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Sample records for linking dwarf galaxies

  1. Redshift, metallicity and size of two extended dwarf Irregular galaxies. A link between dwarf Irregulars and ultra diffuse galaxies?

    NASA Astrophysics Data System (ADS)

    Bellazzini, M.; Belokurov, V.; Magrini, L.; Fraternali, F.; Testa, V.; Beccari, G.; Marchetti, A.; Carini, R.

    2017-01-01

    We present the results of the spectroscopic and photometric follow-up of two field galaxies that were selected as possible stellar counterparts of local high velocity clouds. Our analysis shows that the two systems are distant (D>20 Mpc) dwarf irregular galaxies unrelated to the local H I clouds. However, the newly derived distance and structural parameters reveal that the two galaxies have luminosities and effective radii very similar to the recently identified ultra diffuse galaxies (UDGs). At odds with classical UDGs, they are remarkably isolated, having no known giant galaxy within ˜2.0 Mpc. Moreover, one of them has a very high gas content compared to galaxies of similar stellar mass, with a H I to stellar mass ratio MHI/M⋆ ˜ 90, typical of almost-dark dwarfs. Expanding on this finding, we show that extended dwarf irregulars overlap the distribution of UDGs in the MV vs. log re plane and that the sequence including dwarf spheroidals, dwarf irregulars and UDGs appears as continuously populated in this plane. This may suggest an evolutionary link between dwarf irregulars and UDGs.

  2. Exploring Dwarf Galaxy Evolution

    NASA Astrophysics Data System (ADS)

    Dunn, Jacqueline M.

    2017-01-01

    Dwarf galaxies are the most numerous galaxies in the universe, yet little is definitively understood about their formation and evolution. An evolutionary link has been proposed between dwarf irregular and dwarf elliptical galaxies by previous studies. The nature and existence of so-called dwarf spiral galaxies is still heavily debated. This project explores the properties of dwarf galaxies spanning a range in morphological type, luminosity, physical size, and surrounding environment (i.e. group / field galaxies). The goal of this project is to determine the range of exhibited properties for each type of dwarf galaxy using available ultraviolet, visible, and near-infrared imaging and spectra. Similarities in visible, broadband colors support the proposed evolutionary link dwarf irregular and dwarf elliptical galaxies when the range of brightness of the samples is constrained to the fainter galaxies. Here, comparisons amongst a sub-sample of 59 dwarf irregulars, 12 dwarf ellipticals, and 29 dwarf spirals will be presented using archival ultraviolet, visible, and near-infrared imaging. The effect of constraining the comparisons to the fainter sample members will be explored, as well as the effect of constraining the comparisons to the brighter sample members.

  3. Investigating Dwarf Spiral Galaxies

    NASA Astrophysics Data System (ADS)

    Weerasooriya, Sachithra; Dunn, Jacqueline M.

    2017-01-01

    Several studies have proposed that dwarf elliptical / spheroidal galaxies form through the transformation of dwarf irregular galaxies. Early and late type dwarfs resemble each other in terms of their observed colors and light distributions (each can often be represented by exponential disks), providing reason to propose an evolutionary link between the two types. The existence of dwarf spirals has been largely debated. However, more and more recent studies are using the designation of dwarf spiral to describe their targets of interest. This project seeks to explore where dwarf spirals fit into the above mentioned evolutionary sequence, if at all. Optical colors will be compared between a sample of dwarf irregular, dwarf elliptical, and dwarf spiral galaxies. The dwarf irregular and dwarf elliptical samples have previously been found to overlap in both optical color and surface brightness profile shape when limiting the samples to their fainter members. A preliminary comparison including the dwarf spiral sample will be presented here, along with a comparison of available ultraviolet and near-infrared data. Initial results indicate a potential evolutionary link that merits further investigation.

  4. Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor.

    PubMed

    Frebel, Anna; Kirby, Evan N; Simon, Joshua D

    2010-03-04

    Current cosmological models indicate that the Milky Way's stellar halo was assembled from many smaller systems. On the basis of the apparent absence of the most metal-poor stars in present-day dwarf galaxies, recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt on this conclusion. Verification of the iron-deficiency, however, and measurements of additional elements, such as the alpha-element Mg, are necessary to demonstrate that the same type of stars produced the metals found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars be conclusively linked to early stellar halo assembly. Here we report high-resolution spectroscopic abundances for 11 elements in S1020549, confirming its iron abundance of less than 1/4,000th that of the Sun, and showing that the overall abundance pattern follows that seen in low-metallicity halo stars, including the alpha-elements. Such chemical similarity indicates that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs, and suggests that the early chemical enrichment of all galaxies may be nearly identical.

  5. NGC 5044-N50: a link between blue compact galaxies and dwarf ellipticals

    NASA Astrophysics Data System (ADS)

    Cellone, Sergio A.; Buzzoni, Alberto

    We present new optical observations of the dwarf galaxy N50 in the NGC 5044 Group, showing that this object is probably at an intermediate BCD→dE evolutionary stage, after a realtively recent burst of star formation.

  6. THE METALLICITY OF VOID DWARF GALAXIES

    SciTech Connect

    Kreckel, K.; Groves, B.; Croxall, K.; Pogge, R. W.; Van de Weygaert, R.

    2015-01-01

    The current ΛCDM cosmological model predicts that galaxy evolution proceeds more slowly in lower density environments, suggesting that voids are a prime location to search for relatively pristine galaxies that are representative of the building blocks of early massive galaxies. To test the assumption that void galaxies are more pristine, we compare the evolutionary properties of a sample of dwarf galaxies selected specifically to lie in voids with a sample of similar isolated dwarf galaxies in average density environments. We measure gas-phase oxygen abundances and gas fractions for eight dwarf galaxies (M{sub r} > –16.2), carefully selected to reside within the lowest density environments of seven voids, and apply the same calibrations to existing samples of isolated dwarf galaxies. We find no significant difference between these void dwarf galaxies and the isolated dwarf galaxies, suggesting that dwarf galaxy chemical evolution proceeds independent of the large-scale environment. While this sample is too small to draw strong conclusions, it suggests that external gas accretion is playing a limited role in the chemical evolution of these systems, and that this evolution is instead dominated mainly by the internal secular processes that are linking the simultaneous growth and enrichment of these galaxies.

  7. Tidal Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Houck, James R.; Higdon, Sarah

    2004-09-01

    Tidal Dwarf Galaxies (TDG's) are formed from material stripped from the disks of spiral galaxies, which are undergoing tidal interactions with a nearby companion. These galaxies provide important clues to our understanding of galaxy formation, evolution and cosmic recycling. Using the IRS we will measure the star formation activity in 6 TDG candidates. We will measure the ionization state ( [NeII] 12.8 um, [NeIII] 15.6 um and [NeV] 14.3um and [OIV] 25.9 um), the density in the ionized gas ([SIII] 18.7um/33.5um), the PAH fractions at 5.5-9um and 11-12.2um and possibly (optimistic here!) molecular hydrogen emission form PDRs at H2 (S0) 28um and H2 (S1) at 17um. In addition to the IRS observations we will map both the Guitar and Stephan's Quintet with IRAC. This will enable us to compare the PAH fraction in the dwarf galaxy to that of its parent. Similarly we will compare our observation of the proposed TDG at the southern tip of NGC 4038 with the GT observations of the central region of the Antennae. This program compliments two existing GT programmes: 1) the high-Z program - these observations enable us to observe in fine detail the nearby/present day analogs of galaxy formation in the early universe. 2) Blue Compact Dwarf programme - On first inpsection BCD's and TDG's appear the same: BCDs are similar in size to TDG's, but TDG's may not have a large dark matter halo component (affecting the long term stability of an object) and BCD's typically have a much lower metallicity. We will be able to compare the star formation activity in terms of the ionization state and PAH fraction in the two galaxy types.

  8. Dwarf spheroidal galaxies: Keystones of galaxy evolution

    NASA Technical Reports Server (NTRS)

    Gallagher, John S., III; Wyse, Rosemary F. G.

    1994-01-01

    Dwarf spheroidal galaxies are the most insignificant extragalactic stellar systems in terms of their visibility, but potentially very significant in terms of their role in the formation and evolution of much more luminous galaxies. We discuss the present observational data and their implications for theories of the formation and evolution of both dwarf and giant galaxies. The putative dark-matter content of these low-surface-brightness systems is of particular interest, as is their chemical evolution. Surveys for new dwarf spheroidals hidden behind the stars of our Galaxy and those which are not bound to giant galaxies may give new clues as to the origins of this unique class of galaxy.

  9. Seeing Baby Dwarf Galaxies

    NASA Technical Reports Server (NTRS)

    2009-01-01

    [figure removed for brevity, see original site] [figure removed for brevity, see original site] [figure removed for brevity, see original site] Visible/DSS Click on image for larger version Ultraviolet/GALEX Click on image for larger version Poster Version Click on image for larger version

    The unique ultraviolet vision of NASA's Galaxy Evolution Explorer reveals, for the first time, dwarf galaxies forming out of nothing more than pristine gas likely leftover from the early universe. Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way.

    The forming dwarf galaxies shine in the far ultraviolet spectrum, rendered as blue in the call-out on the right hand side of this image. Near ultraviolet light, also obtained by the Galaxy Evolution Explorer, is displayed in green, and visible light from the blue part of the spectrum here is represented by red. The clumps (in circles) are distinctively blue, indicating they are primarily detected in far ultraviolet light.

    The faint blue overlay traces the outline of the Leo Ring, a huge cloud of hydrogen and helium that orbits around two massive galaxies in the constellation Leo (left panel). The cloud is thought likely to be a primordial object, an ancient remnant of material that has remained relatively unchanged since the very earliest days of the universe. Identified about 25 years ago by radio waves, the ring cannot be seen in visible light.

    Only a portion of the Leo Ring has been imaged in the ultraviolet, but this section contains the telltale ultraviolet signature of recent massive star formation within this ring of pristine gas. Astronomers have previously only seen dwarf galaxies form out of gas that has already been cycled through a galaxy and enriched with metals elements heavier than helium produced as stars evolve.

    The visible data come from the Digitized Sky Survey of the Space Telescope Science Institute in Baltimore, Md. The

  10. Dwarf elliptical galaxies

    NASA Technical Reports Server (NTRS)

    Ferguson, Henry C.; Binggeli, Bruno

    1994-01-01

    Dwarf elliptical (dE) galaxies, with blue absolute magnitudes typically fainter than M(sub B) = -16, are the most numerous type of galaxy in the nearby universe. Tremendous advances have been made over the past several years in delineating the properties of both Local Group satellite dE's and the large dE populations of nearby clusters. We review some of these advances, with particular attention to how well currently availiable data can constrain (a) models for the formation of dE's, (b) the physical and evolutionary connections between different types of galaxies that overlap in the same portion of the mass-spectrum of galaxies, (c) the contribution of dE's to the galaxy luminosity functions in clusters and the field, (d) the star-forming histories of dE's and their possible contribution to faint galaxy counts, and (e) the clustering properties of dE's. In addressing these issues, we highlight the extent to which selection effects temper these constraints, and outline areas where new data would be particularly valuable.

  11. Local Universe Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Carignan, Claude

    2015-08-01

    One of the outstanding problems in cosmology is addressing the "small-scale crisis" and understanding structure formation at the smallest scales. Standard Lambda Cold Dark Matter cosmological simulations of Milky Way-size DM halos predict many more DM sub-halos than the number of dwarf galaxies observed. This is the so-called Missing Satellites Problem. The most popular interpretation of the Missing Satellites Problem is that the smallest dark matter halos in the universe are extremely inefficient at forming stars. The virialized extent of the Milky Way's halo should contain ~500 satellites, while only ˜100 satellites and dwarfs are observed in the whole Local Group. Despite the large amount of theoretical work and new optical observations, the discrepancy, even if reduced, still persists between observations and hierarchical models, regardless of the model parameters. It may be possible to find those isolated ultra-faint missing dwarf galaxies via their neutral gas component, which is one of the goals we are pursuing with the SKA precursor KAT-7 in South Africa, and soon with the SKA pathfinder MeerKAT.

  12. Radial Velocity of the Phoenix Dwarf Galaxy: Linking Stars and H I Gas

    NASA Astrophysics Data System (ADS)

    Gallart, C.; Martínez-Delgado, D.; Gómez-Flechoso, M. A.; Mateo, M.

    2001-05-01

    We present the first radial velocity measurement of the stellar component of the Local Group dwarf galaxy Phoenix, using the FORS1 instrument at the VLT's Unit Telescope 1 (Antu). From the spectra of 31 red giant branch stars, we derive a heliocentric optical radial velocity for Phoenix of Vsolar=-52+/-6 km s-1. On the basis of this velocity, and taking into account the results of a series of semianalytical and numerical simulations, we discuss the possible association of the H I clouds observed in the Phoenix vicinity. We conclude that the characteristics of the H I cloud with heliocentric velocity -23 km s-1 are consistent with this gas having been associated with Phoenix in the past and being lost by the galaxy after the last event of star formation in the galaxy, about 100 Myr ago. Two possible scenarios are discussed: the ejection of the gas by the energy released by the supernovae (SNe) produced in that last event of star formation and a ram pressure stripping scenario. We derive that the kinetic energy necessary to eject the gas is ESNe~2×1051 ergs and that the number of SNe necessary to transfer this amount of kinetic energy to the gas cloud is ~20. This is consistent with the number of SNe expected for the last event of star formation in Phoenix, according to the star formation history derived by Martínez-Delgado, Gallart, & Aparicio. The drawback of this scenario is the regular appearance of the H I cloud and its anisotropic distribution with respect to the stellar component. Another possibility is that the H I gas was stripped as a consequence of ram pressure by the intergalactic medium. In our simulations, the structure of the gas remains quite smooth as it is stripped from Phoenix, keeping a distribution similar to that of the observed H I cloud. Both in the SNe ejection case and in the ram pressure sweeping scenario, the distances and relative velocities imply that the H I cloud is not gravitationally bound to Phoenix, since this would require a

  13. Irregular Dwarf Galaxy IC 1613

    NASA Technical Reports Server (NTRS)

    2005-01-01

    Ultraviolet image (left) and visual image (right) of the irregular dwarf galaxy IC 1613. Low surface brightness galaxies, such as IC 1613, are more easily detected in the ultraviolet because of the low background levels compared to visual wavelengths.

  14. Interaction between the IGM and a dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Lora, V.; Raga, A. C.; Grebel, E. K.

    2015-04-01

    Dwarf Galaxies are the most common objects in the Universe and are believed to contain large amounts of dark matter. There are mainly three morphologic types of dwarf galaxies: dwarf ellipticals, dwarf spheroidals and dwarf irregulars. Dwarf irregular galaxies are particularly interesting in dwarf galaxy evolution, since dwarf spheroidal predecessors could have been very similar to them. Therefore, a mechanism linked to gas-loss in dwarf irregulars should be observed, i.e. ram pressure stripping. In this paper, we study the interaction between the ISM of a dwarf galaxy and a flowing IGM. We derive the weak-shock, plasmon solution corresponding to the balance between the post-bow shock pressure and the pressure of the stratified ISM (which we assume follows the fixed stratification of a gravitationally dominant dark matter halo). We compare our model with previously published numerical simulations and with the observed shape of the HI cloud around the Ho II and Pegasus dwarf irregular galaxies. We show that such a comparison provides a straightforward way for estimating the Mach number of the impinging flow.

  15. Local Group dwarf galaxies: nature and nurture

    NASA Astrophysics Data System (ADS)

    Sawala, Till; Scannapieco, Cecilia; White, Simon

    2012-02-01

    We investigate the formation and evolution of dwarf galaxies in a high-resolution, hydrodynamical cosmological simulation of a Milky Way sized halo and its environment. Our simulation includes gas cooling, star formation, supernova feedback, metal enrichment and ultraviolet heating. In total, 90 satellites and more than 400 isolated dwarf galaxies are formed in the simulation, allowing a systematic study of the internal and environmental processes that determine their evolution. We find that 95 per cent of satellite galaxies are gas free at z= 0, and identify three mechanisms for gas loss: supernova feedback, tidal stripping and photoevaporation due to re-ionization. Gas-rich satellite galaxies are only found with total masses above ˜5 × 109 M⊙. In contrast, for isolated dwarf galaxies, a total mass of ˜109 M⊙ constitutes a sharp transition; less massive galaxies are predominantly gas free at z= 0, more massive, isolated dwarf galaxies are often able to retain their gas. In general, we find that the total mass of a dwarf galaxy is the main factor which determines its star formation, metal enrichment and its gas content, but that stripping may explain the observed difference in gas content between field dwarf galaxies and satellites with total masses close to 109 M⊙. We also find that a morphological transformation via tidal stripping of infalling, luminous dwarf galaxies whose dark matter is less concentrated than their stars cannot explain the high total mass-to-light ratios of the faint dwarf spheroidal galaxies.

  16. Angular Momentum of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Butler, Kirsty M.; Obreschkow, Danail; Oh, Se-Heon

    2017-01-01

    We present measurements of baryonic mass {M}{{b}} and specific angular momentum (sAM) {j}{{b}} in 14 rotating dwarf Irregular (dIrr) galaxies from the LITTLE THINGS sample. These measurements, based on 21 cm kinematic data from the Very Large Array and stellar mass maps from the Spitzer Space Telescope, extend previous AM measurements by more than two orders of magnitude in {M}{{b}}. The dwarf galaxies show systematically higher {j}{{b}} values than expected from the {j}{{b}}\\propto {M}{{b}}2/3 scaling of spiral galaxies, representative of a scale-free galaxy formation scenario. This offset can be explained by decreasing baryon mass fractions {f}{{M}}={M}{{b}}/{M}{dyn} (where {M}{dyn} is the dynamical mass) with decreasing {M}{{b}} (for {M}{{b}}< {10}11 {M}ȯ ). We find that the sAM of neutral atomic hydrogen (H i) alone is about 2.5 times higher than that of the stars. The M–j relation of H i is significantly steeper than that of the stars, as a direct consequence of the systematic variation of the H i fraction with {M}{{b}}.

  17. Central Dark Matter Distribution In Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Oh, Se-Heon; Brook, C.; Governato, F.; Brinks, E.; Mayer, L.; de Blok, E.; Brooks, A.; Walter, F.

    2012-01-01

    Central dark matter distribution in dwarf galaxies Se-Heon Oh, Chris Brook, Fabio Governato, Elias Brinks, Lucio Mayer, W.J.G. de Blok, Alyson Brooks and Fabian Walter We present high-resolution mass models of 7 nearby dwarf galaxies from "The HI Nearby Galaxy Survey” (THINGS) and compare these with those from hydrodynamic simulations of dwarf galaxies assuming a ΛCDM cosmology. The simulations include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating and most importantly, physically motivated gas outflows driven by supernovae (SNe). For the THINGS dwarf galaxies, we derive the mass models for the dark matter component by subtracting the contribution from baryons, derived from our HI observations and using the "Spitzer Infrared Nearby Galaxies Survey” (SINGS) 3.6μm data, from the total kinematics, leaving only the contribution by the Dark Matter halo. In parallel, we perform dark matter mass modeling of the simulated dwarf galaxies in exactly the same way as the observed THINGS dwarf galaxies. From a direct comparison between the observations and simulations, we find that the dark matter rotation curves of the simulated dwarf galaxies rise less steeply in the inner regions than those of dark-matter-only simulations based on the ΛCDM paradigm, and are more consistent with those of the THINGS dwarf galaxies. In addition, the mean value of the logarithmic inner dark matter density slopes, α, of the simulated galaxies is approximately -0.4 ± 0.1, which is in good agreement with α = -0.29 ± -0.07 of the THINGS dwarf galaxies. This shows that the baryonic feedback processes in the simulations are efficient in flattening the initial cusps with α = -1.0 to -1.5 predicted from dark-matter-only simulations, and render the dark matter halo mass distribution more similar to that observed in nearby dwarf galaxies.

  18. Dwarf spheroidal galaxies and resonant orbital coupling

    NASA Technical Reports Server (NTRS)

    Kuhn, J. R.; Miller, R. H.

    1989-01-01

    The structural properties of the dwarf spheroidal satellite galaxies of the Milky Way may be strongly affected by their time-dependent interactions with the 'tidal' field of the Milky Way. A low Q resonance of the tidal driving force with collective oscillation modes of the dwarf system can produce many of the observed properties of the Local Group dwarf spheroidal galaxies, including large velocity dispersions that would normally be interpreted as indicating large dynamical masses.

  19. Dwarf galaxy evolution within the environments of massive galaxies

    NASA Astrophysics Data System (ADS)

    Arraki, Kenza S.; Klypin, Anatoly A.; Ceverino, Daniel; Trujillo-Gomez, Sebastian; Primack, Joel R.

    2016-01-01

    Understanding galaxy evolution depends on connecting large-scale structure determined by the ΛCDM model with, at minimum, the small-scale physics of gas, star formation, and stellar feedback. Formation of galaxies within dark matter halos is sensitive to the physical phenomena occurring within and around the halo. This is especially true for dwarf galaxies, which have the smallest potential wells and are more susceptible to the effects of gas ionization and removal than larger galaxies. At dwarf galaxies scales comparisons of dark matter-only simulations with observations has unveiled various differences including the core-cusp, the missing satellites, and the too-big-to-fail problems. We have run a new suite of hydrodynamical simulations using the ART code to examine the evolution of dwarf galaxies in massive host environments. These are cosmological zoom-in simulations including deterministic star formation and stellar feedback in the form of supernovae feedback, stellar winds, radiation pressure, and photoionization pressure. We simulates galaxies with final halo masses on the order of 1012 M⊙ with high resolution, allowing us to examine the satellite dwarf galaxies and local isolated dwarf galaxies around each primary galaxy. We analyzed the abundance and structure of these dwarfs specifically the velocity function, their star formation rates, core creation and the circumgalactic medium. By reproducing observations of dwarf galaxies in simulations we show how including baryons in simulations relieves tensions seen in comparing dark matter only simulations with observations.

  20. Morphology and Structures of Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Seo, Mira; Ann, HongBae

    2015-08-01

    We performed an analysis of the structure of nearby dwarf galaxies based on a 2-dimensional decomposition of galaxy images using GALFIT. The present sample consists of ~1,100 dwarf galaxies with redshift less than z = 0.01, which is is derived from the morphology catalog of the Visually classified galaxies in the local universe (Ann, Seo, and Ha 2015). In this catalog, dwarf galaxies are divided into 5 subtypes: dS0, dE, dSph, dEbc, dEblue with distinction of the presence of nucleation in dE, dSph, and dS0. We found that dSph and dEblue galaxies are fainter than other subtypes of dwarf galaxies. In most cases, single component, represented by the Sersic profile with n=1~1.5, well describes the luminosity distribution of dwarf galaxies in the present sample. However, a significant fraction of dS0, dEbc, and dEbue galaxies show sub-structures such as spiral arms and rings. We will discuss the morphology dependent evolutionary history of the local dwarf galaxies.

  1. White Dwarfs in the Galaxy's Halo

    NASA Astrophysics Data System (ADS)

    Oppenheimer, B.; Murdin, P.

    2002-12-01

    The Galaxy's large spherical halo (see GALACTICMETAL-POOR HALO and HALO, GALACTIC) may harboras many as several hundred billion WHITE DWARFS, apopulation as large in number as the total number of stars in theGalaxy's disk (see DISK GALAXIES and GALACTIC THIN DISK). Although this assertion iscontroversial, several astronomical surveys provide strong support for it andthe implications affect fields ...

  2. Satellite dwarf galaxies in a hierarchical universe: the prevalence of dwarf-dwarf major mergers

    SciTech Connect

    Deason, Alis; Wetzel, Andrew; Garrison-Kimmel, Shea

    2014-10-20

    Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the Local Group remains poorly understood. We investigate the frequency of major mergers between dwarf galaxies in the Local Group using the ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way- and M31-like host halos. We find that ∼10% of satellite dwarf galaxies with M {sub star} > 10{sup 6} M {sub ☉} that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z = 1, with a lower fraction for lower mass dwarf galaxies. Recent merger remnants are biased toward larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo. Satellite-satellite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group. The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the Local Group are the most likely to have experienced a recent major merger. We discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive black holes in dwarf galaxies.

  3. The Hunt for Dwarf Galaxies' Ancestors

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-01-01

    Dwarf galaxies are typically very faint, and are therefore hard to find. Given that, what are our chances of finding their distant ancestors, located billions of light-years away? A recent study aims to find out.Ancient CounterpartsDwarf galaxies are a hot topic right now, especially as we discover more and more of them nearby. Besides being great places to investigate a variety of astrophysical processes, local group dwarf galaxies are also representative of the most common type of galaxy in the universe. For many of these dwarf galaxies, their low masses and typically old stellar populations suggest that most of their stars were formed early in the universes history, and further star formation was suppressed when the universe was reionized at redshifts of z ~ 610. If this is true, most dwarf galaxies are essentially fossils: theyve evolved little since that point.To test this theory, wed like to find counterparts to our local group dwarf galaxies at these higher redshifts of z = 6 or 7. But dwarf galaxies, since they dont exhibit lots of active star formation, have very low surface brightnesses making them very difficult to detect. What are the chances that current or future telescope sensitivities will allow us to detect these? Thats the question Anna Patej and Abraham Loeb, two theorists at Harvard University, have addressed in a recent study.Entering a New RegimeThe surface brightness vs. size for 73 local dwarf galaxies scaled back to redshifts of z=6 (top) and z=7 (bottom). So far weve been able to observe high-redshift galaxies within the boxed region of the parameter space. JWST will open the shaded region of the parameter space, which includes some of the dwarf galaxies. [Patej Loeb 2015]Starting from observational data for 87 Local-Group dwarf galaxies, Patej and Loeb used a stellar population synthesis code to evolve the galaxies backward in time to redshifts of z = 6 and 7. Next, they narrowed this sample to only those dwarfs for which most star

  4. Dust in Cluster Dwarf Elliptical Galaxies

    NASA Astrophysics Data System (ADS)

    De Looze, I.; Baes, M.; Fritz, J.; Verstappen, J.; Bendo, G. J.; Bianchi, S.; Bomans, D. J.; Boselli, A.; Clemens, M.; Corbelli, E.; Cortese, L.; Dariush, A.; Davies, J. I.; di Serego Alighieri, S.; Fadda, D.; Garcia-Appadoo, D. A.; Gavazzi, G.; Giovanardi, C.; Grossi, M.; Hughes, T. M.; Hunt, L. K.; Jones, A. P.; Madden, S.; Magrini, L.; Pierini, D.; Pohlen, M.; Sabatini, S.; Smith, M. W. L.; Vlahakis, C.; Xilouris, E. M.; Zibetti, S.

