galaxies is correlated with the metallicity gradient in the halo, confirming that stellar halos contain unique information about the build-up and merger histories of galaxies.

  1. A Speeding Binary in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-04-01

    The recent discovery of a hyper-velocity binary star system in the halo of the Milky Way poses a mystery: how was this system accelerated to its high speed?Accelerating StarsUnlike the uniform motion in the Galactic disk, stars in the Milky Ways halo exhibit a huge diversity of orbits that are usually tilted relative to the disk and have a variety of speeds. One type of halo star, so-called hyper-velocity stars, travel with speeds that can approach the escape velocity of the Galaxy.How do these hyper-velocity stars come about? Assuming they form in the Galactic disk, there are multiple proposed scenarios through which they could be accelerated and injected into the halo, such as:Ejection after a close encounter with the supermassive black hole at the Galactic centerEjection due to a nearby supernova explosionEjection as the result of a dynamical interaction in a dense stellar population.Further observations of hyper-velocity stars are necessary to identify the mechanism responsible for their acceleration.J1211s SurpriseModels of J1211s orbit show it did not originate from the Galactic center (black dot). The solar symbol shows the position of the Sun and the star shows the current position of J1211. The bottom two panels show two depictions(x-y plane and r-z plane) of estimated orbits of J1211 over the past 10 Gyr. [Nmeth et al. 2016]To this end, a team of scientists led by Pter Nmeth (Friedrich Alexander University, Erlangen-Nrnberg) recently studied the candidate halo hyper-velocity star SDSS J121150.27+143716.2. The scientists obtained spectroscopy of J1211 using spectrographs at the Keck Telescope in Hawaii and ESOs Very Large Telescope in Chile. To their surprise, they discovered the signature of a companion in the spectra: J1211 is actually a binary!Nmeth and collaborators found that J1211, located roughly 18,000 light-years away, is moving at a rapid ~570 km/s relative to the galactic rest frame. The binary system consists of a hot (30,600 K) subdwarf and a

  2. Massive black holes in galactic halos?

    NASA Technical Reports Server (NTRS)

    Lacey, C. G.; Ostriker, J. P.

    1985-01-01

    In the present attempt to resolve the problems posed by the composition of dark halos and the heating of stellar disks, under the assumption that galaxy halos are composed of massive black holes, it is noted that the black holes must have masses of the order of one million solar masses. The heating mechanism proposed yields predictions for the dependence of the velocity dispersion on time, and for the shape of the velocity ellipsoid, which are in good agreement with observations. Attention is given to the constraints set by dynamical friction causing black holes to spiral to the Galactic center, by the possible presence of dark matter in dwarf spheroidal galaxies, and by the accretion of interstellar gas by the black holes that produce luminous objects in the Galaxy.

  3. Hot Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Adelman, Saul J.; Upgren, Arthur R.; Adelman, Carol J.

    2011-03-01

    Participants; Preface; Foreword; Acknowledgements; Part I. Introductory Papers: 1. What is the galaxy's halo population?; 2. Theoretical properties of horizontal-branch stars; 3. A review of A-type horizontal-branch stars; Part II. Surveys: 4. A progress report on the Edinburgh-Cape object survey; 5. A 300 square degree survey of young stars at high galactic latitudes; 6. The isolation of a new sample of B stars in the halo; 7. A northern catalog of FHB/A stars; 8. Recent progress on a continuing survey of galactic globular clusters for blue stragglers; 9. UV observations with FAUST and the galactic model; 10. Hot stars at the South Galactic Pole; Part III. Clusters: 11. Population II horizontal branches: a photometric study of globular clusters; 12. The period-shift effect in Oosterhoff type II globular clusters; 13. UV photometry of hot stars in omega centauri; 14. Spectroscopic and UBV observations of blue stars at the NGP; 15. Population I horizontal branches: probing the halo-to-disk transition; Part IV. Stars: 16. Very hot subdwarf O stars; 17. Quantitative spectroscopy of the very hot subluminous O-stars: K646, PG1159-035, and KPD0005+5106; 18. Analyzing the helium-rich hot sdO stars in the Palomar Green Survey; 19. Late type companions of hot sd O stars; 20. Hot stars in globular clusters; 21. Faint blue stars from the Hamburg Schmidt Survey; 22. Stellar winds and the evolution of sdB's to sdO's; 23. Halo stars in the Vilnius photometric system; 24. Horizontal branch stars in the geneva photometric system; 25. Zeeman observations of FHB stars and hot subdwarf stars; 26. What does a FHB star's spectrum look like?; 27. A technique for distinguishing FHB stars from A-type stars; 28. eEemental abundances of halo A and interloper stars; 29. The mass of blue horizontal branch stars in the globular cluster NGC6397; 30. IUE observations of blue HB stars in the globular clusters M3 and NGC6752; 31. Metallicities and kinematics of the local RR lyraes: lukewarm stars

  4. Hot Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Adelman, Saul J.; Upgren, Arthur R.; Adelman, Carol J.

    1994-08-01

    Participants; Preface; Foreword; Acknowledgements; Part I. Introductory Papers: 1. What is the galaxy's halo population?; 2. Theoretical properties of horizontal-branch stars; 3. A review of A-type horizontal-branch stars; Part II. Surveys: 4. A progress report on the Edinburgh-Cape object survey; 5. A 300 square degree survey of young stars at high galactic latitudes; 6. The isolation of a new sample of B stars in the halo; 7. A northern catalog of FHB/A stars; 8. Recent progress on a continuing survey of galactic globular clusters for blue stragglers; 9. UV observations with FAUST and the galactic model; 10. Hot stars at the South Galactic Pole; Part III. Clusters: 11. Population II horizontal branches: a photometric study of globular clusters; 12. The period-shift effect in Oosterhoff type II globular clusters; 13. UV photometry of hot stars in omega centauri; 14. Spectroscopic and UBV observations of blue stars at the NGP; 15. Population I horizontal branches: probing the halo-to-disk transition; Part IV. Stars: 16. Very hot subdwarf O stars; 17. Quantitative spectroscopy of the very hot subluminous O-stars: K646, PG1159-035, and KPD0005+5106; 18. Analyzing the helium-rich hot sdO stars in the Palomar Green Survey; 19. Late type companions of hot sd O stars; 20. Hot stars in globular clusters; 21. Faint blue stars from the Hamburg Schmidt Survey; 22. Stellar winds and the evolution of sdB's to sdO's; 23. Halo stars in the Vilnius photometric system; 24. Horizontal branch stars in the geneva photometric system; 25. Zeeman observations of FHB stars and hot subdwarf stars; 26. What does a FHB star's spectrum look like?; 27. A technique for distinguishing FHB stars from A-type stars; 28. eEemental abundances of halo A and interloper stars; 29. The mass of blue horizontal branch stars in the globular cluster NGC6397; 30. IUE observations of blue HB stars in the globular clusters M3 and NGC6752; 31. Metallicities and kinematics of the local RR lyraes: lukewarm stars

  5. Accretion in the galactic halo

    NASA Astrophysics Data System (ADS)

    Stephens, Alex Courtney

    2000-10-01

    The Milky Way disk is enveloped in a diffuse, dynamically-hot collection of stars and star clusters collectively known as the ``stellar halo''. Photometric and chemical analyses suggest that these stars are ancient fossils of the galaxy formation epoch. Yet, little is known about the origin of this trace population. Is this system merely a vestige of the initial burst of star formation within the decoupled proto-Galaxy, or is it the detritus of cannibalized satellite galaxies? In an attempt to unravel the history of the Milky Way's stellar halo, I performed a detailed spectroscopic analysis of 55 metal-poor stars possessing ``extreme'' kinematic properties. It is thought that stars on orbits that either penetrate the remote halo or exhibit large retrograde velocities could have been associated with assimilated (or ``accreted'') dwarf galaxies. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the α-elements (O, Mg, Si, Ca, Ti) with respect to iron, a consequence of sporadic bursts of star formation within the diminutive galaxies. Abundances for a select group of light metals (Li, Na, Mg, Si, Ca, Ti), iron-peak nuclides (Cr, Fe, Ni), and neutron-capture elements (Y, Ba) were calculated using line-strengths measured from high-resolution, high signal-to-noise spectral observations collected with the Keck I 10-m and KPNO 4-m telescopes. The abundances extracted from the spectra reveal: (1)The vast majority of outer halo stars possess supersolar [α/Fe] > 0.0) ratios. (2)The [α/Fe] ratio appears to decrease with increasing metallicity. (3)The outer halo stars have lower ratios of [α/Fe] than inner halo stars at a given metallicity. (4)At the largest metallicities, there is a large spread in the observed [α/Fe] ratios. (5)[α/Fe] anti-correlates with RAPO. (6)Only one star (BD+80° 245) exhibits the peculiar abundances expected of an assimilated star. The general conclusion extracted from these data is that the

  6. Clouds Dominate the Galactic Halo

    NASA Astrophysics Data System (ADS)

    2003-01-01

    Using the exquisite sensitivity of the National Science Foundation's Robert C. Byrd Green Bank Telescope (GBT), astronomer Jay Lockman of the National Radio Astronomy Observatory (NRAO) in Green Bank, W. Va., has produced the best cross-section ever of the Milky Way Galaxy's diffuse halo of hydrogen gas. This image confirms the presence of discrete hydrogen clouds in the halo, and could help astronomers understand the origin and evolution of the rarefied atmosphere that surrounds our Galaxy. Lockman presented his findings at the American Astronomical Society meeting in Seattle, WA. Hydrogen Clouds Graphic Artist's Rendering of the Milky Way (background) with insert showing GBT image of cross-section of neutral atomic Hydrogen Credit: Kirk Woellert/National Science Foundation Patricia Smiley, NRAO. "The first observations with the Green Bank Telescope suggested that the hydrogen in the lower halo, the transition zone between the Milky Way and intergalactic space, is very clumpy," said Lockman. "The latest data confirm these results and show that instead of trailing away smoothly from the Galactic plane, a significant fraction of the hydrogen gas in the halo is concentrated in discrete clouds. There are even some filaments." Beyond the star-filled disk of the Milky Way, there exists an extensive yet diffuse halo of hydrogen gas. For years, astronomers have speculated about the origin and structure of this gas. "Even the existence of neutral hydrogen in the halo has been somewhat of a puzzle," Lockman remarked. "Unlike the Earth's atmosphere, which is hot enough to hold itself up against the force of gravity, the hydrogen in the halo is too cool to support itself against the gravitational pull of the Milky Way." Lockman points out that some additional factor has to be involved to get neutral hydrogen to such large distances from the Galactic plane. "This force could be cosmic rays, a supersonic wind, the blast waves from supernovae, or something we have not thought of

  7. The Anemic Stellar Halo of M101

    NASA Astrophysics Data System (ADS)

    Holwerda, Benne

    2014-10-01

    Models of galaxy formation in a cosmological context predict that massive disk galaxies should have richly-structured extended stellar halos, containing ~10% of a galaxy's stars, originating in large part from the tidal disruption of dwarf galaxies. Observations of a number of nearby disk galaxies have generally agreed with these expectations. Recent new observations in integrated light with a novel array of low scattered-light telephoto lenses have failed to convincingly detect a stellar halo in the nearby massive face-on disk galaxy M101 (van Dokkum et al. 2014). They argue that any halo has to have <0.3% of the mass of the galaxy. This halo would be the least massive of any massive disk galaxy in the local Universe (by factors of several) -- such a halo is not predicted or naturally interpreted by the models, and would present a critical challenge to the picture of ubiquitous stellar halos formed from the debris of disrupting dwarf galaxies.We propose to resolve the stellar populations of this uniquely anemic stellar halo for 6 orbits with HST (ACS and WFC3), allowing us to reach surface brightness limits sufficient to clearly detect and characterize M101's stellar halo if it carries more than 0.1% of M101's mass. With resolved stellar populations, we can use the gradient of stellar populations as a function of radius to separate stellar halo from disk, which is impossible using integrated light observations. The resolved stellar populations will reveal the halo mass to much greater accuracy, measure the halo radial profile, constrain any halo lopsidedness, estimate the halo's stellar metallicity, and permit an analysis of outer disk stellar populations.

  8. SUBSTRUCTURE IN THE STELLAR HALOS OF THE AQUARIUS SIMULATIONS

    SciTech Connect

    Helmi, Amina; Cooper, A. P.; Cole, S.; Frenk, C. S.; White, S. D. M.; Navarro, J. F.

    2011-05-20

    We characterize the substructure in the simulated stellar halos of Cooper et al. which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic halos of the Aquarius project. These stellar halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our halos compared with the value estimated for the Milky Way from main-sequence turnoff stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main-sequence stars, and might suggest that {approx}10% of the Milky Way halo stars have formed in situ.

  9. Modelling galactic conformity with the colour-halo age relation in the Illustris simulation

    NASA Astrophysics Data System (ADS)

    Bray, Aaron D.; Pillepich, Annalisa; Sales, Laura V.; Zhu, Emily; Genel, Shy; Rodriguez-Gomez, Vicente; Torrey, Paul; Nelson, Dylan; Vogelsberger, Mark; Springel, Volker; Eisenstein, Daniel J.; Hernquist, Lars

    2016-01-01

    Comparisons between observational surveys and galaxy formation models find that dark matter haloes' mass can largely explain their galaxies' stellar mass. However, it remains uncertain whether additional environmental variables, known as assembly bias, are necessary to explain other galaxy properties. We use the Illustris simulation to investigate the role of assembly bias in producing galactic conformity by considering 18 000 galaxies with Mstellar > 2 × 109 M⊙. We find a significant signal of galactic conformity: out to distances of about 10 Mpc, the mean red fraction of galaxies around redder galaxies is higher than around bluer galaxies at fixed stellar mass. Dark matter haloes exhibit an analogous conformity signal, in which the fraction of haloes formed at earlier times (old haloes) is higher around old haloes than around younger ones at fixed halo mass. A plausible interpretation of galactic conformity is the combination of the halo conformity signal with the galaxy colour-halo age relation: at fixed stellar mass, particularly towards the low-mass end, Illustris' galaxy colours correlate with halo age, with the reddest galaxies (often satellites) preferentially found in the oldest haloes. We explain the galactic conformity effect with a simple semi-empirical model, assigning stellar mass via halo mass (abundance matching) and galaxy colour via halo age (age matching). Regarding comparison to observations, we conclude that the adopted selection/isolation criteria, projection effects, and stacking techniques can have a significant impact on the measured amplitude of the conformity signal.

  10. THE GALACTIC CENTER S-STARS AND THE HYPERVELOCITY STARS IN THE GALACTIC HALO: TWO FACES OF THE TIDAL BREAKUP OF STELLAR BINARIES BY THE CENTRAL MASSIVE BLACK HOLE?

    SciTech Connect

    Zhang Fupeng; Lu Youjun; Yu Qingjuan

    2013-05-10

    In this paper, we investigate the link between the hypervelocity stars (HVSs) discovered in the Galactic halo and the Galactic center (GC) S-stars, under the hypothesis that they are both the products of the tidal breakup of the same population of stellar binaries by the central massive black hole (MBH). By adopting several hypothetical models for binaries to be injected into the vicinity of the MBH and doing numerical simulations, we realize the tidal breakup processes of the binaries and their follow-up dynamical evolution. We find that many statistical properties of the detected HVSs and GC S-stars could be reproduced under some binary injecting models, and their number ratio can be reproduced if the stellar initial mass function is top-heavy (e.g., with slope {approx} - 1.6). The total number of the captured companions is {approx}50 that have masses in the range {approx}3-7 M{sub Sun} and semimajor axes {approx}< 4000 AU and survive to the present within their main-sequence lifetime. The innermost one is expected to have a semimajor axis {approx}300-1500 AU and a pericenter distance {approx}10-200 AU, with a significant probability of being closer to the MBH than S2. Future detection of such a close star would offer an important test to general relativity. The majority of the surviving ejected companions of the GC S-stars are expected to be located at Galactocentric distances {approx}< 20 kpc, and have heliocentric radial velocities {approx} - 500-1500 km s{sup -1} and proper motions up to {approx}5-20 mas yr{sup -1}. Future detection of these HVSs may provide evidence for the tidal breakup formation mechanism of the GC S-stars.

  11. Dark matter particles in the galactic halo

    SciTech Connect

    Bernabei, R. Belli, P.; Montecchia, F.; Nozzoli, F.; Cappella, F.; D'Angelo, A.; Incicchitti, A.; Prosperi, D.; Cerulli, R.; Dai, C. J.; He, H. L.; Kuang, H. H.; Ma, J. M.; Sheng, X. D.; Ye, Z. P.

    2009-12-15

    Arguments on the investigation of the DarkMatter particles in the galactic halo are addressed. Recent results obtained by exploiting the annual modulation signature are summarized and the perspectives are discussed.

  12. Dark matter particles in the galactic halo

    NASA Astrophysics Data System (ADS)

    Bernabei, R.; Belli, P.; Montecchia, F.; Nozzoli, F.; Cappella, F.; D'Angelo, A.; Incicchitti, A.; Prosperi, D.; Cerulli, R.; Dai, C. J.; He, H. L.; Kuang, H. H.; Ma, J. M.; Sheng, X. D.; Ye, Z. P.

    2009-12-01

    Arguments on the investigation of the DarkMatter particles in the galactic halo are addressed. Recent results obtained by exploiting the annual modulation signature are summarized and the perspectives are discussed.

  13. Resolved Stellar Halos of M87 and NGC 5128: Metallicities from the Red-Giant Branch

    NASA Astrophysics Data System (ADS)

    Bird, Sarah A.

    2016-08-01

    We have searched halo fields of two giant elliptical galaxies: M87, using HST images at 10 kpc from the galactic center, and NGC 5128 (Cen A), using VIMOS VLT images at 65 kpc from the center and archival HST data from 8 to 38 kpc from the center. We have resolved thousands of red-giant-branch (RGB) stars in these stellar halo fields using V and I filters, and, in addition, measured the metallicity using stellar isochrones. The metallicity distribution function (MDF) of the inner stellar halo of M87 is similar to that of NGC 5128's stellar halo.

  14. Studying Stellar Halos with Future Facilities

    NASA Astrophysics Data System (ADS)

    Greggio, Laura; Falomo, Renato; Uslenghi, Michela

    2015-08-01

    Stellar halos around galaxies retain fundamental evidence of the processes which lead to their build up. Sophisticated models of galaxy formation in a cosmological context yield quantitative predictions about various observable characteristics, including the amount of substructure, the slope of radial mass profiles and three dimensional shapes, and the properties of the stellar populations in the galaxies halos. The comparison of such models with the observations leads to constraints on the general picture of galaxy formation in the hierarchical Universe, as well as on the physical processes taking place in the halos formation. With the current observing facilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. In this contribution we illustrate the progress which we expect in this field with the future large aperture ground based telescopes (E-ELT and TNT), and with JWST. In particular we adress the following issues: (I) the characterization of the stellar populations in the halos innermost regions and substructures, (ii) the measurement of the halos profiles and shapes , and the halos mass content, (iii) the study of Globular Clusters inhabiting the halos of distant galaxies. In order to assess the expected capabilities of future facilities we present the results of a set of simulated images to evaluate to which level of accuracy it will be possible to probe the halos of distant galaxies.

  15. THE RESOLVED STELLAR HALO OF NGC 253

    SciTech Connect

    Bailin, Jeremy; Bell, Eric F.; Chappell, Samantha N.; Radburn-Smith, David J.; De Jong, Roelof S.

    2011-07-20

    We have obtained Magellan/IMACS and Hubble Space Telescope (HST)/Advanced Camera for Surveys imaging data that resolve red giant branch stars in the stellar halo of the starburst galaxy NGC 253. The HST data cover a small area, and allow us to accurately interpret the ground-based data, which cover 30% of the halo to a distance of 30 kpc, allowing us to make detailed quantitative measurements of the global properties and structure of a stellar halo outside of the Local Group. The geometry of the halo is significantly flattened in the same sense as the disk, with a projected axis ratio of b/a {approx} 0.35 {+-} 0.1. The total stellar mass of the halo is estimated to be M{sub halo} {approx} (2.5 {+-} 1.5) x 10{sup 9} M{sub sun}, or 6% of the total stellar mass of the galaxy, and has a projected radial dependence that follows a power law of index -2.8 {+-} 0.6, corresponding to a three-dimensional power-law index of {approx} - 4. The total luminosity and profile shape that we measure for NGC 253 are somewhat larger and steeper than the equivalent values for the Milky Way and M31, but are well within the scatter of model predictions for the properties of stellar halos built up in a cosmological context. Structure within the halo is seen at a variety of scales: there is small kpc-scale density variation and a large shelf-like feature near the middle of the field. The techniques that have been developed will be essential for quantitatively comparing our upcoming larger sample of observed stellar halos to models of halo formation.

  16. QUANTIFYING KINEMATIC SUBSTRUCTURE IN THE MILKY WAY'S STELLAR HALO

    SciTech Connect

    Xue Xiangxiang; Zhao Gang; Luo Ali; Rix, Hans-Walter; Bell, Eric F.; Koposov, Sergey E.; Kang, Xi; Liu, Chao; Yanny, Brian; Beers, Timothy C.; Lee, Young Sun; Bullock, James S.; Johnston, Kathryn V.; Morrison, Heather; Rockosi, Constance

    2011-09-01

    We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative 'close pair distribution' as a statistic in the four-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at r{sub gc} < 20 kpc.

  17. Stellar Haloes with the Illustris Simulation: Mock Observations and Assembly

    NASA Astrophysics Data System (ADS)

    Pillepich, Annalisa; Torrey, Paul; Nelson, Dylan; Snyder, Greg; Rodriguez-Gomez, Vicente; Genel, Shy; Vogelsberger, Mark; Hernquist, Lars

    2015-08-01

    Illustris is a state-of-the-art simulation which combines the statistical power of a ˜106 Mpc-side cosmological volume with gasdynamics, prescriptions for star formation, feedback, and kpc resolution. It allows us to analyze about ˜5,000 well-resolved galaxies spanning a variety of morphologies, environments, and halo masses (3×10^11 < Mvir < 10^14 Msun). Illustris therefore provides the most realistic and richest sample of simulated galactic stellar haloes available up to date. Based on the properties of the stellar particles in each simulated galaxy/halo, we have produced synthetic images in different luminosity bands and extracted information about the mass distribution, smoothness, and phase-space structures up to large galactocentric distances at different limits of surface brightness. We can therefore gain insight and provide theoretically-motivated expectations for the build-up and properties of the stellar haloes, and their relation to the underlying DM haloes, their central galaxies, and their halo assembly histories.

  18. MODIFIED GRAVITY SPINS UP GALACTIC HALOS

    SciTech Connect

    Lee, Jounghun; Zhao, Gong-Bo; Li, Baojiu; Koyama, Kazuya

    2013-01-20

    We investigate the effect of modified gravity on the specific angular momentum of galactic halos by analyzing the halo catalogs at z = 0 from high-resolution N-body simulations for a f(R) gravity model that meets the solar-system constraint. It is shown that the galactic halos in the f(R) gravity model tend to acquire significantly higher specific angular momentum than those in the standard {Lambda}CDM model. The largest difference in the specific angular momentum distribution between these two models occurs for the case of isolated galactic halos with mass less than 10{sup 11} h {sup -1} M {sub Sun }, which are likely least shielded by the chameleon screening mechanism. As the specific angular momentum of galactic halos is rather insensitive to other cosmological parameters, it can in principle be an independent discriminator of modified gravity. We speculate a possibility of using the relative abundance of low surface brightness galaxies (LSBGs) as a test of general relativity given that the formation of the LSBGs occurs in fast spinning dark halos.

  19. Static galactic halo and galactic wind

    NASA Technical Reports Server (NTRS)

    Ko, Chung-Ming

    1993-01-01

    Although the exact state of the interstellar medium (ISM) in our Galaxy (other galaxies as well) is not clear at all, the 'common consensus' is that a rough pressure balance (or equipartition of energy) exists between different components and phases: cold, warm, hot phases of the ISM, magnetic field, cosmic rays, etc. If the halo of a galaxy is taken to be an extension of the ISM, then its structure is influenced by various ISM components. A 'complete' description of the halo is evidently very complicated. This paper gives a brief account on cosmic ray halo, which emphasizes the role played by cosmic rays. The interaction between cosmic rays and thermal plasma is facilitated by magnetic field. The cosmic rays are scattered by hydromagnetic waves (e.g., Alfven waves) which in turn can be generated by cosmic ray streaming instability. This constitutes a self-consistent picture. Since we are interested in the structure of the halo, we adopted a hydrodynamic model in which the cosmic rays and waves are described by their pressures. In general there are two classes of halos: static and dynamic.

  20. Magnetized galactic haloes and velocity lags

    NASA Astrophysics Data System (ADS)

    Henriksen, R. N.; Irwin, J. A.

    2016-06-01

    We present an analytic model of a magnetized galactic halo surrounding a Mestel gravitating disc. The magnetic field is taken to be in energy equipartition with the pressure dominant rotating halo gas (not with the cosmic rays), and the whole system is in a steady state. A more flexible `anisotropic equipartition' model is also explored. A definite pressure law is required to maintain the equilibrium, but the halo density is constant. The velocity/magnetic system is scale-free. The objective is to find the rotational velocity lag in such a halo. The magnetic field is not force-free so that angular momentum may be transported from the halo to the intergalactic medium. We find that the `X'-shaped structure observed for halo magnetic fields can be obtained together with a simple analytic formula for the rate of decline of the velocity with height z. The formula also predicts the change in lag with radius, r.

  1. STELLAR POPULATION VARIATIONS IN THE MILKY WAY's STELLAR HALO

    SciTech Connect

    Bell, Eric F.; Xue Xiangxiang; Rix, Hans-Walter; Ruhland, Christine; Hogg, David W.

    2010-12-15

    If the stellar halos of disk galaxies are built up from the disruption of dwarf galaxies, models predict highly structured variations in the stellar populations within these halos. We test this prediction by studying the ratio of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor populations) to main-sequence turn-off stars (MSTO stars; a feature of all populations) in the stellar halo of the Milky Way using data from the Sloan Digital Sky Survey. We develop and apply an improved technique to select BHB stars using ugr color information alone, yielding a sample of {approx}9000 g < 18 candidates where {approx}70% of them are BHB stars. We map the BHB/MSTO ratio across {approx}1/4 of the sky at the distance resolution permitted by the absolute magnitude distribution of MSTO stars. We find large variations of the BHB/MSTO star ratio in the stellar halo. Previously identified, stream-like halo structures have distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo features, e.g., the low-latitude structure, appear to be almost completely devoid of BHB stars, whereas other structures appear to be rich in BHB stars. The Sagittarius tidal stream shows an apparent variation in the BHB/MSTO ratio along its extent, which we interpret in terms of population gradients within the progenitor dwarf galaxy. Our detection of coherent stellar population variations between different stellar halo substructures provides yet more support to cosmologically motivated models for stellar halo growth.

  2. SECULAR DAMPING OF STELLAR BARS IN SPINNING DARK MATTER HALOS

    SciTech Connect

    Long, Stacy; Shlosman, Isaac; Heller, Clayton

    2014-03-01

    We demonstrate using numerical simulations of isolated galaxies that growth of stellar bars in spinning dark matter halos is heavily suppressed in the secular phase of evolution. In a representative set of models, we show that for values of the cosmological spin parameter λ ≳ 0.03, bar growth (in strength and size) becomes increasingly quenched. Furthermore, the slowdown of the bar pattern speed weakens considerably with increasing λ until it ceases completely. The terminal structure of the bars is affected as well, including extent and shape of their boxy/peanut bulges. The essence of this effect lies in the modified angular momentum exchange between the disk and the halo facilitated by the bar. For the first time we have demonstrated that a dark matter halo can emit and not purely absorb angular momentum. Although the halo as a whole is not found to emit, the net transfer of angular momentum from the disk to the halo is significantly reduced or completely eliminated. The paradigm shift implies that the accepted view that disks serve as sources of angular momentum and halos serve as sinks must be revised. Halos with λ ≳ 0.03 are expected to form a substantial fraction, based on the lognormal distribution of λ. The dependence of secular bar evolution on halo spin, therefore, implies profound corollaries for the cosmological evolution of galactic disks.

  3. Secular Damping of Stellar Bars in Spinning Dark Matter Halos

    NASA Astrophysics Data System (ADS)

    Long, Stacy; Shlosman, Isaac; Heller, Clayton

    2014-03-01

    We demonstrate using numerical simulations of isolated galaxies that growth of stellar bars in spinning dark matter halos is heavily suppressed in the secular phase of evolution. In a representative set of models, we show that for values of the cosmological spin parameter λ >~ 0.03, bar growth (in strength and size) becomes increasingly quenched. Furthermore, the slowdown of the bar pattern speed weakens considerably with increasing λ until it ceases completely. The terminal structure of the bars is affected as well, including extent and shape of their boxy/peanut bulges. The essence of this effect lies in the modified angular momentum exchange between the disk and the halo facilitated by the bar. For the first time we have demonstrated that a dark matter halo can emit and not purely absorb angular momentum. Although the halo as a whole is not found to emit, the net transfer of angular momentum from the disk to the halo is significantly reduced or completely eliminated. The paradigm shift implies that the accepted view that disks serve as sources of angular momentum and halos serve as sinks must be revised. Halos with λ >~ 0.03 are expected to form a substantial fraction, based on the lognormal distribution of λ. The dependence of secular bar evolution on halo spin, therefore, implies profound corollaries for the cosmological evolution of galactic disks.

  4. Stellar halos around Local Group galaxies

    NASA Astrophysics Data System (ADS)

    McConnachie, Alan W.

    2016-08-01

    The Local Group is now home to 102 known galaxies and candidates, with many new faint galaxies continuing to be discovered. The total stellar mass range spanned by this population covers a factor of close to a billion, from the faintest systems with stellar masses of order a few thousand to the Milky Way and Andromeda, with stellar masses of order 1011 M ⊙. Here, I discuss the evidence for stellar halos surrounding Local Group galaxies spanning from dwarf scales (with the case of the Andromeda II dwarf spheroidal), though to intermediate mass systems (M33) and finishing with M31. Evidence of extended stellar populations and merging is seen across the luminosity function, indicating that the processes that lead to halo formation are common at all mass scales.

  5. The SEGUE K Giant Survey. III. Quantifying Galactic Halo Substructure

    NASA Astrophysics Data System (ADS)

    Janesh, William; Morrison, Heather L.; Ma, Zhibo; Rockosi, Constance; Starkenburg, Else; Xue, Xiang Xiang; Rix, Hans-Walter; Harding, Paul; Beers, Timothy C.; Johnson, Jennifer; Lee, Young Sun; Schneider, Donald P.

    2016-01-01

    We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125 kpc. These stars have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Survey’s Sloan Extension for Galactic Understanding and Exploration project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using blue horizontal branch (BHB) stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of individual groups, we find that a large fraction (˜33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity.

  6. Analysis of HST/COS spectra of the bare C-O stellar core H1504+65 and a high-velocity twin in the Galactic halo

    NASA Astrophysics Data System (ADS)

    Werner, K.; Rauch, T.

    2015-12-01

    H1504+65 is an extremely hot white dwarf (effective temperature Teff = 200 000 K) with a carbon-oxygen dominated atmosphere devoid of hydrogen and helium. This atmospheric composition was hitherto unique among hot white dwarfs (WDs), and it could be related to recently detected cooler WDs with C or O dominated spectra. The origin of the H and He deficiency in H1504+65 is unclear. To further assess this problem, we performed ultraviolet spectroscopy with the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST). In accordance with previous far-ultraviolet spectroscopy performed with the Far Ultraviolet Spectroscopic Explorer, the most prominent lines stem from C iv, O v-vi, and Ne vi-viii. Archival HST/COS spectra are utilized to prove that the supersoft X-ray source RX J0439.8-6809 is, considering the exotic composition, a twin of H1504+65 that is even hotter (Teff = 250 000 K). In contrast to earlier claims, we find that the star is not located in the Large Magellanic Cloud but a foreground object in the Galactic halo at a distance of 9.2 kpc, 5.6 kpc below the Galactic plane, receding with vrad = +220 km s-1. Based on observations 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 NAS5-26666.

  7. Armchair cartography - A map of the Galactic halo based on observations of local, metal-poor stars

    NASA Astrophysics Data System (ADS)

    Sommer-Larsen, Jesper; Zhen, Chen

    1990-01-01

    The velocity distribution of metal-poor halo stars in the solar neighborhood is studied to extract data on the global spatial and kinematic properties of the Galactic stellar halo. A global model of the solar neighborhood stars is constructed from observed positions and three-dimensional velocity of local, metal-poor halo stars in terms of a discrete sum of orbits. The characteristics of the reconstructed halo are examined and used to study the evolution of the halo subsystems.

  8. Mapping the Galactic Halo. VIII. Quantifying Substructure

    NASA Astrophysics Data System (ADS)

    Starkenburg, Else; Helmi, Amina; Morrison, Heather L.; Harding, Paul; van Woerden, Hugo; Mateo, Mario; Olszewski, Edward W.; Sivarani, Thirupathi; Norris, John E.; Freeman, Kenneth C.; Shectman, Stephen A.; Dohm-Palmer, R. C.; Frey, Lucy; Oravetz, Dan

    2009-06-01

    We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high-latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity, and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of one group and seven pairs of clumped stars. The group, with six members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. using the Sloan Digital Sky Survey data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets, we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided that the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites.

  9. Distant Galactic Halo Substructures Observed by the Palomar Transient Factory

    NASA Astrophysics Data System (ADS)

    Sesar, Branimir

    2013-01-01

    Characterization of Galactic halo substructures is important as their kinematic and chemical properties help constrain the properties of the Galactic dark matter halo, the formation history of the Milky Way, and the galaxy formation process in general. The best practical choice for finding distant halo substructures are pulsating RR Lyrae stars, due to their intrinsic brightness (M_V = 0.6 mag) and distinct light curves. I will present kinematic and chemical properties of two distant halo substructures that were traced using RR Lyrae stars observed by the Palomar Transient Factory. One of these substructures, located at 90 kpc from the Sun in the Cancer constellation, consists of two groups of RR Lyrae stars moving away from the Galaxy at ~80 and ~20 km/s, respectively. The second substructure is located at ~65 kpc from the Sun in the Hercules constellation. The kinematics of RR Lyrae stars tracing this substructure suggest a presence of 2 or 3 stellar streams extending in the similar direction on the sky. Due to their spatial extent, both of these substructures are clearly disrupted and would be very difficult to detect using tradiitonal techniques such as the color-magnitude diagram filtering.

  10. Neutron stars and white dwarfs in galactic halos?

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1990-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  11. Neutron stars and white dwarfs in galactic halos

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1989-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  12. RADIAL VELOCITIES OF GALACTIC HALO STARS IN VIRGO

    SciTech Connect

    Brink, Thomas G.; Mateo, Mario; Martinez-Delgado, David E-mail: mmateo@umich.ed

    2010-11-15

    We present multi-slit radial velocity measurements for 111 stars in the direction of the Virgo Stellar Stream (VSS). The stars were photometrically selected to be probable main-sequence stars in the Galactic halo. When compared with the radial velocity distribution expected for the halo of the Milky Way, as well as the distribution seen in a control field, we observe a significant excess of negative velocity stars in the field, which can likely be attributed to the presence of a stellar stream. This kinematic excess peaks at a Galactic standard of rest radial velocity of -75 km s{sup -1}. A rough distance estimate suggests that this feature extends from {approx}15 kpc out to, and possibly beyond, the {approx}30 kpc limit of the study. The mean velocity of these stars is incompatible with those of the VSS itself (V{sub gsr} {approx} 130 km s{sup -1}), which we weakly detect, but it is consistent with radial velocity measurements of nearby 2MASS M-giants and SDSS+SEGUE K/M-giants and blue horizontal branch stars that constitute the leading tidal tail of the Sagittarius dwarf spheroidal galaxy. Some oblate models for the shape of the Milky Way's dark matter halo predict that the leading arm of the Sagittarius Stream should pass through this volume, and have highly negative (V{sub gsr} {approx}< -200 km s{sup -1}) radial velocities, as it descends down from the northern Galactic hemisphere toward the Galactic plane. The kinematic feature observed in this study, if it is in fact Sagittarius debris, is not consistent with these predictions, and instead, like other leading stream radial velocity measurements, is consistent with a recently published triaxial halo model, or, if axisymmetry is imposed, favors a prolate shape for the Galactic halo potential. However, a rough distance estimate to the observed kinematic feature places it somewhat closer (D {approx} 15-30 kpc) than the Sagittarius models predict (D {approx} 35-45 kpc).

  13. Galactic evolution. II - Disk galaxies with massive halos

    NASA Technical Reports Server (NTRS)

    Ostriker, J. P.; Thuan, T. X.

    1975-01-01

    Models of galactic evolution are computed in which matter shed by dying halo stars accumulates in a smaller, more rapidly rotating disk. The models are simpler and more successful than one-zone (pure disk) models in that (1) the observed absence of low-metal-abundance low-mass dwarfs is expected, not anomalous and (2) the relative birthrate function (or IMF) need not be a strongly variable function of time in agreement with recent interpretations of observed stellar populations and neutral hydrogen in our own and other galaxies. Even a simple 'Salpeter' IMF for both disk and halo will produce an acceptable model. The model with a halo 'Salpeter' IMF, roughly one-quarter of the mass in the secondary disk, and approximately half the metals produced in the halo seems most compatible with observations of the metal abundance in low-mass stars, the deuterium abundance, halo planetary nebulae, and light from Population II stars, as well as with arguments on the stability of the disk.

  14. Haloes light and dark: dynamical models of the stellar halo and constraints on the mass of the Galaxy

    NASA Astrophysics Data System (ADS)

    Williams, A. A.; Evans, N. W.