    Based on single cross-scan data of the Herschel Virgo Cluster Survey, we report the first detections of dust in cluster early-type dwarf galaxies: VCC 209, VCC 781 and VCC 951. All three galaxies have dust masses M d ≈ 105 - 106 M⊙ and average dust temperatures ≈ 16-20 K. Since these three early-type dwarfs reside in densely crowded regions close to the center of the Virgo cluster, and several H I-detected dwarfs in the outskirts of Virgo were not detected by Herschel(implying a dust content < 104 M⊙), this might imply that dust in dwarfs is more closely related to the molecular gas, which is more centrally peaked in a galaxy's potential well and therefore, not easily removed by any stripping mechanism. We conclude that the removal of interstellar dust from these early-type dwarfs appears to be less efficient than the removal of the H I gas.

  5. Mass Modelling of dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Klimentowski, Jarosław; Łokas, Ewa L.; Kazantzidis, Stelios; Prada, Francisco; Mayer, Lucio; Mamon, Gary A.

    2008-05-01

    We study the origin and properties of unbound stars in the kinematic samples of dwarf spheroidal galaxies. For this purpose we have run a high resolution N-body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. We create mock kinematic data sets by observing the dwarf in different directions. When the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. However, most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert. We model the velocity dispersion profiles of the cleaned-up kinematic samples using solutions of the Jeans equation. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and mass-to-light ratio of the dwarf with accuracy typically better than 25%.

  6. Tracking star formation in dwarf cluster galaxies

    NASA Astrophysics Data System (ADS)

    Rude, Cody Millard

    The evolution of galaxies in dense environments can be affected by close encounters with neighboring galaxies and interactions with the intracluster medium (ICM). Dwarf galaxies may be especially susceptible to these effects due to their low mass. The goal of my dissertation research is to look for signs of star formation in cluster dwarf galaxies by measuring and comparing the r- and u-band luminosity functions of 15 low redshift Abell galaxy clusters using archival data from the Canada-France-Hawaii Telescope (CFHT). Luminosity functions, dwarf-to-giant ratios, and blue fractions are measured in four cluster-centric annuli from stacked cluster data. To account for differences in cluster optical richness, each cluster is scaled according to r200, where r200 is the radius of a sphere, centered on the cluster, whose average density is 200 times the critical density of the universe. The outer region of the cluster sample shows an increase in the faint-end slope of the u-band luminosity function relative to the r-band, indicating star formation in dwarf galaxies. The blue fraction for dwarf galaxies steadily rises with increasing cluster-centric radii. The change in the blue fraction of giant galaxies also increases, but at a lower rate. Additionally, the inner regions of clusters ranging from 0.185 < z < 0.7 from the "Cluster Lensing and Supernova survey with Hubble (CLASH)" are used to generate blue- and red-band luminosity functions, dwarf-to-giant ratios, and blue fractions. Comparisons of the inner region of the CLASH and CFHT clusters show an increase in the blue fraction of dwarf galaxies with redshift that is not present in giant galaxies.

  7. Metallic Winds in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Robles-Valdez, F.; Rodríguez-González, A.; Hernández-Martínez, L.; Esquivel, A.

    2017-02-01

    We present results from models of galactic winds driven by energy injected from nuclear (at the galactic center) and non-nuclear starbursts. The total energy of the starburst is provided by very massive young stellar clusters, which can push the galactic interstellar medium and produce an important outflow. Such outflow can be a well or partially mixed wind, or a highly metallic wind. We have performed adiabatic 3D N-Body/Smooth Particle Hydrodynamics simulations of galactic winds using the gadget-2 code. The numerical models cover a wide range of parameters, varying the galaxy concentration index, gas fraction of the galactic disk, and radial distance of the starburst. We show that an off-center starburst in dwarf galaxies is the most effective mechanism to produce a significant loss of metals (material from the starburst itself). At the same time, a non-nuclear starburst produces a high efficiency of metal loss, in spite of having a moderate to low mass loss rate.

  8. Dwarf Galaxies with Active Massive Black Holes

    NASA Astrophysics Data System (ADS)

    Reines, Amy E.; Greene, J. E.; Geha, M. C.

    2014-01-01

    Supermassive black holes (BHs) live at the heart of essentially all massive galaxies with bulges, power AGN, and are thought to be important agents in the evolution of their hosts. However, the birth and growth of the first supermassive BH "seeds" is far from understood. While direct observations of these distant BHs in the infant Universe are unobtainable with current capabilities, massive BHs in present-day dwarf galaxies can place valuable constraints on the masses, formation path, and hosts of supermassive BH seeds. Using optical spectroscopy from the SDSS, we have systematically assembled the largest sample of dwarf galaxies hosting active massive BHs to date. These dwarf galaxies have stellar masses comparable to the Magellanic Clouds and contain some of the least-massive supermassive BHs known.

  9. Efficiency of Metal Mixing in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hirai, Yutaka; Saitoh, Takayuki R.

    2017-04-01

    Metal mixing plays a critical role in the enrichment of metals in galaxies. The abundance of elements such as Mg, Fe, and Ba in metal-poor stars helps us understand the metal mixing in galaxies. However, the efficiency of metal mixing in galaxies is not yet understood. Here we report a series of N-body/smoothed particle hydrodynamics simulations of dwarf galaxies with different efficiencies of metal mixing using a turbulence-induced mixing model. We show that metal mixing apparently occurs in dwarf galaxies from Mg and Ba abundances. We find that a scaling factor for metal diffusion larger than 0.01 is necessary to reproduce the measured abundances of Ba in dwarf galaxies. This value is consistent with the value expected from turbulence theory and experiments. We also find that the timescale of metal mixing is less than 40 Myr. This timescale is shorter than the typical dynamical times of dwarf galaxies. We demonstrate that the determination of a degree of scatters of Ba abundance by the observation will help us to better constrain the efficiency of metal mixing.

  10. The Local Dwarf GALAXIES:BUILDING Blocks of Massive Ones? I.THE Fornax Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Nykytyuk, T. V.

    A chemical evolution of the Local Group dwarf galaxy Fornax is considered in the framework of the merger scenario. We suppose a galactic stellar halo to be formed as separate fragments which then merge; thus, we can calculate the set of such the fragments to reproduce the observed metallicity distribution function of a galaxy. Accordingly, if dwarf galaxies were such the systems, which, once merged, have formed massive galaxies, we need to obtain only one fragment to reproduce the observed metallicity distribution function of a dwarf galaxy. To test this assumption, the stellar metallicity distribution functions of Fornax was calculated in the framework of the merger scenario. The more than one fragment was obtained for galaxy under consideration; thus, it is unlikely the systems similar to Fornax to be building blocks of massive galaxies.

  11. FORMATION OF ULTRA-COMPACT BLUE DWARF GALAXIES AND THEIR EVOLUTION INTO NUCLEATED DWARFS

    SciTech Connect

    Bekki, Kenji

    2015-10-10

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

  12. "Missing Mass" Found in Recycled Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    2007-05-01

    Astronomers studying dwarf galaxies formed from the debris of a collision of larger galaxies found the dwarfs much more massive than expected, and think the additional material is "missing mass" that theorists said should not be present in this kind of dwarf galaxy. Multiwavelength Image of NGC 5291 Multiwavelength image of NGC 5291 and dwarf galaxies around it. CREDIT: P-A Duc, CEA-CNRS/NRAO/AUI/NSF/NASA. Click on image for page of more graphics and full information The scientists used the National Science Foundation's Very Large Array (VLA) radio telescope to study a galaxy called NGC 5291, 200 million light-years from Earth. This galaxy collided with another 360 million years ago, and the collision shot streams of gas and stars outward. Later, the dwarf galaxies formed from the ejected debris. "Our detailed studies of three 'recycled' dwarf galaxies in this system showed that the dwarfs have twice as much unseen matter as visible matter. This was surprising, because they were expected to have very little unseen matter," said Frederic Bournaud, of the French astrophysics laboratory AIM of the French CEA and CNRS. Bournaud and his colleagues announced their discovery in the May 10 online issue of the journal Science. "Dark matter," which astronomers can detect only by its gravitational effects, comes, they believe, in two basic forms. One form is the familiar kind of matter seen in stars, planets, and humans -- called baryonic matter -- that does not emit much light or other type of radiation. The other form, called non-baryonic dark matter, comprises nearly a third of the Universe but its nature is unknown. The visible portion of spiral galaxies, like our own Milky Way, lies mostly in a flattened disk, usually with a bulge in the center. This visible portion, however, is surrounded by a much larger halo of dark matter. When spiral galaxies collide, the material expelled outward by the interaction comes from the galaxies' disks. For this reason, astronomers did

  13. Stellar feedback in dwarf galaxy formation.

    PubMed

    Mashchenko, Sergey; Wadsley, James; Couchman, H M P

    2008-01-11

    Dwarf galaxies pose substantial challenges for cosmological models. In particular, current models predict a dark-matter density that is divergent at the center, which is in sharp contrast with observations that indicate a core of roughly constant density. Energy feedback, from supernova explosions and stellar winds, has been proposed as a major factor shaping the evolution of dwarf galaxies. We present detailed cosmological simulations with sufficient resolution both to model the relevant physical processes and to directly assess the impact of stellar feedback on observable properties of dwarf galaxies. We show that feedback drives large-scale, bulk motions of the interstellar gas, resulting in substantial gravitational potential fluctuations and a consequent reduction in the central matter density, bringing the theoretical predictions in agreement with observations.

  14. Exploring Dwarf Galaxy Evolution through Metallicity Distributions

    NASA Astrophysics Data System (ADS)

    Ross, Teresa

    2015-01-01

    As the most numerous type of galaxy, dwarf galaxies are ideal for examining galactic evolution on small scales. Additional clues to galactic evolution come from the metallicity distribution function (MDF), which is influenced by the star formation, accretion, outflows and galaxy interactions. We derived stellar MDFs for the Leo I, Leo II, IC 1613, and Phoenix dwarf galaxies using HST images in order to examine how their evolution compares as a function of various galaxy properties. These galaxies span a range of different morphologies, masses, SFHs and distances from the MW. We fit a simple evolution model and an accretion model to the MDFs in order to quantify the effect of gas flows and enrichment within the galaxies. The MDFs of Leo II (dSph), Phoenix (dTrans) and IC 1613 (dIr) have similar shapes, though their peak metallicities differ. Additionally, we find the accretion model, over the simple model, is a better fit chemical evolution model for these three galaxies. However these best fit accretion models do not require a significant amount of additional gas to explain the MDF shapes. In contrast the chemical evolution model that best fits the narrow MDF of Leo I implies twice the additional gas accretion. The similarities in the MDF shapes of Leo II, Phoenix and IC 1613, even though these galaxies all have different morphologies, implies that the current morphology is not the driving factor in shaping the MDF of these galaxies.

  15. Dwarf Galaxies Swimming in Tidal Tails

    NASA Technical Reports Server (NTRS)

    2005-01-01

    This false-color infrared image from NASA's Spitzer Space Telescope shows little 'dwarf galaxies' forming in the 'tails' of two larger galaxies that are colliding together. The big galaxies are at the center of the picture, while the dwarfs can be seen as red dots in the red streamers, or tidal tails. The two blue dots above the big galaxies are stars in the foreground.

    Galaxy mergers are common occurrences in the universe; for example, our own Milky Way galaxy will eventually smash into the nearby Andromeda galaxy. When two galaxies meet, they tend to rip each other apart, leaving a trail, called a tidal tail, of gas and dust in their wake. It is out of this galactic debris that new dwarf galaxies are born.

    The new Spitzer picture demonstrates that these particular dwarfs are actively forming stars. The red color indicates the presence of dust produced in star-forming regions, including organic molecules called polycyclic aromatic hydrocarbons. These carbon-containing molecules are also found on Earth, in car exhaust and on burnt toast, among other places. Here, the molecules are being heated up by the young stars, and, as a result, shine in infrared light.

    This image was taken by the infrared array camera on Spitzer. It is a 4-color composite of infrared light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red). Starlight has been subtracted from the orange and red channels in order to enhance the dust features.

  16. The dwarf spheroidal galaxy Andromeda I

    SciTech Connect

    Mould, J.; Kristian, J. Mount Wilson and Las Campanas Observatories, Pasadena, CA )

    1990-05-01

    Images of Andromeda I in the visual and near-infrared show a giant branch characteristic of galactic globular clusters of intermediate metallicity. The distance of the galaxy is estimated from the tip of the giant branch to be 790 + or - 60 kpc. The physical dimensions and luminosity are similar to those of the dwarf spheroidal in Sculptor. There is no evidence for an intermediate age population in Andromeda I, and appropriate upper limits are specified. There is marginal evidence for a color gradient in the galaxy, a phenomenon not previously noted in a dwarf spheroidal. 21 refs.

  17. Massive Star Clusters in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Larsen, Søren S.

    2017-03-01

    Dwarf galaxies can have very high globular cluster specific frequencies, and the GCs are in general significantly more metal-poor than the bulk of the field stars. In some dwarfs, such as Fornax, WLM, and IKN, the fraction of metal-poor stars that belong to GCs can be as high as 20%-25%, an order of magnitude higher than the 1%-2% typical of GCs in halos of larger galaxies. Given that chemical abundance anomalies appear to be present also in GCs in dwarf galaxies, this implies severe difficulties for self-enrichment scenarios that require GCs to have lost a large fraction of their initial masses. More generally, the number of metal-poor field stars in these galaxies is today less than what would originally have been present in the form of low-mass clusters if the initial cluster mass function was a power-law extending down to low masses. This may imply that the initial GC mass function in these dwarf galaxies was significantly more top-heavy than typically observed in present-day star forming environments.

  18. Scaling Stellar Mass Estimates of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Carr, Brandon Michael; McQuinn, Kristen B.; Cannon, John M.; Dalcanton, Julianne; Dolphin, Andrew E.; Skillman, Evan D.; Williams, Benjamin F.; van Zee, Liese

    2017-01-01

    Hubble Space Telescope (HST) optical imaging of resolved stellar populations has been used to constrain the star formation history (SFH) and chemical evolution of many nearby dwarf galaxies. However, even for dwarf galaxies, the angle subtended by nearby systems can be greater than the HST field of view. Thus, estimates of stellar mass from the HST footprint do not accurately represent the total mass of the system, impacting how SFH results can be used in holistic comparisons of galaxy properties. Here, we use the SFHs of dwarfs combined with stellar population synthesis models to determine mass-to-light ratios for individual galaxies, and compare these values with measured infrared luminosities from Spitzer IRAC data. In this way, we determine what fraction of mass is not included in the HST field of view. To test our methodology, we focus on dwarfs whose stellar disks are contained within the HST observations. Then, we also apply this method to galaxies with larger angular sizes to scale the stellar masses accordingly.

  19. GIANT GALAXIES, DWARFS, AND DEBRIS SURVEY. I. DWARF GALAXIES AND TIDAL FEATURES AROUND NGC 7331

    SciTech Connect

    Ludwig, Johannes; Pasquali, Anna; Grebel, Eva K.; Gallagher, John S. III

    2012-12-01

    The Giant GAlaxies, Dwarfs, and Debris Survey (GGADDS) concentrates on the nearby universe to study how galaxies have interacted in groups of different morphology, density, and richness. In these groups, we select the dominant spiral galaxy and search its surroundings for dwarf galaxies and tidal interactions. This paper presents the first results from deep wide-field imaging of NGC 7331, where we detect only four low-luminosity candidate dwarf companions and a stellar stream that may be evidence of a past tidal interaction. The dwarf galaxy candidates have surface brightnesses of {mu}{sub r} Almost-Equal-To 23-25 mag arcsec{sup -2} with (g - r){sub 0} colors of 0.57-0.75 mag in the Sloan Digital Sky Survey filter system, consistent with their being dwarf spheroidal (dSph) galaxies. A faint stellar stream structure on the western edge of NGC 7331 has {mu}{sub g} Almost-Equal-To 27 mag arcsec{sup -2} and a relatively blue color of (g - r){sub 0} = 0.15 mag. If it is tidal debris, then this stream could have formed from a rare type of interaction between NGC 7331 and a dwarf irregular or transition-type dwarf galaxy. We compare the structure and local environments of NGC 7331 to those of other nearby giant spirals in small galaxy groups. NGC 7331 has a much lower ({approx}2%) stellar mass in the form of early-type satellites than found for M31 and lacks the presence of nearby companions like luminous dwarf elliptical galaxies or the Magellanic Clouds. However, our detection of a few dSph candidates suggests that it is not deficient in low-luminosity satellites.

  20. Are dwarf galaxies killed by reionization?

    NASA Astrophysics Data System (ADS)

    Arraki, Kenza S.; Klypin, Anatoly A.; Trujillo-Gomez, Sebastian; Ceverino, Daniel; Primack, Joel R.

    2015-01-01

    The ΛCDM cosmological model has been very successful at predicting the large-scale structure of the Universe. However, for dwarf galaxies, simulations have failed to reproduce the number and structure of satellite and isolated dwarf galaxies. The inclusion of baryons in simulations has been found to alleviate the small-scale issues within ΛCDM, such as the core-cusp, missing satellites, and too-big-to-fail problems. To address these concerns, we analyzed state-of-the-art, high-resolution hydrodynamical simulations of galaxy formation created using the ART code. These simulations model relevant physical processes of star formation and stellar feedback including stellar winds, supernovae feedback, and radiation pressure. We examined 1,000 galaxies from the VELA suite of simulations and find steep velocity functions for satellite galaxies and a large spread in the stellar halo mass relation for a given virial mass or maximum circular velocity. The star formation histories of these galaxies agree with recent observations in that they have an initial burst and then are roughly constant. Reionization does not completely suppress star formation in the majority of these galaxies and only acts to decrease the star formation rate. 73% of galaxies with virial masses greater than 108 M⊙ are luminous, which contributes to a larger abundance of these low mass objects than are observed. Analysis of these kinds of simulations can shed light on the role of baryons in the overabundance and structure problems.

  1. Choirs H I galaxy groups: The metallicity of dwarf galaxies

    SciTech Connect

    Sweet, Sarah M.; Drinkwater, Michael J.; Meurer, Gerhardt; Bekki, Kenji; Dopita, Michael A.; Nicholls, David C.; Kilborn, Virginia

    2014-02-10

    We present a recalibration of the luminosity-metallicity relation for gas-rich, star-forming dwarfs to magnitudes as faint as M{sub R} ∼ –13. We use the Dopita et al. metallicity calibrations to calibrate the relation for all the data in this analysis. In metallicity-luminosity space, we find two subpopulations within a sample of high-confidence Sloan Digital Sky Survey (SDSS) DR8 star-forming galaxies: 52% are metal-rich giants and 48% are metal-medium galaxies. Metal-rich dwarfs classified as tidal dwarf galaxy (TDG) candidates in the literature are typically of metallicity 12 + log(O/H) = 8.70 ± 0.05, while SDSS dwarfs fainter than M{sub R} = –16 have a mean metallicity of 12 + log(O/H) = 8.28 ± 0.10, regardless of their luminosity, indicating that there is an approximate floor to the metallicity of low-luminosity galaxies. Our hydrodynamical simulations predict that TDGs should have metallicities elevated above the normal luminosity-metallicity relation. Metallicity can therefore be a useful diagnostic for identifying TDG candidate populations in the absence of tidal tails. At magnitudes brighter than M{sub R} ∼ –16, our sample of 53 star-forming galaxies in 9 H I gas-rich groups is consistent with the normal relation defined by the SDSS sample. At fainter magnitudes, there is an increase in dispersion of the metallicity of our sample, suggestive of a wide range of H I content and environment. In our sample, we identify three (16% of dwarfs) strong TDG candidates (12 + log(O/H) > 8.6) and four (21%) very metal-poor dwarfs (12 + log(O/H) < 8.0), which are likely gas-rich dwarfs with recently ignited star formation.

  2. FORMATION OF DWARF SPHEROIDAL GALAXIES VIA MERGERS OF DISKY DWARFS

    SciTech Connect

    Kazantzidis, Stelios; Lokas, Ewa L.; Klimentowski, Jaroslaw; Mayer, Lucio; Knebe, Alexander

    2011-10-10

    We perform collisionless N-body simulations to investigate whether binary mergers between rotationally supported dwarfs can lead to the formation of dwarf spheroidal galaxies (dSphs). Our simulation campaign is based on a hybrid approach combining cosmological simulations and controlled numerical experiments. We select merger events from a Constrained Local Universe simulation of the Local Group (LG) and record the properties of the interacting dwarf-sized halos. This information is subsequently used to seed controlled experiments of binary encounters between dwarf galaxies consisting of exponential stellar disks embedded in cosmologically motivated dark matter halos. These simulations are designed to reproduce eight cosmological merger events, with initial masses of the interacting systems in the range {approx}(5-60) x 10{sup 7} M{sub sun}, occurring quite early in the history of the LG, more than 10 Gyr ago. We compute the properties of the merger remnants as a distant observer would and demonstrate that at least three of the simulated encounters produce systems with kinematic and structural properties akin to those of the classic dSphs in the LG. Tracing the history of the remnants in the cosmological simulation to z = 0, we find that two dSph-like objects remain isolated at distances {approx}> 800 kpc from either the Milky Way or M31. These systems constitute plausible counterparts of the remote dSphs Cetus and Tucana which reside in the LG outskirts, far from the tidal influence of the primary galaxies. We conclude that merging of rotationally supported dwarfs represents a viable mechanism for the formation of dSphs in the LG and similar environments.

  3. Color Profile Trends of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Herrmann, Kimberly A.; LITTLE THINGS Team

    2012-01-01

    Radial stellar surface brightness profiles of spiral galaxies can be classified into three types: (I) single exponential, (II) truncated: the light falls off with one exponential out to a break radius and then falls off more steeply, and (III) anti-truncated: the light falls off with one exponential out to a break radius and then falls off less steeply. Stellar surface brightness profile breaks are also found in dwarf disk galaxies, but with an additional category: (FI) flat-inside: the light is roughly constant or increasing and then falls off beyond a break. Additionally, Bakos, Trujillo, & Pohlen (2008) showed that for spirals, each profile type has a characteristic color trend with respect to the break location. Furthermore, color trends reveal information about possible stellar population changes at the breaks. Here we show color trends for the four profile types from a large multi-wavelength photometric study of dwarf disk galaxies (the 141 dwarf parent sample of the LITTLE THINGS galaxies). We explore the similarities and differences between spirals and dwarfs and also between different colors. We gratefully acknowledge funding for this research from the National Science Foundation (AST-0707563).

  4. The transmutation of dwarf galaxies: stellar populations

    NASA Astrophysics Data System (ADS)

    Koleva, Mina; Bouchard, Antoine; Prugniel, Philippe; De Rijcke, Sven; Vauglin, Isabelle

    2013-02-01

    Transition-type dwarf (TTD) galaxies share characteristics of early- and late-type dwarfs. Thus, they are suspected to be the thread that connects them. We selected 19 TTD galaxies in the nearby Universe (cz < 2900 km s-1) from the Sloan Digital Sky Survey. They span the luminosity range from ˜- 14.5 to -19.0 mag in the B band, and are located in different environments. We derive their single stellar population parameters and star formation histories, using the full spectrum fitting technique with two independent population synthesis models. Irrespective of the synthesis models, we find that these dwarfs have a relatively young mean age (around 1-2 Gyr) and low metallicities (˜- 0.7 dex). Moreover, they had approximately constant star formation rates until a few Gyr ago, associated with strong metal enrichment during the first few Gyr of their evolution. We compare these results with the results from Koleva et al., who studied dwarf elliptical (dE) galaxies in the same luminosity range. We find that (1) both samples occupy the same region in the luminosity-metallicity relation, (2) the build-up of the stellar mass in both types of galaxies is very similar, with most of the stars already formed 5 Gyr ago and (3) contrary to the dEs, TTDs are forming stars at present, but after 1 Gyr of passive evolution, their star formation histories would appear identical to that of dEs. As far as the stellar population is concerned, the transformation of TTDs into dEs is definitely possible. A star-forming dwarf galaxy can be stripped of at least a fraction of its gas, and its star formation rate can be reduced to that of the TTDs of the present sample. Continued gas removal may drive a galaxy to the state of a gas-depleted bona fide dE. However, we cannot exclude a scenario where a star-forming galaxy is rapidly transformed into an early type without passing through a noticeable `transition' phase, as suggested by the relatively small fraction of observed dEs with an

  5. Abundances in Blue Compact Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Dewolfe, O. M.; Salzer, J. J.

    1994-12-01

    The nebular abundances in dwarf galaxies provide valuable clues to their star-formation histories. We have been engaged in an extensive observational study of actively star-forming blue compact dwarf galaxies (BCDs). Our program combines optical and NIR imaging, optical spectroscopy, and HI and CO radio data in an attempt to understand the status of star-formation and chemical evolution in these systems. Luminosities of the BCDs in our sample lie in the range M_B = -13 -- -16. We have obtained high-quality optical spectra of 21 BCDs for the purpose of deriving nebular abundances. We present values for the abundances of He, N, O, S, Ne, and Ar for this sample. In addition, we have estimated O and N abundances for a sample of 7 non-dwarf starburst galaxies of higher luminosity. For the BCD sample, we find that the N/O ratio remains constant at a mean value of 0.032 +/- 0.004 with increasing O/H. This result, which extends over a factor of more than 10 in O/H, is indicative of the N being chiefly of primary origin in these systems. The value of N/O is observed to increase in the sample of larger starburst galaxies for log(O/H) above -3.5, consistent with previous studies. S/O and Ne/O are also both constant with increasing O/H, and have mean values of 0.032 +/- 0.007 and 0.135 +/- 0.013 respectively. The BCD and starburst O/H values were used to calibrate an abundance sequence for a number of key emission-line diagnostic ratios which allow us to estimate metallicities for a large sample of emission-line galaxies (ELGs) to an accuracy of 0.1 -- 0.2 dex. The calibrations were applied to spectral data from two large published samples of ELGs from the Michigan and Case surveys. We use these data, representing nearly 200 individual galaxies, to investigate the existence of a metallicity-luminosity relationship for actively star-forming galaxies. We find the ELGs exhibit a relationship very similar to that found for more quiescent dwarf and Magellanic irregular galaxies by

  6. Blue diffuse dwarf galaxies: a clearer picture

    NASA Astrophysics Data System (ADS)

    James, Bethan L.; Koposov, Sergey E.; Stark, Daniel P.; Belokurov, Vasily; Pettini, Max; Olszewski, Edward W.; McQuinn, Kristen B. W.