    2015-11-01

    We develop a flexible set of action-based distribution functions (DFs) for stellar haloes. The DFs have five free parameters, controlling the inner and outer density slope, break radius, flattening, and anisotropy, respectively. The DFs generate flattened stellar haloes with a rapidly varying logarithmic slope in density, as well as a spherically aligned velocity ellipsoid with a long axis that points towards the Galactic Centre - all attributes possessed by the stellar halo of the Milky Way. We use our action-based DF to model the blue horizontal branch stars extracted from the Sloan Digital Sky Survey as stellar halo tracers in a spherical Galactic potential. As the selection function is hard to model, we fix the density law from earlier studies and solve for the anisotropy and gravitational potential parameters. Our best-fitting model has a velocity anisotropy that becomes more radially anisotropic on moving outwards. It changes from β ≈ 0.4 at Galactocentric radius of 15 kpc to ≈0.7 at 60 kpc. This is a gentler increase than is typically found in simulations of stellar haloes built from the multiple accretion of smaller systems. We find the potential corresponds to an almost flat rotation curve with amplitude of ≈200 km s-1 at these distances. This implies an enclosed mass of ≈4.5 × 1011 M⊙ within a spherical shell of radius 50 kpc.

  15. The Information Content of Stellar Halos: Accretion Histories and Stellar Population Gradients in Quiescent Illustris Galaxies

    NASA Astrophysics Data System (ADS)

    Cook, Benjamin A.; Conroy, Charlie; Pillepich, Annalisa; Rodriguez-Gomez, Vicente; Hernquist, Lars

    2016-06-01

    Long dynamical timescales in the outskirts of galaxies are thought to preserve the information content of their accretion histories, in the form of stellar population gradients. We present a detailed analysis of the stellar halo properties of a statistically representative sample of quiescent galaxies from the Illustris simulation, and show that stellar population gradients at large radii can indeed be used to infer galactic accretion histories. We measure metallicity, age, and surface-brightness profiles in the halos of Illustris galaxies ranging from 1010 to 1012 solar masses. We find that the ex-situ mass fraction – the fraction of stars that were accreted from smaller bodies – at large radius is correlated with the gradients of both metallicity and surface-brightness between 2-10 effective radii. There is a tight relation between the two gradients, suggesting that the information content of hierarchical accretion is predominantly the same between the two. The residuals from this mean relation are correlated with the mean (mass-weighted) merger mass ratio, which implies that major and minor mergers leave slightly different signatures in the stellar populations of stellar halos.

  16. Cool Carbon Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Gigoyan, K. S.

    2016-06-01

    In this paper we report current status of search and study for Faint High Latitude Carbon Stars (FHLCs). Data for more than 1800 spectroscopically confirmed FHLCs are known, which are found thanks to objective prism surveys and photometric selections. More than half of the detected objects belongs to group of dwarf Carbon (dC) stars. Many-sided investigations based on modern astrophysical databases are necessary to study the space distribution of different groups of the FHLC stars and their possible origin in the Halo of our Galaxy. We report about the selection of FHLCs by the spectroscopic surveys: First Byurakan Survey (FBS), Hamburg/ESO Survey (HES), LAMOST Pilot Survey and SDSS, as well as by photometric selection: APM Survey for Cool Carbon Stars in the Galactic Halo, SDSS and 2MASS JHK colours.

  17. Inhomogeneous chemical enrichment in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Kobayashi, Chiaki

    2016-08-01

    In a galaxy, chemical enrichment takes place in an inhomogeneous fashion, and the Galactic Halo is one of the places where the inhomogeneous effects are imprinted and can be constrained from observations. I show this using my chemodynamical simulations of Milky Way type galaxies. The scatter in the elemental abundances originate from radial migration, merging/accretion of satellite galaxies, local variation of star formation and chemical enrichment, and intrinsic variation of nucleosynthesis yields. In the simulated galaxies, there is no strong age-metallicity relation. This means that the most metal-poor stars are not always the oldest stars, and can be formed in chemically unevolved clouds at later times. The long-lifetime sources of chemical enrichment such as asymptotic giant branch stars or neutron star mergers can contribute at low metallicities. The intrinsic variation of yields are important in the early Universe or metal-poor systems such as in the Galactic halo. The carbon enhancement of extremely metal-poor (EMP) stars can be best explained by faint supernovae, the low [α/Fe] ratios in some EMP stars naturally arise from low-mass (~ 13 - 15M ⊙) supernovae, and finally, the [α/Fe] knee in dwarf spheroidal galaxies can be produced by subclasses of Type Ia supernovae such as SN 2002cx-like objects and sub-Chandrasekhar mass explosions.

  18. Carbon Stars in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Totten, E. J.; Irwin, M. J.

    1996-04-01

    A byproduct of the APM high redshift quasar survey (Irwin et al. 1991) was the discovery of ~ 20 distant (20-100kpc) cool AGB carbon stars (all N-type) at high Galactic latitude. In August we used the INT+IDS to survey the rest of the high latitude SGC sky visible from La Palma and found 10 more similar carbon stars. Before this work there were only a handful of published faint high latitude cool carbon stars known (eg. Margon et al., 1984, Mould et al., 1985) and there has been speculation as to their origin (eg. Sanduleak, 1980, van den Bergh & Lafontaine, 1984). Intermediate age carbon stars (3 -- 7 Gyrs) seem unlikely to have formed in the halo in isolation from other star forming regions so how did they get there ? One possiblity that we are investigating, is that they arise from either the disruption of tidally captured dSph galaxies or are a manifestion of the long sought after optical component of the Magellanic Stream. Lack of proper motion rules out the possibility of them being dwarf carbon stars (eg. Warren et al., 1992); indeed no N-type carbon stars have been found to be dwarf carbon stars. Our optical spectroscopy confirms their carbon star type (they are indistinguishable from cool AGB carbon stars in nearby dwarf galaxies) and hence probable large distances. We are extending our survey to the NGC region, obtaining radial velocities and good S:N fluxed spectra for all the carbon stars. This will enable us to investigate their kinematics, true spatial distribution and hence their origin. Even, in the event that these objects are somehow an integral part of the Galactic halo, then their velocities and large distances will enable direct studies of the velocity ellipsoid and rotation of the outer halo (eg. Green et al., 1994).

  19. Discy dwarf disruption and the shape of the Galactic halo

    NASA Astrophysics Data System (ADS)

    Gibbons, S. L. J.; Belokurov, V.; Erkal, D.; Evans, N. W.

    2016-05-01

    The shape of the Galactic dark halo can, in principle, be inferred through modelling of stellar tidal streams in the Milky Way halo. The brightest and the longest of these, the Sagittarius stream, reaches out to large Galactocentric distances and hence can deliver the tightest constraints on the Galaxy's potential. In this contribution, we revisit the idea that the Sagittarius Stream was formed from a rotating progenitor. We demonstrate that the angle between the disc's angular momentum and the progenitor's orbital angular momentum does not remain constant throughout the disruption. Instead, it undergoes a dramatic evolution caused, in part, by the changes in the progenitor's moment of inertia tensor. We show that, even in a spherical potential, the streams produced as a result of a discy dwarf disruption appear to be `precessing'. Yet, this debris plane evolution is illusory as it is solely caused by the swaying and wobbling of the progenitor's disc. Stream plane precession is therefore not an unambiguous indicator of asphericity of the dark halo.

  20. Characterising stellar halo populations I: An extended distribution function for halo K giants

    NASA Astrophysics Data System (ADS)

    Das, Payel; Binney, James

    2016-05-01

    We fit an Extended Distribution Function (EDF) to K giants in the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. These stars are detected to radii ˜80 kpc and span a wide range in [Fe/H]. Our EDF, which depends on [Fe/H] in addition to actions, encodes the entanglement of metallicity with dynamics within the Galaxy's stellar halo. Our maximum-likelihood fit of the EDF to the data allows us to model the survey's selection function. The density profile of the K giants steepens with radius from a slope ˜-2 to ˜-4 at large radii. The halo's axis ratio increases with radius from 0.7 to almost unity. The metal-rich stars are more tightly confined in action space than the metal-poor stars and form a more flattened structure. A weak metallicity gradient ˜-0.001 dex/kpc, a small gradient in the dispersion in [Fe/H] of ˜0.001 dex/kpc, and a higher degree of radial anistropy in metal-richer stars result. Lognormal components with peaks at ˜-1.5 and ˜-2.3 are required to capture the overall metallicity distribution, suggestive of the existence of two populations of K giants. The spherical anisotropy parameter varies between 0.3 in the inner halo to isotropic in the outer halo. If the Sagittarius stream is included, a very similar model is found but with a stronger degree of radial anisotropy throughout.

  1. Characterizing stellar halo populations - I. An extended distribution function for halo K giants

    NASA Astrophysics Data System (ADS)

    Das, Payel; Binney, James

    2016-08-01

    We fit an extended distribution function (EDF) to K giants in the Sloan Extension for Galactic Understanding and Exploration survey. These stars are detected to radii ˜80 kpc and span a wide range in [Fe/H]. Our EDF, which depends on [Fe/H] in addition to actions, encodes the entanglement of metallicity with dynamics within the Galaxy's stellar halo. Our maximum-likelihood fit of the EDF to the data allows us to model the survey's selection function. The density profile of the K giants steepens with radius from a slope ˜-2 to ˜-4 at large radii. The halo's axis ratio increases with radius from 0.7 to almost unity. The metal-rich stars are more tightly confined in action space than the metal-poor stars and form a more flattened structure. A weak metallicity gradient ˜-0.001 dex kpc-1, a small gradient in the dispersion in [Fe/H] of ˜0.001 dex kpc-1, and a higher degree of radial anisotropy in metal-richer stars result. Lognormal components with peaks at ˜-1.5 and ˜-2.3 are required to capture the overall metallicity distribution, suggestive of the existence of two populations of K giants. The spherical anisotropy parameter varies between 0.3 in the inner halo to isotropic in the outer halo. If the Sagittarius stream is included, a very similar model is found but with a stronger degree of radial anisotropy throughout.

  2. THE STELLAR HALOS OF MASSIVE ELLIPTICAL GALAXIES

    SciTech Connect

    Greene, Jenny E.; Murphy, Jeremy D.; Comerford, Julia M.; Gebhardt, Karl; Adams, Joshua J.

    2012-05-01

    We use the Mitchell Spectrograph (formerly VIRUS-P) on the McDonald Observatory 2.7 m Harlan J. Smith Telescope to search for the chemical signatures of massive elliptical galaxy assembly. The Mitchell Spectrograph is an integral-field spectrograph with a uniquely wide field of view (107'' Multiplication-Sign 107''), allowing us to achieve remarkably high signal-to-noise ratios of {approx}20-70 pixel{sup -1} in radial bins of 2-2.5 times the effective radii of the eight galaxies in our sample. Focusing on a sample of massive elliptical galaxies with stellar velocity dispersions {sigma}{sub *} > 150 km s{sup -1}, we study the radial dependence in the equivalent widths (EW) of key metal absorption lines. By twice the effective radius, the Mgb EWs have dropped by {approx}50%, and only a weak correlation between {sigma}{sub *} and Mgb EW remains. The Mgb EWs at large radii are comparable to those seen in the centers of elliptical galaxies that are {approx} an order of magnitude less massive. We find that the well-known metallicity gradients often observed within an effective radius continue smoothly to 2.5 R{sub e} , while the abundance ratio gradients remain flat. Much like the halo of the Milky Way, the stellar halos of our galaxies have low metallicities and high {alpha}-abundance ratios, as expected for very old stars formed in small stellar systems. Our observations support a picture in which the outer parts of massive elliptical galaxies are built by the accretion of much smaller systems whose star formation history was truncated at early times.

  3. Resolved Stellar Halos of M87 and NGC 5128

    NASA Astrophysics Data System (ADS)

    Bird, Sarah A.; Harris, William; Flynn, Chris; Blakeslee, John P.; Valtonen, Mauri

    2015-08-01

    We search halo fields of two giant elliptical galaxies: M87, using HST images at 10 kpc from the center, and NGC 5128 (Cen A), using VIMOS VLT images at 65 kpc from the center and archival HST data from 8 to 38 kpc from the center. We resolve thousands of red-giant-branch stars in these stellar halo fields using V and I filters, and, in addition, measure the metallicity using stellar isochrones. In Cen A, we find that the density of metal-rich and metal-poor halo stars falls off with the same slope in the de Vaucouleurs' law profile, from the inner halo of 8 kpc out to 70 kpc, with no sign of a transition to dominance by metal-poor stars. We also find that the metallicity distribution of the inner stellar halo of M87 is most similar to that of NGC 5128's inner stellar halo.

  4. Experimental searches for galactic halo axions.

    PubMed

    van Bibber, Karl A; Kinion, S Darin

    2003-11-15

    A very light axion would be copiously produced during the Big Bang as a zero-temperature Bose gas, and it would possess vanishingly small couplings to matter and radiation. It thus represents an ideal cold dark matter candidate. Galactic halo axions may be detected by their resonant conversion to microwave photons in a high-Q cavity permeated by a strong magnetic field. A large-scale search for the axion is ongoing in the US with sufficient sensitivity to see axions of plausible model couplings. Dramatic breakthroughs in the development of near-quantum limited superconducting quantum interference device amplifiers promise to improve the sensitivity of the experiment by a factor of 30 in the near future. In Japan, a group has been developing a Rydberg atom single-quantum detector as an alternative to linear amplifiers for a microwave-cavity axion experiment. Should the axion be discovered, the predicted fine structure in the axion signal would be rich in information about the history of galactic formation. PMID:14667317

  5. Highly ionized gas in the Galactic halo

    NASA Technical Reports Server (NTRS)

    Shull, J. Michael; Slavin, Jonathan D.

    1994-01-01

    We reexamine the values of electron density n(sub e) and gas pressure P/k in the interstellar medium (ISM) of the Galactic halo, as inferred from C IV emission and absorption lines and using current C IV atomic data. In a homogeneous model with 4.7 less than or equal to log T less than or equal to 5.3, the data are consistent with 0.01 less than or equal to n(sub e) less than or equal to 0.02/cu cm and 2200 less than or equal to P/k less than or equal to 3700/cu cm K, a factor of 2-3 higher than advocated by Martin & Bowyer (1990) and comparable to the thermal pressure in the disk. If some of the C IV absorption arises from nonemitting, photoionized gas, then the inferred density and pressure will increase accordingly. The volume filling factor for homogeneous models ranges from 0.5% to 5%. Because of the constraints arising from filling factor and radiated power, most of the C IV must arise from gas near the peak of the cooling curve, at log t less than or equal to 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants (SNRs) provide significant amounts of halo gas at approximately 10(exp 5.3) K and process 20-40 solar mass/yr with a power of approximately 10(exp 41) ergs/sec. Since the observed C IV and N V absorption scale heights have been reported to differ, at 4.9 kpc and 1.6 kpc, respectively, we examine inhomogeneous models with different exponential scale heights of T, P, and SN energy input. The ISM may change its character with distance above the Galactic plane, as superbubbles and mixing layers dominate over isolated SNRs as the source of the C IV. For appropiate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O IV lambda 1034 diffuse emission line, which can be used as a temperature diagnostic of the hot gas, is predicted to be comparable in strength to that of C IV lambda 1549 (approximately 6000 photons

  6. MAPPING THE STELLAR STRUCTURE OF THE MILKY WAY THICK DISK AND HALO USING SEGUE PHOTOMETRY

    SciTech Connect

    De Jong, Jelte T. A.; Rix, Hans-Walter; Martin, Nicolas F.; Yanny, Brian; Dolphin, Andrew E.; Beers, Timothy C.

    2010-05-01

    We map the stellar structure of the Galactic thick disk and halo by applying color-magnitude diagram (CMD) fitting to photometric data from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. The SEGUE imaging scans allow, for the first time, a comprehensive analysis of Milky Way structure at both high and low latitudes using uniform Sloan Digital Sky Survey photometry. Incorporating photometry of all relevant stars simultaneously, CMD fitting bypasses the need to choose single tracer populations. Using old stellar populations of differing metallicities as templates, we obtain a sparse three-dimensional map of the stellar mass distribution at |Z|>1 kpc. Fitting a smooth Milky Way model comprising exponential thin and thick disks and an axisymmetric power-law halo allows us to constrain the structural parameters of the thick disk and halo. The thick-disk scale height and length are well constrained at 0.75 {+-} 0.07 kpc and 4.1 {+-} 0.4 kpc, respectively. We find a stellar halo flattening within {approx}25 kpc of c/a = 0.88 {+-} 0.03 and a power-law index of 2.75 {+-} 0.07 (for 7 kpc {approx_lt}R{sub GC} {approx_lt} 30 kpc). The model fits yield thick-disk and stellar halo densities at the solar location of {rho}{sub thick,sun} = 10{sup -2.3{+-}0.1} M{sub sun} pc{sup -3} and {rho}{sub halo,sun} = 10{sup -4.20{+-}0.05} M{sub sun} pc{sup -3}, averaging over any substructures. Our analysis provides the first clear in situ evidence for a radial metallicity gradient in the Milky Way's stellar halo: within R {approx_lt} 15 kpc the stellar halo has a mean metallicity of [Fe/H] {approx_equal} -1.6, which shifts to [Fe/H] {approx_equal} -2.2 at larger radii, in line with the two-component halo deduced by Carollo et al. from a local kinematic analysis. Subtraction of the best-fit smooth and symmetric model from the overall density maps reveals a wealth of substructures at all latitudes, some attributable to known streams and overdensities, and some

  7. Black holes in binary stellar systems and galactic nuclei

    NASA Astrophysics Data System (ADS)

    Cherepashchuk, A. M.

    2014-04-01

    In the last 40 years, following pioneering papers by Ya B Zeldovich and E E Salpeter, in which a powerful energy release from nonspherical accretion of matter onto a black hole (BH) was predicted, many observational studies of black holes in the Universe have been carried out. To date, the masses of several dozen stellar-mass black holes (M_BH = (4{-}20) M_\\odot) in X-ray binary systems and of several hundred supermassive black holes (M_BH = (10^{6}{-}10^{10}) M_\\odot) in galactic nuclei have been measured. The estimated radii of these massive and compact objects do not exceed several gravitational radii. For about ten stellar-mass black holes and several dozen supermassive black holes, the values of the dimensionless angular momentum a_* have been estimated, which, in agreement with theoretical predictions, do not exceed the limiting value a_* = 0.998. A new field of astrophysics, so-called black hole demography, which studies the birth and growth of black holes and their evolutionary connection to other objects in the Universe, namely stars, galaxies, etc., is rapidly developing. In addition to supermassive black holes, massive stellar clusters are observed in galactic nuclei, and their evolution is distinct from that of supermassive black holes. The evolutionary relations between supermassive black holes in galactic centers and spheroidal stellar components (bulges) of galaxies, as well as dark-matter galactic haloes are brought out. The launch into Earth's orbit of the space radio interferometer RadioAstron opened up the real possibility of finally proving that numerous discovered massive and highly compact objects with properties very similar to those of black holes make up real black holes in the sense of Albert Einstein's General Relativity. Similar proofs of the existence of black holes in the Universe can be obtained by intercontinental radio interferometry at short wavelengths \\lambda \\lesssim 1 mm (the international program, Event Horizon Telescope).

  8. Probing the Shape and History of the Milky Way Halo with Stellar Orbits

    NASA Astrophysics Data System (ADS)

    Valluri, Monica

    2010-05-01

    One of the key predictions of Lambda-CDM cosmological simulations is that the dark matter halos of galaxies such as the Milky Way are prolate or triaxial. However simulations show that both the shapes and density profiles of dark matter halos can be dramatically altered by the condensation of baryons. I will describe how Laskar's Frequency mapping technique can be used to analyze the phase-space structure of the Milky Way's halo with halo stellar orbits. From Frequency Maps of halo stars it is possible to set constrains, not just on the present shape and phase space distribution of the Milky Way halo, but also on its past shape history. The power of this technique is demonstrated by applying it to a series of controlled simulations in which dynamically realistic disks are grown in isolated triaxial dark matter halos. I will demonstrate that even when the growth of a baryonic disk causes the halo's shape to become oblate or close to spherical, it is possible to determine if it was originally prolate or triaxial as predicted by cosmological N-body simulations. Current and future Galactic surveys (e.g. RAVE, Segue, Gaia and LAMOST) are expected to obtain the full 6 dimensional phase space orbits of hundreds of thousands of halo stars. This technique will be a new and valuable tool that will complement on-going efforts to constrain the shape of the halo with tidal streams. Reliable indicators of the halo's shape can be obtained with as few as 5000-10000 halo stellar orbits.

  9. THE FRACTION OF GLOBULAR CLUSTER SECOND-GENERATION STARS IN THE GALACTIC HALO

    SciTech Connect

    Vesperini, Enrico; McMillan, Stephen L. W.; D'Antona, Francesca; D'Ercole, Annibale

    2010-08-01

    Many observational studies have revealed the presence of multiple stellar generations in Galactic globular clusters. These studies suggest that second-generation stars make up a significant fraction of the current mass of globular clusters, with the second-generation mass fraction ranging from {approx}50% to 80% in individual clusters. In this Letter, we carry out hydrodynamical simulations to explore the dependence of the mass of second-generation stars on the initial mass and structural parameters and stellar initial mass function (IMF) of the parent cluster. We then use the results of these simulations to estimate the fraction f{sub SG,H} of the mass of the Galactic stellar halo composed of second-generation stars that originated in globular clusters. We study the dependence of f{sub SG,H} on the parameters of the IMF of the Galactic globular cluster system. For a broad range of initial conditions, we find that the fraction of mass of the Galactic stellar halo in second-generation stars is always small, f{sub SG,H} < 4%-6% for a Kroupa-1993 IMF and f{sub SG,H} < 7%-9% for a Kroupa-2001 IMF.

  10. The Milky Way, the Galactic Halo, and the Halos of Galaxies

    NASA Astrophysics Data System (ADS)

    Gerhard, Ortwin

    2016-08-01

    The Milky Way, ``our'' Galaxy, is currently the subject of intense study with many ground-based surveys, in anticipation of upcoming results from the Gaia mission. From this work we have been learning about the full three-dimensional structure of the Galactic box/peanut bulge, the distribution of stars in the bar and disk, and the many streams and substructures in the Galactic halo. The data indicate that a large fraction of the Galactic halo has been accreted from outside. Similarly, in many external galaxy halos there is now evidence for tidal streams and accretion of satellites. To study these features requires exquisite, deep photometry and spectroscopy. These observations illustrate how galaxy halos are still growing, and sometimes can be used to ``time'' the accretion events. In comparison with cosmological simulations, the structure of galaxy halos gives us a vivid illustration of the hierarchical nature of our Universe.

  11. New cluster members and halo stars of the Galactic globular cluster NGC 1851

    NASA Astrophysics Data System (ADS)

    Navin, Colin A.; Martell, Sarah L.; Zucker, Daniel B.

    2015-10-01

    NGC 1851 is an intriguing Galactic globular cluster, with multiple stellar evolutionary sequences, light and heavy element abundance variations and indications of a surrounding stellar halo. We present the first results of a spectroscopic study of red giant stars within and outside of the tidal radius of this cluster. Our results identify nine probable new cluster members (inside the tidal radius) with heliocentric radial velocities consistent with that of NGC 1851. We also identify, based on their radial velocities, four probable extratidal cluster halo stars at distances up to ˜3.1 times the tidal radius, which are supportive of previous findings that NGC 1851 is surrounded by an extended stellar halo. Proper motions were available for 12 of these 13 stars and all are consistent with that of NGC 1851. Apart from the cluster members and cluster halo stars, our observed radial velocity distribution agrees with the expected distribution from a Besançon disc/N-body stellar halo Milky Way model generated by the GALAXIA code, suggesting that no other structures at different radial velocities are present in our field. The metallicities of these stars are estimated using equivalent width measurements of the near-infrared calcium triplet absorption lines and are found, within the limitations of this method, to be consistent with that of NGC 1851. In addition we recover 110 red giant cluster members from previous studies based on their radial velocities and identify three stars with unusually high radial velocities.

  12. A stellar feedback origin for neutral hydrogen in high-redshift quasar-mass haloes

    NASA Astrophysics Data System (ADS)

    Faucher-Giguère, Claude-André; Feldmann, Robert; Quataert, Eliot; Kereš, Dušan; Hopkins, Philip F.; Murray, Norman

    2016-09-01

    Observations reveal that quasar host haloes at z ˜ 2 have large covering fractions of cool dense gas (≳60 per cent for Lyman limit systems within a projected virial radius). Most simulations have so far failed to explain these large observed covering fractions. We analyse a new set of 15 simulated massive haloes with explicit stellar feedback from the FIRE project, covering the halo mass range Mh ≈ 2 × 1012 - 1013 M⊙ at z = 2. This extends our previous analysis of the circum-galactic medium of high-redshift galaxies to more massive haloes. Active galactic nuclei (AGN) feedback is not included in these simulations. We find Lyman limit system covering fractions consistent with those observed around quasars. The large H I covering fractions arise from star formation-driven galactic winds, including winds from low-mass satellite galaxies that interact with cosmological filaments. We show that it is necessary to resolve these satellite galaxies and their winds to reproduce the large Lyman limit system covering fractions observed in quasar-mass haloes. Our simulations predict that galaxies occupying dark matter haloes of mass similar to quasars but without a luminous AGN should have Lyman limit system covering fractions comparable to quasars.

  13. Correlating galaxy colour and halo concentration: a tunable halo model of galactic conformity

    NASA Astrophysics Data System (ADS)

    Paranjape, Aseem; Kovač, Katarina; Hartley, William G.; Pahwa, Isha

    2015-12-01

    We extend the halo occupation distribution (HOD) framework to generate mock galaxy catalogues exhibiting varying levels of `galactic conformity', which has emerged as a potentially powerful probe of environmental effects in galaxy evolution. Our model correlates galaxy colours in a group with the concentration of the common parent dark halo through a `group quenching efficiency' ρ which makes older, more concentrated haloes at fixed mass preferentially host redder galaxies. We find that, for a specific value of ρ, this 1-halo conformity matches corresponding measurements in a group catalogue based on the Sloan Digital Sky Survey. Our mocks also display conformity at large separations from isolated objects, potentially an imprint of halo assembly bias. A detailed study - using mocks with assembly bias erased while keeping 1-halo conformity intact - reveals a rather nuanced situation, however. At separations ≲4 Mpc, conformity is mainly a 1-halo effect dominated by the largest haloes and is not a robust indicator of assembly bias. Only at very large separations (≳8 Mpc) does genuine 2-halo conformity, driven by the assembly bias of small haloes, manifest distinctly. We explain all these trends in standard halo model terms. Our model opens the door to parametrized HOD analyses that self-consistently account for galactic conformity at all scales.

  14. Structure and Population of the NGC 55 Stellar Halo from A Subaru/Suprime-Cam Survey

    NASA Astrophysics Data System (ADS)

    Tanaka, Mikito; Chiba, Masashi; Komiyama, Yutaka; Guhathakurta, Puragra; Kalirai, Jason S.

    2011-09-01

    As part of our survey of galactic stellar halos, we investigate the structure and stellar populations of the northern outer part of the stellar halo in NGC 55, a member galaxy of the Sculptor Group, using deep and wide-field V- and I-band images taken with Subaru/Suprime-Cam. Based on the analysis of the color-magnitude diagrams for red giant branch (RGB) stars, we derive a tip of RGB based distance modulus to the galaxy of (m - M)0 = 26.58 ± 0.11(d = 2.1 ± 0.1 Mpc). From the stellar density maps, we detect the asymmetrically disturbed, thick disk structure and two metal-poor overdense substructures in the north region of NGC 55, which may correspond to merger remnants associated with hierarchical formation of NGC 55's halo. In addition, we identify a diffuse metal-poor halo extended out to at least z ~ 16 kpc from the galactic plane. The surface brightness profiles toward the z-direction perpendicular to the galactic plane suggest that the stellar density distribution in the northern outer part of NGC 55 is described by a locally isothermal disk at z <~ 6 kpc and a likely diffuse metal-poor halo with V-band surface brightness of μV >~ 32 mag arcsec-2, where old RGB stars dominate. We derive the metallicity distributions (MDs) of these structures on the basis of the photometric comparison of RGB stars with the theoretical stellar evolutionary models. The MDs of the thick disk structures show the peak and mean metallicity of [Fe/H]peak ~ -1.4 and [Fe/H]mean ~ -1.7, respectively, while the outer substructures show more metal-poor features than the thick disk structure. Combined with the current results with our previous study for M31's halo, we discuss the possible difference in the formation process of stellar halos among different Hubble types. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

  15. Fading features found in the kinematics of the far-reaching Milky Way stellar halo

    NASA Astrophysics Data System (ADS)

    Bird, Sarah A.; Flynn, Chris

    2015-09-01

    We test the long-term kinematical stability of a Galactic stellar halo model, due to Kafle et al. (2012), who study the kinematics of approximately 5000 blue horizontal branch stars in the Sloan Digital Sky Survey. The velocity dispersion σ and anisotropy parameter β of the stars have been determined as functions of Galactocentric radius, over the range 6 < RGC < 25 kpc, and show a strong dip in the anisotropy profile at RGC ˜ 17 kpc. By directly integrating orbits of particles in a 3D model of the Galactic potential with these characteristics, we show that the σ and β profiles quickly evolve on a time-scale of a few × 10 Myr whereas the density ρ profile remains largely unaffected. We suggest that the feature is therefore transient. The origin of such features in the Galactic halo remains unclear.

  16. STELLAR TRANSITS IN ACTIVE GALACTIC NUCLEI

    SciTech Connect

    Beky, Bence; Kocsis, Bence E-mail: bkocsis@cfa.harvard.edu

    2013-01-01

    Supermassive black holes (SMBHs) are typically surrounded by a dense stellar population in galactic nuclei. Stars crossing the line of site in active galactic nuclei (AGNs) produce a characteristic transit light curve, just like extrasolar planets do when they transit their host star. We examine the possibility of finding such AGN transits in deep optical, UV, and X-ray surveys. We calculate transit light curves using the Novikov-Thorne thin accretion disk model, including general relativistic effects. Based on the expected properties of stellar cusps, we find that around 10{sup 6} solar mass SMBHs, transits of red giants are most common for stars on close orbits with transit durations of a few weeks and orbital periods of a few years. We find that detecting AGN transits requires repeated observations of thousands of low-mass AGNs to 1% photometric accuracy in optical, or {approx}10% in UV bands or soft X-ray. It may be possible to identify stellar transits in the Pan-STARRS and LSST optical and the eROSITA X-ray surveys. Such observations could be used to constrain black hole mass, spin, inclination, and accretion rate. Transit rates and durations could give valuable information on the circumnuclear stellar clusters as well. Transit light curves could be used to image accretion disks with unprecedented resolution, allowing us to resolve the SMBH silhouette in distant AGNs.

  17. Stellar Transits in Active Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Béky, Bence; Kocsis, Bence

    2013-01-01

    Supermassive black holes (SMBHs) are typically surrounded by a dense stellar population in galactic nuclei. Stars crossing the line of site in active galactic nuclei (AGNs) produce a characteristic transit light curve, just like extrasolar planets do when they transit their host star. We examine the possibility of finding such AGN transits in deep optical, UV, and X-ray surveys. We calculate transit light curves using the Novikov-Thorne thin accretion disk model, including general relativistic effects. Based on the expected properties of stellar cusps, we find that around 106 solar mass SMBHs, transits of red giants are most common for stars on close orbits with transit durations of a few weeks and orbital periods of a few years. We find that detecting AGN transits requires repeated observations of thousands of low-mass AGNs to 1% photometric accuracy in optical, or ~10% in UV bands or soft X-ray. It may be possible to identify stellar transits in the Pan-STARRS and LSST optical and the eROSITA X-ray surveys. Such observations could be used to constrain black hole mass, spin, inclination, and accretion rate. Transit rates and durations could give valuable information on the circumnuclear stellar clusters as well. Transit light curves could be used to image accretion disks with unprecedented resolution, allowing us to resolve the SMBH silhouette in distant AGNs.

  18. Building Blocks of the Milky Way's Stellar Halo

    NASA Astrophysics Data System (ADS)

    van Oirschot, Pim; Starkenburg, Else; Helmi, Amina; Nelemans, Gijs

    2016-08-01

    We study the assembly history of the stellar halo of Milky Way-like galaxies using the six high-resolution Aquarius dark matter simulations combined with the Munich-Groningen semi-analytic galaxy formation model. Our goal is to understand the stellar population contents of the building blocks of the Milky Way halo, including their star formation histories and chemical evolution, as well as their internal dynamical properties. We are also interested in how they relate or are different from the surviving satellite population. Finally, we will use our models to compare to observations of halo stars in an attempt to reconstruct the assembly history of the Milky Way's stellar halo itself.

  19. Mixing between high velocity clouds and the galactic halo

    SciTech Connect

    Gritton, Jeffrey A.; Shelton, Robin L.; Kwak, Kyujin E-mail: rls@physast.uga.edu

    2014-11-01

    In the Galactic halo, metal-bearing Galactic halo material mixes into high velocity clouds (HVCs) as they hydrodynamically interact. This interaction begins long before the clouds completely dissipate and long before they slow to the velocity of the Galactic material. In order to make quantitative estimates of the mixing efficiency and resulting metal enrichment of HVCs, we made detailed two- and three-dimensional simulations of cloud-interstellar medium interactions. Our simulations track the hydrodynamics and time-dependent ionization levels. They assume that the cloud originally has a warm temperature and extremely low metallicity while the surrounding medium has a high temperature, low density, and substantial metallicity, but our simulations can be generalized to other choices of initial metallicities. In our simulations, mixing between cloud and halo gas noticeably raises the metallicity of the high velocity material. We present plots of the mixing efficiency and metal enrichment as a function of time.

  20. Could wormholes form in dark matter galactic halos?

    NASA Astrophysics Data System (ADS)

    Rahaman, Farook; Shit, G. C.; Sen, Banashree; Ray, Saibal

    2016-01-01

    We estimate expression for velocity as a function of the radial coordinate r by using polynomial interpolation based on the experimental data of rotational velocities at distant outer regions of galaxies. The interpolation technique has been used to estimate fifth degree polynomial followed by cubic spline interpolation. This rotational velocity is used to find the geometry of galactic halo regions within the framework of Einstein's general relativity. In this paper we have analyzed features of galactic halo regions based on two possible choices for the dark matter density profile, viz. Navarro, Frenk & White (NFW) type (Navarro et al. in Astrophys. J. 462:563, 1996) and Universal Rotation Curve (URC) (Castignani et al. in Nat. Sci. 4:265, 2012). It is argued that spacetime of the galactic halo possesses some of the characteristics needed to support traversable wormholes.

  1. Mixing between High Velocity Clouds and the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Gritton, Jeffrey A.; Shelton, Robin L.; Kwak, Kyujin

    2014-11-01

    In the Galactic halo, metal-bearing Galactic halo material mixes into high velocity clouds (HVCs) as they hydrodynamically interact. This interaction begins long before the clouds completely dissipate and long before they slow to the velocity of the Galactic material. In order to make quantitative estimates of the mixing efficiency and resulting metal enrichment of HVCs, we made detailed two- and three-dimensional simulations of cloud-interstellar medium interactions. Our simulations track the hydrodynamics and time-dependent ionization levels. They assume that the cloud originally has a warm temperature and extremely low metallicity while the surrounding medium has a high temperature, low density, and substantial metallicity, but our simulations can be generalized to other choices of initial metallicities. In our simulations, mixing between cloud and halo gas noticeably raises the metallicity of the high velocity material. We present plots of the mixing efficiency and metal enrichment as a function of time.

  2. THE GROWTH OF GALAXY STELLAR MASS WITHIN DARK MATTER HALOS

    SciTech Connect

    Zehavi, Idit; Patiri, Santiago; Zheng Zheng

    2012-02-20

    We study the evolution of stellar mass in galaxies as a function of host halo mass, using the 'MPA' and 'Durham' semi-analytic models, implemented on the Millennium Run simulation. For both models, the stellar mass of the central galaxies increases rapidly with halo mass at the low-mass end and more slowly in halos of larger masses at the three redshifts probed (z {approx} 0, 1, 2). About 45% of the stellar mass in central galaxies in present-day halos less massive than {approx}10{sup 12} h{sup -1} M{sub Sun} is already in place at z {approx} 1, and this fraction increases to {approx}65% for more massive halos. The baryon conversion efficiency into stars has a peaked distribution with halo mass, and the peak location shifts toward lower mass from z {approx} 1 to z {approx} 0. The stellar mass in low-mass halos grows mostly by star formation since z {approx} 1, while in high-mass halos most of the stellar mass is assembled by mergers, reminiscent of 'downsizing'. We compare our findings to empirical results from the Sloan Digital Sky Survey and DEEP2 surveys utilizing galaxy clustering measurements to study galaxy evolution. The theoretical predictions are in qualitative agreement with these phenomenological results, but there are large discrepancies. The most significant one concerns the number of stars already in place in the progenitor galaxies at z {approx} 1, which is about a factor of two larger in both semi-analytic models. We demonstrate that methods studying galaxy evolution from the galaxy-halo connection are powerful in constraining theoretical models and can guide future efforts of modeling galaxy evolution. Conversely, semi-analytic models serve an important role in improving such methods.

  3. Do Not Forget the Forest for the Trees: The Stellar-mass Halo-mass Relation in Different Environments

    NASA Astrophysics Data System (ADS)

    Tonnesen, Stephanie; Cen, Renyue

    2015-10-01

    The connection between dark matter halos and galactic baryons is often not well constrained nor well resolved in cosmological hydrodynamical simulations. Thus, halo occupation distribution models that assign galaxies to halos based on halo mass are frequently used to interpret clustering observations, even though it is well known that the assembly history of dark matter halos is related to their clustering. In this paper we use high-resolution hydrodynamical cosmological simulations to compare the halo and stellar mass growth of galaxies in a large-scale overdensity to those in a large-scale underdensity (on scales of about 20 Mpc). The simulation reproduces assembly bias, in which halos have earlier formation times in overdense environments than in underdense regions. We find that the ratio of stellar mass to halo mass is larger in overdense regions in central galaxies residing in halos with masses between 1011 and 1012.9 M⊙. When we force the local density (within 2 Mpc) at z = 0 to be the same for galaxies in the large-scale over- and underdensities, we find the same results. We posit that this difference can be explained by a combination of earlier formation times, more interactions at early times with neighbors, and more filaments feeding galaxies in overdense regions. This result puts the standard practice of assigning stellar mass to halos based only on their mass, rather than considering their larger environment, into question.

  4. The gamma-ray-flux PDF from galactic halo substructure

    NASA Astrophysics Data System (ADS)

    Lee, Samuel K.; Ando, Shin'ichiro; Kamionkowski, Marc

    2009-07-01

    One of the targets of the recently launched Fermi Gamma-ray Space Telescope is a diffuse gamma-ray background from dark-matter annihilation or decay in the Galactic halo. N-body simulations and theoretical arguments suggest that the dark matter in the Galactic halo may be clumped into substructure, rather than smoothly distributed. Here we propose the gamma-ray-flux probability distribution function (PDF) as a probe of substructure in the Galactic halo. We calculate this PDF for a phenomenological model of halo substructure and determine the regions of the substructure parameter space in which the PDF may be distinguished from the PDF for a smooth distribution of dark matter. In principle, the PDF allows a statistical detection of substructure, even if individual halos cannot be detected. It may also allow detection of substructure on the smallest microhalo mass scales, ~ M⊕, for weakly-interacting massive particles (WIMPs). Furthermore, it may also provide a method to measure the substructure mass function. However, an analysis that assumes a typical halo substructure model and a conservative estimate of the diffuse background suggests that the substructure PDF may not be detectable in the lifespan of Fermi in the specific case that the WIMP is a neutralino. Nevertheless, for a large range of substructure, WIMP annihilation, and diffuse background models, PDF analysis may provide a clear signature of substructure.

  5. The Milky Way, the Galactic halo, and the Halos of Galaxies

    NASA Astrophysics Data System (ADS)

    Gerhard, Ortwin

    2015-08-01

    The Milky Way, "our" Galaxy, is currently the subject of intense study with many ground-based surveys, in anticipation of upcoming results from the GAIA mission. From this work we have been learning about the full three-dimensional structure of the Galactic box/peanut bulge, the distribution of stars in the bar and disk, and the many streams in the Galactic halo. The data tell us that most of the Galactic bulge formed from the disk, and that a large fraction of the Galactic halo has been accreted from outside. Similarly, in many external galaxy halos there is now evidence for tidal streams and accretion of satellites. To see these features requires exquisite data - mostly very deep photometry, but some halo substructures have also been found with kinematic data. These observations illustrate how galaxy halos are still growing, and sometimes can be used to "time" the accretion events. In comparison with cosmological simulations, the structure of galaxy halos gives us a vivid illustration of the hierarchical nature of our Universe.

  6. An extremely primitive star in the Galactic halo.

    PubMed

    Caffau, Elisabetta; Bonifacio, Piercarlo; François, Patrick; Sbordone, Luca; Monaco, Lorenzo; Spite, Monique; Spite, François; Ludwig, Hans-G; Cayrel, Roger; Zaggia, Simone; Hammer, François; Randich, Sofia; Molaro, Paolo; Hill, Vanessa

    2011-09-01

    The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z, is known as 'metallicity'. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z < 1.5 × 10(-5), it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10(-8) to 1.5 × 10(-6) (ref. 8), although competing theories claiming the contrary do exist. (We use 'low-mass' here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10(-7), which is 4.5 × 10(-5) times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars--that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium. PMID:21886158

  7. Compact binary mergers as the origin of r-process elements in the Galactic halo

    SciTech Connect

    Ishimaru, Yuhri; Wanajo, Shinya; Prantzos, Nikos

    2014-05-02

    Compact binary mergers (of double neutron star and black hole-neutron star systems) are suggested to be the major site of the r-process elements in the Galaxy by recent hydrodynamical and nucleosynthesis studies. It has been pointed out, however, that estimated long lifetimes of compact binaries are in conflict with the presence of r-process-enhanced stars at the metallicity [Fe/H] ∼ −3. To resolve this problem, we examine the role of compact binary mergers in the early Galactic chemical evolution on the assumption that our Galactic halo was formed from merging sub-halos. The chemical evolutions are modeled for sub-halos with their total stellar masses between 10{sup 4}M{sub ⊙} and 2 × 10{sup 8}M{sub ⊙}. The lifetimes of compact binaries are assumed to be 100 Myr (95%) and 1 Myr (5%) according to recent binary population synthesis studies. We find that the r-process abundances (relative to iron; [r/Fe]) start increasing at [Fe/H] ≤ −3 if the star formation rates are smaller for less massive sub-halos. Our models also suggest that the star-to-star scatter of [r/Fe]'s observed in Galactic halo stars can be interpreted as a consequence of greater gas outflow rates for less massive sub-halos. In addition, the sub-solar [r/Fe]'s (observed as [Ba/Fe] ∼ −1.5 for [Fe/H] < −3) are explained by the contribution from the short-lived (∼ 1 Myr) binaries. Our result indicates, therefore, that compact binary mergers can be potentially the origin of the r-process elements throughout the Galactic history.

  8. HaloSat- A CubeSat to Study the Hot Galactic Halo

    NASA Astrophysics Data System (ADS)

    Kaaret, Philip

    We propose to develop, build, and fly HaloSat, a CubeSat capable of measuring the oxygen line emission from the hot Galactic halo. A dedicated CubeSat enables an instrument design and observing strategy to maximize the halo signal while minimizing foregrounds from solar wind charge exchange interactions within the solar system. We will use HaloSat to map the distribution of hot gas in the Milky Way and determine whether it fills an extended, and thus massive halo, or whether the halo is compact, and thus does not contribute significantly to the total mass of the Milky Way. HaloSat can be accomplished at modest cost using a CubeSat, a novel platform for space astrophysics missions. We will use a commercially available CubeSat bus and commercially available X-ray detectors to reduce development risk and minimize overall mission cost. HaloSat builds on the initiatives of GSFC/Wallops Flight Facility (WFF) in the development of CubeSats for low cost access to space and relies on the technical expertise of WFF personnel for spacecraft and mission design and operations. The team, from University of Iowa (UI), GSFC, Johns Hopkins, and CNRS (France), contains experts in X-ray detector development and data analysis and the astrophysics of hot plasmas and Galactic structure. The UI team will include a number of junior researchers (undergraduates, graduate students, and a postdoc) and help train them for future leadership roles on NASA space flight missions.

  9. Lithium in halo stars from standard stellar evolution

    NASA Technical Reports Server (NTRS)

    Deliyannis, Constantine P.; Demarque, Pierre; Kawaler, Steven D.

    1990-01-01

    A grid has been constructed of theoretical evolution sequences of models for low-metallicity stars from the premain-sequence to the giant branch phases. The grid is used to study the history of surface Li abundance during standard stellar evolution. The Li-7 observations of halo stars by Spite and Spite (1982) and subsequent observations are synthesized to separate the halo stars by age. The theory of surface Li abundance is illustrated by following the evolution of a reference halo star model from the contracting fully convective premain sequence to the giant branch phase. The theoretical models are compared with observed Li abundances. The results show that the halo star lithium abundances can be explained in the context of standard stellar evolution theory using completely standard assumptions and physics.

  10. Stellar and galactic jets - Theoretical issues

    NASA Technical Reports Server (NTRS)

    Konigl, A.

    1986-01-01

    Theoretical issues pertaining to the modelling of jets in young stellar objects and in active galactic nuclei are reviewed. The strong morphological similarities between these two types of sources are emphasized, and observational constraints on the basic physical mechanisms that may be responsible for the jet phenomenon are outlined. Particular attention is given to the 'momentum-discharge problem' in molecular-cloud outflows and to its possible resolution in terms of a centrifugally driven magnetohydrodynamic wind from an accretion disk. In addition, various propagation effects are discussed, and the relevance to stellar jets of the de Laval collimation mechanism and of the accelerated-clump model for emission knots is assessed. The review concludes with a brief list of potentially useful observational tests.

  11. On the stellar halo metallicity profile of Milky Way-like galaxies in the Auriga simulations

    NASA Astrophysics Data System (ADS)

    Monachesi, Antonela; Gómez, Facundo A.; Grand, Robert J. J.; Kauffmann, Guinevere; Marinacci, Federico; Pakmor, Rüdiger; Springel, Volker; Frenk, Carlos S.

    2016-06-01

    A recent observational study of haloes of nearby Milky Way-like galaxies shows that only half (four out of eight) of the current sample exhibits strong negative metallicity ([Fe/H]) gradients. This is at odds with predictions from hydrodynamical simulations where such gradients are ubiquitous. In this Letter, we use high-resolution cosmological hydrodynamical simulations to study the [Fe/H] distribution of galactic haloes. We find that kinematically selected stellar haloes, including both in situ and accreted particles, have an oblate [Fe/H] distribution. Spherical [Fe/H] radial profiles show strong negative gradients within 100 kpc, in agreement with previous numerical results. However, the projected median [Fe/H] profiles along the galactic disc minor axis, typically obtained in observations, are significantly flatter. The median [Fe/H] values at a given radius are larger for the spherical profiles than for the minor axis profiles by as much as 0.4 dex within the inner 50 kpc. Similar results are obtained if only the accreted stellar component is considered indicating that the differences between spherical and minor axis profiles are not purely driven by heated disc star particles formed in situ. Our study highlights the importance of performing careful comparisons between models and observations of halo [Fe/H] distributions.

  12. MAPPING THE GALACTIC HALO WITH BLUE HORIZONTAL BRANCH STARS FROM THE TWO-DEGREE FIELD QUASAR REDSHIFT SURVEY

    SciTech Connect

    De Propris, Roberto; Harrison, Craig D.; Mares, Peter J.

    2010-08-20

    We use 666 blue horizontal branch stars from the 2Qz Redshift Survey to map the Galactic halo in four dimensions (position, distance, and velocity). We find that the halo extends to at least 100 kpc in Galactocentric distance, and obeys a single power-law density profile of index {approx}-2.5 in two different directions separated by about 150{sup 0} on the sky. This suggests that the halo is spherical. Our map shows no large kinematically coherent structures (streams, clouds, or plumes) and appears homogeneous. However, we find that at least 20% of the stars in the halo reside in substructures and that these substructures are dynamically young. The velocity dispersion profile of the halo appears to increase toward large radii while the stellar velocity distribution is non-Gaussian beyond 60 kpc. We argue that the outer halo consists of a multitude of low luminosity overlapping tidal streams from recently accreted objects.

  13. GALACTIC ARCHEOLOGY AND THE HIGH-REDSHIFT DETECTABILITY OF MILKY WAY HALO PROGENITOR GALAXIES

    SciTech Connect

    Okrochkov, Mikhail; Tumlinson, Jason

    2010-06-10

    Using a simple model of Milky Way halo formation and chemical evolution, we examine the criteria for observing Milky Way-like progenitor galaxies and the stellar populations within them with the James Webb Space Telescope (JWST). We determine that Milky Way progenitors are visible in deep fields (AB = 30-31) up to z = 7 with low dust content and up to z = 9 with no dust content. These models allow us to determine the fraction of Milky Way halo stars formed in galaxies that should be visible with Near-Infrared Camera on JWST, as a function of metallicity. We show that galaxies in which Galactic halo stars of metallicity above [Fe/H] = -2.5 formed can be detected. Galaxies that form stars with higher metallicities should be visible in deep fields and can be studied by combining observations within the Galaxy as well as external galaxies. We find that halo stars with [Fe/H] <-2.8 formed in progenitor galaxies that are not visible to JWST and that examining their remnants within the Milky Way provides the only realistic prospect of studying these ancient stellar populations. The oldest visible stellar populations are shown to be centrally concentrated, with the stars that formed in visible galaxies at z > 3 residing preferentially within the central 10 kpc region of the Galaxy.

  14. VizieR Online Data Catalog: The SEGUE K giant survey. III. Galactic halo (Janesh+, 2016)

    NASA Astrophysics Data System (ADS)

    Janesh, W.; Morrison, H. L.; Ma, Z.; Rockosi, C.; Starkenburg, E.; Xue, X. X.; Rix, H.-W.; Harding, P.; Beers, T. C.; Johnson, J.; Lee, Y. S.; Schneider, D. P.

    2016-03-01

    We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125kpc. These stars have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Survey's Sloan Extension for Galactic Understanding and Exploration (SEGUE) project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using blue horizontal branch (BHB) stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of individual groups, we find that a large fraction (~33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity. (2 data files).

  15. GAS CONDENSATION IN THE GALACTIC HALO

    SciTech Connect

    Joung, M. Ryan; Bryan, Greg L.; Putman, Mary E.

    2012-02-01

    Using adaptive mesh refinement (AMR) hydrodynamic simulations of vertically stratified hot halo gas, we examine the conditions under which clouds can form and condense out of the hot halo medium to potentially fuel star formation in the gaseous disk. We find that halo clouds do not develop from linear isobaric perturbations. This is a regime where the cooling time is longer than the Brunt-Vaeisaelae time, confirming previous linear analysis. We extend the analysis into the nonlinear regime by considering mildly or strongly nonlinear perturbations with overdensities up to 100, also varying the initial height, the cloud size, and the metallicity of the gas. Here, the result depends on the ratio of cooling time to the time required to accelerate the cloud to the sound speed (similar to the dynamical time). If the ratio exceeds a critical value near unity, the cloud is accelerated without further cooling and gets disrupted by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities. If it is less than the critical value, the cloud cools and condenses before disruption. Accreting gas with overdensities of 10-20 is expected to be marginally unstable; the cooling fraction will depend on the metallicity, the size of the incoming cloud, and the distance to the galaxy. Locally enhanced overdensities within cold streams have a higher likelihood of cooling out. Our results have implications on the evolution of clouds seeded by cold accretion that are barely resolved in current cosmological hydrodynamic simulations and absorption line systems detected in galaxy halos.

  16. Stellar halos and the link to galaxy formation

    NASA Astrophysics Data System (ADS)

    Helmi, Amina

    2016-08-01

    I present a brief overview of how stellar halos may be used to constrain the process of galaxy formation. In particular, streams and substructure in stellar halos trace merger events but can also be used to determine the mass distribution of the host galaxy and hence put constraints on the nature of dark matter. Much of the focus of this contribution is on the Milky Way, but I also present an attempt to understand the kinematics of the globular cluster system of M31.

  17. HaloSat - A CubeSat to Study the Hot Galactic Halo

    NASA Astrophysics Data System (ADS)

    Kaaret, Philip

    2016-04-01

    Observations of the nearby universe fail to locate about half of the normal matter (baryons) observed in the early universe. The missing baryons may be in hot galactic halos. HaloSat is a CubeSat designed to map oxygen line emission (O VII and O VIII) around the Milky Way in order to constrain the mass and spatial distribution of hot gas in the halo. HaloSat has a grasp competitive with current X-ray observatories. Its observing program will be optimized to minimize contributions from solar wind charge exchange (SWCX) emission that limit the accuracy of current measurements. We will describe the HaloSat mission concept, progress towards its implementation, and plans for archiving and distribution of the data.

  18. Massive stellar content of some Galactic supershells

    NASA Astrophysics Data System (ADS)

    Kaltcheva, Nadejda; Golev, Valeri

    2015-08-01

    The giant Galactic H II regions provide a unique opportunity to study the OB-star influence on the surrounding interstellar matter. In this contribution, several multi-wavelength surveys (Wisconsin H-α Mapper Northern Sky Survey, Southern H-α Sky Survey Atlas, MSX Mid-IR Galactic Plane Survey, WISE All-Sky Data Release, CO survey of the Milky Way, and the Southern Galactic Plane HI Survey) are combined with available intermediate-band uvbyβ photometry to attempt a precise spatial correlation between the OB-stars and the neutral and ionized material. Our study is focused on the H I supershell GSH 305+01-24 in Centaurus, the Car OB2 supershell, the Cygnus star-forming complex and the GSH 224-01+24 shell toward the GMN 39/Seagull nebula region. We refine the massive stellar content of these star-forming fields and study the energetics of its interaction with the shells’ material.

  19. Resolving the stellar halos of six massive disk galaxies beyond the Local Group

    NASA Astrophysics Data System (ADS)

    Monachesi, Antonela; Bell, Eric F.; Radburn-Smith, David J.; de Jong, Roelof S.; Bailin, Jeremy; Holwerda, Benne; Streich, David

    2016-08-01

    Models of galaxy formation in a hierarchical universe predict substantial scatter in the halo-to-halo stellar properties, owing to stochasticity in galaxies' merger histories. Currently, only few detailed observations of stellar halos are available, mainly for the Milky Way and M31. We present the stellar halo color/metallicity and density profiles of red giant branch stars out to ~60 kpc along the minor axis of six massive nearby Milky Way-like galaxies beyond the Local Group from the Galaxy Halos, Outer disks, Substructure, Thick disks and Star clusters (GHOSTS) HST survey. This enlargement of the sample of galaxies with observations of stellar halo properties is needed to understand the range of possible halo properties, i.e. not only the mean properties but also the halo-to-halo scatter, what a `typical' halo looks like, and how similar the Milky Way halo is to other halos beyond the Local Group.

  20. Building the Galactic halo from globular clusters: evidence from chemically unusual red giants

    NASA Astrophysics Data System (ADS)

    Martell, S. L.; Smolinski, J. P.; Beers, T. C.; Grebel, E. K.

    2011-10-01

    We present a spectroscopic search for halo field stars that originally formed in globular clusters. Using moderate-resolution SDSS-III/SEGUE-2 spectra of 561 red giants with typical halo metallicities (-1.8 ≤ [Fe/H] ≤ -1.0), we identify 16 stars, 3% of the sample, with CN and CH bandstrength behavior indicating depleted carbon and enhanced nitrogen abundances relative to the rest of the data set. Since globular clusters are the only environment known in which stars form with this pattern of atypical light-element abundances, we claim that these stars are second-generation globular cluster stars that have been lost to the halo field via normal cluster mass-loss processes. Extrapolating from theoretical models of two-generation globular cluster formation, this result suggests that globular clusters contributed significant numbers of stars to the construction of the Galactic halo: we calculate that a minimum of 17% of the present-day mass of the stellar halo was originally formed in globular clusters. The ratio of CN-strong to CN-normal stars drops with Galactocentric distance, suggesting that the inner-halo population may be the primary repository of these stars. Full Tables 1 and 3 are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/534/A136

  1. Does SEGUE/SDSS indicate a dual galactic halo?

    SciTech Connect

    Schönrich, Ralph; Asplund, Martin; Casagrande, Luca

    2014-05-01

    We re-examine recent claims of observational evidence for a dual Galactic halo in SEGUE/SDSS data, and trace them back to improper error treatment and neglect of selection effects. In particular, the detection of a vertical abundance gradient in the halo can be explained as a metallicity bias in distance. A similar bias and the impact of disk contamination affect the sample of blue horizontal branch stars. These examples highlight why non-volume complete samples require forward modeling from theoretical models or extensive bias-corrections. We also show how observational uncertainties produce the specific non-Gaussianity in the observed azimuthal velocity distribution of halo stars, which can be erroneously identified as two Gaussian components. A single kinematic component yields an excellent fit to the observed data, when we model the measurement process including distance uncertainties. Furthermore, we show that sample differences in proper motion space are the direct consequence of kinematic cuts and are enhanced when distance estimates are less accurate. Thus, their presence is neither proof of a separate population nor a measure of reliability for the applied distances. We conclude that currently there is no evidence from SEGUE/SDSS that would favor a dual Galactic halo over a single halo that is full of substructure.

  2. A general relativistic approach to the Navarro Frenk White galactic halos

    NASA Astrophysics Data System (ADS)

    Matos, Tonatiuh; Núñez, Darío; Sussman, Roberto A.

    2004-11-01

    Although galactic dark matter halos are basically Newtonian structures, the study of their interplay with large-scale cosmic evolution and with relativistic effects, such as gravitational lenses, quintessence sources or gravitational waves, makes it necessary to obtain adequate relativistic descriptions for these self-gravitating systems. With this purpose in mind, we construct a post-Newtonian fluid framework for the 'Navarro Frenk White' (NFW) models of galactic halos that follow from N-body numerical simulations. Since these simulations are unable to resolve regions very near the halo centre, the extrapolation of the fitting formula leads to a spherically averaged 'universal' density profile that diverges at the origin. We remove this inconvenient feature by replacing a small central region of the NFW halo with an interior Schwarzschild solution with constant density, continuously matched to the remaining NFW spacetime. A model of a single halo, as an isolated object with finite mass, follows by smoothly matching the NFW spacetime to a Schwarzschild vacuum exterior along the virial radius, the physical 'cut-off' customarily imposed, as the mass associated with NFW profiles diverges asymptotically. Numerical simulations assume weakly interacting collisionless particles, hence we suggest that NFW halos approximately satisfy an 'ideal gas' type of equation of state, where mass-density is the dominant rest-mass contribution to matter-energy, with the internal energy contribution associated with an anisotropic kinetic pressure. We show that, outside the central core, this pressure and the mass density roughly satisfy a polytropic relation. Since stellar polytropes are the equilibrium configurations in Tsallis' non-extensive formalism of statistical mechanics, we argue that NFW halos might provide a rough empirical estimate of the free parameter q of Tsallis' formalism.

  3. Stellar Abundances for Galactic Archaeology database for stars in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Suda, T.; Hidaka, J.; Ishigaki, M.; Katsuta, Y.; Yamada, S.; Komiya, Y.; Fujimoto, M. Y.; Aoki, W.

    We present a new database for observed stars in dwarf galaxies in the local group. This is an extension of the Stellar Abundances for Galactic Archaeology (SAGA) database (Suda et al. 2008, PASJ, 60, 1159) that deals with metal-poor Galactic halo stars. The main features of the new database are the same as the database for Galactic halo stars. Users can access and select data based on various criteria, and then inspect the selected data on a diagram with user-specified axes. The database includes more than two hundred stars based on high-resolution spectra for 20 galaxies, while the number of data is more than five thousand by including the data with medium-resolution spectra. We briefly discuss the characteristics of stars in dwarf galaxies using the database.

  4. A skewer survey of the Galactic halo from deep CFHT and INT images

    NASA Astrophysics Data System (ADS)

    Pila-Díez, B.; de Jong, J. T. A.; Kuijken, K.; van der Burg, R. F. J.; Hoekstra, H.

    2015-07-01

    We study the density profile and shape of the Galactic halo using deep multicolour images from the MENeaCS and CCCP projects, over 33 fields selected to avoid overlap with the Galactic plane. Using multicolour selection and point spread function homogenization techniques we obtain catalogues of F stars (near-main sequence turnoff stars) out to Galactocentric distances up to 60 kpc. Grouping nearby lines of sight, we construct the stellar density profiles through the halo in eight different directions by means of photometric parallaxes. Smooth halo models are then fitted to these profiles. We find clear evidence for a steepening of the density profile power law index around R = 20 kpc, from -2.50 ± 0.04 to -4.85 ± 0.04, and for a flattening of the halo towards the poles with best-fit axis ratio 0.79 ± 0.02. Furthermore, we cannot rule out a mild triaxiality (w ≥ 0.88 ± 0.07). We recover the signatures of well-known substructure and streams that intersect our lines of sight. These results are consistent with those derived from wider but shallower surveys, and augur well for upcoming, wide-field surveys of comparable depth to our pencil beam surveys.

  5. Characterizing the X-Ray Spectrum of the Galactic Halo

    NASA Technical Reports Server (NTRS)

    2003-01-01

    The goal of this project is to determine the spectrum of the Galactic halo's soft X-ray emission. These photons are emitted by hot, diffuse gas hundreds to thousands of parsecs from the Galactic plane. Thus, the emission is weak, can be confused with locally produced photons, and must be distinguished from noise. My co-I has made significant progress on determining the background. I have been working on a complementary aspect of the project: computer simulations of the hot gas in the local and distant regions.

  6. A panoramic VISTA of the stellar halo of NGC 253

    NASA Astrophysics Data System (ADS)

    Greggio, L.; Rejkuba, M.; Gonzalez, O. A.; Arnaboldi, M.; Iodice, E.; Irwin, M.; Neeser, M. J.; Emerson, J.

    2014-02-01

    Context. Outskirts of large galaxies contain important information about galaxy formation and assembly. Resolved star count studies can probe the extremely low surface brightness of the outer halos. Aims: NGC 253 is a nearly edge-on disk galaxy in the Sculptor group, of which we resolved the halo stars from ground-based images, with the aim of studying its stellar population content, the structure and the overall extent of the halo. Methods: We use Z and J-band images from the VIRCAM camera mounted on the VISTA telescope to construct the spatially resolved J vs. Z-J color-magnitude diagrams (CMDs). The very deep photometry and the wide area covered allow us to trace the red giant branch (RGB) and asymptotic giant branch (AGB) stars that belong to the halo of NGC 253 out to 50 kpc along the galaxy's minor axis. Results: We confirm the existence of an extra-planar stellar component of the disk, with a very prominent southern shelf and a symmetrical feature on the north side. The only additional visible substructure is an overdensity in the north-west part of the halo ~28 kpc distant from the plane and extending over 20 kpc parallel with the disk of the galaxy. Our data are not deep enough to distinguish its stellar population from that of the surrounding halo, but the excess of stars above the smooth halo traces the mass of the parent population of ~7.5 × 106M⊙. From stellar counts, we measure the transition from the disk to the halo at a radial distance of about 25 kpc with a clear break in the number density profile. The isodensity contours show that the inner halo is a flattened structure that blends with a more extended, diffuse, rounder outer halo. Such external structure can be traced to the very edge of our image out to 50 kpc from the disk plane. The number density profile of the stars in the stellar halo follows a power law with index -1.6, as a function of radius. The CMD shows a very homogeneous stellar population across the field. By comparing

  7. On the alleged duality of the Galactic halo

    NASA Astrophysics Data System (ADS)

    Schönrich, Ralph; Asplund, Martin; Casagrande, Luca

    2011-08-01

    We examine the kinematics of the Galactic halo based on SDSS/SEGUE data by Carollo et al. We find that their claims of a counter-rotating halo are the result of substantial biases in distance estimates (of the order of 50 per cent): the claimed retrograde component, which makes up only a tiny fraction of the entire sample, prone to contaminations, is identified as the tail of distance overestimates. The strong overestimates also result in a lift in the vertical velocity component, which explains the large altitudes those objects were claimed to reach. Errors are worst for the lowest metallicity stars, which explains the metal-poor nature of the artificial component. We also argue that measurement errors were not properly accounted for and that the use of Gaussian fitting on intrinsically non-Gaussian Galactic components invokes the identification of components that are distorted or even artificial. Our evaluation of the data leads to a revision of the estimated velocity ellipsoids and does not yield any reliable evidence for a counter-rotating halo component. If a distinct counter-rotating halo component exists, then it must be far weaker than claimed by Carollo et al. Finally, we note that their revised analysis presented in Beers et al. does not alleviate our main concerns.

  8. Structure Formation inside Triaxial Dark Matter Halos: Galactic Disks, Bulges, and Bars

    NASA Astrophysics Data System (ADS)

    Heller, Clayton H.; Shlosman, Isaac; Athanassoula, E.

    2007-12-01

    We investigate formation and evolution of galactic disks immersed in assembling live DM halos. Models have been evolved from cosmological initial conditions and represent the collapse of an isolated density perturbation. The baryons include gas participating in star formation (SF) and stars with the energy feedback onto the ISM. We find that (1) the triaxial halo figure tumbling is insignificant and the angular momentum (J) is channeled into the internal circulation, while the baryonic collapse is stopped by the centrifugal barrier; (2) density response of the (disk) baryons is out of phase with DM, thus washing out the inner halo ellipticity; (3) the total J is neatly conserved, even in models accounting for stellar feedback; (4) the specific J for DM is nearly constant, while that for baryons is decreasing; (5) early stage of disk formation resembles the cat's cradle-a small amorphous disk fueled via radial string patterns-followed by growing oval disk whose shape varies with its orientation to the halo major axis; (6) the disk gas layer thins when the SF rate drops below ~5 Msolar yr-1 (7) about half of the baryons remain outside the disk SF region or in the halo as a hot gas; (8) rotation curves appear to be flat and account for the observed disk/halo contributions; (9) a range of bulge-dominated to bulgeless disks was obtained, depending on the stellar feedback parameter, ɛSF: smaller ɛSF leads to a larger and earlier bulge; lower density threshold for SF leads to a smaller, thicker disk; gas gravitational softening mimics a number of intrinsic processes within the ISM; (10) models are characterized by an extensive bar-forming activity; (11) nested bars form in response to the gas inflow along the primary bars, as shown by Heller, Shlosman, and Athanassoula.

  9. Post Asymptotic Giant Branch and Central Stars of Planetary Nebulae in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Weston, Simon

    2012-01-01

    Post asymptotic giant branch (post-AGB) stars, central stars of planetary nebulae (CSPNe) and planetary nebulae (PNe) are important phases of stellar evolution as the material they feedback is the seed of subsequent star formation in a galaxy. The majority of low and intermediate mass stars are expected to evolve through these channels, however, it is uncertain how many actually do, and at what rate. The Galactic halo, with its older population, provides a direct test of evolutionary models for low mass stars. Birthrate estimates of PNe are uncertain and worse still, are in contradiction with accepted white dwarf (WD) birthrate estimates. Much of the uncertainty stems from the lack of complete samples and poorly determined distance estimates. New surveys such as the Sloan Digital Sky Survey (SDSS), Galaxy Evolutionary Explorer (GALEX) and the INT Photometric Ha Survey (IPHAS) have discovered many new PNe and have observed the far edges of the Galaxy. Improved methods of determining distances to CSPNe are presented here, using model atmospheres, evolutionary tracks and high resolution reddening maps utilising these revolutionary surveys. Locating the CSPN is non-trivial particularly for evolved PNe, as they are extended with their central star often displaced from the centre of the nebula. Therefore, photometric criteria are required to locate the CSPN in the nebula's field. Synthetic photometry of the CSPNe is derived from spectral energy distributions (SEDs) computed from a grid of model atmospheres covering the parameter range of CSPNe. The SEDs are convolved with filter transmission curves to compute synthetic magnitudes for a given photometric system which are then calibrated with standard stars and WDs. A further project borne out of a search for luminous central stars of faint PNe, resulted in a systematic search for post-AGB stars in the Galactic halo. In this work, new candidate halo post-AGB stars are discovered from a search through the SDSS spectroscopic

  10. The PAndAS field of streams: Stellar structures in the milky way halo toward Andromeda and Triangulum

    SciTech Connect

    Martin, Nicolas F.; Ibata, Rodrigo A.; Rich, R. Michael; Collins, Michelle L. M.; Fardal, Mark A.; Irwin, Michael J.; Lewis, Geraint F.; Bate, Nicholas F.; Conn, Anthony R.; McConnachie, Alan W.; Babul, Arif; Navarro, Julio F.; Chapman, Scott C.; Crnojević, Denija; Ferguson, Annette M. N.; Peñarrubia, Jorge; Mackey, A. Dougal; Tanvir, Nial T.; Valls-Gabaud, David

    2014-05-20

    We reveal the highly structured nature of the Milky Way (MW) stellar halo within the footprint of the Pan-Andromeda Archaeological Survey (PAndAS) photometric survey from blue main sequence (MS) and MS turn-off stars. We map no fewer than five stellar structures within a heliocentric range of ∼5-30 kpc. Some of these are known (the Monoceros Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three well-defined stellar structures that could be, at least partly, responsible for the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In particular, we trace a new faint stellar stream located at a heliocentric distance of ∼17 kpc. With a surface brightness of Σ {sub V} ∼ 32-32.5 mag arcsec{sup –2}, it follows an orbit that is almost parallel to the Galactic plane north of M31 and has so far eluded surveys of the MW halo as these tend to steer away from regions dominated by the Galactic disk. Investigating our follow-up spectroscopic observations of PAndAS, we serendipitously uncover a radial velocity signature from stars that have colors and magnitudes compatible with the stream. From the velocity of eight likely member stars, we show that this stellar structure is dynamically cold, with an unresolved velocity dispersion that is lower than 7.1 km s{sup –1} at the 90% confidence level. Along with the width of the stream (300-650 pc), its dynamics point to a dwarf-galaxy-accretion origin. The numerous stellar structures we can map in the MW stellar halo between 5 and 30 kpc and their varying morphology is a testament to the complex nature of the stellar halo at these intermediate distances.

  11. The FUSE Survey of 0 VI in the Galactic Halo

    NASA Technical Reports Server (NTRS)

    Sonneborn, George; Savage, B. D.; Wakker, B. P.; Sembach, K. R.; Jenkins, E. B.; Moos, H. W.; Shull, J. M.

    2003-01-01

    This paper summarizes the results of the Far-Ultraviolet Spectroscopic Explorer (FUSE) program to study 0 VI in the Milky Way halo. Spectra of 100 extragalactic objects and two distant halo stars are analyzed to obtain measures of O VI absorption along paths through the Milky Way thick disk/halo. Strong O VI absorption over the velocity range from -100 to 100 km/s reveals a widespread but highly irregular distribution of O VI, implying the existence of substantial amounts of hot gas with T approx. 3 x 10(exp 5) K in the Milky Way thick disk/halo. The overall distribution of O VI is not well described by a symmetrical plane-parallel layer of patchy O VI absorption. The simplest departure from such a model that provides a reasonable fit to the observations is a plane-parallel patchy absorbing layer with an average O VI mid-plane density of n(sub 0)(O VI) = 1.7 x 10(exp -2)/cu cm, a scale height of approx. 2.3 kpc, and a approx. 0.25 dex excess of O VI in the northern Galactic polar region. The distribution of O VI over the sky is poorly correlated with other tracers of gas in the halo, including low and intermediate velocity H I, Ha emission from the warm ionized gas at approx. l0(exp 4) K, and hot X-ray emitting gas at approx. l0(exp 6) K . The O VI has an average velocity dispersion, b approx. 60 km/s and standard deviation of 15 km/s. Thermal broadening alone cannot explain the large observed profile widths. A combination of models involving the radiative cooling of hot fountain gas, the cooling of supernova bubbles in the halo, and the turbulent mixing of warm and hot halo gases is required to explain the presence of O VI and other highly ionized atoms found in the halo. The preferential venting of hot gas from local bubbles and superbubbles into the northern Galactic polar region may explain the enhancement of O VI in the North.

  12. THE INTEGRATED STELLAR CONTENT OF DARK MATTER HALOS

    SciTech Connect

    Leauthaud, Alexie; Bundy, Kevin; Tanaka, Masayuki; George, Matthew R.; Behroozi, Peter S.; Wechsler, Risa H.; Tinker, Jeremy; Conroy, Charlie; Finoguenov, Alexis

    2012-02-10

    Measurements of the total amount of stars locked up in galaxies as a function of host halo mass contain key clues about the efficiency of processes that regulate star formation. We derive the total stellar mass fraction f{sub *} (excluding stars in the intracluster light) as a function of halo mass M{sub 500c} from z = 0.2 to z = 1 using two complementary methods. First, we derive f{sub *} using a statistical Halo Occupation Distribution model jointly constrained by data from lensing, clustering, and the stellar mass function. This method enables us to probe f{sub *} over a much wider halo mass range than with group or cluster catalogs. Second, we derive f{sub *} at group scales using a COSMOS X-ray group catalog and show that the two methods agree to within 30%. We quantify the systematic uncertainty on f{sub *} using abundance matching methods and show that the statistical uncertainty on f{sub *} ({approx}10%) is dwarfed by systematic uncertainties associated with stellar mass measurements ({approx}45% excluding initial mass function, IMF, uncertainties). Assuming a Chabrier IMF, we find 0.012 {<=} f{sub *} {<=} 0.025 at M{sub 500c} = 10{sup 13} M{sub Sun} and 0.0057 {<=} f{sub *} {<=} 0.015 at M{sub 500c} = 10{sup 14} M{sub Sun }. These values are significantly lower than previously published estimates. We investigate the cause of this difference and find that previous work has overestimated f{sub *} owing to a combination of inaccurate stellar mass estimators and/or because they have assumed that all galaxies in groups are early-type galaxies with a constant mass-to-light ratio. Contrary to previous claims, our results suggest that the mean value of f{sub *} is always significantly lower than f{sub gas} for halos above 10{sup 13} M{sub Sun }. Combining our results with recently published gas mass fractions, we find a shortfall in f{sub *} + f{sub gas} at R{sub 500c} compared to the cosmic mean. This shortfall varies with halo mass and becomes larger toward

  13. The ionization conditions in the Milky Way halo - Infalling gas toward the North Galactic Pole

    NASA Technical Reports Server (NTRS)

    Danly, Laura

    1992-01-01

    Observations of gas in the Milky Way halo are studied with an eye toward the theoretical predictions of the Galactic Fountain model for the production of halo gas. Data are shown that indicate significant variations in the ionization conditions in infalling halo gas in the northern galactic hemisphere. Understanding the nature of Milky Way halo gas plays a critical role in interpreting QSO absorption lines in the investigation of galaxies at high redshift.