    2017-03-01

    The search for chemically unevolved galaxies remains prevalent in the nearby Universe, mostly because these systems provide excellent proxies for exploring in detail the physics of high-z systems. The most promising candidates are extremely metal-poor galaxies (XMPs), i.e. galaxies with <1/10 solar metallicity. However, due to the bright emission-line-based search criteria traditionally used to find XMPs, we may not be sampling the full XMP population. In 2014, we reoriented this search using only morphological properties and uncovered a population of ∼150 'blue diffuse dwarf (BDD) galaxies', and published a sub-sample of 12 BDD spectra. Here, we present optical spectroscopic observations of a larger sample of 51 BDDs, along with their Sloan Digital Sky Survey (SDSS) photometric properties. With our improved statistics, we use direct-method abundances to confirm that BDDs are chemically unevolved (7.43 < 12 + log(O/H) < 8.01), with ∼20 per cent of our sample classified as being XMP galaxies, and find that they are actively forming stars at rates of ∼1-33 × 10-2 M⊙ yr-1 in H II regions randomly embedded in a blue, low-surface-brightness continuum. Stellar masses are calculated from population synthesis models and estimated to be in the range log (M*/M⊙) ≃ 5-9. Unlike other low-metallicity star-forming galaxies, BDDs are in agreement with the mass-metallicity relation at low masses, suggesting that they are not accreting large amounts of pristine gas relative to their stellar mass. BDD galaxies appear to be a population of actively star-forming dwarf irregular (dIrr) galaxies which fall within the class of low-surface-brightness dIrr galaxies. Their ongoing star formation and irregular morphology make them excellent analogues for galaxies in the early Universe.

  7. POX 186: A Dwarf Galaxy Under Construction?

    NASA Astrophysics Data System (ADS)

    Corbin, M. R.; Vacca, W. D.

    2000-12-01

    We have obtained deep images of the ultracompact ( ~ 3'') blue compact dwarf galaxy POX 186 in the F336W, F555W, and F814W filters of the Planetary Camera of the Hubble Space Telescope. We have additionally obtained a low-resolution near ultraviolet spectrum of the object with STIS and combine this with a ground-based spectrum covering the visible continuum and emission lines. Our images confirm this object to be highly compact, with a maximum projected size of only ~ 240 pc, making it one of the smallest galaxies known. We also confirm that the outer regions of the galaxy consist of an evolved stellar population, ruling out earlier speculations that POX 186 is a protogalaxy. However, the PC images reveal the galaxy to have a highly irregular morphology, with a pronounced tidal arm on its western side. This morphology is strongly suggestive of a recent collision between two smaller components which has in turn triggered the central starburst. The F336W image also shows that the material in this tidal stream is actively star forming. Given the very small ( ~ 100 pc) sizes of the colliding components, POX 186 may be a dwarf galaxy in the early stages of formation, which would be consistent with current ``downsizing'' models of galaxy formation in which the least massive objects are the last to form. This work is supported by NASA and the Space Telescope Science Institute.

  8. Suites of Dwarfs around nearby Giant Galaxies

    NASA Astrophysics Data System (ADS)

    Karachentsev, Igor D.; Kaisina, Elena I.; Makarov, Dmitry I.

    2014-01-01

    The Updated Nearby Galaxy Catalog (UNGC) contains the most comprehensive summary of distances, radial velocities, and luminosities for 800 galaxies located within 11 Mpc from us. The high density of observables in the UNGC makes this sample indispensable for checking results of N-body simulations of cosmic structures on a ~1 Mpc scale. The environment of each galaxy in the UNGC was characterized by a tidal index Θ1, depending on the separation and mass of the galaxy's main disturber (MD). We grouped UNGC galaxies with a common MD in suites, and ranked suite members according to their Θ1. All suite members with positive Θ1 are assumed to be physical companions of the MD. About 58% of the sample are members of physical groups. The distribution of suites by the number of members, n, follows a relation N(n) ~ n -2. The 20 most populated suites contain 468 galaxies, i.e., 59% of the UNGC sample. The fraction of MDs among the brightest galaxies is almost 100% and drops to 50% at MB = -18m. We discuss various properties of MDs, as well as galaxies belonging to their suites. The suite abundance practically does not depend on the morphological type, linear diameter, or hydrogen mass of the MD, the tightest correlation being with the MD dynamical mass. Dwarf galaxies around MDs exhibit well-known segregation effects: the population of the outskirts has later morphological types, richer H I contents, and higher rates of star formation activity. Nevertheless, there are some intriguing cases where dwarf spheroidal galaxies occur at the far periphery of the suites, as well as some late-type dwarfs residing close to MDs. Comparing simulation results with galaxy groups, most studies assume the Local Group is fairly typical. However, we recognize that the nearby groups significantly differ from each other and there is considerable variation in their properties. The suites of companions around the Milky Way and M31, consisting of the Local Group, do not quite seem to be a typical

  9. Pulsar searches in nearby dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Rubio-Herrera, Eduardo; Maccarone, Thomas

    2013-03-01

    We have been undertaking a comprehensive survey for pulsars and fast radio transients in the dwarf spheroidal satellite galaxies of the Milky Way using the Green Bank Radio Telescope operating at a central frequency of 350 MHz. Our search pipeline allows the detection of periodical signals and single dispersed pulses and it is optimized to search for millisecond radio pulsars. Here we present preliminary results of the searches we have conducted in the Ursa Minoris, Draco and Leo I dwarf spheroidal satellite galaxies. Our searches have revealed no periodic signals but a few unconfirmed millisecond single pulses at various dispersion measures, possibly related to neutron stars. Detecting neutron stars in these systems can potentially help to test the existence of haloes of dark matter surrounding these systems as predicted by Dehnen & King (2006).

  10. Missing dark matter in dwarf galaxies?

    NASA Astrophysics Data System (ADS)

    Oman, Kyle A.; Navarro, Julio F.; Sales, Laura V.; Fattahi, Azadeh; Frenk, Carlos S.; Sawala, Till; Schaller, Matthieu; White, Simon D. M.

    2016-08-01

    We use cosmological hydrodynamical simulations of the APOSTLE project along with high-quality rotation curve observations to examine the fraction of baryons in ΛCDM haloes that collect into galaxies. This `galaxy formation efficiency' correlates strongly and with little scatter with halo mass, dropping steadily towards dwarf galaxies. The baryonic mass of a galaxy may thus be used to place a lower limit on total halo mass and, consequently, on its asymptotic maximum circular velocity. A number of observed dwarfs seem to violate this constraint, having baryonic masses up to 10 times higher than expected from their rotation speeds, or, alternatively, rotating at only half the speed expected for their mass. Taking the data at face value, either these systems have formed galaxies with extraordinary efficiency - highly unlikely given their shallow potential wells - or their dark matter content is much lower than expected from ΛCDM haloes. This `missing dark matter' is reminiscent of the inner mass deficit of galaxies with slowly rising rotation curves, but cannot be explained away by star formation-induced `cores' in the dark mass profile, since the anomalous deficit applies to regions larger than the luminous galaxies themselves. We argue that explaining the structure of these galaxies would require either substantial modification of the standard ΛCDM paradigm or else significant revision to the uncertainties in their inferred mass profiles, which should be much larger than reported. Systematic errors in inclination may provide a simple resolution to what would otherwise be a rather intractable problem for the current paradigm.

  11. GLOBAL H I KINEMATICS IN DWARF GALAXIES

    SciTech Connect

    Stilp, Adrienne M.; Dalcanton, Julianne J.; Warren, Steven R.; Skillman, Evan; Ott, Juergen; Koribalski, Baerbel

    2013-03-10

    H I line widths are typically interpreted as a measure of interstellar medium turbulence, which is potentially driven by star formation (SF). In an effort to better understand the possible connections between line widths and SF, we have characterized H I kinematics in a sample of nearby dwarf galaxies by co-adding line-of-sight spectra after removing the rotational velocity to produce average global H I line profiles. These ''superprofiles'' are composed of a central narrow peak ({approx}6-10 km s{sup -1}) with higher-velocity wings to either side that contain {approx}10%-15% of the total flux. The superprofiles are all very similar, indicating a universal global H I profile for dwarf galaxies. We compare characteristics of the superprofiles to various galaxy properties, such as mass and measures of SF, with the assumption that the superprofile represents a turbulent peak with energetic wings to either side. We use these quantities to derive average scale heights for the sample galaxies. When comparing to physical properties, we find that the velocity dispersion of the central peak is correlated with ({Sigma}{sub HI}). The fraction of mass and characteristic velocity of the high-velocity wings are correlated with measures of SF, consistent with the picture that SF drives surrounding H I to higher velocities. While gravitational instabilities provide too little energy, the SF in the sample galaxies does provide enough energy through supernovae, with realistic estimates of the coupling efficiency, to produce the observed superprofiles.

  12. DWARF GALAXIES AND THE COSMIC WEB

    SciTech Connect

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

    2013-02-01

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

  13. Broken Surface Brightness Profiles in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Herrmann, Kimberly A.; Hunter, D. A.; Zhang, H. X.; LITTLE THINGS Team

    2011-01-01

    Recently it has been well shown that there are three different surface brightness profile types in spiral galaxies: (I) the minority, where the light falls off with a single exponential; (II) truncated, the majority, where the light falls off with one exponential to a break radius and then falls off more steeply; and (III) anti-truncated, where the light falls off with a more shallow exponential beyond the break radius. Additionally, Bakos, Trujillo, & Pohlen (2008) showed that each type has a characteristic color trend with respect to the break location. In dwarf disk galaxies, however, there is a fourth type which is perhaps a special Type II case: the light profile is flat on the inside and then falls off exponentially beyond the break radius. We will show the different color trends for these four profile types from a large photometric study of dwarf disk galaxies and explore the ramifications of the differences between spirals and dwarfs. We gratefully acknowledge funding for this research from the National Science Foundation (AST-0707563).

  14. Turbulence and Star Formation in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hollyday, Gigja; Hunter, Deidre Ann; Little Things Team

    2015-01-01

    We are interested in understanding the nature and role of turbulence in the interstellar medium of dwarf irregular galaxies. Turbulence, resulting from a variety of processes, is a potential source for cloud formation, and thus star formation. We have undertaken an indirect analysis of turbulence via the third (skewness) and fourth (kurtosis) moments of the distribution of atomic hydrogen gas densities using the LITTLE THINGS data for a 40-count sample of nearby (<10.3 Mpc) dwarf galaxies. We followed the formulism used by Burkhart et al. (2010) in a study of the SMC. We found that there is evidence of turbulence in dwarf galaxies at a level comparable to that found in the SMC, but we have found no correlation between integrated star formation rates and integrated kurtosis values nor a clear correlation between kurtosis as a function of radius with gas surface density and star formation profiles. We are grateful for a summer internship provided by the Research Experiences for Undergraduates program at Northern Arizona University, run by Dr. Kathy Eastwood and Dr. David Trilling and funded by the National Science Foundation through grant AST-1004107.

  15. Angular Momentum in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Del Popolo, A.

    We study the ``angular momentum catastrophe" in the framework of interaction among baryons and dark matter through dynamical friction. By means of Del Popolo (2009) model we simulate 14 galaxies similar to those investigated by van den Bosch, Burkert and Swaters (2001), and calculate the distribution of their spin parameters and the angular momenta. Our model gives the angular momentum distribution which is in agreement with the van den Bosch et al. observations. Our result shows that the ``angular momentum catastrophe" can be naturally solved in a model that takes into account the baryonic physics and the exchange of energy and angular momentum between the baryonic clumps and dark matter through dynamical friction.

  16. How Typical Are the Local Group Dwarf Galaxies?

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Dolphin, Andrew E.; Dalcanton, Julianne J.; Skillman, Evan D.; Holtzman, Jon; Williams, Benjamin F.; Gilbert, Karoline M.; Seth, Anil C.; Cole, Andrew; Gogarten, Stephanie M.; Rosema, Keith; Karachentsev, Igor D.; McQuinn, Kristen B. W.; Zaritsky, Dennis

    2011-12-01

    We compare the cumulative star formation histories (SFHs) of Local Group (LG) dwarf galaxies with those in the volume-limited ACS Nearby Galaxy Survey Treasury (ANGST) sample (D <~ 4 Mpc), in order to understand how typical the LG dwarf galaxies are relative to those in the nearby universe. The SFHs were derived in a uniform manner from high-quality optical color-magnitude diagrams constructed from Hubble Space Telescope imaging. We find that the mean cumulative SFHs of the LG dwarfs are comparable to the mean cumulative SFHs of the ANGST sample for the three different morphological types (dwarf spheroidals/ellipticals: dSph/dE; dwarf irregulars: dI; transition dwarfs: dTrans). We also discuss effects such as population gradients and systematic uncertainties in the stellar models that may influence the derived SFHs. Both the ANGST and LG dwarf galaxies show a consistent and strong morphology-density relationship, emphasizing the importance of environment in the evolution of dwarf galaxies. Specifically, we confirm that dIs are found at lower densities and higher luminosities than dSphs, within this large sample. We also find that dTrans are located in similar environments to those occupied by dwarf irregular galaxies, but have systematically lower luminosities that are more comparable to those of dwarf spheroidals. The similarity of the SFHs and morphology-density relationships of the LG and ANGST dwarf galaxies suggests that the LG dwarfs are a good representation of dwarf galaxies in the local universe. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive 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.

  17. The RSA survey of dwarf galaxies, 1: Optical photometry

    NASA Technical Reports Server (NTRS)

    Vader, J. Patricia; Chaboyer, Brian

    1994-01-01

    We present detailed surface photometry, based on broad B-band charge coupled device (CCD) images, of about 80 dwarf galaxies. Our sample represents approximately 10% of all dwarf galaxies identified in the vicinity of Revised Shapley-Ames (RSA) galaxies on high resolution blue photographic plates, referred to as the RSA survey of dwarf galaxies. We derive global properties and radial surface brightness profiles, and examine the morphologies. The radial surface brightness profiles of dwarf galaxies, whether early or late type, display the same varieties in shape and complexity as those of classical giant galaxies. Only a few are well described by a pure r(exp 1/4) law. Exponential profiles prevail. Features typical of giant disk galaxies, such as exponential profiles with a central depression, lenses, and even, in one case (IC 2041), a relatively prominent bulge are also found in dwarf galaxies. Our data suggest that the central region evolves from being bulge-like, with an r(exp 1/4) law profile, in bright galaxies to a lens-like structure in dwarf galaxies. We prove detailed surface photometry to be a helpful if not always sufficient tool in investigating the structure of dwarf galaxies. In many cases kinematic information is needed to complete the picture. We find the shapes of the surface brightness profiles to be loosely associated with morphological type. Our sample contains several new galaxies with properties intermediate between those of giant and dwarf ellipticals (but no M32-like objects). This shows that such intermediate galaxies exist so that at least a fraction of early-type dwarf ellipticals is structurally related to early-type giants instead of belonging to a totally unrelated, disjunct family. This supports an origin of early-type dwarf galaxies as originally more massive systems that acquired their current morphology as a result of substantial, presumable supernova-driven, mass loss. On the other hand, several early-type dwarfs in our sample are

  18. Accretion phenomena in nearby star-forming dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Annibali, F.; Tosi, M.; Aloisi, A.; Bellazzini, M.; Buzzoni, A.; Cignoni, M.; Ciotti, L.; Cusano, F.; Nipoti, C.; Sacchi, E.; Paris, D.; Romano, D.

    2017-03-01

    We present two pilot studies for the search and characterization of accretion events in star-forming dwarf galaxies. Our strategy consists of two complementary approaches: i) the direct search for stellar substructures around dwarf galaxies through deep wide-field imaging, and ii) the characterization of the chemical properties in these systems up to large galacto-centric distances. We show our results for two star-forming dwarf galaxies, the starburst irregular NGC 4449, and the extremely metal-poor dwarf DDO 68.

  19. Star formation in proto dwarf galaxies

    NASA Technical Reports Server (NTRS)

    Noriega-Crespo, A.; Bodenheimer, P.; Lin, D. N. C.; Tenorio-Tagle, G.

    1990-01-01

    The effects of the onset of star formation on the residual gas in primordial low-mass Local-Group dwarf spheroidal galaxies is studied by a series of hydrodynamical simulations. The models have concentrated on the effect of photoionization. The results indicate that photoionization in the presence of a moderate gas density gradient can eject most of the residual gas on a time scale of a few 10 to the 7th power years. High central gas density combined with inefficient star formation, however, may prevent mass ejection. The effect of supernova explosions is discussed briefly.

  20. Seeking footprints of the primeval Universe in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Hidalgo, Sebastian L.

    2015-03-01

    We present the star formation histories (SFHs) of four isolated dwarf galaxies, Cetus, Tucana, LGS-3, and Phoenix, as a function of galactocentric radius. Our results suggest that beyond some distance from the center, there are no significative differences in fundamental properties of these galaxies, such as the star formation rate (SFR) or age-metallicity relation (AMR). The stellar content of this region would be composed of old (>~ 10.5 Gyr) metal-poor stars only. In the innermost regions, dwarf galaxies appear to have formed stars during time intervals which duration varies from galaxy to galaxy. This extended star formation produces the dichotomy between dwarf spheroidal (dSph) and dwarf Transition (dTr) galaxy types.

  1. THE OUTER DISKS OF DWARF IRREGULAR GALAXIES

    SciTech Connect

    Hunter, Deidre A.; Massey, Philip; Wilsey, Nick; Riabokin, Malanka; Elmegreen, Bruce G.; Oh, Se-Heon; Anderson, Ed; Nordgren, Tyler E. E-mail: phil.massey@lowell.edu E-mail: riabokin@msu.edu E-mail: seheon-oh@ast.uct.ac.za E-mail: tyler_nordgren@redlands.edu

    2011-10-15

    In order to explore the properties of extreme outer stellar disks, we obtained ultra-deep V and GALEX ultraviolet (UV) images of four dwarf irregular galaxies and one blue compact dwarf galaxy, and ultra-deep B images of three of these. Our V-band surface photometry extends to 29.5 mag arcsec{sup -2}. We convert the FUV and V-band photometry, along with H{alpha} photometry obtained in a larger survey, into radial star formation rate profiles that are sensitive to timescales from 10 Myr to the lifetime of the galaxy. We also obtained H I-line emission data and compare the stellar distributions, surface brightness profiles, and star formation rate profiles to H I-line emission maps, gas surface density profiles, and gas kinematics. Our data lead us to two general observations. First, the exponential disks in these irregular galaxies are extraordinarily regular. We observe that the stellar disks continue to decline exponentially as far as our measurements extend. In spite of lumpiness in the distribution of young stars and H I distributions and kinematics that have significant unordered motions, sporadic processes that have built the disks-star formation, radial movement of stars, and perhaps even perturbations from the outside-have, nevertheless, conspired to produce standard disk profiles. Second, there is a remarkable continuity of star formation throughout these disks over time. In four out of five of our galaxies the star formation rate in the outer disk measured from the FUV tracks that determined from the V-band, to within factors of five, requiring star formation at a fairly steady rate over the galaxy's lifetime. Yet, the H I surface density profiles generally decline with radius more shallowly than the stellar light, and the gas is marginally gravitationally stable against collapse into clouds. Outer stellar disks are challenging our concepts of star formation and disk growth and provide a critical environment in which to understand processes that mold

  2. Evolution of dwarf galaxy properties in local group environments

    NASA Astrophysics Data System (ADS)

    Arraki, Kenza Sigrid

    Understanding galaxy evolution depends on connecting large-scale structures determined by the ACDM model with, at minimum, the small-scale physics of gas, star formation, and stellar feedback. Formation of galaxies within dark matter halos is sensitive to the physical phenomena occurring within and around the halo. This is especially true for dwarf galaxies, which have smaller potential wells and are more susceptible to the effects of tidal stripping and gas ionization and removal than larger galaxies. At dwarf galaxies scales comparisons of dark matter-only simulations with observations has unveiled various differences such as the core-cusp, the missing satellites, and the too big to fail problems. We have run suites of collisionless and hydrodynamical simulations of dwarf galaxies evolution in massive host environments to address these issues. We performed controlled, numerical simulations, which mimic the effects of baryons, in order to examine the assumptions implicitly made by dark matter-only simulations. The too big to fail problem is due to the overabundance of relatively massive, dense satellite galaxies found in simulations of Milky Way-like environments. We found that the removal of a small baryonic component from the central regions of forming dwarf spheroidal galaxies and the inclusion of a disk component in the host galaxy can substantially reduce the central dark matter density of satellites, bringing simulations and observations of satellites into agreement. Additionally, we studied hydrodynamical simulations of massive host galaxies and their surrounding dwarf galaxy populations. The VELA simulation suite of cosmological zoom-in simulations is run with the ART code, stochastic star formation, and stellar feedback (supernovae feedback, stellar winds, radiation pressure, and photoionization pressure). The suite includes host galaxies with Mvir(z=0)=1011-10 12M ȯ and their satellite dwarf galaxies and local isolated dwarf galaxies around each primary

  3. Testing modified gravity with dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Haghi, Hosein; Amiri, Vahid

    2016-12-01

    The observed velocity dispersion of the classical dwarf spheroidal (dSph) galaxies of the Milky Way (MW) requires the Newtonian stellar mass-to-light (M*/L) ratios in the range of about 10 to more than 100 solar units that are well outside the acceptable limit predicted by stellar population synthesis models. Using Jeans analysis, we calculate the line-of-sight velocity dispersion (σlos) of stars in eight MW dSphs in the context of the modified gravity (MOG) theory of Moffat, assuming a constant M*/L ratio without invoking the exotic cold dark matter. First, we use the weak field approximation of MOG and assume the two parameters α and μ of the theory to be constant as has already been inferred from fitting to the observed rotational data of The H I Nearby Galaxy Survey catalogue of galaxies. We find that the derived M*/L ratios for almost all dSphs are too large to be explained by the stellar population values. In order to fit the line-of-sight velocity dispersions of the dSph with reasonable M*/L values, we must vary α and μ on a case by case basis. A common pair of values cannot be found for all dSphs. Comparing with the values found from rotation curve fitting, it appears that μ correlates strongly with galaxy luminosity, shedding doubt on it as a universal constant.

  4. Stellar and Gaseous Properties of Tidal Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Duc, P.-A.; Mirabel, I. F.; Brinks, E.

    Small galaxies may form as a result of interactions between galaxies. This idea is consistent with a number of observations of the outer regions of interacting systems: optical condensations, as luminous as dwarf galaxies, are often found at the end of tidal tails. We will present multi--wavelength observations of a few systems and highlight the existence of a new class of objects, which have been named ``tidal dwarf galaxies'', that are made of material pulled out from colliding galaxies. They are gas--rich, associated with massive HI clouds and, like traditional dwarf irregular and blue compact dwarf galaxies, they host active star formation regions. However, they have a high metallicity for their mass. This is the very result of their peculiar mode of formation which is based on cosmic recycling. Part of their stellar populations consists of young stars formed from the collapse in the intergalactic medium of tidally expelled HI clouds, while another fraction is composed of old stars pulled out from the disks of the parent galaxies. We have found for the first time evidence that some tidal dwarfs might already be gravitationally bound and therefore have become independent galaxies. Systematic studies of the properties of dwarf galaxies in different environments are underway in order to estimate the fraction of recycled objects among them.

  5. Effects of Tides on Milky Way Dwarf Satellite Galaxies

    NASA Astrophysics Data System (ADS)

    Wang, Mei-Yu; Strigari, Louis; Fattahi, Azadeh; Frenk, Carlos S.; Cooper, Andrew; Lovell, Mark; Navarro, Julio F.; Sawala, Till; Zentner, Andrew

    2017-01-01

    Using detailed observations of the Local Group to study wide-ranging questions in galaxy formation and dark matter physics - has become a rich field over the past decade. In this talk, I will present frameworks that address some of these questions by combining high-precision stellar kinematic measurements with state-of-art cosmological hydrodynamical N-body simulations. I will demonstrate that the properties of dark matter subhalo of individual satellite galaxies implied by stellar kinematic data can be linked to the galaxy evolution mechanisms such as infall time and the gravitational tidal interaction with Milky Way potential. In the cold dark matter (CDM) scenario, some dwarf galaxies explicitly require to be shaped under significant gravitational tidal forces, which will leave imprints on their stellar distribution and kinematics. I will discuss how these features could serve as a test to the nature of dark matter or stellar feedback strength. I will also discuss how we can study the tidally “disturbed” or even “destroyed” satellite galaxies as building blocks to our Milky Way stellar halo by understanding the properties of their progenitors and observation limit imposed by current and future surveys.

  6. The Spectral Energy Distributions of Interacting Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Liss, Sandra; Johnson, Kelsey E.; Stierwalt, Sabrina; Kallivayalil, Nitya; Besla, Gurtina; Patton, David R.; Privon, George C.

    2016-01-01

    We present spectral energy distributions (SEDs) for the TiNy Titans survey, the first systematic study of interactions between dwarf galaxies. Galaxy interactions are known to be of fundamental importance to the evolution of massive galaxies -- they have been observed to impact morphology, star formation rates, and ISM composition. Such interactions also occur frequently between low mass dwarf galaxies, but this process is poorly understood and largely overlooked in comparison. Although the majority of mergers at all redshifts are expected to take place between low mass galaxies, until now there have not been comparable systematic studies of dwarf galaxy interactions, leaving open the question of whether interactions between low mass galaxies can strongly affect their own evolution. The TiNy Titans survey, a complete sample of isolated dwarf galaxy pairs selected from the Sloan Digital Sky Survey (SDSS), is specifically designed to address this gap in our understanding of galaxy evolution. The SEDs presented here, generated from archival WISE, SDSS, and GALEX photometric data, allow us to characterize the typical interacting dwarf galaxy, as well as quantify the deviations from this average distribution. We also present trends in the SEDs as a function of projected radial separation, a proxy for interaction stage.

  7. Star Formation in Dwarf Galaxies: Life in a Rough Neighborhood

    SciTech Connect

    Murray, S

    2003-10-16

    Star formation within dwarf galaxies is governed by several factors. Many of these factors are external, including ram-pressure stripping, tidal stripping, and heating by external UV radiation. The latter, in particular, may prevent star formation in the smallest systems. Internal factors include negative feedback in the form of UV radiation, winds and supernovae from massive stars. These act to reduce the star formation efficiency within dwarf systems, which may, in turn, solve several theoretical and observational problems associated with galaxy formation. In this contribution, we discuss our recent work being done to examine the importance of the many factors in the evolution of dwarf galaxies.

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

    PubMed

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

    2003-05-29

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

  9. Characterizing the population of active galactic nuclei in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Baldassare, Vivienne F.; Reines, Amy E.; Gallo, Elena; Greene, Jenny E.

    2017-01-01

    Clues to super-massive black hole (BH) formation and growth reside in the population and properties of BHs in local dwarf galaxies. The masses of BHs in these systems are our best observational constraint on the masses of the first BH "seeds" at high redshift. Moreover, present-day dwarf galaxies are unlikely to have undergone major mergers, making them a relatively pristine testbed for studying triggers of BH accretion. However, in order to find BHs in dwarf galaxies outside the Local Group, it is necessary to search for signatures of accretion, i.e., active galactic nuclei (AGN). Until recently, only a handful of dwarf galaxies were known to contain AGN. However, large surveys such as the SDSS have led to the production of samples of over a hundred dwarf galaxies with AGN signatures (see e.g., Reines et al. 2013). My dissertation work has involved in-depth, multi-wavelength follow-up of nearby (z<0.055) dwarf galaxies with optical spectroscopic AGN signatures in SDSS.I analyzed high resolution spectra of dwarf galaxies with narrow-line AGN, which led to the discovery of a 50,000 MSun BH in the nucleus of RGG 118 - the smallest BH yet reported in a galaxy nucleus (Baldassare et al. 2015). I also used multi-epoch optical spectroscopy to study the nature of broad H-alpha emission in dwarf galaxies. A characteristic signature of dense gas orbiting around a BH, broad emission can also be produced by transient stellar processes. I showed that broad H-alpha in star-forming dwarf galaxies fades over a baseline of 5-10 years, and is likely produced by e.g., a Type II SN as opposed to an AGN. However, broad emission in dwarf galaxies with AGN/composite narrow lines is persistent and consistent across observations, suggesting an AGN origin (Baldassare et al. 2016). Finally, I analyzed X-ray and UV observations of dwarf galaxies with broad and narrow-line AGN signatures. All targets had nuclear X-ray detections at levels significantly higher than expected from X-ray binaries

  10. Chemo-Dynamical Evolution of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hensler, G.

    Dwarf galaxies are characterized by low masses, by their morphological variety, and by low metallicities on average as well as by their scatter in the gas fraction-to-metallicity relation. Because of the low binding energy even under the assumption of a dark matter halo their evolution should be dominated by supernova-driven galactic-wind mass loss which inherently leads to metallicity depletion with preference of particular abundances. Since the gas phases of the interstellar medium evolve dynamically, energetically and chemically differently, their interactions have to be considered by the chemo-dynamical treatment of their evolution. 1d and recent 2d chemo-dynamical models will be presented and compared with particular concern to star-formation episodes, starbursts, star-formation self-regulation, and structural effects which are caused by different environmental conditions and initial parameters. In addition, it is studied to what amount these will affect the abundance ratios.

  11. HI--line Observations of Low Surface Brightness Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Thomas, D. L.; Taylor, C. L.; Brinks, E.; Skillman, E. D.

    1995-12-01

    We present HI--line observations of low--surface brightness (LSB) dwarf galaxies to survey their nearby environment (r <= 100 kpc) for HI bright companions. It is possible that star formation histories in dwarf galaxies can be strongly affected by interactions with nearby companions. If massive star formation can be triggered by galaxy--galaxy interactions, then the absence of such interactions likely explains the lack of star formation in LSB dwarf galaxies relative to their high surface brightness counterparts (HII galaxies). Taylor et al. (1995, ApJS, 99, 427) surveyed a sample of HII galaxies and found that 60% of the sample had nearby HI companions. In our sample of 18 LSB galaxies, we found that only 22% had companions detectable in HI. Furthermore, we find that the peak column densities for the LSB sample are all below {10(21) } HI atoms cm(-2) . This is below the empirically established level needed for star formation to occur in dwarf galaxies as discussed by Skillman (1987, in Star Formation in Galaxies, ed. C.J. Lonsdale Persson (NASA Conf. Pub. CP--2466), p.263), Kennicutt (1989, ApJ, 344, 685), van der Hulst et al. (1993, AJ, 106, 548), and Taylor et al. (1994, AJ, 107, 971).

  12. Cold H I in faint dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Patra, Narendra Nath; Chengalur, Jayaram N.; Karachentsev, Igor D.; Kaisin, Serafim S.; Begum, Ayesha

    2016-03-01

    We present the results of a study of the amount and distribution of cold atomic gas, as well its correlation with recent star formation in a sample of extremely faint dwarf irregular galaxies. Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) and its extension, FIGGS2. We use two different methods to identify cold atomic gas. In the first method, line-of-sight H I spectra were decomposed into multiple Gaussian components and narrow Gaussian components were identified as cold H I. In the second method, the brightness temperature (TB ) is used as a tracer of cold H I. We find that the amount of cold gas identified using the TB method is significantly larger than the amount of gas identified using Gaussian decomposition. We also find that a large fraction of the cold gas identified using the TB method is spatially coincident with regions of recent star formation, although the converse is not true. That is only a small fraction of the regions with recent star formation are also covered by cold gas. For regions where the star formation and the cold gas overlap, we study the relationship between the star formation rate density and the cold H I column density. We find that the star formation rate density has a power-law dependence on the H I column density, but that the slope of this power law is significantly flatter than that of the canonical Kennicutt-Schmidt relation.

  13. On the tidal disruption of dwarf spheroidal galaxies around the galaxy

    NASA Astrophysics Data System (ADS)

    Oh, K. S.; Lin, D. N. C.; Aarseth, S. J.

    1995-03-01

    The Milky Way induces a strong tidal perturbation on its satellite dwarf spheroidal galaxies. We present numerical simulations of tidal interactions between these low-density dwarf spheroidal galaxies and the Milky Way. Our results indicate that dwarf spheroidal galaxies with limiting radius much larger than the theoretical tidal radius are unstable and likely to be tidally disrupted on a Hubble-time. However, dwarf spheroidal galaxies can survive over a Hubble time if their limiting radii are less than twice their tidal radii at perigalacticon. In a galaxy which is undergoing tidal disruption, (1) the projected surface density is flattened in the galaxy's orbital plane and follows a power-law distribution from the galaxy's center, (2) the velocity dispersion is sustained at the current virial equilibrium value, and (3) the central density ofthe residual remnant is maintained even after most of the initial mass is lost. Beyond the tidal radius, the escapers have a radial velocity gradient along the azimuthal direction of the galaxy's motion. When compared with observational data, our theoretical results are consistent with the scenarios that (1) some dwarf spheroidal galaxies contain dark matter, and (2) some dwarf spheroidal galaxies may be part of the debris that was tidally torn from the Large Magellanic Cloud (LMC) by the Galaxy.

  14. The prevalence of dwarf galaxy compact groups over cosmic time

    NASA Astrophysics Data System (ADS)

    Wiens, Christopher

    2017-01-01

    Galaxy interactions are critical to the evolution of the universe, influencing everything from star formation to the structure of the known universe. By studying galaxy interactions through computer simulations, we are instantaneously able to observe processes that normally take billions of years. “Compact groups” are extremely dense assemblies of at least 3 but typically no more than 10 galaxies that are interacting gravitationally. These groups yield much information about galaxy interactions and mergers in dense environments but are difficult to observe at high redshifts. Compact groups of only dwarf galaxies probe a regime of galaxy evolution that has been hypothesized to be common in the early universe. Here we investigate the populations of such dwarf galaxy compact groups in the Millennium II simulation. Millennium II is a massive n-body simulation of cold dark matter particles on a time scale equivalent to the known universe; allowing us to access to high redshift galaxies and the ability to track their descendants. Our preliminary findings indicate that these dwarf galaxy compact groups do exist in the Millennium II simulation. In the simulation, there is a non-inconsequential number of dwarf compact groups with an evolutionary track that mirrors the more massive compact groups with a peak in groups around a redshift of 2.

  15. The origin of dwarf galaxies, cold dark matter, and biased galaxy formation

    NASA Technical Reports Server (NTRS)

    Dekel, A.; Silk, J.

    1986-01-01

    A reexamination is conducted of the formation of dwarf, diffuse, metal-poor galaxies due to supernova-driven winds, in view of data on the systematic properties of dwarfs in the Local Group and Virgo Cluster. The critical condition for global gas loss as a result of the first burst of star formation is that the virial velocity lie below an approximately 100 km/sec critical value. This leads, as observed, to two distinct classes of galaxies, encompassing the diffuse dwarfs, which primarily originate from typical density perturbations, and the normal, brighter galaxies, including compact dwarfs, which can originate only from the highest density peaks. This furnishes a statistical biasing mechanism for the preferential formation of bright galaxies in denser regions, enhancing high surface brightness galaxies' clustering relative to the diffusive dwarfs.

  16. Faint dwarf galaxies in Hickson Compact Group 90*

    NASA Astrophysics Data System (ADS)

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

    2016-12-01

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

  17. THE AROMATIC FEATURES IN VERY FAINT DWARF GALAXIES

    SciTech Connect

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

    2011-04-01

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

  18. Galaxies on Top of Quasars: Probing Dwarf Galaxies in the SDSS

    NASA Astrophysics Data System (ADS)

    Straka, Lorrie; York, D. G.; Noterdaeme, P.; Srianand, R.; Bowen, D. V.; Khare, P.; Bishof, M.; Whichard, Z.; Kulkarni, V. P.

    2013-07-01

    Absorption lines from galaxies at intervening redshifts in quasar spectra are sensitive probes of metals and gas that are otherwise invisible due to distance or low surface brightness. However, in order to determine the environments these absorption lines arise in, we must detect these galaxies in emission as well. Galaxies on top of quasars (GOTOQs) are low-z galaxies found intervening with background quasars in the SDSS. These galaxies have been flagged for their narrow galactic emission lines present in quasar spectra in the SDSS. Typically, the low-z nature of these galaxies allows them to be easily detected in SDSS imaging. However, a number of GOTOQs (about 10%), despite being detected in spectral emission, are NOT seen in SDSS imaging. This implies that these may be dark galaxies, dwarf galaxies, or similarly low surface brightness galaxies. Additionally, about 25% of those detected in imaging are dwarf galaxies according to their L* values. Dwarf galaxies have long been underrepresented in observations compared to theory and are known to have large extents in dark matter. Given their prevalence here in our sample we must ask what role they play in quasar absorption line systems (QSOALS). Recent detections of 21-cm galaxies with few stars imply that aborted star formation in dark matter sub halos may produce QSOALS. Thus, this sub sample of galaxies offers a unique technique for probing dark and dwarf galaxies. The sample and its properties will be discussed, including star formation rates and dust estimates, as well as prospects for the future.

  19. Wave Dark Matter and Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Parry, Alan R.

    We explore a model of dark matter called wave dark matter (also known as scalar field dark matter and boson stars) which has recently been motivated by a new geometric perspective by Bray. Wave dark matter describes dark matter as a scalar field which satisfies the Einstein-Klein-Gordon equations. These equations rely on a fundamental constant Upsilon (also known as the "mass term'' of the Klein-Gordon equation). Specifically, in this dissertation, we study spherically symmetric wave dark matter and compare these results with observations of dwarf spheroidal galaxies as a first attempt to compare the implications of the theory of wave dark matter with actual observations of dark matter. This includes finding a first estimate of the fundamental constant Upsilon. In the introductory Chapter 1, we present some preliminary background material to define and motivate the study of wave dark matter and describe some of the properties of dwarf spheroidal galaxies. In Chapter 2, we present several different ways of describing a spherically symmetric spacetime and the resulting metrics. We then focus our discussion on an especially useful form of the metric of a spherically symmetric spacetime in polar-areal coordinates and its properties. In particular, we show how the metric component functions chosen are extremely compatible with notions in Newtonian mechanics. We also show the monotonicity of the Hawking mass in these coordinates. Finally, we discuss how these coordinates and the metric can be used to solve the spherically symmetric Einstein-Klein-Gordon equations. In Chapter 3, we explore spherically symmetric solutions to the Einstein-Klein-Gordon equations, the defining equations of wave dark matter, where the scalar field is of the form f(t, r) = eiotF(r) for some constant o ∈ R and complex-valued function F(r). We show that the corresponding metric is static if and only if F( r) = h(r)eia for some constant alpha ∈ R and real-valued function h(r). We describe the

  20. Star Formation in Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Dohm-Palmer, Robbie Christopher

    I have explored the star formation histories of the dwarf irregular galaxies Sextans A and GR 8. I measured photometry of individual stars from images taken by the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. With the photometry I constructed color-magnitude diagrams (CMDs) in the B, V, and I. I investigated the errors in the photometry extraction, and conducted artificial star tests to measure the photometric limits. The high resolution of the Hubble Space Telescope allowed photometric measurements that were far more accurate than ground-based observations. For galaxies at these distances (1-2 Mpc), the accuracy of stellar photometry from ground-based observations is limited by crowding of stellar images. The high accuracy photometry showed a clear separation of the main sequence from the massive, blue, core He-burning stars (HeB). These are stars in the bluest extent of the so-called 'blue-loop' phase of stellar evolution. This is the first time this phase of evolution has been clearly identified in a low metallicity system. The distributions of stars in the CMDs agreed very well with stellar evolution model predictions. I have used the CMDs to calculate the recent star formation histories of both galaxies. The main sequence luminosity function provided the star formation rate (SFR) over the past ~50 Myr. I developed a new technique for calculating the SFR from the blue HeB luminosity function. Furthermore, the blue HeB evolutionary phase has a one-to-one relation between age and magnitude. This allowed me to calculate the position, as well as the strength of star formation over the past ~500 Myr. The star formation was found in concentrated regions. These regions are of order 100 pc across and last of order 100 Myr. The regions were found near the highest density HI gas. I estimated the gas-to-star conversion efficiency to be 5-10%. The results from GR 8 suggest that the star forming gas clouds may be self-gravitating, and that each cloud

  1. DISSECTING EARLY-TYPE DWARF GALAXIES INTO THEIR MULTIPLE COMPONENTS

    SciTech Connect

    Janz, J.; Laurikainen, E.; Salo, H.; Lisker, T.; Peletier, R. F.; Den Brok, M.; Niemi, S.-M.; Toloba, E.; Hensler, G.

    2012-02-15

    Early-type dwarf galaxies, once believed to be simple systems, have recently been shown to exhibit an intriguing diversity in structure and stellar content. To analyze this further, we started the SMAKCED project (Stellar content, MAss and Kinematics of Cluster Early-type Dwarfs, http://www.smakced.net) and obtained deep H-band images for 101 early-type dwarf galaxies in the Virgo Cluster in a brightness range of -19 mag {<=} M{sub r} {<=} -16 mag, typically reaching a signal-to-noise ratio of 1 per pixel of {approx}0.''25 at surface brightnesses {approx}22.5 mag arcsec{sup -2} in the H band. Here we present the first results of decomposing their two-dimensional light distributions. This is the first study dedicated to early-type dwarf galaxies using the two-dimensional multi-component decomposition approach, which has been proven to be important for giant galaxies. Armed with this new technique, we find more structural components than previous studies: only a quarter of the galaxies fall into the simplest group, namely, those represented by a single Sersic function, optionally with a nucleus. Furthermore, we find a bar fraction of 18%. We also detect a similar fraction of lenses which appear as shallow structures with sharp outer edges. Galaxies with bars and lenses are found to be more concentrated toward the Virgo galaxy center than the other sample galaxies.

  2. EVOLUTIONARY TRACKS OF TIDALLY STIRRED DISKY DWARF GALAXIES

    SciTech Connect

    Lokas, Ewa L.; Kazantzidis, Stelios; Mayer, Lucio E-mail: stelios@mps.ohio-state.edu

    2011-09-20

    Using collisionless N-body simulations, we investigate the tidal evolution of late-type, rotationally supported dwarfs inside Milky Way sized host galaxies. Our study focuses on a wide variety of dwarf orbital configurations and initial structures. During the evolution, the disky dwarfs undergo strong mass loss, the stellar disks are transformed into spheroids, and rotation is replaced by random motions of the stars. Thus, the late-type progenitors are transformed into early-type dwarfs as envisioned by the tidal stirring model for the formation of dwarf spheroidal (dSph) galaxies in the Local Group. We determine the photometric properties of the dwarfs, including the total visual magnitude, the half-light radius, and the central surface brightness as they would be measured by an observer near the galactic center. Special emphasis is also placed on studying their kinematics and shapes. We demonstrate that the measured values are biased by a number of observational effects including the increasing angle of the observation cone near the orbital pericenter, the fact that away from the pericenter the tidal tails are typically oriented along the line of sight, and the fact that for most of the evolution the stellar components of the dwarfs are triaxial ellipsoids whose major axis tumbles with respect to the line of sight. Finally, we compare the measured properties of the simulated dwarfs to those of dwarf galaxies in the Local Group. The evolutionary tracks of the dwarfs in different parameter planes and the correlations between their different properties, especially the total magnitude and the surface brightness, strongly suggest that present-day dSph galaxies may have indeed formed from late-type progenitors as proposed by the tidal stirring scenario.

  3. Satellite Quenching and the Lifecycle of Dwarf Galaxies.

    NASA Astrophysics Data System (ADS)

    Slater, Colin; Bell, Eric F.

    2015-01-01

    In the past ten years the known population of Local Group dwarf galaxies has expanded substantially, both to greater distances from the Milky Way and to lower dwarf masses. This growing sample allows us to study the dwarf system as a population, and ask if we can see in aggregate the signs of processes that would otherwise be difficult to trace in dwarfs individually. Following this strategy I will discuss how the quenching of dwarf galaxies can be modeled and understood at the population-level, and how we use that to constrain how possible quenching mechanisms must work if they are to reproduce the Local Group system that we see. I show that the distribution of quenched satellites can be reproduced by environmental quenching if and only if a single pericenter passage is sufficient to end star formation in low mass dwarfs. I also show that there is a significant transition in the effectiveness of quenching between low mass dwarfs and dwarfs at Magellanic cloud-like masses, with the higher mass dwarfs much more resilient to quenching. I present both ram pressure and delay time models to try to understand the origin of this transition.

  4. Far-infrared line images of dwarf galaxies

    NASA Technical Reports Server (NTRS)

    Poglitsch, A.; Geis, N.; Herrmann, F.; Madden, S. C.; Stacey, G. J.; Townes, C. H.; Genzel, R.

    1993-01-01

    Irregular dwarf galaxies are about ten times more widespread in the universe than regular spiral galaxies. They are characterized by a relatively low metallicity, i.e., lower abundance of the heavier elements (metals) with respect to hydrogen than in the solar neighborhood. These heavier elements in the form of molecules, atoms, or ions, which have radiative transitions in the infrared play a decisive role in the energy balance of the ISM and thereby for the formation of stars. Dwarf galaxies are thus model cases for the physical conditions in the early phase of the universe. Large Magellanic Cloud: 30 Doradus. The two nearest dwarf galaxies are the Magellanic clouds at a distance approximately 50 kpc. The LMC contains 30 Dor, a region with young, extremely massive stars which strongly interact with the surrounding ISM on account of their stellar winds and intense UV radiation. 30 Dor is the brightest object in the LMC at almost all wavelengths.

  5. Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

    NASA Astrophysics Data System (ADS)

    Wheeler, Coral; Oñorbe, Jose; Bullock, James S.; Boylan-Kolchin, Michael; Elbert, Oliver D.; Garrison-Kimmel, Shea; Hopkins, Philip F.; Kereš, Dušan

    2015-10-01

    We present Feedback in Realistic Environment (FIRE)/GIZMO hydrodynamic zoom-in simulations of isolated dark matter haloes, two each at the mass of classical dwarf galaxies (Mvir ≃ 1010 M⊙) and ultra-faint galaxies (Mvir ≃ 109 M⊙), and with two feedback implementations. The resulting central galaxies lie on an extrapolated abundance matching relation from M⋆ ≃ 106 to 104 M⊙ without a break. Every host is filled with subhaloes, many of which form stars. Each of our dwarfs with M⋆ ≃ 106 M⊙ has 1-2 well-resolved satellites with M⋆ = 3-200 × 103 M⊙. Even our isolated ultra-faint galaxies have star-forming subhaloes. If this is representative, dwarf galaxies throughout the Universe should commonly host tiny satellite galaxies of their own. We combine our results with the Exploring the Local Volume in Simulations (ELVIS) simulations to show that targeting ˜ 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ˜35 per cent compared to random pointings, and specifically identify the region around the Phoenix dwarf galaxy as a good potential target. The well-resolved ultra-faint galaxies in our simulations (M⋆ ≃ 3-30 × 103 M⊙) form within Mpeak ≃ 0.5-3 × 109 M⊙ haloes. Each has a uniformly ancient stellar population ( > 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that Mhalo ≃ 5 × 109 M⊙ is a probable dividing line between haloes hosting reionization `fossils' and those hosting dwarfs that can continue to form stars in isolation after reionization.

  6. Anomalous Cepheids in the Sculptor dwarf galaxy

    SciTech Connect

    Smith, H.A.; Stryker, L.L.

    1986-08-01

    The Sculptor dwarf galaxy contains at least three Cepheids (V25, V26, and V119), each with a period near 1 day and B magnitudes about 1.4 mag brighter than those of the Sculptor RR Lyrae stars. Low-resolution spectra of these so-called anomalous Cepheids were obtained. Metal abundances of the Cepheids have been determined by the Delta-S method and are found to be: Fe/H = -1.9 + or - 0.2, -1.8 + or - 0.2, and -2.2 + or - 0.3 for V25, V26, and V119, respectively. These values are consistent with the metal abundances of Sculptor red giants estimated from the color of the giant branch. Pulsational masses have been estimated for V25 and V26, but there is a need for improved photometry of these stars to obtain accurate results. It cannot be unambiguously established whether the Sculptor anomalous Cepheids are evolved single stars, aged about 3 Gyr, or whether they are created by mass transfer in older binary systems. The occurrence of anomalous Cepheids in other systems is discussed. There is some evidence that most anomalous Cepheids in the Small Magellanic Cloud are evolved single stars. 89 references.

  7. The Star Formation Properties of Void Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Moorman, Crystal; Vogeley, Michael S.

    2016-01-01

    We measure the star formation properties of two large samples of galaxies from the SDSS in large-scale cosmic voids on time scales of 10 Myr and 100 Myr, using Ha emission line strengths and GALEX FUV fluxes, respectively. The first sample consists of 109,818 optically selected galaxies. We find that void galaxies in this sample have higher specific star formation rates (SSFRs; star formation rates per unit stellar mass) than similar stellar mass galaxies in denser regions. The second sample is a subset of the optically selected sample containing 8070 galaxies with reliable S/N HI detections from ALFALFA. For the HI detected sample, SSFRs are similar regardless of large-scale environment. Investigating only the HI detected dwarf galaxies reveals a trend towards higher SSFRs in voids. Furthermore, we estimate the star formation rate per unit HI mass, known as the star formation efficiency (SFE) of a galaxy, as a function of environment. For the overall HI detected population, we notice no environmental dependence. Limiting the sample to dwarf galaxies again reveals a trend towards higher SFEs in voids. These results suggest that void environments provide a nurturing environment for dwarf galaxy evolution.

  8. Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

    NASA Astrophysics Data System (ADS)

    Wheeler, Coral Rose

    2016-06-01

    The high dark matter content and the shallow potential wells of low mass galaxies (10^3 Msun < Mstar < 10^9.5 Msun) make them excellent testbeds for differing theories of galaxy formation. Additionally, the recent up-tick in the number and detail of Local Group dwarf galaxy observations provides a rich dataset for comparison to simulations that attempt to answer important questions in near field cosmology: why are there so few observed dwarfs compared to the number predicted by simulations? What shuts down star formation in ultra-faint galaxies? Why do dwarfs have inverted age gradients and what does it take to convert a dwarf irregular (dIrrs) into a dwarf spheroidal (dSph) galaxy?We to attempt to answer these questions by running ultra-high resolution cosmological FIRE simulations of isolated dwarf galaxies. We predict that many ultra-faint dwarfs should exist as satellites of more massive isolated Local Group dwarfs. The ultra-faints (Mstar < 10^4 Msun) formed in these simulations have uniformly ancient stellar populations (> 10 Gyr), having had their star formation shut down by reionization. Additionally, we show that the kinematics and ellipticities of isolated simulated dwarf centrals are consistent with observed dSphs satellites without the need for harassment from a massive host. We further show that most (but not all) observed *isolated* dIrrs in the Local Volume also have dispersion-supported stellar populations, contradicting the previous view that these objects are rotating. Finally, we investigate the stellar age gradients in dwarfs — showing that early mergers and strong feedback can create an inverted gradient, with the older stars occupying larger galactocentric radii.These results offer an interesting direction in testing models that attempt to solve dark matter problems via explosive feedback episodes. Can the same models that create large cores in simulated dwarfs preserve the mild stellar rotation that is seen in a minority of isolated d

  9. The history of the evolution of dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Lotz, Jennifer Mae

    2003-08-01

    The formation of stars in the smallest galaxies is an important test of the standard paradigm for galaxy formation and evolution. While reasonably successful otherwise, current simulations of hierarchical galaxy formation have great difficulty reproducing the number density, star-formation histories, and structural parameters of local dwarf galaxies. Motivated by these difficulties, we use the observations of both the local dwarf galaxy population and the progenitors of dwarf galaxies in the distant universe, and a new approach to testing galaxy evolution, to trace the evolution and star-formation histories of dwarf galaxies. In the first half of this thesis, we present the results of an HST survey of ˜ 70 dwarf elliptical galaxies (dEs) in the nearby Virgo and Fornax Clusters. By resolving the globular clusters and nuclei from the underlying stars in each dE, we use these three sub- populations to trace the dE star-formation histories. We find that the dE globular cluster candidates are as blue in V I as the metal-poor globular clusters of the Milky Way. The observed correlation of the dE globular cluster systems' V I color with the luminosity of the host dE is strong evidence that the globular clusters were formed within the halos of dEs, and do not have a pre-galactic origin. The blue V I colors of the globular cluster systems and nuclei relative to the dE stellar envelopes require at least two separate star- formation episodes within the dEs. We explore the possibility that many of the dE nuclei are dynamically decayed massive globular clusters. However, we find that dynamical friction appears to be too effective at destroying globular clusters to account for the faint nuclei and the cluster systems observed in low-luminosity dEs, unless the clusters are relatively young or the dEs possess extended dark-matter halos. The extremely blue colors of two nuclei indicate younger ages than the dE stellar halos and globular cluster systems. In the second half of this

  10. THE ACCRETION OF DWARF GALAXIES AND THEIR GLOBULAR CLUSTER SYSTEMS

    SciTech Connect

    Masters, Craig E.; Ashman, Keith M. E-mail: ashmank@umkc.ed

    2010-12-10

    The question of where the low-metallicity globular clusters in early-type galaxies came from has profound implications for the formation of those galaxies. Our work supports the idea that the metal-poor globular cluster systems of giant early-type galaxies formed in dwarf galaxies that have been subsumed by the giants. To support this hypothesis, two linear relations, one involving globular cluster metallicity versus host galaxy luminosity and one involving metallicity versus velocity dispersion were studied. Tentatively, these relations show that the bright ellipticals do not obey the same trend as the dwarfs, suggesting that the low-metallicity globular clusters did not form within their parent bright ellipticals.