  14. Stellar spiral structures in triaxial dark matter haloes

    NASA Astrophysics Data System (ADS)

    Hu, Shaoran; Sijacki, Debora

    2016-09-01

    We employ very high resolution simulations of isolated Milky Way-like galaxies to study the effect of triaxial dark matter haloes on exponential stellar discs. Non-adiabatic halo shape changes can trigger two-armed grand-design spiral structures which extend all the way to the edge of the disc. Their pattern speed coincides with the inner Lindblad resonance indicating that they are kinematic density waves which can persist up to several Gyr. In dynamically cold discs, grand-design spirals are swing amplified and after a few Gyr can lead to the formation of (multi-armed) transient recurrent spirals. Stellar discs misaligned to the principal planes of the host triaxial halo develop characteristic integral shaped warps, but otherwise exhibit very similar spiral structures as aligned discs. For the grand-design spirals in our simulations, their strength dependence with radius is determined by the torque on the disc, suggesting that by studying grand-design spirals without bars it may be possible to set constraints on the tidal field and host dark matter halo shape.

  15. Evidence for recent star formation in the galactic halo

    NASA Astrophysics Data System (ADS)

    Keenan, F. P.

    1986-09-01

    Observational data for PHL 346 obtained with the 2.5 m Issac Newton telescope on August 1985 are studied. Measured stellar Stromgren colors, hydrogen-line profiles, and helium and metal-line equivalent widths are compared with those predicted by local thermodynamic equilibrium model-atmosphere calculations. Effective temperature, surface gravity, microturbulent velocity, and helium and metal abundances for the star are derived. A mass of 13 + or - 2 solar masses, a lifetime of 11 x 10 to the 6th yr, a distance from the galactic plane of 8.7 + or - 1.5 kpc, and a velocity in the z direction of +56 + or - 10 km/s are calculated for the star. The data reveal that the star was not ejected from the galactic plane, but that it formed out of galactic fountain gas at about 6 kpc from the disc.

  16. TESTING GALAXY FORMATION MODELS WITH THE GHOSTS SURVEY: THE COLOR PROFILE OF M81's STELLAR HALO

    SciTech Connect

    Monachesi, Antonela; Bell, Eric F.; Bailin, Jeremy; Radburn-Smith, David J.; Dalcanton, Julianne J.; Vlajic, Marija; De Jong, Roelof S.; Streich, David; Holwerda, Benne W.

    2013-04-01

    We study the properties of the stellar populations in M81's outermost part, which hereafter we will call the stellar halo, using Hubble Space Telescope (HST) Advanced Camera for Surveys observations of 19 fields from the GHOSTS survey. The observed fields probe the stellar halo out to a projected distance of {approx}50 kpc from the galactic center. Each field was observed in both F606W and F814W filters. The 50% completeness levels of the color-magnitude diagrams (CMDs) are typically at 2 mag below the tip of the red giant branch (TRGB). Fields at distances closer than 15 kpc show evidence of disk-dominated populations whereas fields at larger distances are mostly populated by halo stars. The red giant branch (RGB) of the M81's halo CMDs is well matched with isochrones of {approx}10 Gyr and metallicities [Fe/H] {approx} - 1.2 dex, suggesting that the dominant stellar population of M81's halo has a similar age and metallicity. The halo of M81 is characterized by a color distribution of width {approx}0.4 mag and an approximately constant median value of (F606W - F814W) {approx}1 mag measured using stars within the magnitude range 23.7 {approx}< F814W {approx}< 25.5. When considering only fields located at galactocentric radius R > 15 kpc, we detect no color gradient in the stellar halo of M81. We place a limit of 0.03 {+-} 0.11 mag difference between the median color of RGB M81 halo stars at {approx}15 and at 50 kpc, corresponding to a metallicity difference of 0.08 {+-} 0.35 dex over that radial range for an assumed constant age of 10 Gyr. We compare these results with model predictions for the colors of stellar halos formed purely via accretion of satellite galaxies. When we analyze the cosmologically motivated models in the same way as the HST data, we find that they predict no color gradient for the stellar halos, in good agreement with the observations.

  17. WEAK GALACTIC HALO-DWARF SPHEROIDAL CONNECTION FROM RR LYRAE STARS

    SciTech Connect

    Fiorentino, Giuliana; Bono, Giuseppe; Monelli, Matteo; Gallart, Carme; Martínez-Vásquez, Clara E.; Tolstoy, Eline; Salaris, Maurizio; Bernard, Edouard J.

    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 Sagittarius 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.

  18. Galactic Stellar and Substellar Initial Mass Function

    NASA Astrophysics Data System (ADS)

    Chabrier, Gilles

    2003-07-01

    formation at large redshift remains undetermined, but different observational constraints suggest that it does not extend below ~1 Msolar. These results suggest a characteristic mass for star formation that decreases with time, from conditions prevailing at large redshift to conditions characteristic of the spheroid (or thick disk) to present-day conditions. These conclusions, however, remain speculative, given the large uncertainties in the spheroid and early star IMF determinations. These IMFs allow a reasonably robust determination of the Galactic present-day and initial stellar and brown dwarf contents. They also have important galactic implications beyond the Milky Way in yielding more accurate mass-to-light ratio determinations. The mass-to-light ratios obtained with the disk and the spheroid IMF yield values 1.8-1.4 times smaller than for a Salpeter IMF, respectively, in agreement with various recent dynamical determinations. This general IMF determination is examined in the context of star formation theory. None of the theories based on a Jeans-type mechanism, where fragmentation is due only to gravity, can fulfill all the observational constraints on star formation and predict a large number of substellar objects. On the other hand, recent numerical simulations of compressible turbulence, in particular in super-Alfvénic conditions, seem to reproduce both qualitatively and quantitatively the stellar and substellar IMF and thus provide an appealing theoretical foundation. In this picture, star formation is induced by the dissipation of large-scale turbulence to smaller scales through radiative MHD shocks, producing filamentary structures. These shocks produce local nonequilibrium structures with large density contrasts, which collapse eventually in gravitationally bound objects under the combined influence of turbulence and gravity. The concept of a single Jeans mass is replaced by a distribution of local Jeans masses, representative of the lognormal probability

  19. Self-Conistent Dynamical Modeling of the Milky Way Halo with Stellar Orbits

    NASA Astrophysics Data System (ADS)

    Valluri, Monica

    Despite significant advances in understanding our Galaxy in the context of LCDM, important questions remain unanswered. LCDM predicts that dark matter halos are triaxial overall, but oblate in regions where baryons dominate. However recent measurements of the shape of the Milky Way (MW) dark matter (DM) halo find it to be very triaxial with a shape and orientation that are significantly at odds with theoretical predictions. The European Space Agency s Gaia satellite will soon map the entire MW giving us six phase-space coordinates, ages and abundances for hundreds of thousands of stars. The PI and a postdoc will build a novel code (based on the Schwarzschild orbit superposition method and orbital frequency mapping), to determine the global shape of the Milky Way's dark matter halo using field stars from Gaia. Our technique will simultaneously yield the self-consistent phase-space distribution function of the stellar halo in the inner 20-30 kpc region. Detailed analysis of correlations between the chemical abundances, ages and orbits of halo stars in this distribution function will enable us to extract clues to the formation history of the MW that are encoded in orbital properties of halo stars. Our technique is unique since we will use independent sets of observational constraints, at small and large radii, to measure the radial variation in halo shape, in conjunction with a strategy tailored to modeling discrete stellar datasets. We will perform end-to-end tests of our new code with mock catalogs generated from state-of-the art cosmological simulations of MW like disk galaxies. Our team has the ideal combination of expertise in dynamical modeling, observations, simulations, and Gaia data analysis and processing necessary to achieve our goals. PI Valluri has significant prior experience with Schwarzschild modeling and the analysis of phase-space distribution functions of simulated halos, Co-I Loebman recently published the first measurement of the shape of the DM

  20. The Abundance Pattern of Two Barium Stars in the Galactic Halo: HD 104340 and HD 206983

    NASA Astrophysics Data System (ADS)

    Junqueira, S.; Pereira, C. B.

    2001-07-01

    We present the abundance pattern of two barium stars in the Galactic halo, HD 104340 and HD 206983, based on high-resolution optical spectra. We also determined the spectroscopic stellar atmospheric parameters, temperature, and microturbulent velocity, as well as stellar surface gravity from a solution of excitation and ionization equilibria of Fe I and Fe II lines under the assumption of local thermodynamic equilibrium. The abundance analysis reveals HD 104340 to be a metal-poor K giant with [Fe/H]=-1.72 and HD 206983 also a metal-poor K giant with [Fe/H]=-1.43. From a set of Fe I lines, the radial velocity is found to be 263.3+/-0.6 km s-1 and -319.2+/-4.4 km s-1 for HD 104340 and HD 206983, respectively. Their high velocity, low metallicity, and high galactic latitude imply that both stars are members of a Galactic halo population. From our study and by using information from the literature we believe that HD 206983 is another member of a group known as metal-deficient barium stars. We compare the abundance pattern with the abundances of a halo population. We found that the abundances of the iron group, α-elements, manganese, copper, and zinc, as well as sodium and magnesium, of HD 104340 and HD 206983 follow the abundance pattern of a halo population. The heavy element abundance pattern of both stars shows enhancement by a factor of 4-8 with respect to the metal-poor stars with the same metallicity as that analyzed by us. We also discuss the abundances of the s-process elements and compare our results with other objects that display the same degree of enrichment due to neutron capture reactions, binary systems, and AGB stars, through a diagram of metallicity versus neutron exposure given by the [hs/ls] index. Based on the observations made with the 1.52 m telescope at the European Southern Observatory (La Silla, Chile) under agreement with Observatório Nacional (Brazil).

  1. The Milky Way Halo and the First Stars: New Frontiers in Galactic Archaeology

    NASA Astrophysics Data System (ADS)

    Beers, Timothy C.; Tumlinson, Jason; O'Shea, Brian; Peruta, Carolyn; Carollo, Daniela

    2010-11-01

    We discuss plans for a new joint effort between observers and theorists to understand the formation of the Milky Way halo back to the first epochs of chemical evolution. New models based on high-resolution N-body simulations coupled to simple models of Galactic chemical evolution show that surviving stars from the epoch of the first galaxies remain in the Milky Way today and should bear the nucleosynthetic imprint of the first stars. We investigate the key physical influences on the formation of stars in the first galaxies and how they appear today, including the relationship between cosmic reionization and surviving Milky Way stars. These models also provide a physically motivated picture of the formation of the Milky Ways “outer halo,” which has been identified from recent large samples of stars from SDSS. The next steps are to use these models to guide rigorous gas simulations of Milky Way formation, including its disk, and to gradually build up the fully detailed theoretical “Virtual Galaxy” that is demanded by the coming generation of massive Galactic stellar surveys.

  2. V474 Car: A RARE HALO RS CVn BINARY IN RETROGRADE GALACTIC ORBIT

    SciTech Connect

    Bubar, Eric J.; Mamajek, Eric E.; Jensen, Eric L. N.; Walter, Frederick M.

    2011-04-15

    We report the discovery that the star V474 Car is an extremely active, high velocity halo RS CVn system. The star was originally identified as a possible pre-main-sequence star in Carina, given its enhanced stellar activity, rapid rotation (10.3 days), enhanced Li, and absolute magnitude which places it above the main sequence (MS). However, its extreme radial velocity (264 km s{sup -1}) suggested that this system was unlike any previously known pre-MS system. Our detailed spectroscopic analysis of echelle spectra taken with the CTIO 4 m finds that V474 Car is both a spectroscopic binary with an orbital period similar to the photometric rotation period and metal-poor ([Fe/H] {approx_equal}-0.99). The star's Galactic orbit is extremely eccentric (e {approx_equal} 0.93) with a perigalacticon of only {approx}0.3 kpc of the Galactic center-and the eccentricity and smallness of its perigalacticon are surpassed by only {approx}0.05% of local F/G-type field stars. The observed characteristics are consistent with V474 Car being a high-velocity, metal-poor, tidally locked, chromospherically active binary, i.e., a halo RS CVn binary, and one of only a few such specimens known.

  3. Evidence for a Triaxial Milky Way Dark Matter Halo from the Sagittarius Stellar Tidal Stream

    NASA Astrophysics Data System (ADS)

    Law, David R.; Majewski, S. R.; Johnston, K. V.

    2010-01-01

    Observations of the lengthy tidal streams produced by the destruction of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) are capable of providing strong constraints on the shape of the Galactic gravitational potential. However, previous work, based on modeling different stream properties in axisymmetric Galactic models has yielded conflicting results: while the angular precession of the Sgr leading arm is most consistent with a spherical or slightly oblate halo, the radial velocities of stars in this arm are only reproduced by prolate halo models. We demonstrate that this apparent paradox can be resolved by instead adopting a triaxial potential. Our new Galactic halo model, which simultaneously fits all well-established phase space constraints from the Sgr stream, provides the first conclusive evidence for, and tentative measurement of, triaxiality in an individual dark matter halo. In this model, the minor axis of the dark halo is approximately coincident with the Galactic X axis connecting the Sun and the Galactic Center.

  4. Large-scale gas dynamical processes affecting the origin and evolution of gaseous galactic halos

    NASA Technical Reports Server (NTRS)

    Shapiro, Paul R.

    1991-01-01

    Observations of galactic halo gas are consistent with an interpretation in terms of the galactic fountain model in which supernova heated gas in the galactic disk escapes into the halo, radiatively cools and forms clouds which fall back to the disk. The results of a new study of several large-scale gas dynamical effects which are expected to occur in such a model for the origin and evolution of galactic halo gas will be summarized, including the following: (1) nonequilibrium absorption line and emission spectrum diagnostics for radiatively cooling halo gas in our own galaxy, as well the implications of such absorption line diagnostics for the origin of quasar absorption lines in galactic halo clouds of high redshift galaxies; (2) numerical MHD simulations and analytical analysis of large-scale explosions ad superbubbles in the galactic disk and halo; (3) numerical MHD simulations of halo cloud formation by thermal instability, with and without magnetic field; and (4) the effect of the galactic fountain on the galactic dynamo.

  5. The early gaseous and stellar mass assembly of Milky Way-type galaxy haloes

    NASA Astrophysics Data System (ADS)

    Hensler, Gerhard

    2015-08-01

    In cosmological simulations of Cold Dark Matter (CDM) structure formation a vast number of subhalos is expected around massive galaxies like the Milky Way (MW). These DM subhalos are filled with baryons, gas that forms stars very early as observed from the stellar populations in the MW satellite galaxies. Satellite galaxies evolve in the tidal field of their mature galaxy and suffer accretion to the major galaxy and their partly disruption. By this, their mass loss is expected to feed the galaxy halo with stars and gas.From the Via Lactea II simulations we select a massive DM halo with its satellite system which evolves in the simulations to a present-day MW-type galaxy. We follow its evolution from redshift 4.5 to 2.5, i.e. over almost 2 billion years of the most interesting epoch of mass assembly. A high mass resolution allows for even low-mass satellites down to 10^5 Msun, but limits their distance range to the innermost 240 satellites of the system only. The applied chemo-dynamical method includes star formation, stellar energetic and chemical feedback, and gas physical processes.After the onset of the simulation our models demonstrate the action of tidal effects and satellite merging on the star-formation rate of the satellites, their gas loss by means of hot-gas expansion, of ram-pressure and tidal stripping, and the tidal extraction of stars, leading to the formation of the stellar and gaseous galactic halo. We also analyze the evolution of the satellites’ mass function, their baryonic and DM mass distributions, chemical abundances, their compactness, their present-day appearance, etc. with respect to observations and present-day correlations.

  6. Photoionization of the diffuse interstellar medium and galactic halo by OB associtations

    NASA Technical Reports Server (NTRS)

    Dove, James B.; Shull, J. Michael

    1994-01-01

    Assuming smoothly varying H I distributions in te Galactic disk, we have calculated the geometry of diffuse II regions due to OB associations in the Galactic plane. Near the solar circle, OB associations with a Lyman continuum (Lyc) photon luminosity Psi(sub Lyc) = 3.3 x 10(exp 7) cm(exp -2) s(exp -1), produce H II regions that are density bounded in the vertical direction (H II chimneys) allowing Lyc to escape the gaseous disk and penetrate into the Galactic halo. We provide analytic formulae for the Lyc escape fraction as functions of S(sub 0) O-star catalog of Garmany and a new Lyc stellar stellar Lyc stellar flux calibration, we find a production rate of Lyc photons by OB associations within 2.5 kpc of Psi(sub Lyc) = 3.3 x 10(exp 7) cm(exp -2) s(exp -1). Integrating the fraction of Lyc photons that escape the disk over our adopted luminosity function of OB associations, we estimate that approximately 7% of the ionizing photons, or Phi(sub Lyc) = 2.3 x 10(exp 6) cm(exp -2) s(exp -1), escape each side of the H I disk layer and penetrate the diffuse ionized medium ('Reynolds layer'). This flux is sufficient to explain the potoionization of this, although we have not constructed a model for the observed H-alpha emission and pulsar dispersion measures that is fully consistent with the absorption rate of Lyc in the H II layer. Since our quiescent model does not account for the effects of dynamic chimneys and superbubbles, which should enhance Lyc escape, we conclude the O stars are the probable source of ionizing radiation for the Reynolds layer. For a random distribution of OB associations throughout the disk, the Lyc flux is nearly uniform for heights Z is greater than approximately 0.8 kpc above the midplane.

  7. Baryonic dark clusters in galactic halos and their observable consequences

    NASA Technical Reports Server (NTRS)

    Wasserman, Ira; Salpeter, Edwin E.

    1994-01-01

    We consider the possibility that approximately 10% of the mass of a typical galaxy halo is in the form of massive (approximately 10(exp 7) solar masses), compact (escape speeds approximately 100 km/s) baryonic clusters made of neutron stars (approximately 10% by mass), black holes (less than or approximately equal to 1%) and brown dwarfs, asteroids, and other low-mass debris (approximately 90%). These general properties are consistent with several different observational and phenomenological constraints on cluster properties subject to the condition that neutron stars comprise approximately 1% of the total halo mass. Such compact, dark clusters could be the sites of a variety of collisional phenomena involving neutron stars. We find that integrated out to the Hubble distance approximately one neutron star-neutron star or neutron star-black hole collision occurs daily. Of order 0.1-1 asteroid-neutron star collisions may also happen daily in the halo of the Milky Way if there is roughly equal cluster mass per logarithmic particle mass interval between asteroids and brown dwarfs. These event rates are comparable to the frequency of gamma-ray burst detections by the Burst and Transient Source Experiment (BATSE) on the Compton Observatory, implying that if dark halo clusters are the sites of most gamma-ray bursts, perhaps approximately 90% of all bursts are extragalactic, but approximately 10% are galactic. It is possible that dark clusters of the kind discussed here could be detected directly by the Infrared Space Observatory (ISO) or Space Infrared Telescope Facility (SIRTF). If the clusters considered in this paper exist, they should produce spatially correlated gravitational microlensing of stars in the Large Magellanic Cloud (LMC). If 10% of the halo is in the form of dark baryonic clusters, and the remaining 90% is in brown dwarfs and other dark objects which are either unclustered or collected into low-mass clusters, then we expect that two events within

  8. An origin for multiphase gas in galactic winds and haloes

    NASA Astrophysics Data System (ADS)

    Thompson, Todd A.; Quataert, Eliot; Zhang, Dong; Weinberg, David H.

    2016-01-01

    The physical origin of high-velocity cool gas seen in galactic winds remains unknown. Following work by B. Wang, we argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 107-108 K temperature and the shape of the cooling function for T ≲ 107 K imply that outflows with hot gas mass-loss rate relative to star formation rate of β =dot{M}_hot/dot{M}_star ≳ 0.5 cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the β and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At rcool, the gas produces X-ray and then UV/optical line emission with a total power bounded by ˜10-2 L⋆ if the flow is powered by steady-state star formation with luminosity L⋆. The wind is thermally unstable at rcool, potentially leading to a multiphase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic haloes. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing β, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-β hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.

  9. SMASH: Spitzer Merger History and Shape of the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Johnston, Kathryn; Scowcroft, Vicky; Madore, Barry; Freedman, Wendy; Scowcroft, Victoria; Clementini, Gisella; Cioni, Maria-Rosa; van der Marel, Roeland; Udalski, Andrzej; Pietrzynski, Grzegorz; Soszynski, Igor; Nidever, David; Kallivayalil, Nitya; Besla, Gurtina; Majewski, Steve; Monson, Andy; Seibert, Mark; Smith, Horace; Preston, George; Kollmeier, Juna; Bono, Giuseppe; Marengo, Massimo; Persson, Eric; Law, David; Grillmair, Carl; Cohen, Judy; Sesar, Branimir; Price-Whelan, Adrian; Fabrizio, Michele

    2013-10-01

    The existence of a period-luminosity relation for RR Lyrae variables as measured at IRAC mid-infrared wavelengths allows Spitzer to estimate distances to individual stars with 2% errors. The SMASH program will exploit this unprecedented opportunity to precisely map structures throughout the halo of our Galaxy. SMASH will construct the first 3-D map of one of the larger satellites of the Milky Way (Sagittarius), it will determine precise distances to four more satellites (Ursa Minor, Carina, Sculptor & Bootes) and make the only measurements of stars in tidal streams accurate enough to determine their individual positions within the debris. This proposal describes some of the ground-breaking science enabled by this program, from increased accuracy in determining the orbits of satellite galaxies, to revolutionary constraints on the mass, shape and orientation of our Milky Way's dark matter halo. The foundational importance of these data sets cannot be overstated. These Milky Way structures lie far beyond the reach of any current or proposed future direct parallax measurements. Moreover, the combination of the SMASH results with proper motions from ESA's upcoming astrometric mission, Gaia, can effectively stretch Gaia's horizon for full 6D phase-space maps of our Galaxy by nearly four orders of magnitude in volume! These data and the resulting distance measurements will become Spitzer's legacy to the Galactic Astronomy community for years to come.

  10. Identifying Contributions to the Stellar Halo from Accreted, Kicked-out, and In Situ Populations

    NASA Astrophysics Data System (ADS)

    Sheffield, Allyson A.; Majewski, Steven R.; Johnston, Kathryn V.; Cunha, Katia; Smith, Verne V.; Cheung, Andrew M.; Hampton, Christina M.; David, Trevor J.; Wagner-Kaiser, Rachel; Johnson, Marshall C.; Kaplan, Evan; Miller, Jacob; Patterson, Richard J.

    2012-12-01

    We present a medium-resolution spectroscopic survey of late-type giant stars at mid-Galactic latitudes of (30° < |b| < 60°), designed to probe the properties of this population to distances of ~9 kpc. Because M giants are generally metal-rich and we have limited contamination from thin disk stars by the latitude selection, most of the stars in the survey are expected to be members of the thick disk (lang[Fe/H]rang ~ -0.6) with some contribution from the metal-rich component of the nearby halo. Here we report first results for 1799 stars. The distribution of radial velocity (RV) as a function of l for these stars shows (1) the expected thick disk population and (2) local metal-rich halo stars moving at high speeds relative to the disk, which in some cases form distinct sequences in RV-l space. High-resolution echelle spectra taken for 34 of these "RV outliers" reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: 17 of the stars have abundances reminiscent of the populations present in dwarf satellites of the Milky Way, 8 have abundances coincident with those of the Galactic disk and a more metal-rich halo, and 9 of the stars fall on the locus defined by the majority of stars in the halo. The chemical abundance trends of the RV outliers suggest that this sample consists predominantly of stars accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV outlier sample may have been formed in the inner Galaxy and subsequently kicked to higher eccentricity orbits, but the sample is not large enough to distinguish conclusively between this interpretation and the alternative that these stars represent the tail of the velocity distribution of the thick disk. Our data do not rule out the possibility that a minority of the sample could have formed from gas in situ on their current orbits. These results are consistent with scenarios where the stellar halo, at least as probed by M giants, arises from multiple formation mechanisms; however, when

  11. On the shoulders of giants: properties of the stellar halo and the Milky Way mass distribution

    SciTech Connect

    Kafle, Prajwal Raj; Sharma, Sanjib; Lewis, Geraint F.; Bland-Hawthorn, Joss

    2014-10-10

    Halo stars orbit within the potential of the Milky Way, and hence their kinematics can be used to understand the underlying mass distribution. However, the inferred mass distribution depends sensitively on assumptions made on the density and the velocity anisotropy profiles of the tracer population. Also, there is a degeneracy between the parameters of the halo and those of the disk or bulge. Most previous attempts that use halo stars have made arbitrary assumptions about these. In this paper, we decompose the Galaxy into three major components—a bulge, a Miyamoto-Nagai disk, and a Navarro-Frenk-White dark matter halo - and then model the kinematic data of the halo blue horizontal branch and K-giant stars from the Sloan Extension for Galactic Understanding and Exploration. Additionally, we use the gas terminal velocity curve and the Sgr A* proper motion. With the distance of the Sun from the center of the Galaxy R {sub ☉} = 8.5 kpc, our kinematic analysis reveals that the density of the stellar halo has a break at 17.2{sub −1.0}{sup +1.1} kpc and an exponential cutoff in the outer parts starting at 97.7{sub −15.8}{sup +15.6} kpc. Also, we find that the tracer velocity anisotropy is radially biased with β {sub s} = 0.4 ± 0.2 in the outer halo. We measure halo virial mass M {sub vir} to be 0.80{sub −0.16}{sup +0.31}×10{sup 12} M{sub ⊙}, concentration c to be 21.1{sub −8.3}{sup +14.8}, disk mass to be 0.95{sub −0.30}{sup +0.24}×10{sup 11} M{sub ⊙}, disk scale length to be 4.9{sub −0.4}{sup +0.4} kpc, and bulge mass to be 0.91{sub −0.38}{sup +0.31}×10{sup 10} M{sub ⊙}. The halo mass is found to be small, and this has important consequences. The giant stars reveal that the outermost halo stars have low velocity dispersion, but interestingly this suggests a truncation of the stellar halo density rather than a small overall mass of the Galaxy. Our estimates of local escape velocity v{sub esc}=550.9{sub −22.1}{sup +32.4} km s{sup −1} and

  12. The bulge-halo conspiracy in massive elliptical galaxies: implications for the stellar initial mass function and halo response to baryonic processes

    NASA Astrophysics Data System (ADS)

    Dutton, Aaron A.; Treu, Tommaso

    2014-03-01

    Recent studies have shown that massive elliptical galaxies have total mass density profiles within an effective radius that can be approximated as ρ_tot∝ r^{-γ^', with mean slope <γ'> = 2.08 ± 0.03 and scatter σ _{γ ^' } }=0.16± 0.02. The small scatter of the slope (known as the bulge-halo conspiracy) is not generic in Λ cold dark matter (ΛCDM) based models and therefore contains information about the galaxy formation process. We compute the distribution of γ' for ΛCDM-based models that reproduce the observed correlations between stellar mass, velocity dispersion, and effective radius of early-type galaxies in the Sloan Digital Sky Survey. The models have a range of stellar initial mass functions (IMFs) and dark halo responses to galaxy formation. The observed distribution of γ' is well reproduced by a model with cosmologically motivated but uncontracted dark matter haloes, and a Salpeter-type IMF. Other models are on average ruled out by the data, even though they may happen in individual cases. Models with adiabatic halo contraction (and lighter IMFs) predict too small values of γ'. Models with halo expansion, or mass-follows-light predict too high values of γ'. Our study shows that the non-homologous structure of massive early-type galaxies can be precisely reproduced by ΛCDM models if the IMF is not universal and if mechanisms, such as feedback from active galactic nuclei, or dynamical friction, effectively on average counterbalance the contraction of the halo expected as a result of baryonic cooling.

  13. Mapping the inner stellar halo of the Milky Way from 2MASS and SDSS-III/APOGEE survey

    NASA Astrophysics Data System (ADS)

    Fernández-Trincado, J. G.; Robin, A. C.; Reylé, C.

    2015-12-01

    The Besançon Galaxy model was used to compare the infrared colour distribution of synthetic stars with those from 2MASS observations taking the selection function of the data into account, in order to study the shape of the stellar halo of the Milky Way, with complemetary spectroscopic data from SDSS-III/APOGEE survey. Furthermore, we compared the generated mock metallicity distribution of the Besançon Galaxy model, to the intrinsic metallicity distribution with reliable stellar parameters from the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP). The comparison was carried accross a large volume of the inner part of the Galaxy, revealing that a metal-poor population, [M/H]<-1.2 dex, could fill an extended component of the inner galactic halo. With this data set, we are able to model a more realistic mass density distribution of the stellar halo component of the Milky Way, assuming a six-parameters double power-law model, and reconstruct the behaviour of the rotation curve in the inner part of the Galaxy.

  14. Production and Recycling of Carbon in the Early Galactic Halo

    NASA Astrophysics Data System (ADS)

    Andersen, Johannes; Thidemann Hansen, Terese; Nordström, Birgitta

    2015-08-01

    Extremely metal-poor (EMP) stars - [Fe/H] below ~ -3 - are fossil records of the conditions in the early halo. High-resolution 8m-class spectroscopy has shown that the detailed abundance pattern of EMP giant stars is surprisingly uniform and essentially Solar (e.g. Bonifacio+ 2012), apart from the usual α-enhancement in the halo. In the simplest picture, iron is a proxy for both overall metallicity and time, so the EMP stars should form before the oldest and most metal-poor Galactic globular clusters, notably at the lowest metallicities ([Fe/H] ≲ -3.5).It is thus striking that 20-40% of the EMP giants are strongly enhanced in carbon - the CEMP stars (Lucatello+ 2006). This is conventionally ascribed to mass transfer from a former AGB binary companion, and from a limited compilation of data, Lucatello+ (2005) concluded that most or all CEMP stars are indeed binaries, similar to the classical Ba and CH stars (e.g. Jorissen+ 1998). However, most of the sample was of the inner-halo CEMP-s variety (C and s-process elements both enhanced), while CEMP-no stars dominate the outer halo (Carollo+ 2014). Our precise radial velocity monitoring for CEMP stars over 8 years shed light on this issue.Our data suggest a normal binary frequency for the CEMP-no stars; i.e. the C was not produced in a binary companion, but in sites at interstellar distances, e.g. ‘faint’ SNe, and imprinted on the natal clouds of the low-mass stars we observe. This has immediate implications for the formation of dust in primitive, high-redshift galaxies (Watson+ 2015) and the origin of C-enhanced DLAs (Cooke+ 2011, 2012). The CEMP-s binary orbits are also revealing, with periods up to several decades and generally low amplitudes and eccentricities, suggesting that EMP AGB stars have very large radii, facilitating extensive mass loss. More work on faint SNe and EMP AGB envelopes is needed!

  15. F Turnoff Distribution in the Galactic Halo Using Globular Clusters as Proxies

    NASA Astrophysics Data System (ADS)

    Newby, Matthew; Newberg, Heidi Jo; Simones, Jacob; Cole, Nathan; Monaco, Matthew

    2011-12-01

    F turnoff stars are important tools for studying Galactic halo substructure because they are plentiful, luminous, and can be easily selected by their photometric colors from large surveys such as the Sloan Digital Sky Survey (SDSS). We describe the absolute magnitude distribution of color-selected F turnoff stars, as measured from SDSS data, for 11 globular clusters in the Milky Way halo. We find that the Mg distribution of turnoff stars is intrinsically the same for all clusters studied, and is well fit by two half-Gaussian functions, centered at μ = 4.18, with a bright-side σ = 0.36, and with a faint-side σ = 0.76. However, the color errors and detection efficiencies cause the observed σ of the faint-side Gaussian to change with magnitude due to contamination from redder main-sequence stars (40% at 21st magnitude). We present a function that will correct for this magnitude-dependent change in selected stellar populations, when calculating stellar density from color-selected turnoff stars. We also present a consistent set of distances, ages, and metallicities for 11 clusters in the SDSS Data Release 7. We calculate a linear correction function to Padova isochrones so that they are consistent with SDSS globular cluster data from previous papers. We show that our cluster population falls along the Milky Way age-metallicity relationship (AMR), and further find that isochrones for stellar populations on the AMR have very similar turnoffs; increasing metallicity and decreasing age conspire to produce similar turnoff magnitudes and colors for all old clusters that lie on the AMR.

  16. F TURNOFF DISTRIBUTION IN THE GALACTIC HALO USING GLOBULAR CLUSTERS AS PROXIES

    SciTech Connect

    Newby, Matthew; Newberg, Heidi Jo; Simones, Jacob; Cole, Nathan; Monaco, Matthew E-mail: heidi@rpi.edu

    2011-12-20

    F turnoff stars are important tools for studying Galactic halo substructure because they are plentiful, luminous, and can be easily selected by their photometric colors from large surveys such as the Sloan Digital Sky Survey (SDSS). We describe the absolute magnitude distribution of color-selected F turnoff stars, as measured from SDSS data, for 11 globular clusters in the Milky Way halo. We find that the M{sub g} distribution of turnoff stars is intrinsically the same for all clusters studied, and is well fit by two half-Gaussian functions, centered at {mu} = 4.18, with a bright-side {sigma} = 0.36, and with a faint-side {sigma} = 0.76. However, the color errors and detection efficiencies cause the observed {sigma} of the faint-side Gaussian to change with magnitude due to contamination from redder main-sequence stars (40% at 21st magnitude). We present a function that will correct for this magnitude-dependent change in selected stellar populations, when calculating stellar density from color-selected turnoff stars. We also present a consistent set of distances, ages, and metallicities for 11 clusters in the SDSS Data Release 7. We calculate a linear correction function to Padova isochrones so that they are consistent with SDSS globular cluster data from previous papers. We show that our cluster population falls along the Milky Way age-metallicity relationship (AMR), and further find that isochrones for stellar populations on the AMR have very similar turnoffs; increasing metallicity and decreasing age conspire to produce similar turnoff magnitudes and colors for all old clusters that lie on the AMR.

  17. F Turnoff Distribution in the Galactic Halo Using Globular Clusters as Proxies

    NASA Astrophysics Data System (ADS)

    Newby, Matthew; Newberg, H. J.; Simones, J.; Monaco, M.; Cole, N.

    2012-01-01

    F turnoff stars are important tools for studying Galactic halo substructure because they are plentiful, luminous, and can be easily selected by their photometric colors from large surveys such as the Sloan Digital Sky Survey (SDSS). We describe the absolute magnitude distribution of color-selected F turnoff stars, as measured from SDSS data, for eleven globular clusters in the Milky Way halo. We find that the absolute magnitude distribution of turnoff stars is intrinsically the same for all clusters studied, and is well fit by two half Gaussian functions, centered at μ = 4.18, with a bright-side σ = 0.36, and with a faint-side σ = 0.76. However, the color errors and detection efficiencies cause the observed σ of the faint-side Gaussian to change with magnitude due to contamination from redder main sequence stars (40% at 21st magnitude). We present a function that will correct for this magnitude-dependent change in selected stellar populations, when calculating stellar density from color-selected turnoff stars. We also present a consistent set of distances, ages and metallicities for eleven clusters in the SDSS Data Release 7. We calculate a linear correction function to Padova isochrones so that they are consistent with SDSS globular cluster data from previous papers. We show that our cluster population falls along the theoretical Age-Metallicity Relationship (AMR), and further find that isochrones for stellar populations on the AMR have very similar turnoffs; increasing metallicity and decreasing age conspire to produce similar turnoff magnitudes and colors for all old clusters that lie on the AMR. This research was supported by NSF grant AST 10-09670 and the NASA/NY Space Grant.

  18. Rotational signature of the Milky Way stellar halo

    NASA Astrophysics Data System (ADS)

    Fermani, Francesco; Schönrich, Ralph

    2013-07-01

    We measure the rotation of the Milky Way stellar halo on two samples of blue horizontal branch (BHB) field halo stars from the Sloan Digital Sky Survey (SDSS) with four different methods. The two samples comprise 1582 and 2563 stars, respectively, and reach out to ˜50 kpc in galactocentric distance. Two of the methods to measure rotation rely exclusively on line-of-sight (l.o.s.) velocities, namely the popular double power-law model and a direct estimate of the de-projected l.o.s. velocity. The other two techniques use the full 3D motions: the radial velocity based rotation estimator of Schönrich et al. and a simple 3D azimuthal velocity mean. In this context we (a) critique the popular model and (b) assess the reliability of the estimators. All four methods agree on a weakly prograde or non-rotating halo. Further, we observe no duality in the rotation of sub-samples with different metallicities or at different radii. We trace the rotation gradient across metallicity measured by Deason et al. on a similar sample of BHB stars back to the inclusion of regions in the apparent magnitude-surface gravity plane known to be contaminated. In the spectroscopically selected sample of Xue et al., we flag ˜500 hot metal-poor stars for their peculiar kinematics w.r.t. to both their cooler metal-poor counterparts and the metal-rich stars in the same sample. They show a seemingly retrograde behaviour in l.o.s. velocities, which is not confirmed by the 3D estimators. Their anomalous vertical motion hints at either a pipeline problem or a stream-like component rather than a smooth retrograde population.

  19. STELLAR MASS VERSUS STELLAR VELOCITY DISPERSION: WHICH IS BETTER FOR LINKING GALAXIES TO THEIR DARK MATTER HALOS?

    SciTech Connect

    Li Cheng; Wang Lixin; Jing, Y. P.