  11. Unveiling the Secret of a Virgo Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    2000-05-01

    Dwarf galaxies may not be as impressive in appearance as their larger brethren, but they are at least as interesting from a scientific point of view. And sometimes they may have hidden properties that will only be found by means of careful observations, probing the signals of their stars at the faintest level. Such as the entirely unexpected, well developed spiral structure within an otherwise seemingly normal dwarf elliptical galaxy! This is the surprise result of a new study by a team of astronomers [1], headed by Helmut Jerjen from the Australian National University (Canberra) who obtained detailed observations with the ESO Very Large Telescope (VLT) of the dwarf galaxy IC 3328 in the Virgo Cluster of Galaxies, some 50 million light-years away. Dwarf galaxies Dwarf galaxies are present in all major clusters of galaxies and dominate by numbers in the universe. They may contain a few (tens of) millions of stars, as compared to galaxies of normal size with hundreds of billions of stars. About two dozen dwarf galaxies are known in the "Local Group" of galaxies of which the Milky Way galaxy in which we live is also a member. The Large and Small Magellanic Clouds are some of the best known dwarf galaxies - they are of the irregular type - while NGC 147 and NGC 205, two companions to the great Andromeda Galaxy, are of the elliptical type. Dwarf elliptical galaxies are characterized by their smooth appearance. From various studies, it is known that they are tri-axial ellipsoids of different degrees of elongation. Some are almost spherical while others are more pancake- or cigar-shaped. Like the elliptical galaxies of normal size, dwarf ellipticals are almost pure aggregates of stars. In contrast, spiral galaxies also contain clouds of gas and dust. The visible mass of spiral galaxies is in a rotating disk. Dwarf ellipticals generally keep their form because of the random motions of their stars. VLT observations of dwarf elliptical galaxies Using the FORS1 multi

  12. TIDAL DWARF GALAXIES AROUND A POST-MERGER GALAXY, NGC 4922

    SciTech Connect

    Sheen, Yun-Kyeong; Jeong, Hyunjin; Lee, Young-Wook; Ferreras, Ignacio; Lotz, Jennifer M.; Olsen, Knut A. G.; Dickinson, Mark; Barnes, Sydney; Park, Jang-Hyun; Ree, Chang H.; Madore, Barry F.; Barlow, Tom A.; Conrow, Tim; Foster, Karl; Friendman, Peter G.; Martin, D. Christopher; Morrissey, Patrick; Neff, Susan G.; Schiminovich, David; Yi, Sukyoung K.

    2009-12-15

    One possible channel for the formation of dwarf galaxies involves birth in the tidal tails of interacting galaxies. We report the detection of a bright UV tidal tail and several young tidal dwarf galaxy (TDG) candidates in the post-merger galaxy NGC 4922 in the Coma cluster. Based on a two-component population model (combining young and old stellar populations), we find that the light of tidal tail predominantly comes from young stars (a few Myr old). The Galaxy Evolution Explorer ultraviolet data played a critical role in the parameter (age and mass) estimation. Our stellar mass estimates of the TDG candidates are {approx}10{sup 6-7} M {sub sun}, typical for dwarf galaxies.

  13. Numerical Simulations of the Metallicity Distribution in Dwarf Spheroidal Galaxies

    SciTech Connect

    Ripamonti, Emanuele; Tolstoy, E.; Helmi, A.; Battaglia, G.; Abel, T.; /KIPAC, Menlo Park

    2006-12-12

    Recent observations show that the number of stars with very low metallicities in the dwarf spheroidal satellites of the Milky Way is low, despite the low average metallicities of stars in these systems. We undertake numerical simulations of star formation and metal enrichment of dwarf galaxies in order to verify whether this result can be reproduced with ''standard'' assumptions. The answer is likely to be negative, unless some selection bias against very low metallicity stars is present in the observations.

  14. Stellar kinematics and dark matter in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Battaglia, Giuseppina

    2015-08-01

    In this review I will tour through the most recent findings on the internal kinematic properties of Local Group dwarf galaxies, as determined from extensive spectroscopic surveys of their stellar component.I will also discuss the current status on determinations of the dark matter content and distribution in these objects, with particular focus on the Milky Way dwarf spheroidals, for which the available data-sets allow the application of sophisticated mass modeling techniques.

  15. EPISODIC STARBURSTS IN DWARF SPHEROIDAL GALAXIES: A SIMPLE MODEL

    SciTech Connect

    Nichols, Matthew; Bland-Hawthorn, Joss; Lin Doug

    2012-04-01

    Dwarf galaxies in the Local Group appear to be stripped of their gas within 270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however, show clear evidence of episodic star formation ({Delta}t {approx}a few Gyr) over cosmic time. We present a simple model to account for this behavior. Residual gas within the weak gravity field of the dwarf experiences dramatic variations in the gas cooling time around the eccentric orbit. This variation is due to two main effects. The azimuthal compression along the orbit leads to an increase in the gas cooling rate of {approx}([1 + {epsilon}]/[1 - {epsilon}]){sup 2}. The Galaxy's ionizing field declines as 1/R{sup 2} for R > R{sub disk} although this reaches a floor at R {approx} 150 kpc due to the extragalactic UV field ionizing intensity. We predict that episodic star formation is mostly characteristic of dwarfs on moderately eccentric orbits ({epsilon} > 0.2) that do not come too close to the center (R > R{sub disk}) and do not spend their entire orbit far away from the center (R {approx}> 200 kpc). Up to 40% of early infall dwarf spheroidals can be expected to have already had at least one burst since the initial epoch of star formation, and 10% of these dwarf spheroidals experiencing a second burst. Such a model can explain the timing of bursts in the Carina dwarf spheroidal and restrict the orbit of the Fornax dwarf spheroidal. However, this model fails to explain why some dwarfs, such as Ursa Minor, experience no burst post-infall.

  16. GAS, STARS, AND STAR FORMATION IN ALFALFA DWARF GALAXIES

    SciTech Connect

    Huang Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G. E-mail: haynes@astro.cornell.edu E-mail: jarle@strw.leidenuniv.nl E-mail: susan.g.neff@nasa.gov

    2012-06-15

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

  17. Gas, Stars, and Star Formation in Alfalfa Dwarf Galaxies

    NASA Technical Reports Server (NTRS)

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

    2012-01-01

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

  18. MAPPING THE EXTENDED H I DISTRIBUTION OF THREE DWARF GALAXIES

    SciTech Connect

    Hunter, Deidre A.; Zahedy, Fakhri; Bowsher, Emily C.; Wilcots, Eric M.; Kepley, Amanda A.; Gaal, Veronika E-mail: fsz@mit.edu E-mail: ewilcots@astro.wisc.edu

    2011-11-15

    We present large field H I-line emission maps obtained with the single-dish Green Bank Telescope centered on the dwarf irregular galaxies Sextans A, NGC 2366, and WLM. We do not detect the extended skirts of emission associated with the galaxies that were reported from Effelsberg observations. The ratio of H I at 10{sup 19} atoms cm{sup -2} to optical extents of these galaxies is instead 2-3, which is normal for this type of galaxy. There is no evidence for a truncation in the H I distribution {>=}10{sup 19} atoms cm{sup -2}.

  19. DARK SATELLITES AND THE MORPHOLOGY OF DWARF GALAXIES

    SciTech Connect

    Helmi, Amina; Starkenburg, E.; Starkenburg, T. K.; Vera-Ciro, C. A.; Sales, L. V.; De Lucia, G.; Li, Y.-S.

    2012-10-10

    One of the strongest predictions of the {Lambda}CDM cosmological model is the presence of dark satellites orbiting all types of galaxies. We focus here on the dynamical effects of such satellites on disky dwarf galaxies, and demonstrate that these encounters can be dramatic. Although mergers with M{sub sat} > M{sub d} are not very common, because of the lower baryonic content they occur much more frequently on the dwarf scale than for L{sub *} galaxies. As an example, we present a numerical simulation of a 20% (virial) mass ratio merger between a dark satellite and a disky dwarf (akin to the Fornax dwarf galaxy in luminosity) that shows that the merger remnant has a spheroidal morphology. Perturbations by dark satellites thus provide a plausible path for the formation of dSph systems. The transition from disky to the often amorphous, irregular, or spheroidal morphologies of dwarfs could be a natural consequence of the dynamical heating of hitherto unobservable dark satellites.

  20. Dark matter in dwarf galaxies: Correcting inferred galaxy masses for the orbital motion of binary stars

    NASA Astrophysics Data System (ADS)

    Minor, Quinn Eliot

    We introduce a Bayesian methodology for determining the velocity dispersions of dwarf galaxies which takes into account both binarity and contamination by nonmember stars in a self-consistent way. This method can be readily extended to determine masses and related quantities such as the dark matter annihilation cross-section of dwarf galaxies. In addition we show that measured velocity dispersions of dwarf spheroidal galaxies from about 4 to 10 km/s are unlikely to be inflated by more than 30% due to the orbital motion of binary stars, and demonstrate that the intrinsic velocity dispersions can be determined to within a few percent accuracy using multi-epoch observations with 1-2 years as the optimal time interval. This methodology also constrains properties of binary populations (e.g. binary fraction, period distribution) from multi-epoch velocity measurements, and can be applied to both dwarf galaxies as well as star clusters.

  1. HII regions in dwarf irregular galaxies of the local group

    NASA Technical Reports Server (NTRS)

    Hodge, Paul; Lee, Myung Gyoon

    1990-01-01

    Deep, narrowband H alpha Charge Coupled Device (CCD) surveys of HII regions were carried out in several dwarf irregular galaxies in and near the local group. Data are now complete for these galaxies: NGC 6822, GR 8, IC 10, IC 1613, Sextans A, Sextans B, and Sag Irr. Observations are complete for DDO 47, 53, 167, 168 and 187. Details of some of the results for the surveys completed so far are discussed. For NGC 6822, CCD survey at H alpha resulted in the detection of 145 HII regions in the local group irregular galaxy NGC 6822. Most of them are newly detected, faint surface-brightness objects. Positions, maps and dimensions are being published elsewhere. For GR 8, a deep narrowband H alpha imaging of the nearby dwarf irregular galaxy GR 8 revealed a total of 32 HII regions. Positions, H alpha luminosities, and sizes of these objects were determined. The H alpha luminosity function has the same shape as that for more luminous galaxies, except for size of sample effects. Most HII regions detected are at the very low luminosity end of the general luminosity function. For IC 10, a deep CCD narrowband H alpha imaging of the local group dwarf irregular galaxy IC 10 revealed a total of 144 HII regions. Positions, H alpha luminosities, and sizes of these objects were determined. The H alpha luminosity function has the same shape as that for more luminous galaxies.

  2. Tidal dwarf galaxies and the luminosity-metallicity relation .

    NASA Astrophysics Data System (ADS)

    Sweet, S. M.; Drinkwater, M. J.; Meurer, G.; Bekki, K.; Dopita, M. A.; Kilborn, V.; Nicholls, D.

    We present a recalibration of the luminosity-metallicity relation for gas-rich, star-forming dwarfs to magnitudes as faint as M_R˜ -13. We use the \\citet{Dopita2013} metallicity calibrations to calibrate the relation for all of the data in this analysis. Metal-rich dwarfs classified as tidal dwarf galaxy (TDG) candidates in the literature are typically of metallicity 12 + log(O/H) = 8.70 ± 0.05, while SDSS dwarfs fainter than M_R = -16 have a mean metallicity of 12 + log(O/H) = 8.28 ± 0.10, regardless of their luminosity. Our hydrodynamical simuations predict that TDGs should have metallicities elevated above the normal luminosity-metallicity relation. Metallicity can therefore be a useful diagnostic for identifying TDG candidate populations in the absence of tidal tails. At magnitudes brighter than M_R˜ -16 our sample of 53 star-forming galaxies in 9 HI gas-rich groups is consistent with the normal relation defined by the SDSS sample. At fainter magnitudes there is an increase in dispersion in metallicity of our sample. In our sample we identify three (16% of dwarfs) strong TDG candidates (12 + log(O/H) > 8.6), and four (21%) very metal poor dwarfs (12 + log(O/H) < 8.0), which are likely gas-rich dwarfs with recently ignited star formation. Further details of this analysis are available in Sweet et al. (2013, ApJ submitted).

  3. Interaction/merger-induced starbursts in local very metal-poor dwarfs: link to the common SF in high-z young galaxies

    NASA Astrophysics Data System (ADS)

    Pustilnik, S. A.; Ekta; Kniazev, A. Y.; Chengalur, J. N.; Vanzi, L.

    Introduction. Widespread galaxy formation from pregalactic gas took place in the first 1-3 Gyr after the Big Bang, with most of them forming in low-mass halos (i.e., M in the range of 10^7-10^10 Mo). Observations at high redshifts (z=4-7) are, however, mainly limited to the rare massive "tip of the iceberg" objects. Detailed studies of the properties and the evolution of more common lower-mass young galaxies will have to await the next generation mega telescopes. Although the properties of early galaxies are poorly constrained, it seems reasonable to assume that they should share the following three attributes, viz. a) they should be much more metal-poor than typical local galaxies; b) they should be very gas-rich, and c) due to the much higher galaxy density in the early Universe, interaction/merger induced galaxy-wide SF should be one of the main SF modes. Fortunately, there exist in the local Universe a few good analogs of low-mass young galaxies whose properties can serve as a first approximation for understanding those of the main population of high-redshift galaxies. These are the so-called eXtremely Metal-Deficient (XMD, Z = 1/30 - 1/10 Zo) blue compact galaxies (BCGs). Method. We have conducted a multi-wavelength (including optical/NIR morphology/ photometry, HI imaging, and H-alpha-line kinematics) of a sample of such XMD BCGs and find in a large fraction of them clear evidences that strong interactions or mergers with low-mass objects, provide a trigger mechanism for their observed starbursts. Results. We present about 20 such XMD BCGs arranged in a Toomre-like sequence and also the first results of detailed studies of several individual objects from this sample. Discussion. Since both SF (through cooling rate and the IMF) and its feedback (through the massive star evolution and interaction with the ISM) depend substantially on the ISM metallicity, comprehensive multiwavelength studies of local XMD galaxy mergers, coupled with theoretical modelling

  4. METALLICITY DISTRIBUTION FUNCTIONS OF FOUR LOCAL GROUP DWARF GALAXIES

    SciTech Connect

    Ross, Teresa L.; Holtzman, Jon; Saha, Abhijit; Anthony-Twarog, Barbara J. E-mail: holtz@nmsu.edu

    2015-06-15

    We present stellar metallicities in Leo I, Leo II, IC 1613, and Phoenix dwarf galaxies derived from medium (F390M) and broad (F555W, F814W) band photometry using the Wide Field Camera 3 instrument on board the Hubble Space Telescope. We measured metallicity distribution functions (MDFs) in two ways, (1) matching stars to isochrones in color–color diagrams and (2) solving for the best linear combination of synthetic populations to match the observed color–color diagram. The synthetic technique reduces the effect of photometric scatter and produces MDFs 30%–50% narrower than the MDFs produced from individually matched stars. We fit the synthetic and individual MDFs to analytical chemical evolution models (CEMs) to quantify the enrichment and the effect of gas flows within the galaxies. Additionally, we measure stellar metallicity gradients in Leo I and II. For IC 1613 and Phoenix our data do not have the radial extent to confirm a metallicity gradient for either galaxy. We find the MDF of Leo I (dwarf spheroidal) to be very peaked with a steep metal-rich cutoff and an extended metal-poor tail, while Leo II (dwarf spheroidal), Phoenix (dwarf transition), and IC 1613 (dwarf irregular) have wider, less peaked MDFs than Leo I. A simple CEM is not the best fit for any of our galaxies; therefore we also fit the “Best Accretion Model” of Lynden-Bell. For Leo II, IC 1613, and Phoenix we find similar accretion parameters for the CEM even though they all have different effective yields, masses, star formation histories, and morphologies. We suggest that the dynamical history of a galaxy is reflected in the MDF, where broad MDFs are seen in galaxies that have chemically evolved in relative isolation and narrowly peaked MDFs are seen in galaxies that have experienced more complicated dynamical interactions concurrent with their chemical evolution.

  5. Completing the Census of Isolated Dwarf Galaxy Star Formation Histories

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel

    2014-10-01

    We propose to complete our census of the ancient star formation histories (SFHs) of isolated dwarf galaxies by obtaining deep ACS/WFC optical imaging of WLM and Pegasus Dwarf Irregular Galaxy (PegDIG). They are the only two systems without previous deep HST imaging that are isolated yet close enough to guarantee that their oldest main sequence turnoffs are accessible with HST. We will measure their lifetime SFHs with an age resolution of < 1 Gyr at all epochs to address questions about growth of stellar mass, the effects of reionization, radial population gradients, and variable star populations in WLM and PegDIG. This program is a concerted effort between theorists and observers to obtain the best possible observational constraints on the early epochs of star formation in isolated low-mass galaxies, which are essential to the next generation of galaxy simulations. With these new observations we will have completed our efforts to collect precise lifetime SFHs of all nearby isolated dwarfs that are accessible with HST. In combination with archival data, we will create a legacy sample isolated dwarfs with identically derived SFHs, that will be serve as the baseline for the community's understanding of how low-mass galaxies form and evolve over a Hubble time and in the absence of environmental effects of a massive host (e.g., tides, ram pressure).

  6. A search for HI in some peculiar faint dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Begum, Ayesha; Chengalur, Jayaram N.

    2005-09-01

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

  7. Low-acceleration dwarf galaxies as tests of quantised inertia

    NASA Astrophysics Data System (ADS)

    McCulloch, M. E.

    2017-03-01

    Dwarf satellite galaxies of the Milky Way appear to be gravitationally bound, but their stars' orbital motion seems too fast to allow this given their visible mass. This is akin to the larger-scale galaxy rotation problem. In this paper, a modification of inertia called quantised inertia or MiHsC (Modified inertia due to a Hubble-scale Casimir effect) which correctly predicts larger galaxy rotations without dark matter is tested on eleven dwarf satellite galaxies of the Milky Way, for which mass and velocity data are available. Quantised inertia slightly outperforms MoND (Modified Newtonian Dynamics) in predicting the velocity dispersion of these systems, and has the fundamental advantage over MoND that it does not need an adjustable parameter.

  8. SURFACE BRIGHTNESS PROFILES OF DWARF GALAXIES. I. PROFILES AND STATISTICS

    SciTech Connect

    Herrmann, Kimberly A.; Hunter, Deidre A.; Elmegreen, Bruce G. E-mail: dah@lowell.edu

    2013-11-01

    Radial surface brightness profiles of spiral galaxies are classified into three types: (I) single exponential, or the light falls off with one exponential to a break before falling off (II) more steeply, or (III) less steeply. Profile breaks are also found in dwarf disks, but some dwarf Type IIs are flat or increasing out to a break before falling off. Here we re-examine the stellar disk profiles of 141 dwarfs: 96 dwarf irregulars (dIms), 26 Blue Compact Dwarfs (BCDs), and 19 Magellanic-type spirals (Sms). We fit single, double, or even triple exponential profiles in up to 11 passbands: GALEX FUV and NUV, ground-based UBVJHK and Hα, and Spitzer 3.6 and 4.5 μm. We find that more luminous galaxies have brighter centers, larger inner and outer scale lengths, and breaks at larger radii; dwarf trends with M{sub B} extend to spirals. However, the V-band break surface brightness is independent of break type, M{sub B} , and Hubble type. Dwarf Type II and III profiles fall off similarly beyond the breaks but have different interiors and IIs break ∼twice as far as IIIs. Outer Type II and III scale lengths may have weak trends with wavelength, but pure Type II inner scale lengths clearly decrease from the FUV to visible bands whereas Type III inner scale lengths increase with redder bands. This suggests the influence of different star formation histories on profile type, but nonetheless the break location is approximately the same in all passbands. Dwarfs continue trends between profile and Hubble types such that later-type galaxies have more Type II but fewer Type I and III profiles than early-type spirals. BCDs and Sms are over-represented as Types III and II, respectively, compared to dIms.

  9. Dwarf galaxies in the halo of NGC 891

    SciTech Connect

    Schulz, Earl

    2014-07-20

    We report the results of a survey of the region within 40 arcmin of NGC 891, a nearby nearly perfectly edge-on spiral galaxy. Candidate 'non-stars' with diameters greater than 15 arcsec were selected from the GSC 2.3.2 catalog and cross-comparison of observations in several bands using archived GALEX, DSS2, WISE, and Two Micron All Sky Survey images identified contaminating stars, artifacts, and background galaxies, all of which were excluded. The resulting 71 galaxies, many of which were previously uncataloged, comprise a size-limited survey of the region. A majority of the galaxies are in the background of NGC 891 and are for the most part members of the A347 cluster at a distance of about 75 Mpc. The new finds approximately double the known membership of A347, previously thought to be relatively sparse. We identify a total of seven dwarf galaxies, most of which are new discoveries. The newly discovered dwarf galaxies are dim and gas-poor and may be associated with the previously observed arcs of red giant branch halo stars in the halo and the prominent H I filament and the lopsided features in the disk of NGC 891. Several of the dwarfs show signs of disruption, consistent with being remnants of an ancient collision.

  10. Dwarf Galaxy Gives universe A Breath of Fresh Oxygen

    NASA Astrophysics Data System (ADS)

    2002-07-01

    Astronomers have discovered that a nearby dwarf galaxy is spewing oxygen and other "heavy" elements into intergalactic space. This observation from NASA's Chandra X-ray Observatory supports the idea that dwarf galaxies may be responsible for most of the heavy elements between the galaxies. Despite comprising only a very small fraction of the mass of the universe, so-called heavy elements - everything other than hydrogen and helium -- are essential for the formation of planets and can greatly influence astronomical phenomena, including the rate at which galaxies form. A team led by Crystal Martin of the University of California, Santa Barbara, observed the dwarf galaxy NGC 1569 using Chandra. As reported in an article to be published in The Astrophysical Journal, they found that huge quantities of oxygen and other heavy elements are escaping from the galaxy in bubbles of multimillion-degree gases that are thousands of light years in diameter "Dwarf galaxies are much smaller than ordinary galaxies like our Milky Way and much more common," said Martin. "Because of their small mass, they have relatively low gravity and matter can escape more easily from dwarfs than normal galaxies. This makes them very important in understanding how the universe was seeded with various elements." Scientists have speculated that heavy elements escaping from dwarf galaxies in the early universe could play a dominant role in enriching the intergalactic gas from which other galaxies form. Enriched gas cools more quickly, so the rate and manner of formation of new galaxies in the early universe would have been strongly affected by this process. NGC 1569 X-ray/Optical Composite NGC 1569 Composite Optical/X-ray image "With Chandra it was possible to test these ideas," said Henry Kobulnicky of the University of Wisconsin, Madison, a member of the research team. "We could trace the distribution of oxygen and other elements in the galaxy and determine how much of this matter is escaping from the

  11. X-Ray Sources in the Dwarf Spheroidal Galaxy DRACO

    NASA Astrophysics Data System (ADS)

    Sonbas, E.; Dhuga, K.; Rangelov, B.; Kargaltsev, O.

    2016-06-01

    We present the results of a spectral analysis of X - ray sources in Draco, a nearby dwarf spheroidal galaxy recently observed by XMM-Newton. While most of the sources exhibit properties consistent with AGN, few of them possess characteristics of LMXBs and CVs. We also discuss the possibility of the existence of a central IMBH in Draco.

  12. Radial Color and Mass Profile Trends of Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Herrmann, Kimberly A.; Hunter, D. A.; THINGS, LITTLE

    2014-01-01

    Radial stellar surface brightness (SB) profiles of spiral galaxies can be classified into three types: (I) single exponential, (II) truncated: the light falls off with one exponential out to a break radius and then falls off more steeply, and (III) anti-truncated: the light falls off with one exponential out to a break radius and then falls off less steeply. Stellar SB profile breaks are also found in dwarf disk galaxies, but with an additional sub-category of Type II profiles: (FI) flat-inside: the light is roughly constant or increasing and then falls off beyond a break. Additionally, Bakos, Trujillo, & Pohlen (2008) showed that for spirals, each profile type has a characteristic color trend with respect to the break location which can be combined with color mass-to-light ratio relationships to examine radial mass profiles as well. Here we show radial color and mass profile trends for the three main SB types from a large multi-wavelength photometric study of dwarf irregular galaxies (the 141 dwarf parent sample of the LITTLE THINGS galaxies). We explore the similarities and differences between spirals and dwarfs and also between different colors.

  13. Kinematics of dwarf galaxies in gas-rich groups, and the survival and detectability of tidal dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Sweet, Sarah M.; Drinkwater, Michael J.; Meurer, Gerhardt; Kilborn, Virginia; Audcent-Ross, Fiona; Baumgardt, Holger; Bekki, Kenji

    2016-01-01

    We present DEIMOS multi-object spectroscopy (MOS) of 22 star-forming dwarf galaxies located in four gas-rich groups, including six newly discovered dwarfs. Two of the galaxies are strong tidal dwarf galaxy (TDG) candidates based on our luminosity-metallicity relation definition. We model the rotation curves of these galaxies. Our sample shows low mass-to-light ratios (M/L = 0.73 ± 0.39 M⊙/L⊙) as expected for young, star-forming dwarfs. One of the galaxies in our sample has an apparently strongly falling rotation curve, reaching zero rotational velocity outside the turnover radius of rturn = 1.2re. This may be (1) a polar ring galaxy, with a tilted bar within a face-on disc; (2) a kinematic warp. These scenarios are indistinguishable with our current data due to limitations of slit alignment inherent to MOS-mode observations. We consider whether TDGs can be detected based on their tidal radius, beyond which tidal stripping removes kinematic tracers such as Hα emission. When the tidal radius is less than about twice the turnover radius, the expected falling rotation curve cannot be reliably measured. This is problematic for as much as half of our sample, and indeed more generally, galaxies in groups like these. Further to this, the Hα light that remains must be sufficiently bright to be detected; this is only the case for three (14 per cent) galaxies in our sample. We conclude that the falling rotation curves expected of TDGs are intrinsically difficult to detect.

  14. Old stellar populations in star-forming dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Held, Enrico V.; Saviane, Ivo; Momany, Yazan; Rizzi, Luca; Bertelli, Gianpaolo

    We present deep VLT/FORS1 observations of the two distant, isolated Local Group dwarfs Phoenix and Antlia. Our results provide further evidence for the presence of old stars in these star-forming dwarf galaxies. Old stellar populations are known in all of the Local Group dwarf spheroidal galaxies and in some dwarf irregulars, implying that dwarf galaxies started forming stars at a sharply defined early epoch irrespective of their subsequent star formation histories (e.g., Held et al., 2000; Saviane et al., 2000; and references therein). The new color-magnitude diagrams of Phoenix confirm the presence of a spatially extended blue HB population, indicating a conspicuous old component (Held et al., 1999; Martínez-Delgado et al., 1999). A preliminary analysis of stellar variability has led to the discovery of several tens RR Lyrae variables, which can provide clue information on the earliest star formation episode (see, e.g., Siegel and Majewski, 2000). The young main sequence extends down to the limit of our photometry (V=25.5 mag), which suggests that Phoenix underwent nearly continuous star formation in the last 2 Gyr. Our deep color-magnitude diagrams of Antlia have been used to investigate the gradient in the stellar populations of this dwarf irregular/spheroidal galaxy. While the young stars appear to be concentrated in a round central region (Aparicio et al., 1997; Sarajedini et al., 1997), the spatial distribution of the red giant stars defines an extended flattened halo (or disk) 2-3 kpc across.