    2013-01-01

    It was recently suggested that compared to its stellar mass (M{sub *}), the central stellar velocity dispersion ({sigma}{sub *}) of a galaxy might be a better indicator for its host dark matter halo mass. Here we test this hypothesis by estimating the dark matter halo mass for central galaxies in groups as a function of M{sub *} and {sigma}{sub *}. For this we have estimated the redshift-space cross-correlation function (CCF) between the central galaxies at given M{sub *} and {sigma}{sub *} and a reference galaxy sample, from which we determine both the projected CCF, w{sub p} (r{sub p} ), and the velocity dispersion profile. A halo mass is then obtained from the average velocity dispersion within the virial radius. At fixed M{sub *}, we find very weak or no correlation between halo mass and {sigma}{sub *}. In contrast, strong mass dependence is clearly seen even when {sigma}{sub *} is limited to a narrow range. Our results thus firmly demonstrate that the stellar mass of central galaxies is still a good (if not the best) indicator for dark matter halo mass, better than the stellar velocity dispersion. The dependence of galaxy clustering on {sigma}{sub *} at fixed M{sub *}, as recently discovered by Wake et al., may be attributed to satellite galaxies, for which the tidal stripping occurring within halos has stronger effect on stellar mass than on central stellar velocity dispersion.

  20. Formation of a Giant Galactic Gaseous Halo: Metal-Absorption Lines and High-Velocity Clouds

    NASA Astrophysics Data System (ADS)

    Li, Fan

    1992-04-01

    A Galactic gaseous halo formed through the interstellar disk-halo connection is simulated by means of a two-dimensional axisymmetric hydrodynamic code based upon the chimney model of the interstellar medium, a new version of the galactic fountain. Galactic rotation, heating processes by diffuse UV flux, and radiative cooling processes are taken into account. The resulting gaseous halo can be divided into three categories, i.e., wind-type halo, bound-type halo, and cooled-type halo. In this way, we try to reproduce the column densities of C IV, N V, O VI, and Si IV in the observed absorption lines of halo stars. Assuming that the radiatively cooled halo gas condenses into clouds due to thermal instabilities, we can calculate their distribution and ballistic motions in the Galactic gravitational field. These correspond to the high- and intermediate-velocity clouds observed at high Galactic latitudes. We find that a cooled-type halo with a gas temperature between 5 X 10^5 and 10^6 K and a density between 10^-3 and 10^-2 cm^-3 at the disk-halo interface can reproduce the observational facts about our Galaxy. Supposing that the metal-absorption-line systems of QSOs arise from the halos of intervening galaxies formed by similar processes, we calculate features of the Ca II, Mg II, C IV, and Si IV absorption lines in various stages of galactic evolution. We conclude that C IV systems which are greater than 50 kpc in size correspond to the wind-type halo. On the other hand, Mg II and Ca II systems can only be detected in a very restricted region ( Metaxa, SMALL FAINT CLUSTERS IN THE LMC This is a short review of the main results of my Ph.D. thesis concerning some important problems on the dynamical properties of the LMC star clusters. The topic of this thesis was to find and study the dynamical paramters (tidal radius r_t core radius r_c concentration parameters log (r_t/r_c), and total mass M) for a large sample of small LMC clusters and to define their location in the

  1. Dual Stellar Halos in Early-type Galaxies and Formation of Massive Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Myung Gyoon; Jang, In Sung

    2016-08-01

    M105 in the Leo I Group is a textbook example of a standard elliptical galaxy. It is only one of the few elliptical galaxies for which we can study their stellar halos using the resolved stars. It is an ideal target to study the structure and composition of stellar halos in elliptical galaxies. We present photometry and metallicity of the resolved stars in the inner and outer regions of M105. These provide strong evidence that there are two distinct stellar halos in this galaxy, a metal-poor (blue) halo and a metal-rich (red) halo. Then we compare them with those in other early-type galaxies and use the dual halo mode formation scenario to describe how massive galaxies formed.

  2. Mapping stellar content to dark matter haloes - II. Halo mass is the main driver of galaxy quenching

    NASA Astrophysics Data System (ADS)

    Zu, Ying; Mandelbaum, Rachel

    2016-04-01

    We develop a simple yet comprehensive method to distinguish the underlying drivers of galaxy quenching, using the clustering and galaxy-galaxy lensing of red and blue galaxies in Sloan Digital Sky Survey. Building on the iHOD framework developed by Zu & Mandelbaum, we consider two quenching scenarios: (1) a `halo' quenching model in which halo mass is the sole driver for turning off star formation in both centrals and satellites; and (2) a `hybrid' quenching model in which the quenched fraction of galaxies depends on their stellar mass, while the satellite quenching has an extra dependence on halo mass. The two best-fitting models describe the red galaxy clustering and lensing equally well, but halo quenching provides significantly better fits to the blue galaxies above 1011 h-2 M⊙. The halo quenching model also correctly predicts the average halo mass of the red and blue centrals, showing excellent agreement with the direct weak lensing measurements of locally brightest galaxies. Models in which quenching is not tied to halo mass, including an age-matching model in which galaxy colour depends on halo age at fixed M*, fail to reproduce the observed halo mass for massive blue centrals. We find similar critical halo masses responsible for the quenching of centrals and satellites (˜1.5 × 1012 h-1 M⊙), hinting at a uniform quenching mechanism for both, e.g. the virial shock heating of infalling gas. The success of the iHOD halo quenching model provides strong evidence that the physical mechanism that quenches star formation in galaxies is tied principally to the masses of their dark matter haloes rather than the properties of their stellar components.

  3. EVIDENCE FOR A TRIAXIAL MILKY WAY DARK MATTER HALO FROM THE SAGITTARIUS STELLAR TIDAL STREAM

    SciTech Connect

    Law, David R.; Majewski, Steven R.; Johnston, Kathryn V. E-mail: srm4n@virginia.ed

    2009-09-20

    Observations of the lengthy tidal streams produced by the destruction of the Sagittarius dwarf spheroidal (Sgr dSph) are capable of providing strong constraints on the shape of the Galactic gravitational potential. However, previous work, based on modeling different stream properties in axisymmetric Galactic models, has yielded conflicting results: while the angular precession of the Sgr leading arm is most consistent with a spherical or slightly oblate halo, the radial velocities of stars in this arm are reproduced only by prolate halo models. We demonstrate that this apparent paradox can be resolved by instead adopting a triaxial potential. Our new Galactic halo model, which simultaneously fits all well-established phase space constraints from the Sgr stream, provides the first conclusive evidence for, and tentative measurement of, triaxiality in an individual dark matter halo. The Milky Way halo within {approx}60 kpc is best characterized by a minor/major axis ratio of the isovelocity contours c/a {approx} 0.67, intermediate/major axis ratio b/a {approx} 0.83, and triaxiality parameter T {approx} 0.56. In this model, the minor axis of the dark halo is coincident with the Galactic X-axis connecting the Sun and the Galactic center to within {approx}15 deg., while the major axis also lies in the Galactic plane, approximately along the Galactic Y-axis.

  4. Evidence for a Triaxial Milky Way Dark Matter Halo from the Sagittarius Stellar Tidal Stream

    NASA Astrophysics Data System (ADS)

    Law, David R.; Majewski, Steven R.; Johnston, Kathryn V.

    2009-09-01

    Observations of the lengthy tidal streams produced by the destruction of the Sagittarius dwarf spheroidal (Sgr dSph) are capable of providing strong constraints on the shape of the Galactic gravitational potential. However, previous work, based on modeling different stream properties in axisymmetric Galactic models, has yielded conflicting results: while the angular precession of the Sgr leading arm is most consistent with a spherical or slightly oblate halo, the radial velocities of stars in this arm are reproduced only by prolate halo models. We demonstrate that this apparent paradox can be resolved by instead adopting a triaxial potential. Our new Galactic halo model, which simultaneously fits all well-established phase space constraints from the Sgr stream, provides the first conclusive evidence for, and tentative measurement of, triaxiality in an individual dark matter halo. The Milky Way halo within ~60 kpc is best characterized by a minor/major axis ratio of the isovelocity contours c/a ≈ 0.67, intermediate/major axis ratio b/a ≈ 0.83, and triaxiality parameter T ~ 0.56. In this model, the minor axis of the dark halo is coincident with the Galactic X-axis connecting the Sun and the Galactic center to within ~15°, while the major axis also lies in the Galactic plane, approximately along the Galactic Y-axis.

  5. HaloSat: A CubeSat to Map the Distribution of Baryonic Matter in the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Miles, Drew M.

    2016-04-01

    Approximately half of predicted baryonic matter in the Milky Way remains unidentified. One possible explanation for the location of this missing matter is in an extended Galactic halo. HaloSat is a CubeSat that aims to constrain the mass and distribution of the halo’s baryonic matter by obtaining an all-sky map of O VII and O VIII emission in the hot gas associated with the halo of the Milky Way. HaloSat offers an improvement in the quality of measurements of oxygen line emission over existing X-ray observatories and an observation plan dedicated to mapping the hot gas in the Galactic halo. In addition to the missing baryon problem, HaloSat will assign a portion of its observations to the solar wind charge exchange (SWCX) in order to calibrate models of SCWX emission. We present here the current status of HaloSat and the progression of instrument development in anticipation of a 2018 launch.

  6. THE STELLAR-TO-HALO MASS RELATION FOR LOCAL GROUP GALAXIES

    SciTech Connect

    Brook, C. B.; Cintio, A. Di; Knebe, A.; Yepes, G.; Gottlöber, S.; Hoffman, Y.; Garrison-Kimmel, S.

    2014-03-20

    We contend that a single power-law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass-halo mass relation for low-mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated Local Group realizations, which we determine using local volume simulations. For the stellar mass range 10{sup 7} M {sub ☉}stellar mass-halo mass relation follows a power law with slope of 3.1, significantly steeper than most values in the literature. This steep relation between stellar and halo masses would indicate that Local Group dwarf galaxies are hosted by dark matter halos with a small range of mass. Our methodology is robust down to the stellar mass to which the census of observed Local Group galaxies is complete, but the significant uncertainty in the currently measured slope of the stellar-to-halo mass relation will decrease dramatically if the Local Group completeness limit was 10{sup 6.5} M {sub ☉} or below, highlighting the importance of pushing such limit to lower masses and larger volumes.

  7. Massive stellar X-ray sources in the Galactic center

    NASA Astrophysics Data System (ADS)

    Mauerhan, Jon Christian

    2008-06-01

    The purpose of this thesis is to discover unidentified members of the massive stellar population in the Galactic center, using a novel selection technique: the identification of infrared counterparts to hard X-ray sources. This method provides a means of distinguishing a subset of hot, massive stars from the more numerous cool giants that dominate the stellar population of the central Galaxy, providing potential beacons toward undiscovered regions of massive star formation, and the remains of tidally-disrupted stellar clusters. Hard-X-ray selection also highlights exotic species of massive star, including Wolf-Rayet (WR) binaries with colliding supersonic winds, and wind-accreting neutron stars and black holes in high-mass X-ray binaries (HMXBs). Massive stars were sought in the central 300 pc of the Galaxy by cross- correlating X-ray and IR point-source catalogs. Approximately 1% of the 6067 Chandra X-ray sources near the Galactic center have near-infrared matches with K s < 15.6 mag. A spectroscopic campaign was launched to investigate the most promising candidates; 17 new WR/O stars were discovered throughout the inner 300 pc. Most of the massive stars exhibit infrared excess, attributable to free-free and dust emission. In many cases, mid-IR images exhibit strong interaction of the X-ray sources with the Galactic center medium. Most of the newly found sources have no apparent association with a dense stellar cluster, although several stars lie near the Quintuplet cluster and may have escaped from it. The X-ray emission of the massive stars is consistent with thermal emission from plasma at temperatures above 2 keV, not a ubiquitous feature of single massive stars. The X-ray data are consistent with models of strong WR/O winds colliding with the surfaces of binary companions, but are also consistent with known, low-luminosity HMXBs. Future experiments are discussed, aimed at unambiguously determining the masses of the stellar components, and surveying the

  8. A census of stellar mass in ten massive haloes at z ~ 1 from the GCLASS Survey

    NASA Astrophysics Data System (ADS)

    van der Burg, R. F. J.; Muzzin, A.; Hoekstra, H.; Wilson, G.; Lidman, C.; Yee, H. K. C.

    2014-01-01

    Aims: We study the stellar mass content of massive haloes in the redshift range 0.86 < z < 1.34, by measuring (1) the stellar mass in the central galaxy versus total dynamical halo mass; (2) the total stellar mass (including satellites) versus total halo mass; and (3) the radial stellar mass and number density profiles for the ensemble halo. Methods: We use a Ks-band selected catalogue for the 10 clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS), with photometric redshifts and stellar masses measured from 11-band SED fitting. Combining the photometric catalogues with the deep spectroscopic component of GCLASS, we correct the cluster galaxy sample for interlopers. We also perform a dynamical analysis of the cluster galaxies to estimate the halo mass M200 for each cluster based on a measurement of its velocity dispersion. Results: (1) We find that the central galaxy stellar mass fraction decreases with total halo mass and that this is in reasonable, quantitative agreement with measurements from abundance matching studies at z ~ 1. (2) The total stellar mass fractions of these systems decrease with halo mass, indicating that lower mass systems are more efficient at transforming baryons into stars. We find the total stellar mass to be a good proxy for total halo mass, with a small intrinsic scatter. When we compare these results from GCLASS with literature measurements, we find that the stellar mass fraction at fixed halo mass shows no significant evolution in the range 0 < z < 1. (3) We measure a relatively high NFW concentration parameter cg ~ 7 for the stellar mass distribution in these clusters, and debate a possible scenario for explaining the evolution of the stellar mass distribution from the GCLASS sample to their likely descendants at lower redshift. Conclusions: The stellar mass measurements in the z ~ 1 haloes provided by GCLASS puts constraints on the stellar mass assembly history of clusters observed in the local Universe. A simple

  9. CALET's sensitivity to Dark Matter annihilation in the galactic halo

    NASA Astrophysics Data System (ADS)

    Motz, H.; Asaoka, Y.; Torii, S.; Bhattacharyya, S.

    2015-12-01

    CALET (Calorimetric Electron Telescope), installed on the ISS in August 2015, directly measures the electron+positron cosmic rays flux up to 20 TeV. With its proton rejection capability of 1 : 105 and an aperture of 1200 cm2· sr, it will provide good statistics even well above one TeV, while also featuring an energy resolution of 2%, which allows it to detect fine structures in the spectrum. Such structures may originate from Dark Matter annihilation or decay, making indirect Dark Matter search one of CALET's main science objectives among others such as identification of signatures from nearby supernova remnants, study of the heavy nuclei spectra and gamma astronomy. The latest results from AMS-02 on positron fraction and total electron+positron flux can be fitted with a parametrization including a single pulsar as an extra power law source with exponential cut-off, which emits an equal amount of electrons and positrons. This single pulsar scenario for the positron excess is extrapolated into the TeV region and the expected CALET data for this case are simulated. Based on this prediction for CALET data, the sensitivity of CALET to Dark Matter annihilation in the galactic halo has been calculated. It is shown that CALET could significantly improve the limits compared to current data, especially for those Dark Matter candidates that feature a large fraction of annihilation directly into e+ + e-, such as the LKP (Lightest Kaluza-Klein particle).

  10. Stellar populations in Active Galactic Nuclei III

    NASA Astrophysics Data System (ADS)

    Boisson, C.; Joly, M.; Pelat, D.; Ward, M. J.

    2004-12-01

    In this paper we apply the stellar population synthesis method previously described in Boisson et al. (\\cite{Boisson2000}) to five more AGN. The analysis of these new data strengthen our previous conclusions: i) homogeneity of the stellar population within a class of nuclear activity regardless of the morphological type of the host galaxy; ii) populations within the nuclear regions of LINERs and Seyfert 2s are different: LINERs have a very old metal-rich population while in the Seyfert 2s a contribution of a weak burst of star formation is observed together with the old high metallicity component; iii) in the circum-nuclar region (200 pc ≤D≤1 kpc) of all the active galaxies in our sample, except for NGC 2992, we detect an old burst of star formation (0.2-1 Gyr),which is contrary to what is observed in normal galaxies. We note that the broad OIλ8446 Å emission line detected in the spectrum of the nucleus of NGC 2992 confirms its classification as a Seyfert 1. Based on observations collected at the New Technology Telescope of the European Southern Observatory, La Silla, Chile.

  11. Dynamics and X-Ray Emission of a Galactic Superwind Interacting with Disk and Halo Gas

    NASA Astrophysics Data System (ADS)

    Suchkov, A. A.; Balsara, D. S.; Heckman, T. M.; Leitherer, C.

    1993-12-01

    There is a general agreement that the conspicuous extranuclear x-ray, optical-line, and radio-continuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However, despite the significant advances in observational studies of superwinds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the starburst superwind regions. To address this issue, we have conducted two-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to represent the ISM of the starburst galaxy, we analyse the interaction of the wind with a two-component disk--halo ambient interstellar medium. This two-component representation is argued to be crucial for adequate modelling of starbursts. The emphasis of this study is on the geometry and structure of the wind region and the x-ray emission arising in the wind material and the shocked gas in the disk and the halo of the galaxy. The simulation results have shown that a clear-cut bipolar wind can easily develop under a range of very different conditions. On the other hand, a complex `filamentary' structure associated with the entrained dense disk material is found to arise within the hot bubble blown out by the wind. The flow pattern within the bubble is dominated equally by the central biconic outflow and a system of whirling motions related to the origin and development of the `filaments'. The `filament' parameters make them a good candidate for optical-emission-line filamentary gas observed in starburst halos. We find that the history of mass and energy deposition in the starburst region of the galaxy is crucial for wind dynamics. A ``mild'' early wind, which arises as a result of the cumulative effect of stellar winds from massive stars, produces a bipolar vertical cavity in the disk

  12. Stellar feedback in molecular clouds and galactic discs

    NASA Astrophysics Data System (ADS)

    Walch, S.

    2016-05-01

    Feedback from massive stars in the form of stellar winds, ionizing radiation, and Supernova explosions shapes the structure and dynamics of the multi-phase interstellar medium. Here, I will briefly summarise some of our recent studies and findings on these subjects. I will discuss the effects of ionizing radiation and stellar winds in individual molecular clouds. Furthermore, I will shortly introduce the SILCC project, in which we model the evolution of the Supernova- and wind-driven, multi-phase ISM in stratified galactic discs.

  13. Global Properties of M31's Stellar Halo from the SPLASH Survey. II. Metallicity Profile

    NASA Astrophysics Data System (ADS)

    Gilbert, Karoline M.; Kalirai, Jason S.; Guhathakurta, Puragra; Beaton, Rachael L.; Geha, Marla C.; Kirby, Evan N.; Majewski, Steven R.; Patterson, Richard J.; Tollerud, Erik J.; Bullock, James S.; Tanaka, Mikito; Chiba, Masashi

    2014-12-01

    We present the metallicity distribution of red giant branch (RGB) stars in M31's stellar halo, derived from photometric metallicity estimates for over 1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from 38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to 175 kpc in projected distance from M31's center, and includes 52 confirmed M31 halo stars beyond 100 kpc. While a wide range of metallicities is seen throughout the halo, the metal-rich peak of the metallicity distribution function becomes significantly less prominent with increasing radius. The metallicity profile of M31's stellar halo shows a continuous gradient from 9 to ~100 kpc, with a magnitude of ~ - 0.01 dex kpc-1. The stellar velocity distributions in each field are used to identify stars that are likely associated with tidal debris features. The removal of tidal debris features does not significantly alter the metallicity gradient in M31's halo: a gradient is maintained in fields spanning 10-90 kpc. We analyze the halo metallicity profile, as well as the relative metallicities of stars associated with tidal debris features and the underlying halo population, in the context of current simulations of stellar halo formation. We argue that the large-scale gradient in M31's halo implies M31 accreted at least one relatively massive progenitor in the past, while the field to field variation seen in the metallicity profile indicates that multiple smaller progenitors are likely to have contributed substantially to M31's outer halo. 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.

  14. Global properties of M31's stellar halo from the splash survey. II. Metallicity profile

    SciTech Connect

    Gilbert, Karoline M.; Kalirai, Jason S.; Guhathakurta, Puragra; Geha, Marla C.; Tollerud, Erik J.; Kirby, Evan N.; Bullock, James S.; Tanaka, Mikito; Chiba, Masashi

    2014-12-01

    We present the metallicity distribution of red giant branch (RGB) stars in M31's stellar halo, derived from photometric metallicity estimates for over 1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from 38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to 175 kpc in projected distance from M31's center, and includes 52 confirmed M31 halo stars beyond 100 kpc. While a wide range of metallicities is seen throughout the halo, the metal-rich peak of the metallicity distribution function becomes significantly less prominent with increasing radius. The metallicity profile of M31's stellar halo shows a continuous gradient from 9 to ∼100 kpc, with a magnitude of ∼ – 0.01 dex kpc{sup –1}. The stellar velocity distributions in each field are used to identify stars that are likely associated with tidal debris features. The removal of tidal debris features does not significantly alter the metallicity gradient in M31's halo: a gradient is maintained in fields spanning 10-90 kpc. We analyze the halo metallicity profile, as well as the relative metallicities of stars associated with tidal debris features and the underlying halo population, in the context of current simulations of stellar halo formation. We argue that the large-scale gradient in M31's halo implies M31 accreted at least one relatively massive progenitor in the past, while the field to field variation seen in the metallicity profile indicates that multiple smaller progenitors are likely to have contributed substantially to M31's outer halo.

  15. The large-scale structure of the halo of the Andromeda galaxy. I. Global stellar density, morphology and metallicity properties

    SciTech Connect

    Ibata, Rodrigo A.; Martin, Nicolas F.; Lewis, Geraint F.; McConnachie, Alan W.; Irwin, Michael J.; Ferguson, Annette M. N.; Bernard, Edouard J.; Peñarrubia, Jorge; Babul, Arif; Navarro, Julio; Chapman, Scott C.; Collins, Michelle; Fardal, Mark; Mackey, A. D.; Rich, R. Michael; Tanvir, Nial; Widrow, Lawrence

    2014-01-10

    We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H]<−1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f {sub stream} = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f {sub stream} becoming as high as 86% for [Fe/H]>−0.6. The space density of the smooth metal-poor component has a global power-law slope of γ = –3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to ∼300 kpc. The total stellar mass in the halo at distances beyond 2° is ∼1.1 × 10{sup 10} M {sub ☉}, while that of the smooth component is ∼3 × 10{sup 9} M {sub ☉}. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ∼8 × 10{sup 9} M {sub ☉}. We detect a substantial metallicity gradient, which declines from ([Fe/H]) = –0.7 at R = 30 kpc to ([Fe/H]) = –1.5 at R = 150 kpc for the full sample, with the smooth halo being ∼0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as

  16. Contributions to the accreted stellar halo: an atlas of stellar deposition

    NASA Astrophysics Data System (ADS)

    Amorisco, N. C.

    2016-09-01

    The accreted component of stellar halos is composed of the contributions of several satellites, falling onto their host with their different masses, at different times, on different orbits. This work uses a suite of idealised, collisionless N-body simulations of minor mergers and a particle tagging technique to understand how these different ingredients shape each contribution to the accreted halo, in both density and kinematics. I find that more massive satellites deposit their stars deeper into the gravitational potential of the host, with a clear segregation enforced by dynamical friction. Earlier accretion events contribute more to the inner regions of the halo; more concentrated subhaloes sink deeper through increased dynamical friction. The orbital circularity of the progenitor at infall is only important for low-mass satellites: dynamical friction efficiently radialises the most massive minor mergers erasing the imprint of the infall orbit for satellite-to-host virial mass ratios ≳ 1/20. The kinematics of the stars contributed by each satellite is also ordered with satellite mass: low-mass satellites contribute fast-moving populations, in both ordered rotation and radial velocity dispersion. In turn, contributions by massive satellites have lower velocity dispersion and lose their angular momentum to dynamical friction, resulting in a strong radial anisotropy.

  17. Gusty, gaseous flows of FIRE: galactic winds in cosmological simulations with explicit stellar feedback

    NASA Astrophysics Data System (ADS)

    Muratov, Alexander L.; Kereš, Dušan; Faucher-Giguère, Claude-André; Hopkins, Philip F.; Quataert, Eliot; Murray, Norman

    2015-12-01

    We present an analysis of the galaxy-scale gaseous outflows from the Feedback in Realistic Environments (FIRE) simulations. This suite of hydrodynamic cosmological zoom simulations resolves formation of star-forming giant molecular clouds to z = 0, and features an explicit stellar feedback model on small scales. Our simulations reveal that high-redshift galaxies undergo bursts of star formation followed by powerful gusts of galactic outflows that eject much of the interstellar medium and temporarily suppress star formation. At low redshift, however, sufficiently massive galaxies corresponding to L* progenitors develop stable discs and switch into a continuous and quiescent mode of star formation that does not drive outflows far into the halo. Mass-loading factors for winds in L* progenitors are η ≈ 10 at high redshift, but decrease to η ≪ 1 at low redshift. Although lower values of η are expected as haloes grow in mass over time, we show that the strong suppression of outflows with decreasing redshift cannot be explained by mass evolution alone. Circumgalactic outflow velocities are variable and broadly distributed, but typically range between one and three times the circular velocity of the halo. Much of the ejected material builds a reservoir of enriched gas within the circumgalactic medium, some of which could be later recycled to fuel further star formation. However, a fraction of the gas that leaves the virial radius through galactic winds is never regained, causing most haloes with mass Mh ≤ 1012 M⊙ to be deficient in baryons compared to the cosmic mean by z = 0.

  18. Are ancient dwarf satellites the building blocks of the Galactic halo?

    NASA Astrophysics Data System (ADS)

    Spitoni, E.; Vincenzo, F.; Matteucci, F.; Romano, D.

    2016-05-01

    According to the current cosmological cold dark matter paradigm, the Galactic halo could have been the result of the assemblage of smaller structures. Here we explore the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the Milky Way have been the building blocks of the Galactic halo by comparing their [α/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones observed in Galactic halo stars. The α elements deviate substantially from the observed abundances in the Galactic halo stars for [Fe/H] values larger than -2 dex, while they overlap for lower metallicities. On the other hand, for the [Ba/Fe] ratio, the discrepancy is extended at all [Fe/H] values, suggesting that the majority of stars in the halo are likely to have been formed in situ. Therefore, we suggest that [Ba/Fe] ratios are a better diagnostic than [α/Fe] ratios. Moreover, for the first time we consider the effects of an enriched infall of gas with the same chemical abundances as the matter ejected and/or stripped from dwarf satellites of the Milky Way on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the assumed infall time-scale, and the presence of a threshold in the gas for star formation. In particular, in models with an infall time-scale for the halo around 0.8 Gyr coupled with a threshold in the surface gas density for the star formation (4 M⊙ pc-2), and the enriched infall from dwarf spheroidal satellites, the first halo stars formed show [Fe/H]>-2.4 dex. In this case, to explain [α/Fe] data for stars with [Fe/H]<-2.4 dex, we need stars formed in dSph systems.

  19. Chemical and kinematic correlations in the galactic halo

    NASA Astrophysics Data System (ADS)

    James, Carol Renee

    2000-11-01

    Abundances and kinematics have been determined for sixty-one low metallicity ([Fe/H] < -1.30) stars in an attempt to find correlations between stellar chemistry and dynamics. Distances for program stars have been re-derived spectroscopically, and the computed stellar distances agree with the parallaxes reported by Hipparcos (ESA 1997). Stellar orbits were calculated using the Galactic mass model of Bahcall et al. (1983). We find that the general kinematic results (U, V, W, Rapo, Z max, eccentricity) are more extreme than previously thought, but in good agreement with recent studies of this nature. We have also adopted the chemical and kinematic results for 34 additional stars from the work of others to supplement our sample. Our LTE abundance results concur with previous studies. Specifically, we have found that the alpha-capture elements (Mg, Ca, and Ti) are consistently overabundant by 0.4 dex (i.e. [alpha/Fe] = +0.4) except in the cases of four stars: HD 134439, HD 134440, BD+80 245 and G4-36. Additionally, we have found a typical oxygen overabundance of +0.70 dex, and a sodium abundance of [Na/Fe] = -0.60 dex after an empirical correction for NLTE effects. We find no statistically sound correlations between various kinematic parameters and abundances. We confirm the unusual abundance signatures of HD 134439/HD 134440 and BD+80 245. The former stars are a proper-motion pair with mildly deficient alpha-capture abundances. The latter star possesses a pronounced alpha-capture deficiency combined with an extremely low Ba abundance. Its Na and O are apparently normal. We have also discovered a star with extremely unusual abundances: G4-36 possesses marked deficiencies in O, Mg, Al, Ca, Sr, and Ba, but overabundances of Fe-peak elements Mn, Ni, and Zn. We suggest that the nucleosynthetic origin of G4-36 is substantially different from that of BD+80 245 and the proper motion pair HD 134439/134440. All four unusual stars are in retrograde orbits, as are half the stars

  20. Bright gamma-ray Galactic Center excess and dark dwarfs: Strong tension for dark matter annihilation despite Milky Way halo profile and diffuse emission uncertainties

    NASA Astrophysics Data System (ADS)

    Abazajian, Kevork N.; Keeley, Ryan E.

    2016-04-01

    We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, two of the most precise empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses for single-channel dark matter annihilation models. The GCE and dwarf tension can be alleviated if: one, the halo is very highly concentrated or strongly contracted; two, the dark matter annihilation signal differentiates between dwarfs and the GC; or, three, local stellar density measures are found to be significantly lower, like that from recent stellar counts, increasing the local dark matter density.

  1. The kinematic footprints of five stellar streams in Andromeda's halo

    NASA Astrophysics Data System (ADS)

    Chapman, S. C.; Ibata, R.; Irwin, M.; Koch, A.; Letarte, B.; Martin, N.; Collins, M.; Lewis, G. F.; McConnachie, A.; Peñarrubia, J.; Rich, R. M.; Trethewey, D.; Ferguson, A.; Huxor, A.; Tanvir, N.

    2008-11-01

    We present a spectroscopic analysis of five stellar streams (`A', `B', `Cr', `Cp' and `D') as well as the extended star cluster, EC4, which lies within Stream`C', all discovered in the halo of M31 from our Canada-France-Hawaii Telescope/MegaCam survey. These spectroscopic results were initially serendipitous, making use of our existing observations from the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, and thereby emphasizing the ubiquity of tidal streams that account for ~70 per cent of the M31 halo stars in the targeted fields. Subsequent spectroscopy was then procured in Stream`C' and Stream`D' to trace the velocity gradient along the streams. Nine metal-rich ([Fe/H] ~ -0.7) stars at vhel = -349.5kms-1,σv,corr ~ 5.1 +/- 2.5km s-1 are proposed as a serendipitous detection of Stream`Cr', with follow-up kinematic identification at a further point along the stream. Seven metal-poor ([Fe/H] ~-1.3) stars confined to a narrow, 15 km s-1 velocity bin centred at vhel = -285.6, σv,corr = 4.3+1.7-1.4 km s-1 represent a kinematic detection of Stream`Cp', again with follow-up kinematic identification further along the stream. For the cluster EC4, candidate member stars with average [Fe/H] ~-1.4, are found at vhel = -282 suggesting it could be related to Stream`Cp'. No similarly obvious cold kinematic candidate is found for Stream`D', although candidates are proposed in both of two spectroscopic pointings along the stream (both at ~ -400km s-1). Spectroscopy near the edge of Stream`B' suggests a likely kinematic detection at vhel ~ -330, σv,corr ~ 6.9km s-1, while a candidate kinematic detection of Stream`A' is found (plausibly associated to M33 rather than M31) with vhel ~ -170, σv,corr = 12.5km s-1. The low dispersion of the streams in kinematics, physical thickness and metallicity makes it hard to reconcile with a scenario whereby these stream structures as an ensemble are related to the giant southern stream. We conclude that the M31 stellar

  2. HUBBLE'S SEARCH FOR FAINT FIELD STARS IN GALACTIC HALO

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Left A NASA Hubble Space Telescope image of a randomly selected area of sky taken to search for faint red stars that might constitute dark matter in our Milky Way Galaxy. (Dark matter is material of an unknown type that makes up most of the mass of our galaxy). If the dark matter in our Galaxy was made of faint red stars -- as many scientists have previously conjectured -- then about 38 such stars should have been visible in this HST image. The simulated stars (diamond-shaped symbols), based on theoretical calculations, illustrate what scientists would have seen if the dark matter were locked-up in faint red stars. These surprising results rule out dim stars as an explanation for dark matter in our Galaxy. Right The unmodified HST image shows the region is actually so devoid of stars that far more distant background galaxies can easily be seen. The field is in the constellation Eridanus, far outside the plane of our Milky Way Galaxy. This region was chosen to highlight stars in the galactic halo, where dark matter exists, and to avoid the contribution of faint stars in the plane of the Galaxy. Technical Information: The image was constructed from seven exposures totaling almost three hours of searching by HST. The field shown is about 1.5 arc-minutes across. The image was taken in near-infrared light (814 nm) with the Wide Field Planetary Camera 2, on Feb 8, 1994. This observation is part of the HST parallel observing program. Credit: J Bahcall, Institute for Advance Study, Princeton and NASA

  3. THE DUAL ORIGIN OF STELLAR HALOS. II. CHEMICAL ABUNDANCES AS TRACERS OF FORMATION HISTORY

    SciTech Connect

    Zolotov, Adi; Hogg, David W.; Willman, Beth; Brooks, Alyson M.; Shen, Sijing; Wadsley, James E-mail: bwillman@haverford.ed

    2010-09-20

    Fully cosmological, high-resolution N-body+smooth particle hydrodynamic simulations are used to investigate the chemical abundance trends of stars in simulated stellar halos as a function of their origin. These simulations employ a physically motivated supernova feedback recipe, as well as metal enrichment, metal cooling, and metal diffusion. As presented in an earlier paper, the simulated galaxies in this study are surrounded by stellar halos whose inner regions contain both stars accreted from satellite galaxies and stars formed in situ in the central regions of the main galaxies and later displaced by mergers into their inner halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within 10 kpc of a solar-like observer are analyzed. We find that for galaxies which have not experienced a recent major merger, in situ stars at the high [Fe/H] end of the metallicity distribution function are more [{alpha}/Fe]-rich than accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at a given [Fe/H] results from the different potential wells within which in situ and accreted halo stars form. These results qualitatively match recent observations of local Milky Way halo stars. It may thus be possible for observers to uncover the relative contribution of different physical processes to the formation of stellar halos by observing such trends in the halo populations of the Milky Way and other local L{sup *} galaxies.

  4. The Dual Origin of Stellar Halos. II. Chemical Abundances as Tracers of Formation History

    NASA Astrophysics Data System (ADS)

    Zolotov, Adi; Willman, Beth; Brooks, Alyson M.; Governato, Fabio; Hogg, David W.; Shen, Sijing; Wadsley, James

    2010-09-01

    Fully cosmological, high-resolution N-body+smooth particle hydrodynamic simulations are used to investigate the chemical abundance trends of stars in simulated stellar halos as a function of their origin. These simulations employ a physically motivated supernova feedback recipe, as well as metal enrichment, metal cooling, and metal diffusion. As presented in an earlier paper, the simulated galaxies in this study are surrounded by stellar halos whose inner regions contain both stars accreted from satellite galaxies and stars formed in situ in the central regions of the main galaxies and later displaced by mergers into their inner halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within 10 kpc of a solar-like observer are analyzed. We find that for galaxies which have not experienced a recent major merger, in situ stars at the high [Fe/H] end of the metallicity distribution function are more [α/Fe]-rich than accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at a given [Fe/H] results from the different potential wells within which in situ and accreted halo stars form. These results qualitatively match recent observations of local Milky Way halo stars. It may thus be possible for observers to uncover the relative contribution of different physical processes to the formation of stellar halos by observing such trends in the halo populations of the Milky Way and other local Lsstarf galaxies.

  5. Are Pressure-Confined Clouds in Galactic Halos Possible for a Model of Lyman Alpha Clouds?

    NASA Astrophysics Data System (ADS)

    Miyahata, K.; Ikeuchi, S.

    Lanzetta et al (1995) found that most luminous galaxies at low redshifts produce Ly alpha absorptions at the mean impact parameter ~160h-1 kpc. Motivated by this observation, we propose the two-component protogalaxy model as a model for the Ly alpha cloud besed upon the previous work (MI 1995). In our model, the Ly alpha clouds are supposed to be stable cold clouds confined by the pressure of ambient hot gas in galactic halo. We determine the properties of these cold clouds and hot gas on the basis of theoretical and observational constraints. Especially, we take into account the stability of cold cloud in the galactic halo in addition to the general stability conditions in a two-component medium, and compare the derived quantities of Ly alpha cloud both cases in galactic halo and in intergalactic medium. We conclude that the condition that cloud is stable against both evaporation and tidal destruction by a hot galactic halo is very severe. As a result, at z ~0.5, it is concluded that a pressure-confined, stable spherical Ly alpha cloud of typical column density NHI = 1014 cm-2 cannot survive in the galactic halo, although much higher column density clouds of NHI = 1017 cm-2 can do there. We discuss how our result constraints an alternative model for a Ly alpha cloud which associates with galaxy observed by Lanzetta et.al.

  6. Detecting the Disruption of Dark-Matter Halos with Stellar Streams.

    PubMed

    Bovy, Jo

    2016-03-25

    Narrow stellar streams in the Milky Way halo are uniquely sensitive to dark-matter subhalos, but many of these subhalos may be tidally disrupted. I calculate the interaction between stellar and dark-matter streams using analytical and N-body calculations, showing that disrupting objects can be detected as low-concentration subhalos. Through this effect, we can constrain the lumpiness of the halo as well as the orbit and present position of individual dark-matter streams. This will have profound implications for the formation of halos and for direct- and indirect-detection dark-matter searches. PMID:27058067

  7. Impact of galactic and intergalactic dust on the stellar EBL

    NASA Astrophysics Data System (ADS)

    Vavryčuk, V.