  15. The Local Tully–Fisher Relation for Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Karachentsev, Igor D.; Kaisina, Elena I.; Kashibadze (Nasonova, Olga G.

    2017-01-01

    We study different incarnations of the Tully–Fisher (TF) relation for the Local Volume (LV) galaxies taken from Updated Nearby Galaxy Catalog. The UNGC sample contains 656 galaxies with W50 H i-line-width estimates, mostly belonging to low-mass dwarfs. Of them, 296 objects have distances measured with accuracies better than 10%. For the sample of 331 LV galaxies having baryonic masses {log}{M}{bar}> 5.8{log} {M}ȯ , we obtain a relation {log}{M}{bar}=2.49{log}{W}50+3.97 with an observed scatter of 0.38 dex. The largest factors affecting the scatter are observational errors in K-band magnitudes and W50 line widths for the tiny dwarfs, as well as uncertainty of their inclinations. We find that accounting for the surface brightness of the LV galaxies or their gas fraction, specific star-formation rate, or isolation index does not essentially reduce the observed scatter on the baryonic TF diagram. We also notice that a sample of 71 dSph satellites of the Milky Way and M31 with a known stellar velocity dispersion σ* tends to follow nearly the same bTF relation, having slightly lower masses than that of late-type dwarfs.

  16. THE PRIMEVAL POPULATIONS OF THE ULTRA-FAINT DWARF GALAXIES

    SciTech Connect

    Brown, Thomas M.; Tumlinson, Jason; Kalirai, Jason S.; Avila, Roberto J.; Ferguson, Henry C. E-mail: tumlinson@stsci.edu E-mail: avila@stsci.edu; and others

    2012-07-01

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

  17. The universal rotation curve of dwarf disc galaxies

    NASA Astrophysics Data System (ADS)

    Karukes, E. V.; Salucci, P.

    2017-03-01

    We use the concept of the spiral rotation curves universality to investigate the luminous and dark matter properties of the dwarf disc galaxies in the local volume (size ∼11 Mpc). Our sample includes 36 objects with rotation curves carefully selected from the literature. We find that, despite the large variations of our sample in luminosities (∼2 of dex), the rotation curves in specifically normalized units, look all alike and lead to the lower mass version of the universal rotation curve of spiral galaxies found in Persic et al. We mass model the double normalized universal rotation curve V(R/Ropt)/Vopt of dwarf disc galaxies: the results show that these systems are totally dominated by dark matter whose density shows a core size between 2 and 3 stellar disc scalelengths. Similar to galaxies of different Hubble types and luminosities, the core radius r0 and the central density ρ0 of the dark matter halo of these objects are related by ρ0r0 ∼ 100 M⊙ pc-2. The structural properties of the dark and luminous matter emerge very well correlated. In addition, to describe these relations, we need to introduce a new parameter, measuring the compactness of light distribution of a (dwarf) disc galaxy. These structural properties also indicate that there is no evidence of abrupt decline at the faint end of the baryonic to halo mass relation. Finally, we find that the distributions of the stellar disc and its dark matter halo are closely related.

  18. Dust emission in simulated dwarf galaxies using GRASIL-3D

    NASA Astrophysics Data System (ADS)

    Santos-Santos, I. M.; Domínguez-Tenreiro, R.; Granato, G. L.; Brook, C. B.; Obreja, A.

    2017-03-01

    Recent Herschel observations of dwarf galaxies have shown a wide diversity in the shapes of their IR-submm spectral energy distributions as compared to more massive galaxies, presenting features that cannot be explained with the current models. In order to understand the physics driving these differences, we have computed the emission of a sample of simulated dwarf galaxies using the radiative transfer code GRASIL-3D. This code separately treats the radiative transfer in dust grains from molecular clouds and cirri. The simulated galaxies have masses ranging from 10^6-10^9 M_⊙ and have evolved within a Local Group environment by using CLUES initial conditions. We show that their IR band luminosities are in agreement with observations, with their SEDs reproducing naturally the particular spectral features observed. We conclude that the GRASIL-3D two-component model gives a physical interpretation to the emission of dwarf galaxies, with molecular clouds (cirri) as the warm (cold) dust components needed to recover observational data.

  19. Suppression of dwarf galaxy formation by cosmic reionization.

    PubMed

    Wyithe, J Stuart B; Loeb, Abraham

    2006-05-18

    A large number of faint galaxies, born less than a billion years after the Big Bang, have recently been discovered. Fluctuations in the distribution of these galaxies contributed to a scatter in the ionization fraction of cosmic hydrogen on scales of tens of megaparsecs, as observed along the lines of sight to the earliest known quasars. Theoretical simulations predict that the formation of dwarf galaxies should have been suppressed after cosmic hydrogen was reionized, leading to a drop in the cosmic star-formation rate. Here we report evidence for this suppression. We show that the post-reionization galaxies that produced most of the ionizing radiation at a redshift z approximately 5.5 must have had a mass in excess of approximately 10(10.9 +/- 0.5) solar masses (M(o)) or else the aforementioned scatter would have been smaller than observed. This limiting mass is two orders of magnitude larger than the galaxy mass that is thought to have dominated the reionization of cosmic hydrogen (approximately 10(8) M(o)). We predict that future surveys with space-based infrared telescopes will detect a population of smaller galaxies that reionized the Universe at an earlier time, before the epoch of dwarf galaxy suppression.

  20. Gas and Dust Properties in Dwarf Irregular Galaxies

    NASA Technical Reports Server (NTRS)

    Jones, A. P.; Madden, S. C.; Colgan, S. W. J.; Geis, N.; Haas, M.; Maloney, P.; Nikola, T.; Poglitsch, A.

    1997-01-01

    We present a study of the 158 (micron)meter [C II] fine structure emission line from a sample of 11 low metallicity irregular galaxies using the NASA Kuiper Airborne Observatory (KAO). Our preliminary results demonstrate that the ratio of the 158 (micron)meter [C II] emission to the CO-12(1 yields 0) emission ranges from 6,000 to 46,000. These ratios are significantly enhanced relative to clouds within the Galaxy and to normal metallicity galaxies, which typically have values in the range 2,000 to 6,300. We also find that the [C II] emission in dwarf irregular galaxies can be up to 5% of the far-infrared (FIR) emission, a higher fraction of the FIR than in normal metallicity galaxies. We discuss these results for the dwarf irregular galaxies and compare them to those observed in normal metallicity galaxies. The enhanced 158 (micron)meter [C II] emission relative to CO-12(1 yields 0) emission can be understood in terms of the increased penetration depth of ultraviolet (UV) photons into the clouds in low metallicity environments.

  1. Cosmological simulations of dwarf galaxies with cosmic ray feedback

    NASA Astrophysics Data System (ADS)

    Chen, Jingjing; Bryan, Greg L.; Salem, Munier

    2016-08-01

    We perform zoom-in cosmological simulations of a suite of dwarf galaxies, examining the impact of cosmic rays (CRs) generated by supernovae, including the effect of diffusion. We first look at the effect of varying the uncertain CR parameters by repeatedly simulating a single galaxy. Then we fix the comic ray model and simulate five dwarf systems with virial masses range from 8 to 30 × 1010 M⊙. We find that including CR feedback (with diffusion) consistently leads to disc-dominated systems with relatively flat rotation curves and constant star formation rates. In contrast, our purely thermal feedback case results in a hot stellar system and bursty star formation. The CR simulations very well match the observed baryonic Tully-Fisher relation, but have a lower gas fraction than in real systems. We also find that the dark matter cores of the CR feedback galaxies are cuspy, while the purely thermal feedback case results in a substantial core.

  2. New Ultra-Compact Dwarf Galaxies in Clusters

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2017-02-01

    How do ultra-compact dwarf galaxies (UCDs) galaxies that are especially small and dense form and evolve? Scientists have recently examined distant galaxy clusters, searching for more UCDs to help us answer this question.Origins of DwarfsIn recent years we have discovered a growing sample of small, very dense galaxies. Galaxies that are tens to hundreds of light-years across, with masses between a million and a billion solar masses, fall into category of ultra-compact dwarfs (UCDs).An example of an unresolved compact object from the authors survey that is likely an ultra-compact dwarf galaxy. [Adapted from Zhang Bell 2017]How do these dense and compact galaxies form? Two possibilities are commonly suggested:An initially larger galaxy was tidally stripped during interactions with other galaxies in a cluster, leaving behind only its small, dense core as a UCD.UCDs formed as compact galaxies at very early cosmic times. The ones living in a massive dark matter halo may have been able to remain compact over time, evolving into the objectswe see today.To better understand which of these formation scenarios applies to which galaxies, we need a larger sample size! Our census of UCDs is fairly limited and because theyare small and dim, most of the ones weve discovered are in the nearby universe. To build a good sample, we need to find UCDs at higher redshifts as well.A New SampleIn a recent study, two scientists from University of Michigan have demonstrated how we might find more UCDs. Yuanyuan Zhang (also affiliated with Fermilab) and Eric Bell used the Cluster Lensing and Supernova Survey with Hubble (CLASH) to search 17 galaxy clusters at intermediate redshifts of 0.2 z 0.6, looking for unresolved objects that might be UCDs.The mass and size distributions of the UCD candidates reported in this study, in the context of previously known nuclear star clusters, globular clusters (GCs), UCDs, compact elliptical galaxies (cEs), and dwarf galaxies. [Zhang Bell 2017]Zhang and

  3. Solo dwarfs I: survey introduction and first results for the Sagittarius dwarf irregular galaxy

    NASA Astrophysics Data System (ADS)

    Higgs, C. R.; McConnachie, A. W.; Irwin, M.; Bate, N. F.; Lewis, G. F.; Walker, M. G.; Côté, P.; Venn, K.; Battaglia, G.

    2016-05-01

    We introduce the Solitary Local dwarfs survey (Solo), a wide-field photometric study targeting every isolated dwarf galaxy within 3 Mpc of the Milky Way. Solo is based on (u)gi multiband imaging from Canada-France-Hawaii Telescope/MegaCam for northern targets, and Magellan/Megacam for southern targets. All galaxies fainter than MV ≃ -18 situated beyond the nominal virial radius of the Milky Way and M31 (≳300 kpc) are included in this volume-limited sample, for a total of 42 targets. In addition to reviewing the survey goals and strategy, we present results for the Sagittarius dwarf irregular galaxy (Sag DIG), one of the most isolated, low-mass galaxies, located at the edge of the Local Group. We analyse its resolved stellar populations and their spatial distributions. We provide updated estimates of its central surface brightness and integrated luminosity, and trace its surface brightness profile to a level fainter than 30 mag arcsec-2. Sag DIG is well described by a highly elliptical (disc-like) system following a single component Sérsic model. However, a low-level distortion is present at the outer edges of the galaxy that, were Sag DIG not so isolated, would likely be attributed to some kind of previous tidal interaction. Further, we find evidence of an extremely low level, extended distribution of stars beyond ˜5 arcmin (>1.5 kpc) that suggests Sag DIG may be embedded in a very low-density stellar halo. We compare the stellar and H I structures of Sag DIG, and discuss results for this galaxy in relation to other isolated, dwarf irregular galaxies in the Local Group.

  4. Gas-rich dwarf galaxies in dense and sparse environments

    NASA Technical Reports Server (NTRS)

    Hoffman, G. Lyle

    1993-01-01

    Dwarf irregular galaxies (generically labelled Im for the present purposes) pose an enigma to students of galaxy evolution. In nearby groups and the Virgo cluster, Im galaxies are at least as abundant as spiral galaxies, and their low surface brightnesses and high gas-to-stars ratios suggest that (at least in the stochastic self-propagating star formation scenario) there should be significant numbers of HI clouds with masses approaching 10(exp 8) solar mass which have undergone very little or no star formation. To date, however, no clouds with so little star formation that they would not be recognized as Im galaxies on high-quality photographic plates have been identified. There have been suggestions that such dwarfs may be tidally disrupted in regions of high galactic density, but may be prevalent in low density regions. We offer data from three parallel programs relevant to this issue. (1) A large number of Im galaxies throughout the Local Supercluster have been mapped in the HI spectral line using the Arecibo Radiotelescope, and we can establish the frequency with which HI disks much more extended than their optically visible portions are found. (2) Our extensive mapping of spiral and dwarf galaxies in the Virgo cluster allows us to set stringent limits on the density of star-free Hi clouds in that cluster. (3) We have conducted a sampling of the void in the distribution of galaxies toward the super galactic pole, optimized for finding low-mass HI clouds at redshifts out to approximately 2000 km/s.

  5. NGC 5291: Implications for the Formation of Dwarf Galaxies

    NASA Technical Reports Server (NTRS)

    Malphrus, Benjamin K.; Simpson, Caroline E.; Gottesman, S. T.; Hawarden, Timothy G.

    1997-01-01

    The possible formation and evolution of dwarf irregular galaxies from material derived from perturbed evolved galaxies is addressed via an H I study of a likely example, the peculiar system NGC 5291. This system, located in the western outskirts of the cluster Abell 3574, contains the lenticular galaxy NGC 5291 which is in close proximity to a disturbed companion and is flanked by an extensive complex of numerous knots extending roughly 4 min north and 4 min south of the galaxy. In an initial optical and radio study, Longmore et al. (1979, MNRAS, 188, 285) showed that these knots have the spectra of vigorous star-forming regions, and suggested that some may in fact be young dwarf irregular galaxies. High resolution 21-cm line observations taken with the VLA are presented here and reveal that the H I distribution associated with this system encompasses not only the entire N-S complex of optical knots, but also forms an incomplete ring or tail that extends approximately 3 min to the west. The H I associated with NGC 5291 itself shows a high velocity range; the Seashell is not detected. The formation mechanism for this unusual system is unclear and two models - a large, low-luminosity ram-swept disk, and a ram-swept interaction-are discussed. The H I in the system contains numerous concentrations, mostly along the N-S arc of the star-forming complexes, which generally coincide with one or more optical knots; the larger H I features contain several x 10(exp 9) solar mass of gas. Each of the knots is compared to a set of criteria designed to determine if these objects are bound against their own internal kinetic energy and are tidally stable relative to the host galaxy. An analysis of the properties of the H I concentrations surrounding the optical star-forming complexes indicates that at least the largest of these is a bound system; it also possesses a stellar component. It is suggested that this object is a genuinely young dwarf irregular galaxy that has evolved from

  6. New low surface brightness dwarf galaxies in the Centaurus group

    NASA Astrophysics Data System (ADS)

    Müller, Oliver; Jerjen, Helmut; Binggeli, Bruno

    2017-01-01

    Context. The distribution of satellite galaxies around the Milky Way and Andromeda and their correlation in phase space pose a major challenge to the standard ΛCDM model of structure formation. Other nearby groups of galaxies are now being scrutinized to test for the ubiquity of the phenomenon. Aims: We conducted an extensive CCD imaging survey for faint, unresolved dwarf galaxies of very low surface brightness in the whole Centaurus group region, encompassing the Cen A and M 83 subgroups lying at a distance of roughly 4 and 5 Mpc, respectively. The aim is to significantly increase the sample of known Centaurus group members down to a fainter level of completeness, serving as a basis for future studies of the 3D structure of the group. Methods: Following our previous survey of 60 square degrees covering the M 83 subgroup, we extended and completed our survey of the Centaurus group region by imaging another 500 square degrees area in the g and r bands with the wide-field Dark Energy Survey camera at the 4 m Blanco telescope at CTIO. The surface brightness limit reached for unresolved dwarf galaxies is μr ≈ 29 mag arcsec-2. The faintest suspected Centaurus members found have mr ≈ 19.5 mag or Mr ≈ -8.8 mag at the mean distance of the group. The images were enhanced using different filtering techniques. Results: We found 41 new dwarf galaxy candidates, which together with the previously discovered 16 dwarf candidates in the M 83 subgroup amounts to almost a doubling of the number of known galaxies in the Centaurus complex, if the candidates are confirmed. We carried out surface photometry in g and r, and report the photometric parameters derived therefrom, for all new candidates as well as previously known members in the surveyed area. The photometric properties of the candidates, when compared to those of Local Group dwarfs and previously known Centaurus dwarfs, suggest membership in the Centaurus group. The sky distribution of the new objects is generally

  7. White dwarfs, the Galaxy and Dirac's cosmology

    NASA Technical Reports Server (NTRS)

    Stothers, R.

    1976-01-01

    The additive and multiplicative versions of Dirac's cosmological hypothesis relating the gravitational constant variation with elapsed time and number of particles populating the universe is invoked to account for the deficiency or absence of white dwarfs fainter than about 0.0001 solar luminosity. An estimate is made of white dwarf luminosity in accordance with the two evolutionary models, and it is conjectured that some old white dwarfs with high space velocities may be on the verge of gravitational collapse. Lack of a special mechanism to produce the vast numbers of black holes or other dead stars accounting for 'missing matter' in the vicinity of the sun and in the galactic halo is noted in Dirac's multiplicative model. Results indicate that either Dirac's theory is untenable, or that radiation and heating are of some unknown nature, or that the process of creation of new matter requires a corresponding input of energy.

  8. Tidal dwarf galaxies in gas-rich groups

    NASA Astrophysics Data System (ADS)

    Sweet, Sarah M.

    2014-09-01

    I develop new methods for identifying and measuring tidal dwarf galaxies, using a sample of galaxies within Hi-rich groups that have no evidence of advanced major mergers. These groups are taken from the Survey of Ionization in Neutral Gas Galaxies (SINGG, Meurer et al., 2006), an optical follow-up survey to the HI Parkes All Sky Survey (HIPASS, Barnes et al., 2001). Fifteen of the fields contain four or more emission line galaxies and are named Choir groups. I detect new dwarf galaxies that are too small to be individually detectable in HIPASS; they are detectable in the SINGG narrow-band imaging because of their star formation and membership of these HI-rich groups. The Choir groups are compact, with a mean projected separation between the two brightest members of 190 kpc. They have comparable star formation efficiency (the ratio of star formation rate to HI mass) to the remaining SINGG fields. The Choir member galaxies also match the wider SINGG sample in their radii, Hα equivalent width and surface brightness. I define a new, more robust calibration for the metallicity diagnostic for identifying tidal dwarf galaxy candidates in the absence of tidal tails, based on the luminosity-metallicity relation with a consistent metallicity definition. Using that calibration, SDSS dwarfs fainter than MR = -16 have a mean metallicity of 12 + log(O/H) = 8.28 (±) 0.10, regardless of their luminosity. Tidal dwarf galaxy candidates in the literature are elevated above this at 12 + log(O/H) = 8.70 (±) 0.05 on average. Our hydrodynamical simulations also predict that tidal dwarf galaxies should have metallicities elevated above the normal luminosity-metallicity relation. I compare 53 star-forming galaxies in 9 of the Hi gas-rich Choir groups and find those brighter than MR ~ -16 to be consistent with the normal relation defined by the SDSS sample. At fainter magnitudes my sample has a wide range in metallicity, suggestive of varying Hi content and environment. Three (16%) of

  9. Interstellar Medium Abundances in the Pegasus Dwarf Irregular Galaxy

    NASA Astrophysics Data System (ADS)

    Skillman, Evan D.; Bomans, Dominik J.; Kobulnicky, Henry A.

    1997-01-01

    We report Hα imaging and optical spectrophotometry of a faint H II region in the Pegasus dwarf irregular galaxy, a low-luminosity, possible Local Group member. We find that the emission-line spectrum is consistent with excitation by a single star with a temperature of less than 35,000 K, in agreement with Aparicio & Gallart (1995). Although we cannot measure the electron temperature of the ionized gas directly, we can infer a narrow range in electron temperature by comparing the emission-line spectrum with photoionization models. Then we derive a relatively low oxygen abundance of approximately 10% of the solar value, in accordance with the well-known metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 6%, which is higher than the typical value of 3% found in more actively star-forming dwarf irregular galaxies. By comparing this high N/O ratio with the recent star formation history of the Pegasus dwarf irregular derived by Aparicio & Gallart, we hypothesize that this may be evidence of delayed N production.

  10. Galactic Building Blocks: Dwarf Galaxies Near and Far

    NASA Astrophysics Data System (ADS)

    Lipnicky, Andrew; Chakrabarti, Sukanya

    2017-01-01

    The work I have done during my thesis has consisted of both observational and theoretical projects involving dwarf galaxies and Cold Dark Matter (CDM) sub-structure both around the Milky Way and in redshift z˜0.1 galaxies. The dwarf galaxies around the Milky Way are distributed in a so-called vast polar structure (VPOS) that may be in conflict with ΛCDM simulations. For this project, we seek to investigate two key questions to determine if the VPOS poses a serious challenge to the ΛCDM paradigm on galactic scales. First, we ask which dwarf galaxy satellites drive the fit to the VPOS and create planar structure. Second, we ask if the VPOS remains coherent as a function of time. Using the measured HST proper motions and associated uncertainties, we integrate the orbits of the classical Milky Way satellites backwards in time and find that for the mean of the measured HST proper motions, the VPOS deteriorates in less than a dynamical time and resembles an isotropic structure. We also explore the effect of the uncertainties on the HST proper motions on the coherence of the VPOS as a function of time. We find that nine of the eleven classical dwarfs have reliable proper motions; for these nine, the VPOS also deteriorates in less than a dynamical time, indicating that the VPOS is not a dynamically stable structure. I will also briefly discuss the observational work that I have done during my thesis, including HI observations of lensed spiral galaxies to constrain CDM sub-structure.

  11. The Distribution of Alpha Elements in Andromeda Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Vargas, Luis C.; Geha, Marla C.; Tollerud, Erik J.

    2014-07-01

    We present alpha to iron abundance ratios for 226 individual red giant branch stars in nine dwarf galaxies of the Andromeda (M31) satellite system. The abundances are measured from the combined signal of Mg, Si, Ca, and Ti lines in Keck/DEIMOS medium-resolution spectra. This constitutes the first large sample of alpha abundance ratios measured in the M31 satellite system. The dwarf galaxies in our sample exhibit a variety of alpha abundance ratios, with the average values in each galaxy ranging from approximately solar ([α/Fe] ~ + 0.0) to alpha-enhanced ([α/Fe] ~ + 0.5). These variations do not show a correlation with internal kinematics, environment, or stellar density. We confirm radial gradients in the iron abundance of two galaxies out of the five with sufficient data (NGC 185 and And II). There is only tentative evidence for an alpha abundance radial gradient in NGC 185. We homogeneously compare our results to the Milky Way classical dwarf spheroidals, finding evidence for wider variation in average alpha abundance. In the absence of chemical abundances for the M31 stellar halo, we compare to the Milky Way stellar halo. A stellar halo comprised of disrupted M31 satellites is too metal-rich and inconsistent with the Milky Way halo alpha abundance distribution even if considering only satellites with predominantly old stellar populations. The M31 satellite population provides a second system in which to study chemical abundances of dwarf galaxies and reveals a wider variety of abundance patterns than the Milky Way.

  12. The distribution of alpha elements in Andromeda dwarf galaxies

    SciTech Connect

    Vargas, Luis C.; Geha, Marla C.; Tollerud, Erik J.

    2014-07-20

    We present alpha to iron abundance ratios for 226 individual red giant branch stars in nine dwarf galaxies of the Andromeda (M31) satellite system. The abundances are measured from the combined signal of Mg, Si, Ca, and Ti lines in Keck/DEIMOS medium-resolution spectra. This constitutes the first large sample of alpha abundance ratios measured in the M31 satellite system. The dwarf galaxies in our sample exhibit a variety of alpha abundance ratios, with the average values in each galaxy ranging from approximately solar ([α/Fe] ∼ + 0.0) to alpha-enhanced ([α/Fe] ∼ + 0.5). These variations do not show a correlation with internal kinematics, environment, or stellar density. We confirm radial gradients in the iron abundance of two galaxies out of the five with sufficient data (NGC 185 and And II). There is only tentative evidence for an alpha abundance radial gradient in NGC 185. We homogeneously compare our results to the Milky Way classical dwarf spheroidals, finding evidence for wider variation in average alpha abundance. In the absence of chemical abundances for the M31 stellar halo, we compare to the Milky Way stellar halo. A stellar halo comprised of disrupted M31 satellites is too metal-rich and inconsistent with the Milky Way halo alpha abundance distribution even if considering only satellites with predominantly old stellar populations. The M31 satellite population provides a second system in which to study chemical abundances of dwarf galaxies and reveals a wider variety of abundance patterns than the Milky Way.

  13. The Extremely Metal-Poor Dwarf Galaxy AGC 198691

    NASA Astrophysics Data System (ADS)

    Hirschauer, Alec S.; Salzer, John Joseph; Cannon, John M.; Skillman, Evan D.; SHIELD II Team

    2016-01-01

    We present spectroscopic observations of the nearby dwarf irregular galaxy AGC 198691. This object is part of the Survey of HI in Extremely Low-Mass Dwarfs (SHIELD) sample, which consists of ultra-low HI mass galaxies discovered by the Arecibo Legacy Fast-Acting ALFA (ALFALFA) survey. SHIELD is a multi-configuration Expanded Very Large Array (EVLA) study of the neutral gas content and dynamics of galaxies with HI masses in the range of 106-107 M⊙. Our spectral data were obtained using the new high-throughput KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m telescope as part of a systematic study of the nebular abundances in the SHIELD galaxy sample. These observations enable measurement of the temperature sensitive [OIII]λ4363 line and hence the determination of a "direct" oxygen abundance for AGC 198691. We find this system to be an extremely metal-deficient (XMD) galaxy with an oxygen abundance comparable to such objects as I Zw 18, SBS 0335-052W, Leo P, and DDO 68 - the lowest metallicity star-forming systems known. It is worth noting that two of the five lowest-abundance galaxies currently recognized were discovered via the ALFALFA blind HI survey. These XMD galaxies are potential analogues to the first star-forming systems, which through hierarchical accretion processes built up the large galaxies we observe today in the local Universe. Detailed analysis of such XMD systems offers observational constraint to models of galactic evolution and star formation histories to allow a better understanding of the processes that govern the chemical evolution of low-mass galaxies.