    2016-06-01

    Current theories assume that the low intensity of the stellar extragalactic background light (stellar EBL) is caused by finite age of the Universe because the finite-age factor limits the number of photons that have been pumped into the space by galaxies and thus the sky is dark in the night. We oppose this opinion and show that two main factors are responsible for the extremely low intensity of the observed stellar EBL. The first factor is a low mean surface brightness of galaxies, which causes a low luminosity density in the local Universe. The second factor is light extinction due to absorption by galactic and intergalactic dust. Dust produces a partial opacity of galaxies and of the Universe. The galactic opacity reduces the intensity of light from more distant background galaxies obscured by foreground galaxies. The inclination-averaged values of the effective extinction AV for light passing through a galaxy is about 0.2 mag. This causes that distant background galaxies become apparently faint and do not contribute to the EBL significantly. In addition, light of distant galaxies is dimmed due to absorption by intergalactic dust. Even a minute intergalactic opacity of 1 × 10^{-2} mag per Gpc is high enough to produce significant effects on the EBL. As a consequence, the EBL is comparable with or lower than the mean surface brightness of galaxies. Comparing both extinction effects, the impact of the intergalactic opacity on the EBL is more significant than the obscuration of distant galaxies by partially opaque foreground galaxies by factor of 10 or more. The absorbed starlight heats up the galactic and intergalactic dust and is further re-radiated at IR, FIR and micro-wave spectrum. Assuming static infinite universe with no galactic or intergalactic dust, the stellar EBL should be as high as the surface brightness of stars. However, if dust is considered, the predicted stellar EBL is about 290 nW m^{-2} sr^{-1}, which is only 5 times higher than the observed

  8. STELLAR POPULATIONS IN THE OUTER HALO OF THE MASSIVE ELLIPTICAL M49

    SciTech Connect

    Mihos, J. Christopher; Harding, Paul; Rudick, Craig S.; Feldmeier, John J. E-mail: paul.harding@case.edu E-mail: jjfeldmeier@ysu.edu

    2013-02-20

    We use deep surface photometry of the giant elliptical M49 (NGC 4472), obtained as part of our survey for diffuse light in the Virgo Cluster, to study the stellar populations in its outer halo. Our data trace M49's stellar halo out to {approx}100 kpc (7r{sub e}), where we find that the shallow color gradient seen in the inner regions becomes dramatically steeper. The outer regions of the galaxy are quite blue (B - V {approx} 0.7); if this is purely a metallicity effect, it argues for extremely metal-poor stellar populations with [Fe/H] < -1. We also find that the extended accretion shells around M49 are distinctly redder than the galaxy's surrounding halo, suggesting that we are likely witnessing the buildup of both the stellar mass and metallicity in M49's outer halo due to late time accretion. While such growth of galaxy halos is predicted by models of hierarchical accretion, this growth is thought to be driven by more massive accretion events which have correspondingly higher mean metallicity than inferred for M49's halo. Thus the extremely metal-poor nature of M49's extended halo provides some tension against current models for elliptical galaxy formation.

  9. BROKEN AND UNBROKEN: THE MILKY WAY AND M31 STELLAR HALOS

    SciTech Connect

    Deason, A. J.; Belokurov, V.; Evans, N. W.; Johnston, K. V.

    2013-02-15

    We use the Bullock and Johnston suite of simulations to study the density profiles of L*-type galaxy stellar halos. Observations of the Milky Way and M31 stellar halos show contrasting results: the Milky Way has a 'broken' profile, where the density falls off more rapidly beyond {approx}25 kpc, while M31 has a smooth profile out to 100 kpc with no obvious break. Simulated stellar halos, built solely by the accretion of dwarf galaxies, also exhibit this behavior: some halos have breaks, while others do not. The presence or absence of a break in the stellar halo profile can be related to the accretion history of the galaxy. We find that a break radius is strongly related to the buildup of stars at apocenters. We relate these findings to observations, and find that the 'break' in the Milky Way density profile is likely associated with a relatively early ({approx}6-9 Gyr ago) and massive accretion event. In contrast, the absence of a break in the M31 stellar halo profile suggests that its accreted satellites have a wide range of apocenters. Hence, it is likely that M31 has had a much more prolonged accretion history than the Milky Way.

  10. Clump formation through colliding stellar winds in the Galactic Center

    NASA Astrophysics Data System (ADS)

    Calderón, Diego

    2016-07-01

    The gas cloud G2 is currently being tidally disrupted by the Galactic Center super-massive black hole, Sgr A*. The region around the black hole is populated by ˜30 Wolf-Rayet stars, which produce strong outflows. Following an analytical approach, we explore the possibility that gas clumps, such as G2, originate from the collision of identical stellar winds via the Non-Linear Thin Shell Instability. We have found that the collision of relatively slow (<750 km s^{-1}) and strong (˜10^{-5} M_{⊙} yr^{-1}) stellar winds from stars at short separations (<2000 AU) is a process that indeed could produce clumps of G2's mass and above. Such short separation encounters of single stars along their orbits are not common in the Galactic Centre, however close binaries, such as IRS 16SW, are promising clump sources (see Calderón et al. 2016). We also present the first results of 2D models of colliding wind systems using the hydrodynamics adaptive mesh refinement code RAMSES, aiming to obtain a clump mass function, and the rate of clump formation and ejection to the ISM. We study the effect of parameters such as wind properties, stellar separation and orbital motion, in order to understand how likely the formation of G2 is in this context.

  11. Stellar haloes of simulated Milky-Way-like galaxies: chemical and kinematic properties

    NASA Astrophysics Data System (ADS)

    Tissera, Patricia B.; Scannapieco, Cecilia; Beers, Timothy C.; Carollo, Daniela

    2013-07-01

    We investigate the chemical and kinematic properties of the diffuse stellar haloes of six simulated Milky-Way-like galaxies from the Aquarius Project. Binding energy criteria are adopted to define two dynamically distinct stellar populations: the diffuse inner and outer haloes, which comprise different stellar subpopulations with particular chemical and kinematic characteristics. Our simulated inner- and outer-halo stellar populations have received contributions from debris stars (formed in subgalactic systems while they were outside the virial radius of the main progenitor galaxies) and endo-debris stars (those formed in gas-rich subgalactic systems inside the dark matter haloes of the main progenitor galaxy). The inner haloes possess an additional contribution from disc-heated stars, in the range ˜3-30 per cent, with a mean of ˜20 per cent. Disc-heated stars might exhibit signatures of kinematical support, in particular among the youngest ones. Endo-debris plus disc-heated stars define the so-called in situ stellar populations. In both the inner- and outer-halo stellar populations, we detect contributions from stars with moderate to low [α/Fe] ratios, mainly associated with the endo-debris or disc-heated subpopulations. The observed abundance gradients in the inner-halo regions are influenced by both the level of chemical enrichment and the relative contributions from each stellar subpopulation. Steeper abundance gradients in the inner-halo regions are related to contributions from the disc-heated and endo-debris stars, which tend to be found at lower binding energies than debris stars. In the case of the outer-halo regions, although [Fe/H] gradients are relatively mild, the steeper profiles arise primarily due to contributions from stars formed in more massive satellites, which sink farther into the main halo system, and tend to have higher levels of chemical enrichment and lower energies. Our findings support the existence of (at least) two distinct diffuse

  12. Highly-Ionized Gas in the Galactic Halo: A FUSE Survey of O 6 Absorption toward 22 Halo Stars

    NASA Astrophysics Data System (ADS)

    Zsargo, J.; Sembach, K. R.; Howk, J. C.; Savage, B. D.

    2002-12-01

    Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactic halo stars are studied to determine the amount of O 6 in the Galactic halo between ~0.3 and ~10 kpc from the Galactic mid-plane. Strong O 6 λ 1031.93 absorption was detected toward 21 stars, and a reliable 3 σ upper limit was obtained toward HD 97991. The weaker member of the O 6 doublet at 1037.62 Å could be studied toward only six stars. The observed columns are reasonably consistent with a patchy exponential O 6 distribution with a mid-plane density of 1.7x10-8 cm-3 and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in O 6 absorption along the Galactic sight lines, which indicates the general absence of high velocity O 6 within 2-5 kpc of the Galactic mid-plane. The correlation between the H 1 and O 6 intermediate velocity absorption is also poor. The O 6 velocity dispersions are much larger than the value of ~18 km/s expected from thermal broadening for gas at T ~ 3x105 K, the temperature at which O 6 is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the O 6 profiles. Kinematical comparisons of O 6 with Ar 1 reveal that 9 of 21 sight lines are closely aligned in LSR velocity (|Δ VLSR| <=5 km/s ), while 8 of 21 exhibit significant velocity differences (|Δ VLSR| >= 15 km/s ). This dual behavior may indicate the presence of two different types of O 6-bearing environments toward the Galactic sight lines. Comparison of O 6 with other highly-ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow, and that turbulent mixing also has a significant contribution. The role of turbulent mixing is most important toward sight lines that sample supernova remnants like Loop I and IV. We are also able to show that the O 6 enhancement toward the Galactic center region that was observed in the FUSE

  13. GHOSTS | Bulges, Halos, and the Resolved Stellar Outskirts of Massive Disk Galaxies

    NASA Astrophysics Data System (ADS)

    de Jong, R. S.; Radburn-Smith, D. J.; Sick, J. N.

    2008-10-01

    In hierarchical galaxy formation the stellar halos of galaxies are formed by the accretion of minor satellites and therefore contain valuable information about the (early) assembly process of galaxies. Our GHOSTS survey measures the stellar envelope properties of 14 nearby disk galaxies by imaging their resolved stellar populations with HST/ACS and WFPC2. Most of the massive galaxies in the sample (V_{rot}>200 km s^{-1}) have very extended stellar envelopes with μ(r) ˜ r^{-2.5} power law profiles in the outer regions. For these massive galaxies there is some evidence that the stellar surface density of the profiles correlates with Hubble type and bulge-to-disk ratio, begging the question whether these envelopes are more related to bulges than to a Milky Way-type stellar halo. Smaller galaxies (V_{rot}˜100 km s^{-1}) have much smaller stellar envelopes, but depending on geometry, they could still be more luminous than expected from satellite remnants in hierarchical galaxy formation models. Alternatively, they could be created by disk heating through the bombardment of small dark matter sub-halos. We find that galaxies show varying amounts of halo substructure.

  14. Studies of the Hot Gas in the Galactic halo and Local Bubble

    NASA Technical Reports Server (NTRS)

    Shelton, Robin L.

    2003-01-01

    This paper presents a report on the progress made on Studies of the Hot Gas in the Galactic halo and Local Bubble at Johns Hopkins University. The broad goals of this project are to determine the physical conditions and history of the hot phase of the Galaxy's interstellar medium. Such gas resides in the Galactic halo, the Local Bubble surrounding the solar neighborhood, other bubbles, and supernova remnants. A better understanding of the hot gas and the processes occurring within it requires several types of work, including ultraviolet and X-ray data analyses and computer modeling.

  15. Wormhole solutions for f(G) gravity in galactic halo region

    NASA Astrophysics Data System (ADS)

    Sharif, M.; Ismat Fatima, H.

    2016-04-01

    In this paper, we study static spherically symmetric wormhole solutions in galactic halo region. Two observational results, Navarro-Frenk-White energy density profile in standard cosmological model and the observed flat rotational curves, are used to discuss traversable wormholes supported by galactic halo in modified Gauss-Bonnet gravity. We explore these solutions either by considering a viable f(G) model to construct shape function or by specifying the shape function to deduce f(G) model. We explore energy conditions and find physically acceptable wormhole solutions threaded by normal matter for all values of r. Finally, we investigate stability of the resulting wormhole solutions.

  16. Exploring the Hot Galactic Halo Using Shadows of High Latitude Clouds

    NASA Technical Reports Server (NTRS)

    Juda, M.; Petre, Robert (Technical Monitor)

    2001-01-01

    The objective of this proposal was to measure variations in the 1/4 keV emission from the galactic halo, using ROSAT (x-ray astronomy satellite) Position Sensitive Proportional Counters (PSPC) observations toward known enhancements in the absorbing column density along the line-of-sight out of the Galaxy. Target directions were selected to have a low total hydrogen column density but to also show significant gradients in the amount of absorbing material, as traced by Infrared Astronomical Satellite (IRAS) 100 micron emission, on angular scales that would be contained within the PSPC field of view. In addition, we restricted the galactic latitude of the target directions to be greater than 60 degrees or less than -60 degrees in order to enable a cleaner separation of Galactic halo emission from that of the Galactic disk. The observations would also provide a measurement of the brightness of the emission from the Local Bubble.

  17. Numerical simulation of the plasma and gravitational dynamics of a galactic magnetic monopole halo

    NASA Astrophysics Data System (ADS)

    Farouki, R.; Shapiro, S. L.; Wasserman, I.

    1984-09-01

    Numerical and analytical calculations designed to assess the viability of a Galactic magnetic monopole halo are presented. The dynamical interplay between magnetic currents and the Galactic magnetic field is treated self-consistently, taking full account of the temporal evolution of currents and fields. It is found that in general self-consistent bounds on the Galactic magnetic monopole flux based on the survival of the Galactic magnetic field are far less restrictive than the Parker limit. The halo models are consistent with monopole masses m(M) approximately equal to or less than 7 x 10 to the 19th GeV, and monopole fluxes in the range F(M) approximately equal to or greater than 3 x 10 to the -13th per sq cm per s. Topics for future, more detailed studies are enumerated and discussed.

  18. Contributions to the Galactic halo from in-situ, kicked-out, and accreted stars

    NASA Astrophysics Data System (ADS)

    Sheffield, Allyson A.; Johnston, Kathryn V.; Cunha, Katia; Smith, Verne V.; Majewski, Steven R.

    2016-08-01

    We report chemical abundances for a sample of 66 M giants with high S/N high-resolution spectroscopy in the inner halo of the Milky Way. The program giant stars have radial velocities that vary significantly from those expected for stars moving on uniform circular orbits in the Galactic disk. Thus, based on kinematics, we expect a sample dominated by halo stars. Abundances are derived for α-elements and neutron capture elements. By analyzing the multi-dimensional abundance space, the formation site of the halo giants - in-situ or accreted - can be assessed. Of particular interest are a class of stars that form in-situ, deep in the Milky Way's gravitational potential well, but are ``kicked out'' of the disk into the halo due to a perturbation event. We find: (1) our sample is dominated by accreted stars and (2) tentative evidence of a small kicked-out population in our Milky Way halo sample.

  19. Subhalo statistics of galactic haloes: beyond the resolution limit

    NASA Astrophysics Data System (ADS)

    Cautun, Marius; Hellwing, Wojciech A.; van de Weygaert, Rien; Frenk, Carlos S.; Jones, Bernard J. T.; Sawala, Till

    2014-12-01

    We study the substructure population of Milky Way (MW)-mass haloes in the Λ cold dark matter (ΛCDM) cosmology using a novel procedure to extrapolate subhalo number statistics beyond the resolution limit of N-body simulations. The technique recovers the mean and the variance of the subhalo abundance, but not its spatial distribution. It extends the dynamic range over which precise statistical predictions can be made by the equivalent of performing a simulation with 50 times higher resolution, at no additional computational cost. We apply this technique to MW-mass haloes, but it can easily be applied to haloes of any mass. We find up to 20 per cent more substructures in MW-mass haloes than found in previous studies. Our analysis lowers the mass of the MW halo required to accommodate the observation that the MW has only three satellites with a maximum circular velocity Vmax ≥ 30 km s- 1 in the ΛCDM cosmology. The probability of having a subhalo population similar to that in the MW is 20 per cent for a virial mass, M200 = 1 × 1012 M⊙ and practically zero for haloes more massive than M200 = 2 × 1012 M⊙.

  20. Systematic problems with using dark matter simulations to model stellar halos

    SciTech Connect

    Bailin, Jeremy; Bell, Eric F.; Valluri, Monica; Stinson, Greg S.; Debattista, Victor P.; Couchman, H. M. P.; Wadsley, James

    2014-03-10

    The limits of available computing power have forced models for the structure of stellar halos to adopt one or both of the following simplifying assumptions: (1) stellar mass can be 'painted' onto dark matter (DM) particles in progenitor satellites; (2) pure DM simulations that do not form a luminous galaxy can be used. We estimate the magnitude of the systematic errors introduced by these assumptions using a controlled set of stellar halo models where we independently vary whether we look at star particles or painted DM particles, and whether we use a simulation in which a baryonic disk galaxy forms or a matching pure DM simulation that does not form a baryonic disk. We find that the 'painting' simplification reduces the halo concentration and internal structure, predominantly because painted DM particles have different kinematics from star particles even when both are buried deep in the potential well of the satellite. The simplification of using pure DM simulations reduces the concentration further, but increases the internal structure, and results in a more prolate stellar halo. These differences can be a factor of 1.5-7 in concentration (as measured by the half-mass radius) and 2-7 in internal density structure. Given this level of systematic uncertainty, one should be wary of overinterpreting differences between observations and the current generation of stellar halo models based on DM-only simulations when such differences are less than an order of magnitude.

  1. The Origin of the Hot Gas in the Galactic Halo: Testing Galactic Fountain Models' X-Ray Emission

    NASA Astrophysics Data System (ADS)

    Henley, David B.; Shelton, Robin L.; Kwak, Kyujin; Hill, Alex S.; Mac Low, Mordecai-Mark

    2015-02-01

    We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find that taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to the halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission.

  2. CLUMPY STREAMS FROM CLUMPY HALOS: DETECTING MISSING SATELLITES WITH COLD STELLAR STRUCTURES

    SciTech Connect

    Yoon, Joo Heon; Johnston, Kathryn V.; Hogg, David W.

    2011-04-10

    Dynamically cold stellar streams are ideal probes of the gravitational field of the Milky Way. This paper re-examines the question of how such streams might be used to test for the presence of {sup m}issing satellites{sup -}the many thousands of dark-matter subhalos with masses 10{sup 5}-10{sup 7} M{sub sun} which are seen to orbit within Galactic-scale dark-matter halos in simulations of structure formation in {Lambda}CDM cosmologies. Analytical estimates of the frequency and energy scales of stream encounters indicate that these missing satellites should have a negligible effect on hot debris structures, such as the tails from the Sagittarius dwarf galaxy. However, long cold streams, such as the structure known as GD1 or those from the globular cluster Palomar 5 (Pal 5), are expected to suffer many tens of direct impacts from missing satellites during their lifetimes. Numerical experiments confirm that these impacts create gaps in the debris' orbital energy distribution, which will evolve into degree- and sub-degree-scale fluctuations in surface density over the age of the debris. Maps of Pal 5's own stream contain surface density fluctuations on these scales. The presence and frequency of these inhomogeneities suggests the existence of a population of missing satellites in numbers predicted in the standard {Lambda}CDM cosmologies.

  3. Clump formation through colliding stellar winds in the Galactic Centre

    NASA Astrophysics Data System (ADS)

    Calderón, D.; Ballone, A.; Cuadra, J.; Schartmann, M.; Burkert, A.; Gillessen, S.

    2016-02-01

    The gas cloud G2 is currently being tidally disrupted by the Galactic Centre supermassive black hole, Sgr A*. The region around the black hole is populated by ˜30 Wolf-Rayet stars, which produce strong outflows. We explore the possibility that gas clumps, such as G2, originate from the collision of stellar winds via the non-linear thin shell instability. Following an analytical approach, we study the thermal evolution of slabs formed in the symmetric collision of winds, evaluating whether instabilities occur, and estimating possible clump masses. We find that the collision of relatively slow (≲750 km s-1) and strong (˜10-5 M⊙ yr-1) stellar winds from stars at short separations (<10 mpc) is a process that indeed could produce clumps of G2's mass and above. Such short separation encounters of single stars along their known orbits are not common in the Galactic Centre, making this process a possible but unlikely origin for G2. We also discuss clump formation in close binaries such as IRS 16SW and in asymmetric encounters as promising alternatives that deserve further numerical study.

  4. The Abundance of Deuterium in the Warm Neutral Medium of the Lower Galactic Halo

    NASA Astrophysics Data System (ADS)

    Savage, Blair D.; Lehner, Nicolas; Fox, Andrew; Wakker, Bart; Sembach, Kenneth

    2007-04-01

    We use high-resolution ultraviolet spectra to obtain Milky Way interstellar column densities of H I, D I, O I, S II, Fe II, and P II toward the QSO HE 0226-4110 in the Galactic direction l=253.4deg and b=-65.77deg. We obtain D/H=21+8-6 ppm from an analysis of the strong damped Lyα line of H I and the weak higher Lyman series absorption of D I. Correcting for a small amount of foreground contamination from D and H in the Local Bubble we obtain D/H=22+8-6 for the warm neutral medium of the lower Galactic halo. The medium sampled has [O/H]=0.12+0.41-0.20 and [Fe/H]=-1.01+0.10-0.09. This suggests the abundances in the gas in the halo toward HE 0226-4110 are not affected by the infall of low-metallicity gas and that the gas originates in the disk and is elevated into the halo by energetic processes that erode but do not totally destroy the dust grains. We compare our result to measured values of D/H in other astrophysical sites. The value we measure in the halo gas is consistent with the hypothesis that for many Galactic disk lines of sight D is incorporated into dust. The high average value of D/H=23.1+/-2.4(1 σ) ppm measured along five sight lines through disk gas in the solar neighborhood is similar to D/H in the lower Galactic halo. These disk and halo observations imply the abundance of deuterium in the Galaxy has only been reduced by a factor of 1.12+/-0.13 since its formation. In contrast, current galactic chemical evolution models predict the astration reduction factor should be in the range from 1.39 to 1.83.

  5. Pathways to massive black holes and compact star clusters in pre-galactic dark matter haloes with virial temperatures >~10000K

    NASA Astrophysics Data System (ADS)

    Regan, John A.; Haehnelt, Martin G.

    2009-06-01

    Large dynamic range numerical simulations of atomic cooling driven collapse of gas in pre-galactic dark matter haloes with Tvir ~ 10000 K show that the gas loses 90 per cent and more of its angular momentum before rotational support sets in. In a fraction of these haloes where the metallicity is low and ultraviolet (UV) radiation suppresses H2 cooling, conditions are thus very favourable for the rapid build-up of massive black holes. Depending on the progression of metal enrichment, the continued suppression of H2 cooling by external and internal UV radiation and the ability to trap the entropy produced by the release of gravitational energy, the gas at the centre of the halo is expected to form a supermassive star, a stellar-mass black hole accreting at super-Eddington accretion rates or a compact star-cluster undergoing collisional run-away of massive stars at its centre. In all three cases, a massive black hole of initially modest mass finds itself at the centre of a rapid inflow of gas with inflow rates of >~1Msolaryr-1. The massive black hole will thus grow quickly to a mass of 105- 106Msolar until further inflow is halted either by consumption of gas by star formation or by the increasing energy and momentum feedback from the growing massive black hole. Conditions for the formation of massive seed black holes in this way are most favourable in haloes with Tvir ~ 15000K and Vvir ~ 20 km s-1 with less massive haloes not allowing collapse of gas by atomic cooling and more massive haloes being more prone to fragmentation. This should imprint a characteristic mass on the mass spectrum of an early population of massive black hole seeds in pre-galactic haloes which will later grow into the observed population of supermassive black holes in galactic bulges.

  6. Assembly history of subhalo populations in galactic and cluster sized dark haloes

    NASA Astrophysics Data System (ADS)

    Xie, Lizhi; Gao, Liang

    2015-12-01

    We make use of two suits of ultrahigh resolution N-body simulations of individual dark matter haloes from the Phoenix and the Aquarius Projects to investigate systematics of assembly history of subhaloes in dark matter haloes differing by a factor of 1000 in the halo mass. We have found that real progenitors which built up present-day subhalo population are relatively more abundant for high-mass haloes, in contrast to previous studies claiming a universal form independent of the host halo mass. That is mainly because of repeated counting of the `re-accreted' (progenitors passed through and were later re-accreted to the host more than once) and inclusion of the `ejected' progenitor population (progenitors were accreted to the host in the past but no longer members at present day) in previous studies. The typical accretion time for all progenitors vary strongly with the host halo mass, which is typical about z ˜ 5 for the galactic Aquarius and about z ˜ 3 for the cluster sized Phoenix haloes. Once these progenitors start to orbit their parent haloes, they rapidly lose their original mass but not their identifiers, more than 55 (50) per cent of them survive to present day for the Phoenix (Aquarius) haloes. At given redshift, survival fraction of the accreted subhalo is independent of the parent halo mass, whilst the mass-loss of the subhalo is more efficient in high-mass haloes. These systematics results in similarity and difference in the subhalo population in dark matter haloes of different masses at present day.

  7. Recent Results from the SPLASH Survey: Chemical Abundances and Kinematics of Andromeda's Stellar Halo

    NASA Astrophysics Data System (ADS)

    Gilbert, Karoline

    2015-08-01

    Large scale surveys of Andromeda's resolved stellar populations have revolutionized our view of this galaxy over the past decade. The combination of large-scale, contiguous photometric surveys and pointed spectroscopic surveys has been particularly powerful for discovering and following up new substructures and disentangling the structural components of Andromeda. The SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) survey consists of broad- and narrow-band imaging and spectroscopy of red giant branch stars in lines of sight throughout the M31 system, ranging in distance from 3 kpc to more than 200 kpc from Andromeda's center. I will present recent results from the SPLASH survey on the structure of Andromeda's stellar halo and the origin of tidal debris features, including measurements of the kinematics and chemical abundances of Andromeda's halo stars.

  8. GLOBAL PROPERTIES OF M31'S STELLAR HALO FROM THE SPLASH SURVEY. I. SURFACE BRIGHTNESS PROFILE

    SciTech Connect

    Gilbert, Karoline M.; Guhathakurta, Puragra; Bullock, James; Tollerud, Erik J.; Geha, Marla C.; Kalirai, Jason S.; Kirby, Evan N.; Tanaka, Mikito; Chiba, Masashi

    2012-11-20

    We present the surface brightness profile of M31's stellar halo out to a projected radius of 175 kpc. The surface brightness estimates are based on confirmed samples of M31 red giant branch stars derived from Keck/DEIMOS spectroscopic observations. A set of empirical spectroscopic and photometric M31 membership diagnostics is used to identify and reject foreground and background contaminants. This enables us to trace the stellar halo of M31 to larger projected distances and fainter surface brightnesses than previous photometric studies. The surface brightness profile of M31's halo follows a power law with index -2.2 {+-} 0.2 and extends to a projected distance of at least {approx}175 kpc ({approx}2/3 of M31's virial radius), with no evidence of a downward break at large radii. The best-fit elliptical isophotes have b/a = 0.94 with the major axis of the halo aligned along the minor axis of M31's disk, consistent with a prolate halo, although the data are also consistent with M31's halo having spherical symmetry. The fact that tidal debris features are kinematically cold is used to identify substructure in the spectroscopic fields out to projected radii of 90 kpc and investigate the effect of this substructure on the surface brightness profile. The scatter in the surface brightness profile is reduced when kinematically identified tidal debris features in M31 are statistically subtracted; the remaining profile indicates that a comparatively diffuse stellar component to M31's stellar halo exists to large distances. Beyond 90 kpc, kinematically cold tidal debris features cannot be identified due to small number statistics; nevertheless, the significant field-to-field variation in surface brightness beyond 90 kpc suggests that the outermost region of M31's halo is also comprised to a significant degree of stars stripped from accreted objects.

  9. GHOSTS: the age and structure of stellar halos around nearby disk galaxies

    NASA Astrophysics Data System (ADS)

    de Jong, Roelof S.; Bailin, Jeremy; Bell, Eric; Holwerda, Benne Willem; Monachesi, Antonela; Streich, David

    2015-08-01

    The stars in the halos around galaxies are thought to be for a significant fraction the result of tidally disrupted infalling smaller galaxies, an important process in the mass build up of galaxies. However, in recent year the realization has grown that a fraction of halo stars may have formed in situ or in the disk of the host galaxy.To understand the nature of stellar halos, the GHOSTS project has used the Hubble Space Telescope to observe the resolved stellar populations in the outskirts of 18 nearby massive galaxies, with a range of masses and seen from face-on to edge-on. From two filter observations (F814W and F606W) with both WFC3 and ACS, we map the different giant populations in these haloes: typically Asymptotic, Red, and He-burning Giants (AGB, RGB and HeB). By studying the distribution of these stellar populations, the GHOSTS project aims to characterize on a statistical basis the size, shape, age, amount of sub-structure, and chemical composition of stellar halos, thereby allowing us to constrain models of the hierarchical galaxy formation process.In this presentation we will in particular show the structural parameters of the observed stellar halos out to 70 kpc and present the rather small range in color gradients present in the RGB population indicative in small metallicity gradients in these halos. We also present the discovery of a surprisingly large population of AGB stars with ages <2 Gyr seen to ~20 kpc above the disks of many edge-on galaxies and speculations on the origin of these young populations at a location where no current star formation is seen.

  10. Benchmark stars for cross-calibration of Galactic stellar surveys

    NASA Astrophysics Data System (ADS)

    Heiter, U.; Jofré, P.; Gustafsson, B.; Thévenin, F.; Korn, A.; Soubiran, C.; Blanco-Cuaresma, S.

    2014-07-01

    Various Galactic stellar spectroscopic surveys are currently underway, and each is expected to achieve high internal accuracy in terms of stellar parameters and abundances. A number of questions related to the formation and evolution of the Galaxy may be addressed based on samples of stars observed within each survey. In addition, complementary samples of stars may be constructed by combining data from different surveys. The Gaia FGK Benchmark Stars provide the necessary link to bring the quantities measured from different spectra with different methods onto the same scale. We selected 28 FGK stars and 5 M giants with available angular diameter θ, bolometric flux Fbol, and distance. We used the fundamental relation Teff ∝ F/θ0.5 to determine a reference effective temperature. We used the fundamental relation g ∝ M/R2, where M is the mass and R the radius, to determine a reference surface gravity. A homogeneous analysis of a high resolution and high signal-to-noise spectral library provides a reference metallicity (Blanco-Cuaresma et al. 2014A&A...566A..98B, Jofré et al. 2014A&A...564A.133J). These stars serve as a reference for testing and homogenizing large stellar surveys such as the Gaia mission and the Gaia-ESO public spectroscopic survey and for improving models of FGK-star atmospheres. A detailed discussion of the fundamental Teff and logg values will be presented in Heiter et al. (to be submitted).

  11. The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo: High-Velocity Clouds (HVCs) Originated in the Galactic Fountain

    NASA Astrophysics Data System (ADS)

    Kwak, Kyujin; Shelton, Robin L.; Raley, Elizabeth A.

    2009-07-01

    In the Galactic fountain scenario, supernovae and/or stellar winds propel material into the Galactic halo. As the material cools, it condenses into clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we model and study the dynamical evolution of these gas clouds after they form and begin to fall toward the Galactic plane. In our simulations, we assume that the gas clouds form at a height of z = 5 kpc above the Galactic midplane, then begin to fall from rest. We investigate how the cloud's evolution, dynamics, and interaction with the interstellar medium (ISM) are affected by the initial mass of the cloud. We find that clouds with sufficiently large initial densities (n >= 0.1 H atoms cm-3) accelerate sufficiently and maintain sufficiently large column densities as to be observed and identified as high-velocity clouds (HVCs) even if the ISM is weakly magnetized (1.3 μG). However, the ISM can provide noticeable resistance to the motion of a low-density cloud (n <= 0.01 H atoms cm-3) thus making it more probable that a low-density cloud will attain the speed of an intermediate-velocity cloud rather than the speed of an HVC. We also investigate the effects of various possible magnetic field configurations. As expected, the ISM's resistance is greatest when the magnetic field is strong and perpendicular to the motion of the cloud. The trajectory of the cloud is guided by the magnetic field lines in cases where the magnetic field is oriented diagonal to the Galactic plane. The model cloud simulations show that the interactions between the cloud and the ISM can be understood via analogy to the shock tube problem which involves shock and rarefaction waves. We also discuss accelerated ambient gas, streamers of material ablated from the clouds, and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped object.

  12. Lyman-Alpha Absorption at Low Redshifts and Hot Gas in Galactic Haloes

    NASA Astrophysics Data System (ADS)

    Mo, H. J.

    1994-08-01

    Motivated by the recent observation of Lanzetta et al. that most luminous galaxies at low redshifts produce Lyα absorptions at impact parameter l <~ 160 h^-1^ kpc, we propose that these absorbers are clouds confined by the pressure of ambient hot gas in galactic haloes. We determine the properties of this hot gas and of the absorption systems on the basis of observational and theoretical constraints. The absorbing clouds need to be replenished on a time-scale of about one orbital time (~10^9^ yr) in the galactic halo. The pressure and temperature of the gas at radius r~100 kpc are P = (10-100) cm^-3^ K and T = 10^5.5-6.5^ K. The model requires that about 10 per cent of the gas in low-redshift galactic haloes be in the hot phase. Such gas in galactic haloes emits X-rays with a bolometric luminosity of the order of 10^37-40^1 erg s^-1^. The likelihood that such gas exists in current models of galaxy formation is discussed.

  13. IC 1257: A New Globular Cluster in the Galactic Halo

    NASA Technical Reports Server (NTRS)

    Harris, W. E.; Phelps, R. L.; Madore, B. F.; Pevunova, O.; Skiff, B. A.; Crute, C.; Wilson, B.

    1996-01-01

    New CCD photometry of the faint, compact star cluster IC 1257 (L = 17? = +/- 15?obtained with the Palomar 5m telescope, reveals that it is a highly reddened globular cluster well beyond the Galactic center.

  14. The stability of stellar discs in Milky Way-sized dark matter haloes

    NASA Astrophysics Data System (ADS)

    Yurin, Denis; Springel, Volker

    2015-09-01

    We employ an improved methodology to insert live stellar discs into high-resolution dark matter simulations of Milky Way-sized haloes, allowing us to investigate the fate of thin stellar discs in the tumultuous environment of cold dark matter structures. We study a set of eight different haloes, drawn from the Aquarius simulation project, in which stellar discs are adiabatically grown with a prescribed structure, and then allowed to self-consistently evolve. The initial velocity distribution is set-up in very good equilibrium with the help of the GALIC code. We find that the residual triaxiality of the haloes leads to significant disc tumbling, qualitatively confirming earlier work. We show that the disc turning motion is unaffected by structural properties of the galaxies such as the presence or absence of a bulge or bar. In typical Milky Way-sized dark matter haloes, we expect an average turning of the discs by about 40°between z = 1 and 0, over the course of 7.6 Gyr. We also investigate the impact of the discs on substructures, and conversely, the disc heating rate caused by the dark matter halo substructures. The presence of discs reduces the central subhalo abundance by a about a factor of 2, due to an increased evaporation rate by gravitational shocks from disc passages. We find that substructures are important for heating the outer parts of stellar discs but do not appear to significantly affect their inner parts.

  15. Prospects for detecting supersymmetric dark matter in the Galactic halo.

    PubMed

    Springel, V; White, S D M; Frenk, C S; Navarro, J F; Jenkins, A; Vogelsberger, M; Wang, J; Ludlow, A; Helmi, A

    2008-11-01

    Dark matter is the dominant form of matter in the Universe, but its nature is unknown. It is plausibly an elementary particle, perhaps the lightest supersymmetric partner of known particle species. In this case, annihilation of dark matter in the halo of the Milky Way should produce gamma-rays at a level that may soon be observable. Previous work has argued that the annihilation signal will be dominated by emission from very small clumps (perhaps smaller even than the Earth), which would be most easily detected where they cluster together in the dark matter haloes of dwarf satellite galaxies. Here we report that such small-scale structure will, in fact, have a negligible impact on dark matter detectability. Rather, the dominant and probably most easily detectable signal will be produced by diffuse dark matter in the main halo of the Milky Way. If the main halo is strongly detected, then small dark matter clumps should also be visible, but may well contain no stars, thereby confirming a key prediction of the cold dark matter model. PMID:18987737

  16. The Longest Stellar Stream in M31's Halo

    NASA Astrophysics Data System (ADS)

    Fardal, Mark A.; PAndAS Collaboration

    2016-01-01

    We present updated data and dynamical modeling of several tidal features in the M31 halo. We focus on the NW Stream, a nearly radial feature extending outwards from M31. Using new distance estimates fromevolved stars and embedded globular clusters, we find this stream extends to a greater distance from its host than any other known tidal stream. We update the stream velocity profile with new measures from resolved stars and globular clusters. We use Bayesian dynamical modeling to discover the stream's implications for M31's halo mass and the life history of its progenitor galaxy.

  17. Galactic flows and the formation of stellar clusters

    NASA Astrophysics Data System (ADS)

    Smilgys, Romas; Bonnell, Ian

    2015-08-01

    We investigate the formation of stellar clusters from a Galactic scale SPH simulation. The simulation traces star formation over a 5 Myr timescale, with local gravitational instabilities resulting in ˜ 105 solar masses of star formation in the form of sink particles. The large scale flow dominates the compression from low densities before self-gravity takes over in higher density regions. We investigate the time evolution of the physical properties of the forming clusters including their half-mass radii, their energies and the depletion time of the gas.We show that the more massive clusters (up to ˜ 2 × 104 solar masses) gather their material from of order 10 pc due to the large scale motions associated with the spiral arm passage and shock. The bulk of the gas becomes gravitationally bound near 1-2 Myr before sink formation, and in the absence of feedback, significant accretion ongoing on longer timescales.