  14. The mass dependence of dwarf satellite galaxy quenching

    SciTech Connect

    Slater, Colin T.; Bell, Eric F. E-mail: ericbell@umich.edu

    2014-09-10

    We combine observations of the Local Group with data from the NASA-Sloan Atlas to show the variation in the quenched fraction of satellite galaxies from low-mass dwarf spheroidals and dwarf irregulars to more massive dwarfs similar to the Magellanic Clouds. While almost all of the low-mass (M {sub *} ≲ 10{sup 7} M {sub ☉}) dwarfs are quenched, at higher masses the quenched fraction decreases to approximately 40%-50%. This change in the quenched fraction is large and suggests a sudden change in the effectiveness of quenching that correlates with satellite mass. We combine this observation with models of satellite infall and ram pressure stripping to show that the low-mass satellites must quench within 1-2 Gyr of pericenter passage to maintain a high quenched fraction, but that many more massive dwarfs must continue to form stars today even though they likely fell into their host >5 Gyr ago. We also characterize how the susceptibility of dwarfs to ram pressure must vary as a function of mass if it is to account for the change in quenched fractions. Though neither model predicts the quenching effectiveness a priori, this modeling illustrates the physical requirements that the observed quenched fractions place on possible quenching mechanisms.

  15. H ii REGIONS WITHIN A COMPACT HIGH VELOCITY CLOUD. A NEARLY STARLESS DWARF GALAXY?

    SciTech Connect

    Bellazzini, M.; Magrini, L.; Mucciarelli, A.; Fraternali, F.; Ibata, R.; Martin, N.; Battaglia, G.; Testa, V.; Fumana, M.; Marchetti, A.; Correnti, M.

    2015-02-10

    Within the SECCO survey we identified a candidate stellar counterpart to the Ultra Compact High Velocity Cloud (UCHVC) HVC274.68+74.70-123 that was suggested by Adams et al. to be a possible mini halo within the Local Group of galaxies. The spectroscopic follow-up of the brightest sources within the candidate reveals the presence of two H ii regions whose radial velocity is compatible with a physical association with the UVHVC. The available data do not allow us to give a definite answer on the nature of the newly identified system. A few alternative hypotheses are discussed. However, the most likely possibility is that we have found a new faint dwarf galaxy residing in the Virgo cluster of galaxies, which we name SECCO 1. Independently of its actual distance, SECCO 1 displays a ratio of neutral hydrogen mass to V luminosity of M{sub H} {sub I}/L{sub V}≳20, by far the largest among local dwarfs. Hence, it appears to be a nearly starless galaxy and it may be an example of the missing links between normal dwarfs and the dark mini halos that are predicted to exist in large numbers according to the currently accepted cosmological model.

  16. Studying the dwarf galaxies in nearby groups of galaxies: Spectroscopic and photometric data

    NASA Astrophysics Data System (ADS)

    Hopp, U.; Vennik, J.

    2014-11-01

    Galaxy evolution by interaction-driven transformation is probably highly efficient in groups of galaxies. Dwarf galaxies with their shallow potential are expected to reflect the interaction most prominently in their observable structure. The major aim of this series of papers is to establish a data base which allows to study the impact of group interaction onto the morphology and star-forming properties of dwarf galaxies. Firstly, we present our selection rules for target groups and the morphological selection method of target dwarf member candidates. Secondly, the spectroscopic follow-up observations with the HET are presented. Thirdly, we applied own reduction methods based on adaptive filtering to derive surface photometry of the candidates. The spectroscopic follow-up indicate a dwarf identification success rate of roughly 55 %, and a group member success rate of about 33 %. A total of 17 new low surface-brightness members is presented. For all candidates, total magnitudes, colours, and light distribution parameters are derived and discussed in the context of scaling relations. We point out short comings of the SDSS standard pipeline for surface photometry for these dim objects. We conclude that our selection strategy is rather efficient to obtain a sample of dim, low surface brightness members of groups of galaxies within the Virgo super-cluster. The photometric scaling relation in these X-ray dim, rather isolated groups does not significantly differ from those of the galaxies within the local volume.

  17. Metallicity Distribution Functions of Four Local Group Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Ross, Teresa L.; Holtzman, Jon; Saha, Abhijit; Anthony-Twarog, Barbara J.

    2015-06-01

    We present stellar metallicities in Leo I, Leo II, IC 1613, and Phoenix dwarf galaxies derived from medium (F390M) and broad (F555W, F814W) band photometry using the Wide Field Camera 3 instrument on board the Hubble Space Telescope. We measured metallicity distribution functions (MDFs) in two ways, (1) matching stars to isochrones in color-color diagrams and (2) solving for the best linear combination of synthetic populations to match the observed color-color diagram. The synthetic technique reduces the effect of photometric scatter and produces MDFs 30%-50% narrower than the MDFs produced from individually matched stars. We fit the synthetic and individual MDFs to analytical chemical evolution models (CEMs) to quantify the enrichment and the effect of gas flows within the galaxies. Additionally, we measure stellar metallicity gradients in Leo I and II. For IC 1613 and Phoenix our data do not have the radial extent to confirm a metallicity gradient for either galaxy. We find the MDF of Leo I (dwarf spheroidal) to be very peaked with a steep metal-rich cutoff and an extended metal-poor tail, while Leo II (dwarf spheroidal), Phoenix (dwarf transition), and IC 1613 (dwarf irregular) have wider, less peaked MDFs than Leo I. A simple CEM is not the best fit for any of our galaxies; therefore we also fit the “Best Accretion Model” of Lynden-Bell. For Leo II, IC 1613, and Phoenix we find similar accretion parameters for the CEM even though they all have different effective yields, masses, star formation histories, and morphologies. We suggest that the dynamical history of a galaxy is reflected in the MDF, where broad MDFs are seen in galaxies that have chemically evolved in relative isolation and narrowly peaked MDFs are seen in galaxies that have experienced more complicated dynamical interactions concurrent with their chemical evolution. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is

  18. Formation of massive clouds and dwarf galaxies during tidal encounters

    NASA Technical Reports Server (NTRS)

    Kaufman, Michele; Elmegreen, Bruce G.; Thomasson, Magnus; Elmegreen, Debra M.

    1993-01-01

    Gerola et al. (1983) propose that isolated dwarf galaxies can form during galaxy interactions. As evidence of this process, Mirabel et al. (1991) find 10(exp 9) solar mass clouds and star formation complexes at the outer ends of the tidal arms in the Antennae and Superantennae galaxies. We describe observations of HI clouds with mass greater than 10(exp 8) solar mass in the interacting galaxy pair IC 2163/NGC 2207. This pair is important because we believe it represents an early stage in the formation of giant clouds during an encounter. We use a gravitational instability model to explain why the observed clouds are so massive and discuss a two-dimensional N-body simulation of an encounter that produces giant clouds.

  19. Dynamical mass modeling of dispersion-supported dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Wolf, Joseph

    The currently favored cold dark matter cosmology (LCDM) has had much success in reproducing the large scale structure of the universe. However, on smaller scales there are some possible discrepancies when attempting to match galactic observations with properties of halos in dissipationless LCDM simulations. One advantageous method to test small scale simulations with observations is through dynamical mass modeling of nearby dwarf spheroidal galaxies (dSphs). The stellar tracers of dSphs are dispersion-supported, which poses a significant challenge in accurately deriving mass profiles. Unlike rotationally-supported galaxies, the dynamics of which can be well-approximated by one-dimensional physics, modeling dispersion-supported systems given only line-of-sight data results in a well-known degeneracy between the mass profile and the velocity dispersion anisotropy. The core of this dissertation is rooted in a new advancement which we have discovered: the range of solutions allowed by the mass-anisotropy degeneracy varies as a function of radius, with a considerable minimal near the deprojected half-light radius of almost all observed dispersion-supported galaxies. This finding allows for a wide range of applications in galaxy formation scenarios to be explored in an attempt to address, amongst other hypotheses, whether the LCDM framework needs to be modified in order to reproduce observations on the small scale. This thesis is comprised of both the derivation of this finding, and its applicability to all dispersion-supported systems, ranging from dwarfs galaxies consisting of a few hundred stars to systems of 'intracluster light', containing over a trillion stars. Rarely does one have the privilege of working with systems that span such a large range in luminosity (or any intrinsic property) in a short graduate career. Although the large applicability of this scale-free finding allows for discussion in many subfields, this thesis will mainly focus on one topic: dwarf

  20. Dwarf galaxies in the dynamically evolved NGC 1407 Group

    NASA Astrophysics Data System (ADS)

    Trentham, Neil; Tully, R. Brent; Mahdavi, Andisheh

    2006-07-01

    The NGC 1407 Group stands out among nearby structures by its properties that suggest it is massive and evolved. It shares properties with entities that have been called fossil groups: the 1.4m differential between the dominant elliptical galaxy and the second brightest galaxy comes close to satisfying the definition that has been used to define the fossil class. There are few intermediate-luminosity galaxies, but a large number of dwarfs in the group. We estimate there are 250 group members to the depth of our survey. The slope of the faint end of the luminosity function (reaching MR = -12) is α = -1.35. Velocities for 35 galaxies demonstrate that this group with one dominant galaxy has a mass of 7 × 1013Msolar and M/LR = 340Msolar/Lsolar. Two galaxies in close proximity to NGC 1407 have very large blueshifts. The most notable is the second brightest galaxy, NGC 1400, with a velocity of -1072 km s-1 with respect to the group mean. We report the detection of X-ray emission from this galaxy and from the group.

  1. Indirect dark matter detection for flattened dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Sanders, Jason L.; Evans, N. Wyn; Geringer-Sameth, Alex; Dehnen, Walter

    2016-09-01

    Gamma-ray experiments seeking to detect evidence of dark matter annihilation in dwarf spheroidal galaxies require knowledge of the distribution of dark matter within these systems. We analyze the effects of flattening on the annihilation (J) and decay (D) factors of dwarf spheroidal galaxies with both analytic and numerical methods. Flattening has two consequences: first, there is a geometric effect as the squeezing (or stretching) of the dark matter distribution enhances (or diminishes) the J-factor; second, the line of sight velocity dispersion of stars must hold up the flattened baryonic component in the flattened dark matter halo. We provide analytic formulas and a simple numerical approach to estimate the correction to the J- and D-factors required over simple spherical modeling. The formulas are validated with a series of equilibrium models of flattened stellar distributions embedded in flattened dark-matter distributions. We compute corrections to the J- and D-factors for the Milky Way dwarf spheroidal galaxies under the assumption that they are all prolate or all oblate and find that the hierarchy of J-factors for the dwarf spheroidals is slightly altered (typical correction factors for an ellipticity of 0.4 are 0.75 for the oblate case and 1.6 for the prolate case). We demonstrate that spherical estimates of the D-factors are very insensitive to the flattening and introduce uncertainties significantly less than the uncertainties in the D-factors from the other observables for all the dwarf spheroidals (for example, +10 per cent/-3 per cent for a typical ellipticity of 0.4). We conclude by investigating the spread in correction factors produced by triaxial figures and provide uncertainties in the J-factors for the dwarf spheroidals using different physically motivated assumptions for their intrinsic shape and axis alignments. We find that the uncertainty in the J-factors due to triaxiality increases with the observed ellipticity and, in general, introduces

  2. Chemical abundances in metal-poor stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim; Norris, John; Shetrone, Matthew

    2015-08-01

    Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. Not only are alpha-poor stars found at lower metallicities, but also r-process challenged stars, and a disparate fraction of carbon-enhanced metal-poor stars. A more pristine and chemically inhomogeneous interstellar medium, combined with stoichastic star formation in a metal-poor environment, is thought to cause these detectable differences in the early SN II contributions. We are also now finding stars in dwarf galaxies that appear to be iron-enhanced, i.e., stars that have formed in pockets of SN Ia enriched gas. A comparison of their chemical abundances with individual SN Ia models can provide unique constraints on the SN Ia progenitors.

  3. RBS 1032: A TIDAL DISRUPTION EVENT IN ANOTHER DWARF GALAXY?

    SciTech Connect

    Maksym, W. Peter; Lin, Dacheng; Irwin, Jimmy A.

    2014-09-10

    RBS 1032 is a supersoft (Γ ∼ 5), luminous (∼10{sup 43} erg s{sup –1}) ROSAT PSPC source which has been associated with an inactive dwarf galaxy at z = 0.026, SDSS J114726.69+494257.8. We have analyzed an XMM-Newton observation that confirms that RBS 1032 is indeed associated with the dwarf galaxy. Moreover, RBS 1032 has undergone a factor of ∼100-300 decay since 1990 November. This variability suggests that RBS 1032 may not be a steadily accreting intermediate-mass black hole, but rather an accretion flare from the tidal disruption of a star by the central black hole (which may or may not be intermediate-mass). We suggest that additional tidal disruption events may remain unidentified in archival ROSAT data, such that disruption rate estimates based upon ROSAT All-Sky Survey data may need reconsideration.

  4. RBS 1032: A Tidal Disruption Event in Another Dwarf Galaxy?

    NASA Astrophysics Data System (ADS)

    Maksym, W. Peter; Lin, Dacheng; Irwin, Jimmy A.

    2014-09-01

    RBS 1032 is a supersoft (Γ ~ 5), luminous (~1043 erg s-1) ROSAT PSPC source which has been associated with an inactive dwarf galaxy at z = 0.026, SDSS J114726.69+494257.8. We have analyzed an XMM-Newton observation that confirms that RBS 1032 is indeed associated with the dwarf galaxy. Moreover, RBS 1032 has undergone a factor of ~100-300 decay since 1990 November. This variability suggests that RBS 1032 may not be a steadily accreting intermediate-mass black hole, but rather an accretion flare from the tidal disruption of a star by the central black hole (which may or may not be intermediate-mass). We suggest that additional tidal disruption events may remain unidentified in archival ROSAT data, such that disruption rate estimates based upon ROSAT All-Sky Survey data may need reconsideration.

  5. Dust Content of Virgo Star-Forming Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Hunt, L. K.; Madden, S.; Vlahakis, C.; Bomans, D. J.; Baes, M.; Bendo, G. J.; Bianchi, S.; Boselli, A.; Clemens, M.; Corbelli, E.; Cortese, L.; Dariush, A.; Davies, J. I.; De Looze, I.; di Serego Alighieri, S.; Fadda, D.; Fritz, J.; Garcia-Appadoo, D. A.; Gavazzi, G.; Giovanardi, C.; Hughes, T. M.; Jones, A. P.; Pierini, D.; Pohlen, M.; Sabatini, S.; Smith, M. W. L.; Verstappen, J.; Xilouris, E. M.; Zibetti, S.

    We investigate the dust properties of a small sample of Virgo cluster dwarf galaxies drawn from the science demonstration phase data set of the Herschel Virgo Cluster Survey (HeViCS). These galaxies have low metallicities (7.8 < 12 + log(O/H) < 8.3) and star formation rates ≲ 0.1 M⊙ yr-1. We measure the spectral energy distribution (SED) from 100 to 500 μm and derive dust temperatures and masses. The SEDs are fitted by a cool component with T ≲ 20 K, implying dust masses around 105 M⊙ and dust-to-gas ratios (D) within the range 10-3-10-2.

  6. A constant intrinsic thickness for dwarf irregular galaxies?

    NASA Astrophysics Data System (ADS)

    Johnson, Megan C.; Hunter, Deidre A.; Kamphuis, Peter; Wang, Jing

    2017-02-01

    We assess the intrinsic thickness parameter, qo, for a sample of dwarf irregular galaxies and compare to larger, more massive spiral galaxy systems. We use optical photometry to determine b/a (minor-to-major axial ratio), and we use the H I kinematic inclination angle to derive qo. We find that qo ranges from 0.1 to 0.8 for the dwarfs. We find a trend in qo with luminosity, indicating that fainter dwarfs are thicker than brighter ones, similar to previous studies. However, we also find a trend in qo with H I kinematic inclination, which indicates that either the radius at which we measure b/a may be too small, or, that the stellar and gas discs have different inclinations. Because we selected only those objects that have nearly identical morphological and kinematic position angles, we find the latter reason unlikely. We find a weak trend in qo with hz/RD (ratio of stellar scaleheight to scalelength), which points to b/a not fully representing the stellar disc distribution. We conclude that a constant qo may not be appropriate for dwarf irregulars.

  7. Dwarf galaxies: quantity and varietyÂ

    NASA Astrophysics Data System (ADS)

    Cellone, S. A.; Buzzoni, A.

    The structural properties and stellar populations of 79 low- and intermediate-luminosity galaxies in the NGC5044 Group are analized. The galaxies in the sample are re-classified into different morphological subgroups, with emphasis on the identification of objects showing a bulge+disk structure. The behaviour of their properties against their (projected) position within the group is addressed, looking for evidences for possible environmental effects. The observations were obtained at ESO (1999-2000) and CASLEO (1996-1999). Nearly 50% of the data presented here are new. FULL TEXT IN SPANISH

  8. Accretion phenomena onto star-forming dwarf-galaxies.

    NASA Astrophysics Data System (ADS)

    Annibali, Francesca

    2017-01-01

    I will present our recent discovery (Annibali et al. 2016, ApJL, 826 L27), based on the combination of deep wide-field LBT imaging from the ground and HST data, of a stellar stream and substructures associated to the very metal-poor star-forming dwarf galaxy DDO 68, located in a Void at ~12.7 Mpc from us. DDO 68 is very light (only 108 Msun in stars), yet it shows evidence for the accretion of at least two smaller satellites. DDO 68 is one of the very few cases where the hierarchical formation process is caught in action at such small galactic scales. This study is part of a large ongoing project based on an approved 2-year strategic program with LBT to search for stellar streams around a sample of ~50 nearby star-forming dwarf galaxies. Our result demonstrates the high potential of wide-field instrumentation at 8-10 m telescopes in combination with HST (and with JWST in the near future) for the study of accretion phenomena onto dwarf-galaxies.

  9. No WIMP mini-spikes in dwarf spheroidal galaxies

    SciTech Connect

    Wanders, Mark; Bertone, Gianfranco; Weniger, Christoph; Volonteri, Marta E-mail: g.bertone@uva.nl E-mail: c.weniger@uva.nl

    2015-04-01

    The formation of black holes inevitably affects the distribution of dark and baryonic matter in their vicinity, leading to an enhancement of the dark matter density, called spike, and if dark matter is made of WIMPs, to a strong enhancement of the dark matter annihilation rate. Spikes at the center of galaxies like the Milky Way are efficiently disrupted by baryonic processes, but mini-spikes can form and survive undisturbed at the center of dwarf spheroidal galaxies. We show that Fermi LAT satellite data allow to set very stringent limits on the existence of mini-spikes in dwarf galaxies: for thermal WIMPs with mass between 100 GeV and 1 TeV, we obtain a maximum black hole mass between 100 and 1000 M{sub ⊙}, ruling out black holes masses extrapolated from the M-σ relationship in a large region of the parameter space. We also performed Monte Carlo simulations of merger histories of black holes in dwarf spheroidals in a scenario where black holes form from the direct collapse of primordial gas in early halos, and found that this specific formation scenario is incompatible at the 84% CL with dark matter being in the form of thermal WIMPs.

  10. HERSCHEL SPECTROSCOPIC OBSERVATIONS OF LITTLE THINGS DWARF GALAXIES

    SciTech Connect

    Cigan, Phil; Young, Lisa; Cormier, Diane; Lebouteiller, Vianney; Madden, Suzanne; Brinks, Elias; Elmegreen, Bruce; Schruba, Andreas; Heesen, Volker; Collaboration: LITTLE THINGS Team

    2016-01-15

    We present far-infrared (FIR) spectral line observations of five galaxies from the Little Things sample: DDO 69, DDO 70, DDO 75, DDO 155, and WLM. While most studies of dwarfs focus on bright systems or starbursts due to observational constraints, our data extend the observed parameter space into the regime of low surface brightness dwarf galaxies with low metallicities and moderate star formation rates. Our targets were observed with Herschel at the [C ii] 158 μm, [O i] 63 μm, [O iii] 88 μm, and [N ii] 122 μm emission lines using the PACS Spectrometer. These high-resolution maps allow us for the first time to study the FIR properties of these systems on the scales of larger star-forming complexes. The spatial resolution in our maps, in combination with star formation tracers, allows us to identify separate photodissociation regions (PDRs) in some of the regions we observed. Our systems have widespread [C ii] emission that is bright relative to continuum, averaging near 0.5% of the total infrared (TIR) budget—higher than in solar-metallicity galaxies of other types. [N ii] is weak, suggesting that the [C ii] emission in our galaxies comes mostly from PDRs instead of the diffuse ionized interstellar medium (ISM). These systems exhibit efficient cooling at low dust temperatures, as shown by ([O i]+[C ii])/TIR in relation to 60 μm/100 μm, and low [O i]/[C ii] ratios which indicate that [C ii] is the dominant coolant of the ISM. We observe [O iii]/[C ii] ratios in our galaxies that are lower than those published for other dwarfs, but similar to levels noted in spirals.

  11. Probing Globular Cluster Formation in Low Metallicity Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Johnson, Kelsey E.; Hunt, Leslie K.; Reines, Amy E.

    2008-12-01

    The ubiquitous presence of globular clusters around massive galaxies today suggests that these extreme star clusters must have been formed prolifically in the earlier universe in low-metallicity galaxies. Numerous adolescent and massive star clusters are already known to be present in a variety of galaxies in the local universe; however most of these systems have metallicities of 12 + log(O/H) > 8, and are thus not representative of the galaxies in which today's ancient globular clusters were formed. In order to better understand the formation and evolution of these massive clusters in environments with few heavy elements, we have targeted several low-metallicity dwarf galaxies with radio observations, searching for newly-formed massive star clusters still embedded in their birth material. The galaxies in this initial study are HS 0822+3542, UGC 4483, Pox 186, and SBS 0335-052, all of which have metallicities of 12 + log(O/H) < 7.75. While no thermal radio sources, indicative of natal massive star clusters, are found in three of the four galaxies, SBS 0335-052 hosts two such objects, which are incredibly luminous. The radio spectral energy distributions of these intense star-forming regions in SBS 0335-052 suggest the presence of ~12,000 equivalent O-type stars, and the implied star formation rate is nearing the maximum starburst intensity limit.

  12. The Origin of Dwarf Early-Type Galaxies

    NASA Astrophysics Data System (ADS)

    Toloba, Elisa

    2012-10-01

    Abridge. We have conducted a spectrophotometric study of dwarf early-type galaxies (dEs) in the Virgo cluster and in regions of lower density. We have found that these galaxies show many properties in common with late-type galaxies but not with more massive early-types (E/S0). The properties of the dEs in Virgo show gradients within the cluster. dEs in the outer parts of the Virgo cluster are kinematically supported by rotation, while those in the center are supported by the random motions of their stars (i.e. pressure supported). The rotationally supported dEs have disky isophotes and faint underlying spiral/irregular substructures, they also show younger ages than those pressure supported, which have boxy isophotes and are smooth and regular, without any substructure. We compare the position of these dEs with massive early-type galaxies in the Faber-Jackson and Fundamental Plane relations, and we find that, although there is no difference between the position of rotationally and pressure supported dEs, both deviate from the relations of massive early-type galaxies in the direction of dwarf spheroidal systems (dSphs). We have used their offset with respect to the Fundamental Plane of E/S0 galaxies to estimate their dark matter fraction. All the properties studied in this work agree with a ram pressure stripping scenario, where late-type galaxies infall into the cluster, their interaction with the intergalactic medium blows away their gas and, as a result, they are quenched in a small amount of time. However, those dEs in the center of the cluster seem to have been fully transformed leaving no trace of their possible spiral origin, thus, if that is the case, they must have experienced a more violent mechanism in combination with ram pressure stripping.

  13. Chemodynamics of Compact Stellar Systems in NGC 5128: How Similar are Globular Clusters, Ultra-Compact Dwarfs, and Dwarf Galaxies?

    NASA Astrophysics Data System (ADS)

    Taylor, Matthew A.; Puzia, Thomas H.; Harris, Gretchen L.; Harris, William E.; Kissler-Patig, Markus; Hilker, Michael

    2010-04-01

    Velocity dispersion measurements are presented for several of the most luminous globular clusters (GCs) in NGC 5128 (Centaurus A) derived from high-resolution spectra obtained with the UVES echelle spectrograph on the 8.2 m ESO/Very Large Telescope. The measurements are made utilizing a penalized pixel-fitting method that parametrically recovers line-of-sight velocity dispersions. Combining the measured velocity dispersions with surface photometry and structural parameter data from the Hubble Space Telescope enables both dynamical masses and mass-to-light ratios to be derived. The properties of these massive stellar systems are similar to those of both massive GCs contained within the Local Group and nuclear star clusters and ultra-compact dwarf galaxies (UCDs). The fundamental plane relations of these clusters are investigated in order to fill the apparent gap between the relations of Local Group GCs and more massive early-type galaxies. It is found that the properties of these massive stellar systems match those of nuclear clusters in dwarf elliptical galaxies and UCDs better than those of Local Group GCs, and that all objects share similarly old (gsim8 Gyr) ages, suggesting a possible link between the formation and evolution of nuclear star clusters in dwarf elliptical galaxies (dE,Ns), UCDs, and massive GCs. We find a very steep correlation between dynamical mass-to-light ratio and dynamical mass of the form Υ _{V}^dyn ∝ M_dyn^{0.24± 0.02} above M}_dyn ≈ 2× 10^6 M sun. Formation scenarios are investigated with a chemical abundance analysis using absorption-line strengths calibrated to the Lick/IDS index system. The results lend support to two scenarios contained within a single general formation scheme. Old, massive, super-solar [α/Fe] systems are formed on short (lsim100 Myr) timescales through the merging of single-collapse GCs which themselves are formed within single, giant molecular clouds. More intermediate- and old-aged (~3-10 Gyr), solar- to sub

  14. Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

    NASA Astrophysics Data System (ADS)

    Mauron, N.; Gigoyan, K. S.; Berlioz-Arthaud, P.; Klotz, A.

    2014-02-01

    The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Hα in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that it is very different from the C stars in the solar neighbourhood. There is a larger proportion of short-period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars. Based on observations made with the NTT and 3.6 m telescope at the European Southern Observatory (La Silla, Chile; programs 084.D-0302 and 070.D-0203), with the TAROT telescopes at La Silla and at Observatoire de la Côte d'Azur (France), and on the exploitation of the Catalina Sky Survey and the LINEAR variability databases.Appendix A is available in electronic form at http://www.aanda.org

  15. Chemodynamic subpopulations of the Carina dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Kordopatis, G.; Amorisco, N. C.; Evans, N. W.; Gilmore, G.; Koposov, S. E.

    2016-04-01

    We study the chemodynamical properties of the Carina dwarf spheroidal by combining an intermediate spectroscopic resolution data set of more than 900 red giant and red clump stars, with high-precision photometry to derive the atmospheric parameters, metallicities and age estimates for our targets. Within the red giant branch population, we find evidence for the presence of three distinct stellar subpopulations with different metallicities, spatial distributions, kinematics and ages. As in the Fornax and Sculptor dwarf spheroidals, the subpopulation with the lowest average metallicity is more extended and kinematically hotter than all other populations. However, we identify an inversion in the parallel ordering of metallicity, kinematics and characteristic length-scale in the two most metal-rich subpopulations, which therefore do not contribute to a global negative chemical gradient. Contrary to common trends in the chemical properties with radius, the metal richest population is more extended and mildly kinematically hotter than the main component of intermediate metallicity. More investigations are required to ascertain the nature of this inversion, but we comment on the mechanisms that might have caused it.