  18. The effect of active galactic nuclei feedback on the halo mass function

    NASA Astrophysics Data System (ADS)

    Cui, Weiguang; Borgani, Stefano; Murante, Giuseppe

    2014-06-01

    We investigate baryon effects on the halo mass function (HMF), with emphasis on the role played by active galactic nuclei (AGN) feedback. Haloes are identified with both friends-of-friends (FoF) and spherical overdensity (SO) algorithms. We embed the standard SO algorithm into a memory-controlled frame program and present the Python spherIcAl Overdensity code - PIAO (Chinese character: ). For both FoF and SO haloes, the effect of AGN feedback is that of suppressing the HMFs to a level even below that of dark matter (DM) simulations. The ratio between the HMFs in the AGN and in the DM simulations is ˜0.8 at overdensity Δc = 500, a difference that increases at higher overdensity Δc = 2500, with no significant redshift and mass dependence. A decrease of the halo masses ratio with respect to the DM case induces the decrease of the HMF in the AGN simulation. The shallower inner density profiles of haloes in the AGN simulation witnesses that mass reduction is induced by the sudden displacement of gas induced by thermal AGN feedback. We provide fitting functions to describe halo mass variations at different overdensities, which can recover the HMFs with a residual random scatter ≲5 per cent for halo masses larger than 1013 h-1 M⊙.

  19. The Segue K giant survey. II. A catalog of distance determinations for the Segue K giants in the galactic halo

    SciTech Connect

    Xue, Xiang-Xiang; Rix, Hans-Walter; Ma, Zhibo; Morrison, Heather L.; Harding, Paul; Beers, Timothy C.; Ivans, Inese I.; Jacobson, Heather R.; Johnson, Jennifer; Lee, Young Sun; Lucatello, Sara; Rockosi, Constance M.; Yanny, Brian; Zhao, Gang; Allende Prieto, Carlos

    2014-04-01

    We present an online catalog of distance determinations for 6036 K giants, most of which are members of the Milky Way's stellar halo. Their medium-resolution spectra from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration are used to derive metallicities and rough gravity estimates, along with radial velocities. Distance moduli are derived from a comparison of each star's apparent magnitude with the absolute magnitude of empirically calibrated color-luminosity fiducials, at the observed (g – r){sub 0} color and spectroscopic [Fe/H]. We employ a probabilistic approach that makes it straightforward to properly propagate the errors in metallicities, magnitudes, and colors into distance uncertainties. We also fold in prior information about the giant-branch luminosity function and the different metallicity distributions of the SEGUE K-giant targeting sub-categories. We show that the metallicity prior plays a small role in the distance estimates, but that neglecting the luminosity prior could lead to a systematic distance modulus bias of up to 0.25 mag, compared to the case of using the luminosity prior. We find a median distance precision of 16%, with distance estimates most precise for the least metal-poor stars near the tip of the red giant branch. The precision and accuracy of our distance estimates are validated with observations of globular and open clusters. The stars in our catalog are up to 125 kpc from the Galactic center, with 283 stars beyond 50 kpc, forming the largest available spectroscopic sample of distant tracers in the Galactic halo.

  20. Looking for building blocks of the Galactic halo: variable stars in the Fornax, Bootes I, Canes Venatici II dwarfs and in NGC 2419

    NASA Astrophysics Data System (ADS)

    Greco, Claudia; Clementini, Gisella; Held, E. V.; Poretti, E.; Catelan, M.; Federici, L.; Maio, M.; Gullieuszik, M.; Ripepi, V.; Dall'Ora, M.; Di Fabrizio, L.; Kinemuchi, K.; Di Crescienzo, M.; Marconi, M.; Musella, I.; Pritzl, B.; Rest, A.; De Lee, N.; Smith, H.

    2010-01-01

    Λ cold-dark-matter hierarchical models of galaxy formation suggest that the halo of the Milky Way (MW) has been assembled, at least in part, through accretion of protogalactic fragments partially resembling the present-day dwarf spheroidal (dSph) satellites of the MW. Investigation of the stellar populations of the MW's globular clusters (GCs) and dSph companions can thus provide excellent tests to infer the dominant Galaxy-formation scenario, whether merger/accretion or cloud collapse. Pulsating variable stars offer a very powerful tool in this context, since variables of different types allow tracing the different stellar generations in a galaxy and to reconstruct the galaxy's star-formation history and assembly back to the first epochs of galaxy formation. In particular, the RR Lyrae stars, belonging to the old population (t > 10 Gyr), witnessed the epoch of halo formation, and thus hold a crucial role to identify the MW satellites that may have contributed to build up the Galactic halo. In the MW, most GCs with an RR Lyrae population sharply divide into two distinct groups (Oosterhoff types I and II) based on the mean periods and relative proportion of fundamental-mode (RRab) and first-overtone (RRc) RR Lyrae stars. On the other hand, the Galactic-halo field RR Lyrae stars show a dominance of Oosterhoff I properties. Here, we investigate the Oosterhoff properties of a number of different stellar systems, starting from relatively undisturbed dwarf galaxies (the Fornax dSph and its globular clusters), through distorted and tidally disrupting ones (the Bootes and Canes Venatici II dSphs), to possible final relics of the disruption process (the Galactic globular cluster NGC 2419). We are addressing the crucial question of whether the RR Lyrae pulsation properties in these systems conform to the Oosterhoff dichotomy characterizing the MW variables. If they do not, the Galaxy's halo cannot have been assembled by dSph-like protogalactic fragments resembling the

  1. Tracing the stellar halo of an early type galaxy out to 25 effective radii

    NASA Astrophysics Data System (ADS)

    Rejkuba, Marina

    2016-08-01

    We have used ACS and WFC3 cameras on board HST to resolve stars in the halo of NGC 5128 out to 140 kpc (25 effective radii, R eff) along the major axis and 70 kpc (13 R eff) along the minor axis. This dataset provides an unprecedented radial coverage of stellar halo properties in any galaxy. Color-magnitude diagrams clearly reveal the presence of the red giant branch stars belonging to the halo of NGC 5128 even in the most distant fields. The V-I colors of the red giants enable us to measure the metallicity distribution in each field and so map the metallicity gradient over the sampled area. The stellar metallicity follows a shallow gradient and even out at 140 kpc (25 R eff) its median value does not go below [M/H]~-1 dex. We observe significant field-to-field metallicity and stellar density variations. The star counts are higher along the major axis when compared to minor axis field located 90 kpc from the galaxy centre, indicating flattening in the outer halo. These observational results provide new important constraints for the assembly history of the halo and the formation of this gE galaxy.

  2. Stellar Feedback: A Multiphase Interstellar Medium and Galactic Outflows

    NASA Astrophysics Data System (ADS)

    Ceverino, D.

    2009-12-01

    I am presenting new results in our ongoing effort of improving the theory of galaxy formation in a ΛCDM Universe. I pay a special attention to the role of supernova explosions and stellar winds in the galaxy assembly. These processes happen at very small scales, they affect the interstellar medium (ISM) at galactic scales and regulate the formation of a whole galaxy. Previous attempts of mimicking these effects in simulations of galaxy formation use very simplified assumptions. I develop a much more realistic prescription for modeling the feedback, which minimizes any ad hoc sub-grid physics. I start with developing high resolution models of the ISM and formulate the conditions required for its realistic functionality: formation of multi-phase medium with hot chimneys, super-bubbles, cold molecular phase, and very slow consumption of gas. Once these effects are resolved in cosmological simulations, galaxy formation proceeds more realistically. For example, I do not have the overcooling problem. The angular momentum problem (resulting in a too massive bulge) is also reduced substantially: the rotation curves are nearly flat. The galaxy formation also becomes more violent. At high redshift, I routinely find substantial gas outflows from star-forming galaxies. I describe several scaling relations between outflow properties and galaxy properties: maximum velocity, mass and kinetic energy versus stellar mass and SFR. The simulations reproduce this picture only if the resolution is very high: better than 70 pc.

  3. A NEW DELIVERY ROUTE TO GALACTIC NUCLEI: WARM HALO CLOUD IMPACTS

    SciTech Connect

    McKernan, Barry; Ford, K. E. Saavik; Maller, Ariyeh

    2010-08-01

    We propose a new mechanism for the delivery of gas to the heart of galactic nuclei. We show that warm halo clouds (WHCs) must periodically impact galactic centers and potentially deliver a large ({approx}10{sup 4}-10{sup 6} M{sub sun}) mass of gas to the galactic nucleus in a singular event. The impact of an accreting WHC originating far in the galactic halo can, depending on mixing, produce a nuclear starburst of low-metallicity stars as well as low-luminosity accretion onto the central black hole. Based on multiphase cooling around a {Lambda}CDM distribution of halos, we calculate the nuclear impact rate, the mass captured by the central black hole, and the fraction of active nuclei for impacting cloud masses in the range 10{sup 4}-10{sup 6} M{sub sun}. If there is moderate braking during cloud infall, our model predicts an average fraction of low-luminosity active nuclei consistent with observations.

  4. Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc

    NASA Astrophysics Data System (ADS)

    Deason, A. J.; Belokurov, V.; Koposov, S. E.; Rockosi, C. M.

    2014-05-01

    We use A-type stars selected from Sloan Digital Sky Survey data release 9 photometry to measure the outer slope of the Milky Way stellar halo density profile beyond 50 kpc. A likelihood-based analysis is employed that models the ugr photometry distribution of blue horizontal branch and blue straggler stars. In the magnitude range 18.5 < g < 20.5, these stellar populations span a heliocentric distance range of: 10 <~ D BS/kpc <~ 75, 40 <~ D BHB/kpc <~ 100. Contributions from contaminants, such as QSOs, and the effect of photometric uncertainties, are also included in our modeling procedure. We find evidence for a very steep outer halo profile, with power-law index α ~ 6 beyond Galactocentric radii r = 50 kpc, and even steeper slopes favored (α ~ 6-10) at larger radii. This result holds true when stars belonging to known overdensities, such as the Sagittarius stream, are included or excluded. We show that, by comparison to numerical simulations, stellar halos with shallower slopes at large distances tend to have more recent accretion activity. Thus, it is likely that the Milky Way has undergone a relatively quiet accretion history over the past several gigayears. Our measurement of the outer stellar halo profile may have important implications for dynamical mass models of the Milky Way, where the tracer density profile is strongly degenerate with total mass estimates.

  5. Touching the void: A striking drop in stellar halo density beyond 50 kpc

    SciTech Connect

    Deason, A. J.; Rockosi, C. M.; Belokurov, V.; Koposov, S. E.

    2014-05-20

    We use A-type stars selected from Sloan Digital Sky Survey data release 9 photometry to measure the outer slope of the Milky Way stellar halo density profile beyond 50 kpc. A likelihood-based analysis is employed that models the ugr photometry distribution of blue horizontal branch and blue straggler stars. In the magnitude range 18.5 < g < 20.5, these stellar populations span a heliocentric distance range of: 10 ≲ D {sub BS}/kpc ≲ 75, 40 ≲ D {sub BHB}/kpc ≲ 100. Contributions from contaminants, such as QSOs, and the effect of photometric uncertainties, are also included in our modeling procedure. We find evidence for a very steep outer halo profile, with power-law index α ∼ 6 beyond Galactocentric radii r = 50 kpc, and even steeper slopes favored (α ∼ 6-10) at larger radii. This result holds true when stars belonging to known overdensities, such as the Sagittarius stream, are included or excluded. We show that, by comparison to numerical simulations, stellar halos with shallower slopes at large distances tend to have more recent accretion activity. Thus, it is likely that the Milky Way has undergone a relatively quiet accretion history over the past several gigayears. Our measurement of the outer stellar halo profile may have important implications for dynamical mass models of the Milky Way, where the tracer density profile is strongly degenerate with total mass estimates.

  6. The early gaseous and stellar mass assembly of Milky Way-type galaxy halos

    NASA Astrophysics Data System (ADS)

    Hensler, Gerhard; Petrov, Mykola

    2016-08-01

    How the Milky Way has accumulated its mass over the Hubble time, whether significant amounts of gas and stars were accreted from satellite galaxies, or whether the Milky Way has experienced an initial gas assembly and then evolved more-or-less in isolation is one of the burning questions in modern astronomy, because it has consequences for our understanding of galaxy formation in the cosmological context. Here we present the evolutionary model of a Milky Way-type satellite system zoomed into a cosmological large-scale simulation. Embedded into Dark Matter halos and allowing for baryonic processes these chemo-dynamical simulations aim at studying the gas and stellar loss from the satellites to feed the Milky Way halo and the stellar chemical abundances in the halo and the satellite galaxies.

  7. THE DARK MATTER HALO CONCENTRATION AND STELLAR INITIAL MASS FUNCTION OF A CASSOWARY GROUP

    SciTech Connect

    Deason, A. J.; Auger, M. W.; Belokurov, V.; Evans, N. W.

    2013-08-10

    We exploit the group environment of the CAmbridge Sloan Survey Of Wide ARcs in the skY z = 0.3 lens J2158+0257 to measure the group dynamical mass as a complement to the central dynamical and lensing mass constraints. Follow-up spectroscopy of candidate group members is performed using VLT/FORS2. From the resulting N = 21 confirmed members, we measure the group dynamical mass by calibrating an analytic tracer mass estimator with cosmological simulations. The luminosity-weighted line-of-sight velocity dispersion and the Einstein radius of the lens are used as mass probes in the inner regions of the galaxy. Combining these three observational probes allows us to independently constrain the mass and concentration of the dark matter halo, in addition to the total stellar mass of the central galaxy. We find a dark matter halo in remarkably good agreement with simulations (log{sub 10} M{sub 200}/M{sub Sun} = 14.2 {+-} 0.2, c{sub 200}= 4.4{sup +1.6}{sub -1.4}) and a stellar mass-to-light ratio which favors a Salpeter initial mass function ((M/L)* = 5.7 {+-} 1.2). Our measurement of a normal halo concentration suggests that there is no discrepancy between simulations and observations at the group mass scale. This is in contrast to the cluster mass scale for which a number of studies have claimed over-concentrated halos. While the halo mass is robustly determined, and the halo concentration is not significantly affected by systematics, the resulting stellar mass-to-light ratio is sensitive to the choice of stellar parameters, such as density profile and velocity anisotropy.

  8. Anisotropies at Ultra High Energies and the Galactic Halo

    NASA Astrophysics Data System (ADS)

    Wolfendale, Arnold

    1999-08-01

    A measure of consistency is appearing in measurements of the anisotropy of arrival directions above 1017 eV and these show a Galactic Plane Enhancement and a S-N excess, to about 3.1018 eV. The implication is that Galactic particles predominate here. At higher energies, where an Extragalactic origin is preferred, a contender for the `sources' is exotic dark matter particles. However, an analysis of the anisotropy at the highest energies, or, rather, the lack of it, makes this interpretation highly unlikely. Instead, `bottom-up' acceleration, in galaxy-systems is preferred; the likely mass mixture of the primaries, above 1018 eV, helps to explain why strong clustering of arrival is not observed.

  9. Centaurus A: Stellar Metallicity Transition in the Halo

    NASA Astrophysics Data System (ADS)

    Bird, Sarah; Flynn, C.; Harris, W. E.; Valtonen, M.

    2013-01-01

    The very earliest stars in giant galaxies - the most metal-poor halo stars and globular clusters - may have formed before the onset of hierarchical merging, within small pregalactic dwarfs that populated the large-scale dark-matter potential well. Today, these relic stars should be found in a sparse and extremely extended “outermost-halo” component. Finding clear traces of this component in other giant galaxies, and deconvolving it from the more obvious and metal-rich spheroid component generated later by mergers, has been extraordinarily difficult. Now, striking new evidence discovered in M 31 and NGC 3379 suggests that the metal-poor outermost halo can be isolated at very large radii, R > 12Reff . We now have a new deep imaging study with ESO VLT of the nearest giant elliptical and merger remnant, Centaurus A, to search for this extended remnant of the galaxy’s earliest history.

  10. X-ray detection of warm ionized matter in the Galactic halo

    NASA Astrophysics Data System (ADS)

    Nicastro, F.; Senatore, F.; Gupta, A.; Guainazzi, M.; Mathur, S.; Krongold, Y.; Elvis, M.; Piro, L.

    2016-03-01

    We report on a systematic investigation of the cold and mildly ionized gaseous baryonic metal components of our Galaxy, through the analysis of high-resolution Chandra and XMM-Newton spectra of two samples of Galactic and extragalactic sources. The comparison between lines of sight towards sources located in the disc of our Galaxy and extragalactic sources allows us for the first time to clearly distinguish between gaseous metal components in the disc and halo of our Galaxy. We find that a warm ionized metal medium (WIMM) permeates a large volume above and below the Galaxy's disc, perhaps up to the circum-galactic space. This halo WIMM imprints virtually the totality of the O I and O II absorption seen in the spectra of our extragalactic targets, has a temperature of T_{WIMM}^{Halo}=2900 ± 900 K, a density < n_H > _{WIMM}^{Halo} = 0.023 ± 0.009 cm-3 and a metallicity Z_{WIMM}^{Halo} = (0.4 ± 0.1) Z⊙. Consistently with previous works, we also confirm that the disc of the Galaxy contains at least two distinct gaseous metal components, one cold and neutral (the CNMM: cold neutral metal medium) and one warm and mildly ionized, with the same temperature of the halo WIMM, but higher density (< n_H > _{WIMM}^{Disc} = 0.09 ± 0.03 cm-3) and metallicity (Z_{WIMM}^{Disc} = 0.8 ± 0.1 Z⊙). By adopting a simple disc+sphere geometry for the Galaxy, we estimate masses of the CNMM and the total (disc + halo) WIMM of MCNMM ≲ 8 × 108 M⊙ and MWIMM ≃ 8.2 × 109 M⊙.

  11. The dark matter haloes of moderate luminosity X-ray AGN as determined from weak gravitational lensing and host stellar masses

    NASA Astrophysics Data System (ADS)

    Leauthaud, Alexie; J. Benson, Andrew; Civano, Francesca; L. Coil, Alison; Bundy, Kevin; Massey, Richard; Schramm, Malte; Schulze, Andreas; Capak, Peter; Elvis, Martin; Kulier, Andrea; Rhodes, Jason

    2015-01-01

    Understanding the relationship between galaxies hosting active galactic nuclei (AGN) and the dark matter haloes in which they reside is key to constraining how black hole fuelling is triggered and regulated. Previous efforts have relied on simple halo mass estimates inferred from clustering, weak gravitational lensing, or halo occupation distribution modelling. In practice, these approaches remain uncertain because AGN, no matter how they are identified, potentially live a wide range of halo masses with an occupation function whose general shape and normalization are poorly known. In this work, we show that better constraints can be achieved through a rigorous comparison of the clustering, lensing, and cross-correlation signals of AGN hosts to the fiducial stellar-to-halo mass relation (SHMR) derived for all galaxies, irrespective of nuclear activity. Our technique exploits the fact that the global SHMR can be measured with much higher accuracy than any statistic derived from AGN samples alone. Using 382 moderate luminosity X-ray AGN at z < 1 from the COSMOS field, we report the first measurements of weak gravitational lensing from an X-ray-selected sample. Comparing this signal to predictions from the global SHMR, we find that, contrary to previous results, most X-ray AGN do not live in medium size groups - nearly half reside in relatively low mass haloes with M200b ˜ 1012.5 M⊙. The AGN occupation function is well described by the same form derived for all galaxies but with a lower normalization - the fraction of haloes with AGN in our sample is a few per cent. The number of AGN satellite galaxies scales as a power law with host halo mass with a power-law index α = 1. By highlighting the relatively `normal' way in which moderate luminosity X-ray AGN hosts occupy haloes, our results suggest that the environmental signature of distinct fuelling modes for luminous quasars compared to moderate luminosity X-ray AGN is less obvious than previously claimed.

  12. The Resolved Stellar Halo and Dwarf Satellite Population of NGC 3109

    NASA Astrophysics Data System (ADS)

    Hargis, Jonathan R.; Crnojevic, Denija; Sand, David J.; Willman, Beth; Spekkens, Kristine; Grillmair, Carl J.; Strader, Jay

    2016-01-01

    The stellar halo and halo substructure of dwarf galaxies provides an important window into both LCDM cosmology and galaxy formation theory on the smallest scales. We are undertaking a deep, wide-field imaging survey of nearby, isolated sub-Milky Way mass galaxies in order to (1) map the substructure, spatial extent, and metallicity of their stellar halos in resolved stars, and (2) search for faint dwarf satellite companions (i.e., the ``dwarfs of dwarfs"). These studies will allow us to explore the role of in-situ versus accretion processes in forming stellar halos in dwarfs, as well as constrain the faint end of the satellite galaxy luminosity function. This work presents a preliminary analysis of the pilot galaxy in our survey: NGC 3109 (Mv = -15 mag), a nearby (d = 1.3 Mpc) dwarf irregular, approximately 1/6th the stellar mass of the SMC. We imaged ~40 sq. deg around NGC 3109 (projected radius of ~100 kpc) using CTIO 4m/DECam to depths ~2 mag below the TRGB. We disovered a new gas-rich dwarf satellite of NGC 3109, dubbed Antlia B (Mv = -9.7 mag), similar to the recently-discovered Leo P. We also discovered five candidate dwarf satellites, with sizes (~100 pc) and luminosities (Mv ~ -6 mag) consistent with being ultra-faint dwarfs at the distance of NGC 3109. Lastly, we present stellar halo maps of resolved RGB stars on both large and small scales. We discuss the various substructures found in these maps and the future directions of our survey. This work was supported by NSF AST-1151462.

  13. Group Finding in the Stellar Halo Using Photometric Surveys: Current Sensitivity and Future Prospects

    NASA Astrophysics Data System (ADS)

    Sharma, Sanjib; Johnston, Kathryn V.; Majewski, Steven R.; Bullock, James; Muñoz, Ricardo R.

    2011-02-01

    The Sloan Digital Sky Survey (SDSS) and the Two Micron All Sky Survey (2MASS) provided the first deep and global photometric catalogs of stars in our halo and not only clearly mapped its structure but also demonstrated the ubiquity of substructure within it. Future surveys promise to push such catalogs to ever increasing depths and larger numbers of stars. This paper examines what can be learned from current and future photometric databases using group-finding techniques. We compare groups recovered from a sample of M-giants from 2MASS with those found in synthetic surveys of simulated ΛCDM stellar halos that were built entirely from satellite accretion events and demonstrate broad consistency between the properties of the two sets. We also find that these recovered groups are likely to represent the majority of high-luminosity (L > 5 × 106 L sun) satellites accreted within the last 10 Gyr and on orbits with apocenters within 100 kpc. However, the sensitivity of the M-giant survey to accretion events that were either ancient from low-luminosity objects or those on radial orbits is limited because of the low number of stars, bias toward high-metallicity stars, and the shallow depth (distance explored only out to 100 kpc from the Sun). We examine the extent to which these limitations are addressed by current and future surveys, in particular catalogs of main-sequence turnoff (MSTO) stars from SDSS and the Large Synoptic Survey Telescope (LSST), and of RR Lyrae stars from LSST or PanSTARRS. The MSTO and RR Lyrae surveys are more sensitive to low-luminosity events (L ~ 105 L sun or less) than the 2MASS M-giant sample. Additionally, RR Lyrae surveys, with superior depth, are also good at detecting events on highly eccentric orbits whose debris tends to lie beyond 100 kpc. When combined we expect these photometric surveys to provide a comprehensive picture of the last 10 Gyr of Galactic accretion. Events older than this are too phase mixed to be discovered. Pushing

  14. On planetary nebulae as sources of carbon dust: Infrared emission from planetary nebulae of the galactic halo

    NASA Technical Reports Server (NTRS)

    Dinerstein, Harriet L.; Lester, Daniel F.

    1990-01-01

    Planetary nebulae of the galactic disk are generally seen to emit a thermal continuum due to dust grains heated by stellar and nebular photons. This continuum typically peaks between 25 and 60 micron m, so that the total power emitted by the dust is sampled well by the broad-band measurements made by IRAS. Researchers examine here the characteristics of the infrared emission from the four planetary nebulae which are believed on the basis of their low overall metallicities to belong to the halo population. These nebulae are of particular interest because they are the most metal-poor ionized nebulae known in our Galaxy, and offer the opportunity to probe possible dependences of the dust properties on nebular composition. Researchers present fluxes extracted from co-addition of the IRAS data, as well as ground-based near infrared measurements. Each of the four halo objects, including the planetary nebula in the globular cluster M15, is detected in at least one infrared band. Researchers compare the estimated infrared excesses of these nebulae (IRE, the ratio of measured infrared power to the power available in the form of resonantly-trapped Lyman alpha photons) to those of disk planetary nebulae with similar densities but more normal abundances. Three of the halo planetaries have IRE values similar to those of the disk nebulae, despite the fact that their Fe- and Si-peak gas phase abundances are factors of 10 to 100 lower. However, these halo nebulae have normal or elevated C/H ratios, due to nuclear processing and mixing in their red giant progenitors. Unlike the other halo planetaries, DDDM1 is deficient in carbon as well as in the other light metals. This nebula has a substantially lower IRE than the other halo planetaries, and may be truly dust efficient. Researchers suggest that the deficiency is due to a lack of the raw material for producing carbon-based grains, and that the main bulk constituent of the dust in these planetary nebulae is carbon.

  15. Soft X-Ray Observations of the Galactic Halo

    NASA Technical Reports Server (NTRS)

    Shelton, Robin; Kuntz, K. D.

    2003-01-01

    In this project, my co-I (K.D. Kuntz) and I plan to extract the soft X-ray spectrum emitted by the hot gas along a high latitude line of sight. We plan to subtract off the local component (garnered from other observations) in order to isolate the halo component. We then plan to combine this spectral information with the ultraviolet resonance line emission produced by slightly cooler gas along the line of sight and use the two observations as a constraint on models. My co-I, K.D., Kuntz has been working on the determination of the instrumental background. I have not yet drawn any of the funds for this project. I have just moved from J h s Hopkins University to the University of Georgia and anticipate finishing the project while at the University of Georgia.

  16. The Stellar Population Structure of the Galactic Disk

    NASA Astrophysics Data System (ADS)

    Bovy, Jo; Rix, Hans-Walter; Schlafly, Edward F.; Nidever, David L.; Holtzman, Jon A.; Shetrone, Matthew; Beers, Timothy C.

    2016-05-01

    The spatial structure of stellar populations with different chemical abundances in the Milky Way (MW) contains a wealth of information on Galactic evolution over cosmic time. We use data on 14,699 red-clump stars from the APOGEE survey, covering 4 {kpc}≲ R≲ 15 {kpc}, to determine the structure of mono-abundance populations (MAPs)—stars in narrow bins in [α /{Fe}] and [{Fe}/{{H}}]—accounting for the complex effects of the APOGEE selection function and the spatially variable dust obscuration. We determine that all MAPs with enhanced [α /{Fe}] are centrally concentrated and are well-described as exponentials with a scale length of 2.2+/- 0.2 {kpc} over the whole radial range of the disk. We discover that the surface-density profiles of low-[α /{Fe}] MAPs are complex: they do not monotonically decrease outwards, but rather display a peak radius ranging from ≈ 5 to ≈ 13 {kpc} at low [{Fe}/{{H}}]. The extensive radial coverage of the data allows us to measure radial trends in the thickness of each MAP. While high-[α /{Fe}] MAPs have constant scale heights, low-[α /{Fe}] MAPs flare. We confirm, now with high-precision abundances, previous results that each MAP contains only a single vertical scale height and that low-[{Fe}/{{H}}], low-[α /{Fe}] and high-[{Fe}/{{H}}], high-[α /{Fe}] MAPs have intermediate ({h}Z≈ 300{--}600 {pc}) scale heights that smoothly bridge the traditional thin- and thick-disk divide. That the high-[α /{Fe}], thick disk components do not flare is strong evidence against their thickness being caused by radial migration. The correspondence between the radial structure and chemical-enrichment age of stellar populations is clear confirmation of the inside-out growth of galactic disks. The details of these relations will constrain the variety of physical conditions under which stars form throughout the MW disk.

  17. Measuring the Stellar Halo Velocity Anisotropy With 3D Kinematics

    NASA Astrophysics Data System (ADS)

    Cunningham, Emily C.; Deason, Alis J.; Guhathakurta, Puragra; Rockosi, Constance M.; van der Marel, Roeland P.; Sohn, S. Tony

    2016-08-01

    We present the first measurement of the anisotropy parameter β using 3D kinematic information outside of the solar neighborhood. Our sample consists of 13 Milky Way halo stars with measured proper motions and radial velocities in the line of sight of M31. Proper motions were measured using deep, multi-epoch HST imaging, and radial velocities were measured from Keck II/DEIMOS spectra. We measure β = -0.3-0.9 +0.4, which is consistent with isotropy, and inconsistent with measurements in the solar neighborhood. We suggest that this may be the kinematic signature of a relatively early, massive accretion event, or perhaps several such events.

  18. Constraints on baryonic dark matter in the Galactic halo and Local Group

    NASA Technical Reports Server (NTRS)

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

    1992-01-01

    A four-color method and deep CCD data are used to search for very faint metal-poor stars in the direction of the south Galactic pole. The results make it possible to limit the contribution of ordinary old, metal-poor stars to the dynamical halo of the Galaxy or to the Local Group. The ratio of the mass of the halo to its ordinary starlight must be more than about 2000, unless the halo is very small. For the Local Group, this ratio is greater than about 400. If this local dark matter is baryonic, the process of compact-object formation must produce very few 'impurities' in the form of stars similar to those found in globular clusters. The expected number of unbound stars with MV not greater than 6 within 100 pc of the sun is less than 1 based on the present 90-percent upper limit to the Local Group starlight.

  19. Small-scale clumps in the Galactic halo

    SciTech Connect

    Berezinsky, V. S.; Dokuchaev, V. I. Eroshenko, Yu. N.

    2010-01-15

    A mass function of small-scale dark matter clumps is calculated. We take into account the tidal destruction of clumps at early stages of structure formation starting from a time of clump detachment from the Universe expansion. Only a small fraction of these clumps, {approx}0.1%, in each logarithmic mass interval {Delta} log M {approx} 1 survives the stage of hierarchical clustering. We calculate the probability of surviving of the remnants of dark matter clumps in the Galaxy by modelling the tidal destruction of the small-scale clumps by disk and stars. It is demonstrated that a substantial fraction of clump remnants may survive through the tidal destruction during the lifetime of the Galaxy if a radius of core is rather small. The resulting mass spectrum of survived clumps is extended down to the mass of the core of the cosmologically produced clumps with a minimal mass. The survived dense remnants of tidally destructed clumps provides a large contribution to the annihilation signal in the Galaxy. We describe the anisotropy of dark matter clump distribution caused by tidal destruction of clumps in the Galactic disk. A corresponding annihilation of dark matter particles in small-scale clumps produces the anisotropic gamma-ray signal with respect to the Galactic disk.

  20. Dynamics and X-ray emission of a galactic superwind interacting with disk and halo gas

    NASA Astrophysics Data System (ADS)

    Suchkov, Anatoly A.; Balsara, Dinshaw S.; Heckman, Timothy M.; Leitherer, Claus

    1994-08-01

    There is a general agreement that the conspicuous extranuclear X-ray, optical-line, and radio-contiuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However, despite the significant advances in observational studies of superwinds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the starburst superwind regions. To address this issue, we have conducted two-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to represent the ISM of the starburst galaxy, we analyze the interaction of the wind with a two-component disk-halo ambient interstellar medium and argue that this two-component representation is crucial for adequate modeling of starbursts. The emphasis of this study is on the geometry and structure of the wind region and the X-ray emission arising in the wind material and the shocked gas in the disk and the halo of the galaxy. The simulation results have shown that a clear-cut bipolar wind can easily develop under a range of very different conditions. On the other hand, a complex 'filamentary' structure associated with the entrained dense disk material is found to arise within the hot bubble blown out by the wind. The flow pattern within the bubble is dominated equally by the central biconic outflow and a system of whirling motions r elated to the origin and development of the 'filaments'. The filament parameters make them a good candidate for optical-emission-line filamentary gas observed in starburst halos. We find that the history of mass and energy deposition in the starburst region of the galaxy is crucial for wind dynamics. A 'mild' early wind, which arises as a result of the cumulative effect of stellar winds from massive stars, produces a bipolar vertical cavity in the disk and

  1. Dynamics and X-ray emission of a galactic superwind interacting with disk and halo gas

    NASA Technical Reports Server (NTRS)

    Suchkov, Anatoly A.; Balsara, Dinshaw S.; Heckman, Timothy M.; Leitherner, Claus

    1994-01-01

    There is a general agreement that the conspicuous extranuclear X-ray, optical-line, and radio-contiuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However, despite the significant advances in observational studies of superwinds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the starburst superwind regions. To address this issue, we have conducted two-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to represent the ISM of the starburst galaxy, we analyze the interaction of the wind with a two-component disk-halo ambient interstellar medium and argue that this two-component representation is crucial for adequate modeling of starbursts. The emphasis of this study is on the geometry and structure of the wind region and the X-ray emission arising in the wind material and the shocked gas in the disk and the halo of the galaxy. The simulation results have shown that a clear-cut bipolar wind can easily develop under a range of very different conditions. On the other hand, a complex 'filamentary' structure associated with the entrained dense disk material is found to arise within the hot bubble blown out by the wind. The flow pattern within the bubble is dominated equally by the central biconic outflow and a system of whirling motions r elated to the origin and development of the 'filaments'. The filament parameters make them a good candidate for optical-emission-line filamentary gas observed in starburst halos. We find that the history of mass and energy deposition in the starburst region of the galaxy is crucial for wind dynamics. A 'mild' early wind, which arises as a result of the cumulative effect of stellar winds from massive stars, produces a bipolar vertical cavity in the disk and

  2. Implications of Stellar Migration for the Properties of Galactic Disks

    NASA Astrophysics Data System (ADS)

    Roskar, Rok; Debattista, V. P.; Quinn, T. R.; Stinson, G. S.; Wadsley, J.

    2010-01-01

    Recent theoretical work suggests that it may be common for stars in the disks of spiral galaxies to migrate radially across significant distances. Such migrations are a result of spiral corotation resonance scattering and move the guiding centers of the stars while preserving the circularities of their orbits.Migration can therefore efficiently mix stars in all parts of the disk. Therefore, if migration does indeed occur in real disks, it requires that disks be thought of as fully inter-connected structures with a common history rather then a set of autonomous regions. In the extreme, radial migration allows the evolution of the innermost regions to contribute significantly to the outermost parts of the disk. I will discuss the results from a suite of idealized N-body/SPH simulations of disk formation and evolution, spanning a range in the parameter space of galaxy properties. I will focus on the insight we can gain from simulations when interpreting observational data of a full range of stellar systems, including the solar neighborhood, the thick and thin disks of the Galaxy, as well as external disks, in particular their outermost regions. I will demonstrate that radial migration needs to be considered in studies of galactic disk evolution, and discuss some of our recent attempts to do so with observational data from SDSS and HST.

  3. Does the dark-matter halo induce chaos in the galactic disk?

    NASA Astrophysics Data System (ADS)

    Weinberg, Martin D.

    2013-07-01

    A three-dimensional galaxy disk will not have regular orbits (tori) everywhere. Therefore, there will be irregular, chaotic regions in addition to regular regions in the disk's phase space. In addition, galaxy formation in the standard LCDM scenario suggests a transition between disk-dominated and halo-dominated gravity at several disk scale lengths. These simple observations raise the following questions: 1) are there generic features of a exponential stellar disk induced by breaking of invariant tori? 2) do the chaotic regions in disks enable significant structural evolution in a galaxy lifetime? 3) does the orbital structure of disks indirectly depend on the dark matter halo especially in the transition region between the disk and dark-matter halo? In this talk, I present early results in my attempt to answer these questions using a new developed numerical technique based the mathematical construction used to prove the KAM theorem.

  4. The galactic halo in mixed dark matter cosmologies

    SciTech Connect

    Anderhalden, D.; Diemand, J.; Schneider, A.; Bertone, G.; Macciò, A.V. E-mail: diemand@physik.uzh.ch E-mail: maccio@mpia.de

    2012-10-01

    A possible solution to the small scale problems of the cold dark matter (CDM) scenario is that the dark matter consists of two components, a cold and a warm one. We perform a set of high resolution simulations of the Milky Way halo varying the mass of the WDM particle (m{sub WDM}) and the cosmic dark matter mass fraction in the WDM component ( f-bar {sub W}). The scaling ansatz introduced in combined analysis of LHC and astroparticle searches postulates that the relative contribution of each dark matter component is the same locally as on average in the Universe (e.g. f{sub W,s}un = f-bar {sub W}). Here we find however, that the normalised local WDM fraction (f{sub W,s}un / f-bar {sub W}) depends strongly on m{sub WDM} for m{sub WDM} < 1 keV. Using the scaling ansatz can therefore introduce significant errors into the interpretation of dark matter searches. To correct this issue a simple formula that fits the local dark matter densities of each component is provided.

  5. Stellar orbital properties as diagnostics of the origin of the stellar halo

    NASA Astrophysics Data System (ADS)

    Valluri, Monica; Loebman, Sarah R.; Bailin, Jeremy; Clarke, Adam; Debattista, Victor P.; Stinson, Greg

    2016-08-01

    We examine metallicities, ages and orbital properties of halo stars in a Milky-Way like disk galaxy formed in the cosmological hydrodynamical MaGICC simulations. Halo stars were either accreted from satellites or they formed in situ in the disk or bulge of the galaxy and were then kicked up into the halo (``in situ/kicked-up'' stars). Regardless of where they formed both types show surprisingly similar orbital properties: the majority of both types are on short-axis tubes with the same sense of rotation as the disk - implying that a large fraction of satellites are accreted onto the halo with the same sense of angular momentum as the disk.

  6. STRUCTURE AND POPULATION OF THE ANDROMEDA STELLAR HALO FROM A SUBARU/SUPRIME-CAM SURVEY

    SciTech Connect

    Tanaka, Mikito; Chiba, Masashi; Komiyama, Yutaka; Iye, Masanori; Guhathakurta, Puragra

    2010-01-10

    We present a photometric survey of the stellar halo of the nearest giant spiral galaxy, Andromeda (M31), using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age. These are used to isolate and characterize different components of the M31 halo: (1) the giant southern stream (GSS); (2) several other substructures; and (3) the smooth halo. First, the GSS is characterized by a broad red giant branch (RGB) and a metal-rich/intermediate-age red clump (RC). The I magnitude of the well-defined tip of the RGB suggests that the distance to the observed GSS field is (m - M){sub 0} = 24.73 +- 0.11 (883 +- 45 kpc) at a projected radius of R approx 30 kpc from M31's center. The GSS shows a high metallicity peaked at [Fe/H]approx>-0.5 with a mean (median) of -0.7 (-0.6), estimated via comparison with theoretical isochrones. Combined with the luminosity of the RC, we estimate the mean age of its stellar population to be approx8 Gyr. The mass of its progenitor galaxy is likely in the range of 10{sup 7}-10{sup 9} M{sub sun}. Second, we study M31's halo substructure along the northwest/southeast minor axis out to R approx 100 kpc and the southwest major-axis region at R approx 60 kpc. We confirm two substructures in the southeast halo reported by Ibata et al. and discover two overdense substructures in the northwest halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin, so its outer halo has experienced at least this many accretion events involving dwarf satellites with mass 10{sup 7}-10{sup 9} M{sub sun

  7. INTERPLAY BETWEEN STELLAR SPIRALS AND THE INTERSTELLAR MEDIUM IN GALACTIC DISKS

    SciTech Connect

    Wada, Keiichi; Baba, Junichi; Saitoh, Takayuki R. E-mail: baba.junichi@nao.ac.jp

    2011-07-01

    We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a 'live' stellar disk. We focus especially on spiral structures excited in an isolated galactic disk without a stellar bar. Using high-resolution, three-dimensional N-body/smoothed particle hydrodynamic simulations, we found that the spiral features of the gas in galactic disks are formed by essentially different mechanisms from the galactic shock in stellar density waves. The stellar spiral arms and the interstellar matter on average corotate in a galactic potential at any radii. Unlike the stream motions in the galactic shock, the interstellar matter flows into the local potential minima with irregular motions. The flows converge to form dense gas clouds/filaments near the bottom of the stellar spirals, whose global structures resemble dust lanes seen in late-type spiral galaxies. The stellar arms are non-steady; they are wound and stretched by the galactic shear, and thus local densities of the arm change on a timescale of {approx}100 Myr, due to bifurcating or merging with other arms. This makes the gas spirals associated with the stellar arms non-steady. The association of dense gas clouds is eventually dissolved into inter-arm regions with non-circular motions. Star clusters are formed from the cold, dense gases, whose ages are less than {approx}30 Myr, and they are roughly associated with the background stellar arms without a clear spatial offset between gas spiral arms and distribution of young stars.

  8. Collision of the Smith Cloud and its dark matter halo with the magnetized Galactic disk

    NASA Astrophysics Data System (ADS)

    Galyardt, Jason; Shelton, Robin L.

    2015-01-01

    The Smith Cloud is a massive High Velocity Cloud (HVC) that may have passed through the Milky Way disk in the recent past. Previous studies using hydrodynamic simulations suggest that a dark matter halo may have provided the confinement neccessary for the Smith Cloud to survive passage through the Galactic corona and disk. However, the models of the Galaxy that were used in these studies did not include a magnetic field, while magnetic fields are known to have confining properties. Other studies have shown that the Galactic magnetic field can inhibit mass exchange between the corona and the disk due to magnetic field compression. We extend upon these studies via FLASH magnetohydrodynamic simulations to consider the effects of a Galactic magnetic field on an infalling, dark matter confined HVC.

  9. Advective and diffusive cosmic ray transport in galactic haloes

    NASA Astrophysics Data System (ADS)

    Heesen, Volker; Dettmar, Ralf-Jürgen; Krause, Marita; Beck, Rainer; Stein, Yelena

    2016-05-01

    We present 1D cosmic ray transport models, numerically solving equations of pure advection and diffusion for the electrons and calculating synchrotron emission spectra. We find that for exponential halo magnetic field distributions advection leads to approximately exponential radio continuum intensity profiles, whereas diffusion leads to profiles that can be better approximated by a Gaussian function. Accordingly, the vertical radio spectral profiles for advection are approximately linear, whereas for diffusion they are of `parabolic' shape. We compare our models with deep Australia Telescope Compact Array observations of two edge-on galaxies, NGC 7090 and 7462, at λλ 22 and 6 cm. Our result is that the cosmic ray transport in NGC 7090 is advection dominated with V=150^{+80}_{-30} km s^{-1}, and that the one in NGC 7462 is diffusion dominated with D=3.0± 1.0 × 10^{28}E_GeV^{0.5} cm^2 s^{-1}. NGC 7090 has both a thin and thick radio disc with respective magnetic field scale heights of hB1 = 0.8 ± 0.1 kpc and hB2 = 4.7 ± 1.0 kpc. NGC 7462 has only a thick radio disc with hB2 = 3.8 ± 1.0 kpc. In both galaxies, the magnetic field scale heights are significantly smaller than what estimates from energy equipartition would suggest. A non-negligible fraction of cosmic ray electrons can escape from NGC 7090, so that this galaxy is not an electron calorimeter.

  10. STELLAR MASS-GAP AS A PROBE OF HALO ASSEMBLY HISTORY AND CONCENTRATION: YOUTH HIDDEN AMONG OLD FOSSILS

    SciTech Connect

    Deason, A. J.; Conroy, C.; Wetzel, A. R.; Tinker, J. L.

    2013-11-10

    We investigate the use of the halo mass-gap statistic—defined as the logarithmic difference in mass between the host halo and its most massive satellite subhalo—as a probe of halo age and concentration. A cosmological N-body simulation is used to study N ∼ 25, 000 group/cluster-sized halos in the mass range 10{sup 12.5} < M{sub halo}/M{sub ☉} < 10{sup 14.5}. In agreement with previous work, we find that halo mass-gap is related to halo formation time and concentration. On average, older and more highly concentrated halos have larger halo mass-gaps, and this trend is stronger than the mass-concentration relation over a similar dynamic range. However, there is a large amount of scatter owing to the transitory nature of the satellite subhalo population, which limits the use of the halo mass-gap statistic on an object-by-object basis. For example, we find that 20% of very large halo mass-gap systems (akin to {sup f}ossil groups{sup )} are young and have likely experienced a recent merger between a massive satellite subhalo and the central subhalo. We relate halo mass-gap to the observable stellar mass-gap via abundance matching. Using a galaxy group catalog constructed from the Sloan Digital Sky Survey Data Release 7, we find that the star formation and structural properties of galaxies at fixed mass show no trend with stellar mass-gap. This is despite a variation in halo age of ≈2.5 Gyr over ≈1.2 dex in stellar mass-gap. Thus, we find no evidence to suggest that the halo formation history significantly affects galaxy properties.

  11. KINEMATICS OF THE STELLAR HALO AND THE MASS DISTRIBUTION OF THE MILKY WAY USING BLUE HORIZONTAL BRANCH STARS

    SciTech Connect

    Kafle, Prajwal R.; Sharma, Sanjib; Lewis, Geraint F.; Bland-Hawthorn, Joss

    2012-12-20

    Here, we present a kinematic study of the Galactic halo out to a radius of {approx}60 kpc, using 4664 blue horizontal branch stars selected from the SDSS/SEGUE survey to determine key dynamical properties. Using a maximum likelihood analysis, we determine the velocity dispersion profiles in spherical coordinates ({sigma}{sub r}, {sigma}{sub {theta}}, {sigma}{sub {phi}}) and the anisotropy profile ({beta}). The radial velocity dispersion profile ({sigma}{sub r}) is measured out to a galactocentric radius of r {approx} 60 kpc, but due to the lack of proper-motion information, {sigma}{sub {theta}}, {sigma}{sub {phi}}, and {beta} could only be derived directly out to r {approx} 25 kpc. From a starting value of {beta} Almost-Equal-To 0.5 in the inner parts (9 < r/kpc < 12), the profile falls sharply in the range r Almost-Equal-To 13-18 kpc, with a minimum value of {beta} = -1.2 at r = 17 kpc, rising sharply at larger radius. In the outer parts, in the range 25 < r/kpc < 56, we predict the profile to be roughly constant with a value of {beta} Almost-Equal-To 0.5. The newly discovered kinematic anomalies are shown not to arise from halo substructures. We also studied the anisotropy profile of simulated stellar halos formed purely by accretion and found that they cannot reproduce the sharp dip seen in the data. From the Jeans equation, we compute the stellar rotation curve (v{sub circ}) of the Galaxy out to r {approx} 25 kpc. The mass of the Galaxy within r {approx}< 25 kpc is determined to be 2.1 Multiplication-Sign 10{sup 11} M{sub Sun }, and with a three-component fit to v{sub circ}(r), we determine the virial mass of the Milky Way dark matter halo to be M{sub vir} = 0.9{sup +0.4}{sub -0.3} Multiplication-Sign 10{sup 12} M{sub Sun} (R{sub vir} = 249{sup +34}{sub -31} kpc).

  12. Origin of strong magnetic fields in Milky Way-like galactic haloes

    NASA Astrophysics Data System (ADS)

    Beck, A. M.; Lesch, H.; Dolag, K.; Kotarba, H.; Geng, A.; Stasyszyn, F. A.

    2012-05-01

    An analytical model predicting the growth rates, the absolute growth times and the saturation values of the magnetic field strength within galactic haloes is presented. The analytical results are compared to cosmological magnetohydrodynamics (MHD) simulations of Milky Way-like galactic halo formation performed with the N-body/SPMHD code GADGET. The halo has a mass of ≈3 × 1012 M⊙ and a virial radius of ≈270 kpc. The simulations in a Λ cold dark matter (ΛCDM) cosmology also include radiative cooling, star formation, supernova feedback and the description of non-ideal MHD. A primordial magnetic seed field ranging from 10-10 to 10-34 G in strength agglomerates together with the gas within filaments and protohaloes. There, it is amplified within a couple of hundred million years up to equipartition with the corresponding turbulent energy. The magnetic field strength increases by turbulent small-scale dynamo action. The turbulence is generated by the gravitational collapse and by supernova feedback. Subsequently, a series of halo mergers leads to shock waves and amplification processes magnetizing the surrounding gas within a few billion years. At first, the magnetic energy grows on small scales and then self-organizes to larger scales. Magnetic field strengths of ≈10-6 G are reached in the centre of the halo and drop to ≈10-9 G in the intergalactic medium. Analysing the saturation levels and growth rates, the model is able to describe the process of magnetic amplification notably well and confirms the results of the simulations.

  13. Origin of strong magnetic fields in Milky Way-like galactic haloes

    NASA Astrophysics Data System (ADS)

    Beck, Alexander; Dolag, Klaus; Lesch, Harald

    2015-08-01

    An analytical model predicting the growth rates, the absolute growth times and the saturation values of the magnetic field strength within galactic haloes is presented. The analytical results are compared to cosmological MHD simulations of Milky Way-like galactic halo formation performed with the N-body / SPMHD code GADGET. The halo has a mass of approximately 3*10^{12} solar masses and a virial radius of approximately 270 kpc. The simulations in a LCDM cosmology also include radiative cooling, star formation, supernova feedback and the description of non-ideal MHD. A primordial magnetic seed field ranging from 10^{-10} to 10^{-34} G in strength agglomerates together with the gas within filaments and protohaloes.There, it is amplified within a couple of hundred million years up to equipartition with the corresponding turbulent energy. The magnetic field strength increases by turbulent small-scale dynamo action. The turbulence is generated by the gravitational collapse and by supernova feedback. Subsequently, a series of halo mergers leads to shock waves and amplification processes magnetizing the surrounding gas within a few billion years. At first, the magnetic energy grows on small scales and then self-organizes to larger scales.Magnetic field strengths of microG are reached in the center of the halo and drop to nG in the IGM. Analyzing the saturation levels and growth rates, the model is able to describe the process of magnetic amplification notably well and confirms the results of the simulations. Additionally, we investigate magnetic seed fields created self-consistently by supernova explosions naturally occuring during the star formation in galaxies. Within starforming regions and given typical dimensions and magnetic field strengths in canonical SN remnants, we inject a dipole-shape magnetic field at a rate of nG/Gyr.In our model for the evolution of galactic magnetic fields, the seed magnetic field determined self-consistently by the star formation process

  14. Characterizing stellar halo populations II: The age gradient in blue horizontal-branch stars

    NASA Astrophysics Data System (ADS)

    Das, Payel; Williams, Angus; Binney, James

    2016-08-01

    The distribution of Milky Way halo blue horizontal-branch (BHB) stars is examined using action-based extended distribution functions (EDFs) that describe the locations of stars in phase space, metallicity, and age. The parameters of the EDFs are fitted using stars observed in the Sloan Extension for Galactic Understanding and Exploration-II (SEGUE-II) survey that trace the phase-space kinematics and chemistry out to ˜70 kpc. A maximum a posteriori probability (MAP) estimate method and a Markov Chain Monte Carlo method are applied, taking into account the selection function in positions, distance, and metallicity for the survey. The best-fit EDF declines with actions less steeply at actions characteristic of the inner halo than at the larger actions characteristic of the outer halo, and older ages are found at smaller actions than at larger actions. In real space, the radial density profile steepens smoothly from -2 at ˜2 kpc to -4 in the outer halo, with an axis ratio ˜0.7 throughout. There is no indication for rotation in the BHBs, although this is highly uncertain. A moderate level of radial anisotropy is detected, with βs varying from isotropic to between ˜0.1 and ˜0.3 in the outer halo depending on latitude. The BHB data are consistent with an age gradient of -0.03 Gyr kpc-1, with some uncertainty in the distribution of the larger ages. These results are consistent with a scenario in which older, larger systems contribute to the inner halo, whilst the outer halo is primarily comprised of younger, smaller systems.

  15. Neutron-capture element and Sc abundances in low- and high-alpha Galactic halo stars

    NASA Astrophysics Data System (ADS)

    Yong, David; Fishlock, Cherie; Karakas, Amanda

    2015-08-01

    Nissen & Schuster (2010) identified two samples of Galactic halo stars with distinct kinematic and chemical properties. The "high-alpha" population is associated with the dissipative monolithic collapse of a proto-Galactic gas cloud while the "low-alpha" population was likely accreted from dwarf galaxies having experienced slower star formation rates. For a subset of these stars, we measured precise abundances of Sc, Zr, La, Ce, Nd and Eu. We find differences in the abundance ratios of [Sc/Fe], [Zr/Fe], and [La/Zr] between the low- and high-alpha groups. The most intriguing result is that the low-alpha stars appear to have higher [Eu/Fe] ratios than the high-alpha stars, in contrast to the expectation that Eu should follow the alpha elements. These data challenge the hypothesis that the high-alpha stars formed in regions only enriched by massive stars and that the low-alpha received additional enrichment from SNeIa and low-mass AGB stars. This work has three main consequences for galaxy halos: 1. The new Eu data could be explained by different IMFs for the two halo populations; 2. The low [alpha/Fe] ratios in some, and perhaps all, dwarf galaxies may be driven (in part of in whole) by different IMFs rather than SNeIa contributions; 3. These data may provide important new constraints on the origin of Eu.

  16. Beyond halo mass: galactic conformity as a smoking gun of central galaxy assembly bias

    NASA Astrophysics Data System (ADS)

    Hearin, Andrew P.; Watson, Douglas F.; van den Bosch, Frank C.

    2015-09-01

    Quenched central galaxies tend to reside in a preferentially quenched large-scale environment, a phenomenon that has been dubbed galactic conformity. Remarkably, this tendency persists out to scales far larger than the virial radius of the halo hosting the central. Therefore, conformity manifestly violates the widely adopted assumption that the dark matter halo mass Mvir exclusively governs galaxy occupation statistics. This paper is the first in a series studying the implications of the observed conformity signal for the galaxy-dark matter connection. We show that recent measurements of conformity on scales r ˜ 1-5 Mpc imply that central galaxy quenching statistics cannot be correctly predicted with the knowledge of Mvir alone. We also demonstrate that ejected (or `backsplash') satellites cannot give rise to the signal. We then invoke the age matching model, which is predicated on the co-evolution of galaxies and haloes. We find that this model produces a strong signal, and that central galaxies are solely responsible. We conclude that large-scale `two-halo' conformity represents a smoking gun of central galaxy assembly bias, and indicates that contemporary models of satellite quenching have systematically overestimated the influence of post-infall processes.

  17. Hidden Galactic Accretion: The Discovery of Low-Velocity Halo Clouds

    NASA Astrophysics Data System (ADS)

    Peek, J. E. G.; Putman, M. E.; Sommer-Larsen, J.; Heiles, C. E.; Stanimirovic, S.; Douglas, K.; Gibson, S.; Korpela, E.

    2007-12-01

    High-Velocity Clouds (HVCs) have been thought to be part of the Galactic accretion process since their discovery more than 40 years ago. Two modes through which HVCs may be generated and contribute to the ongoing growth of our Galaxy are (1) the tidal stripping of satellite galaxies and (2) the fragmented condensation of the Galaxy's hot baryonic halo. We have run cosmological Tree-SPH simulations of a Milky-Way sized galaxy, in which we can resolve clouds down to 10^5 M⊙, in an attempt to probe the cooling halo accretion process. The simulations show that this HVC generation mechanism can indeed reproduce the characteristics of observed population of HVCs, including the flux, velocity and cloud clustering properties. These simulations also predict an equally large population of halo clouds moving at lower radial velocities: Low-Velocity Halo Clouds (LVHCs). These clouds would not be observed as HVCs, but would rather be confused with local disk gas. Taking advantage of the known empirical result that HVCs have undetectably low infrared dust flux compared to their 21cm column, we search for these clouds in the preliminary GALFA-HI survey and IRAS. We announce the discovery of the first examples of these clouds, and describe their properties. This work was supported in part by NSF grant AST 04-06987 and NSF grant AST 07-09347.

  18. CHEMICAL ABUNDANCES OF THE MILKY WAY THICK DISK AND STELLAR HALO. II. SODIUM, IRON-PEAK, AND NEUTRON-CAPTURE ELEMENTS

    SciTech Connect

    Ishigaki, M. N.; Aoki, W.; Chiba, M. E-mail: aoki.wako@nao.ac.jp

    2013-07-01

    We present chemical abundance analyses of sodium, iron-peak, and neutron-capture elements for 97 kinematically selected thick disk, inner halo, and outer halo stars with metallicities -3.3 < [Fe/H] <-0.5. The main aim of this study is to examine chemical similarities and differences among metal-poor stars belonging to these old Galactic components as a clue to determine their early chemodynamical evolution. In our previous paper, we obtained abundances of {alpha} elements by performing a one-dimensional LTE abundance analysis based on the high-resolution (R {approx} 50, 000) spectra obtained with the Subaru/HDS. In this paper, a similar analysis is performed to determine abundances of an additional 17 elements. We show that, in metallicities below [Fe/H] {approx}-2, the abundance ratios of many elements in the thick disk, inner halo, and outer halo subsamples are largely similar. In contrast, in higher metallicities ([Fe/H] {approx}> -1.5), differences in some of the abundance ratios among the three subsamples are identified. Specifically, the [Na/Fe], [Ni/Fe], [Cu/Fe], and [Zn/Fe] ratios in the inner and outer halo subsamples are found to be lower than those in the thick disk subsample. A modest abundance difference between the two halo subsamples in this metallicity range is also seen for the [Na/Fe] and [Zn/Fe] ratios. In contrast to that observed for [Mg/Fe] in our previous paper, [Eu/Fe] ratios are more enhanced in the two halo subsamples rather than in the thick disk subsample. The observed distinct chemical abundances of some elements between the thick disk and inner/outer halo subsamples with [Fe/H] >-1.5 support the hypothesis that these components formed through different mechanisms. In particular, our results favor the scenario that the inner and outer halo components formed through an assembly of multiple progenitor systems that experienced various degrees of chemical enrichments, while the thick disk formed through rapid star formation with an

  19. Assembling the Largest, Most Distant Sample of Halo Wide Binaries for Galactic Structure and Dynamics

    NASA Astrophysics Data System (ADS)

    Coronado, J.; Chanamé, J.

    2015-10-01

    Samples of wide binaries (agtrsim \\ 100\\ AU) are a gold mine for Galactic studies. They have been used on a large list of applications in a diversity of fields. In the dynamical arena, wide binaries provided the first meaningful constraints on the mass and nature of disk dark matter and, more recently, they were used to close the remaining parameter space of MACHO-like halo dark matter not accessible to the micro-lensing campaigns. All these applications were possible when samples of these objects became large enough to not be dominated by random, chance alignments of two unrelated stars projected on the sky. Nevertheless, still today the largest available sample of the particularly valuable halo wide binaries free from selection biases, contains not much more than 100 systems, and conclusions on dark matter are very sensitive to this fact.

  20. NEW CONSTRAINTS ON THE GALACTIC HALO MAGNETIC FIELD USING ROTATION MEASURES OF EXTRAGALACTIC SOURCES TOWARD THE OUTER GALAXY

    SciTech Connect

    Mao, S. A.; McClure-Griffiths, N. M.; Gaensler, B. M.; Brown, J. C.; Van Eck, C. L.; Stil, J. M.; Taylor, A. R.; Haverkorn, M.; Kronberg, P. P.; Shukurov, A.

    2012-08-10

    We present a study of the Milky Way disk and halo magnetic field, determined from observations of Faraday rotation measure (RM) toward 641 polarized extragalactic radio sources in the Galactic longitude range 100 Degree-Sign -117 Degree-Sign , within 30 Degree-Sign of the Galactic plane. For |b| < 15 Degree-Sign , we observe a symmetric RM distribution about the Galactic plane. This is consistent with a disk field in the Perseus arm of even parity across the Galactic mid-plane. In the range 15 Degree-Sign < |b| < 30 Degree-Sign , we find median RMs of -15 {+-} 4 rad m{sup -2} and -62 {+-} 5 rad m{sup -2} in the northern and southern Galactic hemispheres, respectively. If the RM distribution is a signature of the large-scale field parallel to the Galactic plane, then this suggests that the halo magnetic field toward the outer Galaxy does not reverse direction across the mid-plane. The variation of RM as a function of Galactic latitude in this longitude range is such that RMs become more negative at larger |b|. This is consistent with an azimuthal magnetic field of strength 2 {mu}G (7 {mu}G) at a height 0.8-2 kpc above (below) the Galactic plane between the local and the Perseus spiral arm. We propose that the Milky Way could possess spiral-like halo magnetic fields similar to those observed in M51.

  1. A Rich Globular Cluster System in Dragonfly 17: Are Ultra-diffuse Galaxies Pure Stellar Halos?

    NASA Astrophysics Data System (ADS)

    Peng, Eric W.; Lim, Sungsoon

    2016-05-01

    Observations of nearby galaxy clusters at low surface brightness have identified galaxies with low luminosities, but sizes as large as L ⋆ galaxies, leading them to be dubbed “ultra-diffuse galaxies” (UDGs). The survival of UDGs in dense environments like the Coma cluster suggests that UDGs could reside in much more massive dark halos. We report the detection of a substantial population of globular clusters (GCs) around a Coma UDG, Dragonfly 17 (DF17). We find that DF17 has a high GC specific frequency of S N = 26 ± 13. The GC system is extended, with an effective radius of 12″ ± 2″, or 5.6 ± 0.9 kpc at Coma distance, 70% larger than the galaxy itself. We also estimate the mean of the GC luminosity function to infer a distance of {97}-14+17 Mpc, providing redshift-independent confirmation that one of these UDGs is in the Coma cluster. The presence of a rich GC system in DF17 indicates that, despite its low stellar density, star formation was intense enough to form many massive star clusters. If DF17's ratio of total GC mass to total halo mass is similar to those in other galaxies, then DF17 has an inferred total mass of ˜1011 M ⊙, only ˜10% the mass of the Milky Way, but extremely dominated by dark matter, with M/L V ≈ 1000. We suggest that UDGs like DF17 may be “pure stellar halos,” i.e., galaxies that formed their stellar halo components, but then suffered an early cessation in star formation that prevented the formation of any substantial central disk or bulge. Based on observations 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.

  2. RESOLVE and ECO: The Halo Mass-dependent Shape of Galaxy Stellar and Baryonic Mass Functions

    NASA Astrophysics Data System (ADS)

    Eckert, Kathleen D.; Kannappan, Sheila J.; Stark, David V.; Moffett, Amanda J.; Berlind, Andreas A.; Norris, Mark A.

    2016-06-01

    In this work, we present galaxy stellar and baryonic (stars plus cold gas) mass functions (SMF and BMF) and their halo mass dependence for two volume-limited data sets. The first, RESOLVE-B, coincides with the Stripe 82 footprint and is extremely complete down to baryonic mass M bary ∼ 109.1 M ⊙, probing the gas-rich dwarf regime below M bary ∼ 1010 M ⊙. The second, ECO, covers a ∼40× larger volume (containing RESOLVE-A) and is complete to M bary ∼ 109.4 M ⊙. To construct the SMF and BMF we implement a new “cross-bin sampling” technique with Monte Carlo sampling from the full likelihood distributions of stellar or baryonic mass. Our SMFs exhibit the “plateau” feature starting below M star ∼ 1010 M ⊙ that has been described in prior work. However, the BMF fills in this feature and rises as a straight power law below ∼1010 M ⊙, as gas-dominated galaxies become the majority of the population. Nonetheless, the low-mass slope of the BMF is not as steep as that of the theoretical dark matter halo MF. Moreover, we assign group halo masses by abundance matching, finding that the SMF and BMF, separated into four physically motivated halo mass regimes, reveal complex structure underlying the simple shape of the overall MFs. In particular, the satellite MFs are depressed below the central galaxy MF “humps” in groups with mass <1013.5 M ⊙ yet rise steeply in clusters. Our results suggest that satellite destruction and stripping are active from the point of nascent group formation. We show that the key role of groups in shaping MFs enables reconstruction of a given survey’s SMF or BMF based on its group halo mass distribution.

  3. RESOLVE and ECO: The Halo Mass-dependent Shape of Galaxy Stellar and Baryonic Mass Functions

    NASA Astrophysics Data System (ADS)

    Eckert, Kathleen D.; Kannappan, Sheila J.; Stark, David V.; Moffett, Amanda J.; Berlind, Andreas A.; Norris, Mark A.

    2016-06-01

    In this work, we present galaxy stellar and baryonic (stars plus cold gas) mass functions (SMF and BMF) and their halo mass dependence for two volume-limited data sets. The first, RESOLVE-B, coincides with the Stripe 82 footprint and is extremely complete down to baryonic mass M bary ˜ 109.1 M ⊙, probing the gas-rich dwarf regime below M bary ˜ 1010 M ⊙. The second, ECO, covers a ˜40× larger volume (containing RESOLVE-A) and is complete to M bary ˜ 109.4 M ⊙. To construct the SMF and BMF we implement a new “cross-bin sampling” technique with Monte Carlo sampling from the full likelihood distributions of stellar or baryonic mass. Our SMFs exhibit the “plateau” feature starting below M star ˜ 1010 M ⊙ that has been described in prior work. However, the BMF fills in this feature and rises as a straight power law below ˜1010 M ⊙, as gas-dominated galaxies become the majority of the population. Nonetheless, the low-mass slope of the BMF is not as steep as that of the theoretical dark matter halo MF. Moreover, we assign group halo masses by abundance matching, finding that the SMF and BMF, separated into four physically motivated halo mass regimes, reveal complex structure underlying the simple shape of the overall MFs. In particular, the satellite MFs are depressed below the central galaxy MF “humps” in groups with mass <1013.5 M ⊙ yet rise steeply in clusters. Our results suggest that satellite destruction and stripping are active from the point of nascent group formation. We show that the key role of groups in shaping MFs enables reconstruction of a given survey’s SMF or BMF based on its group halo mass distribution.

  4. ISOTHERMAL DISTRIBUTIONS IN MONDian GRAVITY AS A SIMPLE UNIFYING EXPLANATION FOR THE UBIQUITOUS{rho}{proportional_to}r {sup -3} DENSITY PROFILES IN TENUOUS STELLAR HALOS

    SciTech Connect

    Hernandez, X.; Jimenez, M. A.; Allen, C.

    2013-06-20

    That the stellar halo of the Milky Way has a density profile which, to first approximation, satisfies{rho}{proportional_to}r {sup -3} and has been known for a long time. More recently, it has become clear that M31 also has such an extended stellar halo, which approximately follows the same radial scaling. Studies of distant galaxies have revealed the same phenomenology. Also, we now know that the density profiles of the globular cluster systems of our Galaxy and Andromeda to first approximation follow{rho}{proportional_to}r {sup -3},{Sigma}{proportional_to}R {sup -2} in projection. Recently, diffuse populations of stars have been detected spherically surrounding a number of Galactic globular clusters, extending much beyond the Newtonian tidal radii, often without showing any evidence of tidal features. Within the standard Newtonian and general relativity scenario, numerous and diverse particular explanations have been suggested, individually tailored to each of the different classes of systems described above. Here we show that in a MONDian gravity scenario any isothermal tenuous halo of tracer particles forming a small perturbation surrounding a spherically symmetric mass distribution will have an equilibrium configuration which to first approximation satisfies a{rho}{proportional_to}r {sup -3} scaling.

  5. Are Stellar Over-Densities in Dwarf Galaxies the ``Smoking Gun'' of Triaxial Dark Matter Haloes?

    NASA Astrophysics Data System (ADS)

    Peñarrubia, Jorge; Walker, Matthew G.; Gilmore, Gerard

    2010-06-01

    We use N-body simulations to study the tidal evolution of globular clusters (GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically motivated scenario in which the dSph is approximated by a static NFW halo with a triaxial shape. For a large set of orbits and projection angles we examine the spatial and velocity distribution of stellar debris deposited during the complete disruption of stellar clusters. Our simulations show that such debris appears as shells, isolated clumps and elongated over-densities at low surface brightness (>=26 mag/arcsec2), reminiscent of substructure observed in several MW dSphs. Such features arise from the triaxiality of the galaxy potential and do not dissolve in time. Stellar over-densities reported in several MW dSphs may thus be the telltale evidence of dark matter haloes being triaxial in shape. We explore a number of kinematical signatures that would help to validate (or falsify) this scenario. The mean angular momentum of the cluster debris associated with box and resonant orbits, which are absent in spherical potentials, is null. As a result, we show that the line-of-sight velocity distribution may exhibit a characteristic ``double-peak'' depending on the oriention of the viewing angle with respect to the progenitor's orbital plane. Kinematic surveys of dSphs may help to detect and identify substructures associated with the disruption of stellar clusters, as well as to address the shape of the dark matter haloes in which dSphs are embedded.

  6. Inflow of halo gas from the direction of the Galactic north pole

    NASA Technical Reports Server (NTRS)

    De Boer, K. S.; Savage, B. D.

    1984-01-01

    A far-UV echelle spectrum has been obtained of the UV-bright star vZ1128 in the globular cluster M3. This cluster lies in a direction 11 deg off the Galactic north pole at a distance of about 10 kpc. Only the usually strong interstellar lines are recognizable in the faint spectrum, and they show absorption at velocities from near +40 km/s to -100 km/s. Since in the direction observed the line-of-sight component of the Milky Way rotation is negligible, the detected velocities demonstrate for this direction the motion of gas from the Milky Way halo toward the disk. Gas returning to the disk in a galactic fountainlike flow may explain the observations.

  7. Enhanced tidal stripping of satellites in the galactic halo from dark matter self-interactions

    NASA Astrophysics Data System (ADS)

    Dooley, Gregory A.; Peter, Annika H. G.; Vogelsberger, Mark; Zavala, Jesús; Frebel, Anna

    2016-09-01

    We investigate the effects of self-interacting dark matter (SIDM) on the tidal stripping and evaporation of satellite galaxies in a Milky Way-like host. We use a suite of five zoom-in, dark-matter-only simulations, two with velocity-independent SIDM cross-sections, two with velocity-dependent SIDM cross-sections, and one cold dark matter (CDM) simulation for comparison. After carefully assigning stellar mass to satellites at infall, we find that stars are stripped at a higher rate in SIDM than in CDM. In contrast, the total bound dark matter mass-loss rate is minimally affected, with subhalo evaporation having negligible effects on satellites for viable SIDM models. Centrally located stars in SIDM haloes disperse out to larger radii as cores grow. Consequently, the half-light radius of satellites increases, stars become more vulnerable to tidal stripping, and the stellar mass function is suppressed. We find that the ratio of core radius to tidal radius accurately predicts the relative strength of enhanced SIDM stellar stripping. Velocity-independent SIDM models show a modest increase in the stellar stripping effect with satellite mass, whereas velocity-dependent SIDM models show a large increase in this effect towards lower masses, making observations of ultrafaint dwarfs prime targets for distinguishing between and constraining SIDM models. Due to small cores in the largest satellites of velocity-dependent SIDM, no identifiable imprint is left on the all-sky properties of the stellar halo. While our results focus on SIDM, the main physical mechanism of enhanced tidal stripping of stars apply similarly to satellites with cores formed via other means.

  8. Dual Stellar Halos in the Standard Elliptical Galaxy M105 and Formation of Massive Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Myung Gyoon

    2015-08-01

    M105 in the Leo I Group is a textbook example of a standard elliptical galaxy. Old red giant stars in the halo of M105 are easily resolved in the HST images so that it is an ideal target to study the structure and composition of stellar halos in elliptical galaxies. It is only one of the few elliptical galaxies for which we can study their stellar halos using the resolved stars. We present photometry of the resolved stars in its inner region at R~4 arcmin, obtained from F606W and F814W images in the Hubble Space Telescope archive. Then we combine this with photometry of the remote outer region at R~12 arcmin studied before. Deep color-magnitude diagrams of the resolved stars in the inner region show a prominent red giant branch (RGB) with a large color range, while those for the outer region show better a narrow blue RGB. We derive the metallicity of the RGB stars using the isochrones. The metallicity distribution function of the RGB stars shows the existence of two distinct subpopulations: a dominant metal-rich population and a much weaker metal-poor population. The peak metallicity of the metal-rich population changes little as galactocentric distance increases, while the fraction of the metal-poor population increases. The radial number density profile of the metal-poor RGB stars is flatter in the outer region than that of the metal-rich RGB stars. These provide strong evidence that there are two distinct stellar halos in this galaxy, blue (metal-poor) and red (metal-rich) halos, which is consistent with the results based on the study of the globular cluster systems in bright early-type galaxies (Park \\& Lee 2013,ApJ,773, 27). We discuss the implications of these results with regard to the formation of massive early-type galaxies in the dual halo mode formation scenario as well as in the two-phase formation scenario based on similuations.

  9. Emission from the Local Galactic Halo in the 1/4 keV Band

    NASA Astrophysics Data System (ADS)

    Juda, M.

    1994-12-01

    Pointed observations with the ROSAT PSPC toward clouds at high galactic latitude provide a unique opportunity to probe emission from the local galactic halo in the 1/4 keV band. I present data from five fields toward clouds at |b| > 60(deg) identified through their IRAS 100 microns emission, two in the north galactic hemisphere and three in the south. In four of the five fields significant shadows are detected (2 north, 2 south). The derivation of the brightness of the shadowed component depends strongly on the assumed location and amount of absorbing material. Scaling the IRAS 100 microns emission by 10(20) H atom cm(-2) /MJy sr(-1) and correcting for the difference from the observed average 21 cm derived column density, the implied brightness of the distant emitting component is the same for the two northern latitude fields at { ~ 1.2*E(-3) counts s(-1) arcmin(-2}) ; this brightness is lower than that seen in the direction of Draco (Burrows & Mendenhall 1991, Snowden et al. 1991) and higher than in Ursa Major (Snowden et al. 1994). The two southern fields also have the same derived distant brightness at nearly the same level as the northern fields, { ~ 1.0*E(-3) counts s(-1) arcmin(-2}) . Approximately 20% of this emission may be attributed to an extragalactic background (Hasinger et al. 1993). The remaining emission, { ~ 0.8*E(-3) counts s(-1) arcmin(-2}) , would be provided by the local galactic halo. If these x-rays arise from a collisionally excited plasma at a temperature of 10(6) K the required emission measure is { ~ 0.0033 cm(-6) pc}. Burrows & Mendenhall 1991, Nature, 351, 629. Hasinger et al. 1993, A&A, 275, 1. Snowden et al. 1991, Science, 252, 1529. Snowden et al. 1994, ApJ, 430, 601.

  10. Recent Results from SPLASH: Chemical Abundances and Kinematics of Andromeda's Stellar Halo

    NASA Astrophysics Data System (ADS)

    Gilbert, Karoline M.; Beaton, Rachael; Dorman, Claire

    2016-08-01

    Large scale surveys of Andromeda's resolved stellar populations have revolutionized our view of this galaxy over the past decade. The combination of large-scale, contiguous photometric surveys and pointed spectroscopic surveys has been particularly powerful for discovering substructure and disentangling the structural components of Andromeda. The SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) survey consists of broad- and narrow-band imaging and spectroscopy of red giant branch stars in lines of sight ranging in distance from 2 kpc to more than 200 kpc from Andromeda's center. The SPLASH data reveal a power-law surface brightness profile extending to at least two-thirds of Andromeda's virial radius (Gilbert et al. 2012), a metallicity gradient extending to at least 100 kpc from Andromeda's center (Gilbert et al. 2014), and evidence of a significant population of heated disk stars in Andromeda's inner halo (Dorman et al. 2013). We are also using the velocity distribution of halo stars to measure the tangential motion of Andromeda (Beaton et al., in prep).