  16. TiNy Titans: The Role of Dwarf-Dwarf Interactions in Low-mass Galaxy Evolution

    NASA Astrophysics Data System (ADS)

    Stierwalt, S.; Besla, G.; Patton, D.; Johnson, K.; Kallivayalil, N.; Putman, M.; Privon, G.; Ross, G.

    2015-05-01

    We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. Here we present the first results from a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the spectroscopic portion of the Sloan Digital Sky Survey and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M*,1/M*,2 < 10, projected separations <50 kpc, and pair member masses of 7 < log({{M}*}/{{M}⊙ }) < 9.7. The dwarf-dwarf merger sequence, as defined by TNT at z = 0, demonstrates conclusively and for the first time that the star formation enhancement observed for massive galaxy pairs also extends to the dwarf mass range. Star formation is enhanced in paired dwarfs in otherwise isolated environments by a factor of 2.3 (±0.7) at pair separations <50 kpc relative to unpaired analogs. The enhancement decreases with increasing pair separation and extends out to pair separations as large as 100 kpc. Starbursts, defined by Hα EQW >100 Å, occur in 20% of the TNT dwarf pairs, regardless of environment, compared to only 6%-8% of the matched unpaired dwarfs. Starbursts can be triggered throughout the merger (i.e., out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation and triggered starbursts, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs of the same stellar mass. Thus, there may be significant reservoirs of diffuse, non-star-forming neutral gas surrounding the dwarf pairs, or the gas consumption timescales may be long in the starburst phase. The only TNT dwarf pairs with low gas fractions (fgas\\lt 0.4) and the only dwarfs, either paired or unpaired, with Hα EQW < 2 Å are found near massive galaxy hosts. We conclude that dwarf-dwarf interactions are significant drivers of galaxy evolution at the low-mass end, but

  17. Dwarf galaxies in the Coma cluster - I. Velocity dispersion measurements

    NASA Astrophysics Data System (ADS)

    Kourkchi, E.; Khosroshahi, H. G.; Carter, D.; Karick, A. M.; Mármol-Queraltó, E.; Chiboucas, K.; Tully, R. B.; Mobasher, B.; Guzmán, R.; Matković, A.; Gruel, N.

    2012-03-01

    We present the study of a large sample of early-type dwarf galaxies in the Coma cluster observed with DEIMOS on the Keck II to determine their internal velocity dispersion. We focus on a subsample of 41 member dwarf elliptical galaxies for which the velocity dispersion can be reliably measured, 26 of which were studied for the first time. The magnitude range of our sample is -21 < MR < -15 mag. This paper (Paper I) focuses on the measurement of the velocity dispersion and their error estimates. The measurements were performed using penalized pixel fitting (PPXF) and using the calcium triplet absorption lines. We use Monte Carlo bootstrapping to study various sources of uncertainty in our measurements, namely statistical uncertainty, template mismatch and other systematics. We find that the main source of uncertainty is the template mismatch effect which is reduced by using templates with a range of spectral types. Combining our measurements with those from the literature, we study the Faber-Jackson relation (L∝σα) and find that the slope of the relation is α= 1.99 ± 0.14 for galaxies brighter than MR≃-16 mag. A comprehensive analysis of the results combined with the photometric properties of these galaxies is reported in Paper II.

  18. METALS REMOVED BY OUTFLOWS FROM MILKY WAY DWARF SPHEROIDAL GALAXIES

    SciTech Connect

    Kirby, Evan N.; Martin, Crystal L.; Finlator, Kristian

    2011-12-15

    The stars in the dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are significantly more metal-poor than would be expected from a closed box model of chemical evolution. Gas outflows likely carried away most of the metals produced by the dSphs. Based on previous Keck/DEIMOS observations and models, we calculate the mass in Mg, Si, Ca, and Fe expelled from each of eight dSphs. Essentially, these masses are the differences between the observed amount of metals present in the dSphs' stars today and the inferred amount of metals produced by supernovae. We conclude that the dSphs lost 96% to >99% of the metals their stars manufactured. We apply the observed mass function of Milky Way dSphs to the ejected mass function to determine that a single large dSph, like Fornax, lost more metals over 10 Gyr than all smaller dSphs combined. Therefore, small galaxies like dSphs are not significant contributors to the metal content of the intergalactic medium. Finally, we compare our ejected mass function to previous X-ray measurements of the metal content of the winds from the post-starburst dwarf irregular galaxy NGC 1569. Remarkably, the most recent starburst in that galaxy falls exactly on the ejected-mass-stellar-mass relation defined by the Milky Way dSphs.

  19. IN-SPIRALING CLUMPS IN BLUE COMPACT DWARF GALAXIES

    SciTech Connect

    Elmegreen, Bruce G.; Zhang Hongxin; Hunter, Deidre A.

    2012-03-10

    Giant star formation clumps in dwarf irregular galaxies can have masses exceeding a few percent of the galaxy mass enclosed inside their orbital radii. They can produce sufficient torques on dark matter halo particles, halo stars, and the surrounding disk to lose their angular momentum and spiral into the central region in 1 Gyr. Pairs of giant clumps with similarly large relative masses can interact and exchange angular momentum to the same degree. The result of this angular momentum loss is a growing central concentration of old stars, gas, and star formation that can produce a long-lived starburst in the inner region, identified with the blue compact dwarf (BCD) phase. This central concentration is proposed to be analogous to the bulge in a young spiral galaxy. Observations of star complexes in five local BCDs confirm the relatively large clump masses that are expected for this process. The observed clumps also seem to contain old field stars, even after background light subtraction, in which case the clumps may be long-lived. The two examples with clumps closest to the center have the largest relative clump masses and the greatest contributions from old stars. An additional indication that the dense central regions of BCDs are like bulges is the high ratio of the inner disk scale height to the scale length, which is comparable to 1 for four of the galaxies.

  20. Comparison of Alternative Gravity Models in Dwarf Galaxy Rotation Curves

    NASA Astrophysics Data System (ADS)

    Harrington, Justin; Saintable, Taylor; O'Brien, James

    2017-01-01

    Galactic rotation curves have proven to be the testing ground for dark matter bounds in spiral galaxies of all morphologies. Dwarf Galaxies serve as an increasingly interesting testing ground of rotation curve dynamics due to their increased stellar formation and typically rising rotation curve. These galaxies usually are not dominated by typical stellar structure and mostly terminate at small radial distances. This, coupled with the fact that Cold Dark Matter theories such as NFW (∧ CDM) struggle with the universality of galactic rotation curves, allow for exclusive features of alternative gravitational models to be analyzed. Here, we present a thorough application of alternative gravitational models (conformal gravity and MOND) to a 2010 dwarf galaxy sample from Swaters et al. An analysis and discussion of the results of the fitting procedure of the two alternative gravitational models are explored. We posit here that both the Conformal Gravity and MOND can provide an accurate description of the galactic dynamics without the need for copious dark matter.

  1. Structural analysis of the Sextans dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Roderick, T. A.; Jerjen, H.; Da Costa, G. S.; Mackey, A. D.

    2016-07-01

    We present wide-field g- and i-band stellar photometry of the Sextans dwarf spheroidal galaxy and its surrounding area out to four times its half-light radius (rh = 695 pc), based on images obtained with the Dark Energy Camera at the 4-m Blanco telescope at CTIO. We find clear evidence of stellar substructure associated with the galaxy, extending to a distance of 82 arcmin (2 kpc) from its centre. We perform a statistical analysis of the overdensities and find three distinct features, as well as an extended halo-like structure, to be significant at the 99.7 per cent confidence level or higher. Unlike the extremely elongated and extended substructures surrounding the Hercules dwarf spheroidal galaxy, the overdensities seen around Sextans are distributed evenly about its centre, and do not appear to form noticeable tidal tails. Fitting a King model to the radial distribution of Sextans stars yields a tidal radius rt = 83.2 arcmin ± 7.1 arcmin (2.08 ± 0.18 kpc), which implies the majority of detected substructure is gravitationally bound to the galaxy. This finding suggests that Sextans is not undergoing significant tidal disruption from the Milky Way, supporting the scenario in which the orbit of Sextans has a low eccentricity.

  2. HI Gas in Disk and Dwarf Galaxies in the Semi-analytic Models of Galaxy Formation†

    NASA Astrophysics Data System (ADS)

    Fu, Jian; Wang, Jing; Luo, Yu

    We construct the radially-resolved semi-analytic models of galaxy formation based on the L-Galaxies model framework, which include both atomic and molecular gas phase in ISM. The models run on the halo outputs of ΛCDM cosmology N-body simulation. Our models can reproduce varies observations of HI gas in nearby galaxies, e.g. the HI mass function, the HI-to-star ratio vs stellar mass and stellar surface density, universal HI radial surface density profile in outer disks etc. We also give the physical origin of HI size-mass relation. Based on our model results for local dwarf galaxies, we show that the ``missing satellite problem'' also exists in the HI component, i.e., the models over-predict dwarf galaxies with low HI mass around the Milky Way. That is a shortcoming of current ΛCDM cosmology framework. Future survey for HI gas in local dwarf galaxies (e.g. MeerKAT, SKA & FAST) can help to verify the nature of dark matter (cold or warm).

  3. A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy.

    PubMed

    Ibata, Rodrigo A; Lewis, Geraint F; Conn, Anthony R; Irwin, Michael J; McConnachie, Alan W; Chapman, Scott C; Collins, Michelle L; Fardal, Mark; Ferguson, Annette M N; Ibata, Neil G; Mackey, A Dougal; Martin, Nicolas F; Navarro, Julio; Rich, R Michael; Valls-Gabaud, David; Widrow, Lawrence M

    2013-01-03

    Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. It has previously been suspected that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams of H I emission, and may form coplanar groups. These suspicions are supported by recent analyses. It has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence of a planar subgroup of satellites in the Andromeda galaxy (M 31), comprising about half of the population. The structure is at least 400 kiloparsecs in diameter, but also extremely thin, with a perpendicular scatter of less than 14.1 kiloparsecs. Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way's disk and with the vector between the Milky Way and Andromeda.

  4. Variations in the Dust-to-Gas Ratio in Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Schruba, Andreas; Walter, F.; Leroy, A.; Kennicutt, R.; Lee, J.; Gil de Paz, A.; Hunter, D.; Ott, J.

    2009-05-01

    We present the first part of a systematic study of the relationship between dust and gas in a large set of nearby dwarf irregular galaxies. These systems are rich in gas but poor in dust and heavy elements, a contrast that sets them apart from the Milky Way and other nearby spiral galaxies. The central role of dust in the creation and shielding of star-forming gas and its close link to the enrichment of the ISM make the dust-to-gas (DGR) ratio a key observable to understand star formation in these systems. We are combining maps of dust emission taken by Spitzer as part of the Local Volume Legacy (LVL) with atomic gas observations by the VLA large programs LITTLE THINGS, VLA-ANGST, and THINGS to measure the DGR as a function of position in a large sample of dwarf galaxies. This will populate the low end of the DGR vs. metallicity relation and allow us to measure the enrichment of the extended HI envelopes that surround many dwarf irregulars. By comparing the DGR in regions of active star formation to the DGR in nearby quiescent, we can also place an upper limit on the amount of H2 that can be present near the star forming peaks, a quantity that is otherwise difficult to constrain.

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

    NASA Astrophysics Data System (ADS)

    Kang, Hoyoung; Ricotti, Massimo

    2016-01-01

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

  6. Northern dwarf and low surface brightness galaxies. I - The Arecibo neutral hydrogen survey

    NASA Technical Reports Server (NTRS)

    Schneider, Stephen E.; Thuan, Trinh X.; Magri, Christopher; Wadiak, James E.

    1990-01-01

    Neutral hydrogen observations of a large sample of dwarf and other low surface brightness galaxies are presented. Nearly all galaxies classified in the Uppsala General Catalogue as dwarf, irregular, Sd-m, or later and in the declination range of the Arecibo telescope have now been observed; here observations for 762 galaxies are reported. About 40 percent of these galaxies have no previously published detections. In total, counting previous detections, over 90 percent of these late-type systems are detected at Arecibo. The galaxies are examined for potential confusion with nearby galaxies, and substantially better SNR are derived for many previously detected galaxies.

  7. Ultracompact Blue Dwarfs: Galaxy Formation in the Local Universe?

    NASA Astrophysics Data System (ADS)

    Corbin, Michael

    2004-07-01

    Recent observations suggest that very low-mass galaxies in the local universe are still in the process of formation. To investigate this issue we propose to obtain deep ACS HRC images in the U, V and I bands of a sample of 11 "ultracompact" blue dwarf galaxies {UCBDs} identified in the Sloan Digital Sky Survey. These objects are nearby {z < 0.009}, actively star-forming, and have extremely small angular and physical sizes {d < 6" and D < 1 kpc}. They also tend to reside in voids. Our WFPC2 images of the prototype object of this class, POX 186, reveal this tiny object to have a highly disturbed morphlogy indicative of a recent {within 10^8 yr} collision between two small { 100 pc} clumps of stars that could represent the long-sought building blocks predicted by the Press-Schechter model of hierarchical galaxy formation. This collision has also triggered the formation of a "super" star cluster {SSC} at the object's core that may be the progenitor of a globular cluster. POX 186 thus appears to be a very small dwarf galaxy in the process of formation. This exciting discovery strongly motivates HST imaging of a full sample of UCBDs in order to determine if they have morphologies similar to POX 186. HST images are essential for resolving the structure of these objects, including establishing the presence of SSCs. HST also offers the only way to determine their morphologies in the near UV. The spectra of the objects available from the SDSS will also allow us to measure their star formation rates, dust content and metallicities. In addition to potentially providing the first direct evidence of Press-Schechter building blocks, these data could yield insight into the relationship between galaxy and globular cluster formation, and will serve as a test of the recent "downsizing" model of galaxy formation in which the least massive objects are the last to form.

  8. WFPC2 Observations of the URSA Minor Dwarf Spheroidal Galaxy

    NASA Technical Reports Server (NTRS)

    Mighell, Kenneth J.; Burke, Christopher J.

    1999-01-01

    We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F555W (approximately V) and F814W (approximately I) of the central region of the Ursa Minor dwarf spheroidal galaxy. The V versus V - I color-magnitude diagram features a sparsely populated blue horizontal branch, a steep thin red giant branch, and a narrow subgiant branch. The main sequence reaches approximately 2 magnitudes below the main-sequence turnoff (V(sup UMi, sub TO) approximately equals 23.27 +/- 0.11 mag) of the median stellar population. We compare the fiducial sequence of the Galactic globular cluster M92 (NGC 6341). The excellent match between Ursa Minor and M92 confirms that the median stellar population of the UMi dSph galaxy is metal poor ([Fe/H](sub UMi) approximately equals [Fe/H](sub M92) approximately equals -2.2 dex) and ancient (age(sub UMi)approximately equalsage(sub M92) approximately equals 14 Gyr). The B - V reddening and the absorption in V are estimated to be E(B - V) = 0.03 +/- 0.01 mag and A(sup UMi, sub V) = 0.09 +/- 0.03 mag. A new estimate of the distance modulus of Ursa Minor, (m - M)(sup UMi, sub 0) = 19.18 +/- 0.12 mag, has been derived based on fiducial-sequence fitting M92 [DELTA.V(sub UMi - M92) = 4.60 +/- 0.03 mag and DELTA(V - I)(sub UMi - M92) = 0.010 +/- 0.005 mag] and the adoption of the apparent V distance modulus for M92 of (m - M)(sup M92, sub V) = 14.67 +/- 0.08 mag (Pont et al. 1998, A&A, 329, 87). The Ursa Minor dwarf spheroidal galaxy is then at a distance of 69 +/- 4 kpc from the Sun. These HST observations indicate that Ursa Minor has had a very simple star formation history consisting mainly of a single major burst of star formation about 14 Gyr ago which lasted approximately < 2 Gyr. While we may have missed minor younger stellar populations due to the small field-of-view of the WFPC2 instrument, these observations clearly show that most of the stars in the central region Ursa Minor dwarf

  9. A New Milky Way dwarf galaxy in Ursa Major

    SciTech Connect

    Willman, Beth; Dalcanton, Julianne J.; Martinez-Delgado, David; West, Andrew A.; Blanton, Michael R.; Hogg, David W.; Barentine, J.C.; Brewington, Howard J.; Harvanek, Michael; Kleinman, S.J.; Krzesinski, Jurek; Long, Dan; Neilsen, Eric H., Jr.; Nitta, Atsuko; Snedden, Stephanie A.; /CCPP, New York /Washington U., Seattle, Astron. Dept. /IAA, Granada /Heidelberg, Max Planck Inst. Astron. /Apache Point Observ. /Mt. Suhora Observ., Cracow /Fermilab

    2005-03-01

    In this Letter, we report the discovery of a new dwarf satellite to the Milky Way, located at ({alpha}{sub 2000}, {delta}{sub 2000}) = (158.72,51.92) in the constellation of Ursa Major. This object was detected as an overdensity of red, resolved stars in Sloan Digital Sky Survey data. The color-magnitude diagram of the Ursa Major dwarf looks remarkably similar to that of Sextans, the lowest surface brightness Milky Way companion known, but with approximately an order of magnitude fewer stars. Deeper follow-up imaging confirms this object has an old and metal-poor stellar population and is {approx} 100 kpc away. We roughly estimate M{sub V} = -6.75 and r{sub 1/2} = 250 pc for this dwarf. Its luminosity is several times fainter than the faintest known Milky Way dwarfs. However, its physical size is typical for dSphs. Even though its absolute magnitude and size are presently quite uncertain, Ursa Major is likely the lowest luminosity and lowest surface brightness galaxy yet known.

  10. POX 186: A Dwarf Galaxy in the Process of Formation?

    NASA Astrophysics Data System (ADS)

    Corbin, Michael R.; Vacca, William D.

    2002-12-01

    We present deep U-, V-, and I-band images of the ``ultracompact'' blue dwarf galaxy POX 186 obtained with the Planetary Camera 2 of the Hubble Space Telescope. We have also obtained a near-ultraviolet spectrum of the object with the Space Telescope Imaging Spectrograph and combine this with a new ground-based optical spectrum. The images confirm the galaxy to be extremely small, with a maximum extent of only 300 pc, a luminosity of ~10-4L*, and an estimated mass of ~107 Msolar. Its morphology is highly asymmetric, with a tail of material on its western side that may be tidal in origin. The U-band image shows this tail to be part of a stream of material in which stars have recently formed. Most of the star formation in the galaxy is, however, concentrated in a central, compact (d~10-15 pc) star cluster. We estimate this cluster to have a total mass of ~105 Msolar, to be forming stars at a rate of less than 0.05 yr-1, and to have a maximum age of a few million years. The outer regions of the galaxy are significantly redder than the cluster, with V-I colors consistent with a population dominated by K and M stars. From our analysis of the optical spectrum we find the galaxy to have a metallicity Z~=0.06 Zsolar and to contain a significant amount of internal dust [E(B-V)~=0.28] both values agree with previous estimates. While these results rule out earlier speculation that POX 186 is a protogalaxy, its morphology, mass, and active star formation suggest that it represents a recent (within ~108 yr) collision between two clumps of stars of subgalactic size (~100 pc). POX 186 may thus be a very small dwarf galaxy that, dynamically speaking, is still in the process of formation. This interpretation is supported by the fact that it resides in a void, so its morphology cannot be explained as the result of an encounter with a more massive galaxy. Clumps of stars this small may represent the building blocks required by hierarchical models of galaxy formation, and these results

  11. Deep JH Imaging of the LITTLE THINGS Galaxies: Stellar Mass Assembly in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Zhang, Hongxin; Hunter, Deidre; Herrmann, Kim; LITTLE THINGS Team

    2011-02-01

    We propose to obtain deep broadband JH images for 6 dwarf irregular galaxies (dIm) which are part of a larger sample assembled by the LITTLE THINGS project (a VLA Large Proposal). Using the NIR data requested here and other multi-band data we have collected, we will, for the first time, construct high-quality spectral energy distributions (SEDs) covering the whole spectral range of stellar emission for a representative sample of dIm galaxies. dIm galaxies numerically dominate the local Universe, yet our understanding of the star formation processes in dIm galaxies is very poor. For example, the star formation rate profile often correlates closely with the stellar surface brightness profile of the older stars, but not with the gas, and we do not know why. Also, abrupt changes in the slope of the stellar exponential profile imply a change in the star formation process at the surface brightness breaks according to some models, but this has not been shown observationally. With the SEDs constructed as a function of radius within each galaxy and our stellar population synthesis technique, we will answer the questions: 1) How is the stellar mass across the disks assembled throughout the lifetime of dwarf irregular galaxies? 2) Are there corresponding surface mass density breaks at the surface brightness breaks seen in many dIm galaxies? And is there any difference in the stellar populations before and beyond the surface brightness breaks?

  12. Deep JH Imaging of the LITTLE THINGS Galaxies: Stellar Mass Assembly in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Zhang, Hongxin; Hunter, Deidre; Herrmann, Kim; Little Things Team

    2011-08-01

    We propose to obtain deep broadband JH images for 2 dwarf irregular galaxies (dIm) which are part of a larger sample assembled by the LITTLE THINGS project (a VLA Large Proposal). Using the NIR data requested here and other multi-band data we have collected, we will, for the first time, construct high-quality spectral energy distributions (SEDs) covering the whole spectral range of stellar emission for a representative sample of dIm galaxies. dIm galaxies numerically dominate the local universe, yet our understanding of the star formation processes in dIm galaxies is very poor. For example, the star formation rate profile often correlates closely with the stellar surface brightness profile of the older stars, but not with the gas, and we do not know why. Also, abrupt changes in the slope of the stellar exponential profile imply a change in the star formation process at the surface brightness breaks according to some models, but this has not been shown observationally. With the SEDs constructed as a function of radius within each galaxy and our stellar population synthesis technique, we will answer the questions: 1) How is the stellar mass across the disks assembled throughout the lifetime of dwarf irregular galaxies? 2) Are there corresponding surface mass density breaks at the surface brightness breaks seen in many dIm galaxies? 3) Is there any difference in the stellar populations before and beyond the surface brightness breaks?

  13. Dark Matter Searches with Cherenkov Telescopes: Nearby Dwarf Galaxies or Local Galaxy Clusters?

    SciTech Connect

    Sanchez-Conde, Miguel A.; Cannoni, Mirco; Zandanel, Fabio; Gomez, Mario E.; Prada, Francisco; /IAA, Granada

    2012-06-06

    In this paper, we compare dwarf galaxies and galaxy clusters in order to elucidate which object class is the best target for gamma-ray DM searches with imaging atmospheric Cherenkov telescopes (IACTs). We have built a mixed dwarfs+clusters sample containing some of the most promising nearby dwarf galaxies (Draco, Ursa Minor, Wilman 1 and Segue 1) and local galaxy clusters (Perseus, Coma, Ophiuchus, Virgo, Fornax, NGC 5813 and NGC 5846), and then compute their DM annihilation flux profiles by making use of the latest modeling of their DM density profiles. We also include in our calculations the effect of DM substructure. Willman 1 appears as the best candidate in the sample. However, its mass modeling is still rather uncertain, so probably other candidates with less uncertainties and quite similar fluxes, namely Ursa Minor and Segue 1, might be better options. As for galaxy clusters, Virgo represents the one with the highest flux. However, its large spatial extension can be a serious handicap for IACT observations and posterior data analysis. Yet, other local galaxy cluster candidates with more moderate emission regions, such as Perseus, may represent good alternatives. After comparing dwarfs and clusters, we found that the former exhibit annihilation flux profiles that, at the center, are roughly one order of magnitude higher than those of clusters, although galaxy clusters can yield similar, or even higher, integrated fluxes for the whole object once substructure is taken into account. Even when any of these objects are strictly point-like according to the properties of their annihilation signals, we conclude that dwarf galaxies are best suited for observational strategies based on the search of point-like sources, while galaxy clusters represent best targets for analyses that can deal with rather extended emissions. Finally, we study the detection prospects for present and future IACTs in the framework of the constrained minimal supersymmetric standard model. We

  14. Exploring stellar metallicities in dwarf galaxies and their implications

    NASA Astrophysics Data System (ADS)

    Ross, Teresa Lynn

    In this thesis I discuss issues involving stellar metallicities in dwarf galaxies. Stars reflect the gas composition at the time they formed, thereby making the metallicity distribution function (MDF -- the relative number of stars as a function of metallicity) a record of the chemical evolution within a galaxy. I measure photometric metallicities using Wide Field Camera 3 (WFC3) observations aboard the Hubble Space Telescope. Advantages of photometric metallicities include measuring every star in the field down to fainter magnitudes than allowed by spectroscopy. I quantified and calibrated the metallicity and temperature sensitivities of colors derived from nine WFC3 filters using Dartmouth isochrones and Kurucz model atmospheres. The photometric metallicities were tested and calibrated with five well studied Galactic clusters spanning three orders of magnitude in metallicity: M92, NGC 6752, NGC 104, NGC 5927, and NGC 6791. The greatest accuracy in assigning metallicity was found using the (F390M--F555W) color, with the main advantage being the increased color sensitivity at low metallicity. MDFs for a population, along with chemical evolution models provide evolutionary information about gas flows and enrichment within that galaxy. I measured photometric metallicities in Leo I, Leo II, IC 1613, and Phoenix, and analytical chemical evolution models were fit to their MDFs. The MDF shapes, chemical evolution models and dynamic histories suggest that the galactic conditions during periods of star formation influenced the metallicities. I find that the narrower MDFs are indicative of interactions occurring in concert with star formation, while a broader MDF indicates a passive evolution. Additionally, I explore ways to combine chemical evolution models and star formation histories (SFH), to quantify the metallicity evolution with time. The SFHs of Weisz et al. (2014) are assessed for their potential to determine MDFs for 40 Local Group dwarf galaxies. The SFH

  15. Dissipative dark matter and the rotation curves of dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Foot, R.

    2016-07-01

    There is ample evidence from rotation curves that dark matter halos around disk galaxies have nontrivial dynamics. Of particular significance are: a) the cored dark matter profile of disk galaxies, b) correlations of the shape of rotation curves with baryonic properties, and c) Tully-Fisher relations. Dark matter halos around disk galaxies may have nontrivial dynamics if dark matter is strongly self interacting and dissipative. Multicomponent hidden sector dark matter featuring a massless `dark photon' (from an unbroken dark U(1) gauge interaction) which kinetically mixes with the ordinary photon provides a concrete example of such dark matter. The kinetic mixing interaction facilitates halo heating by enabling ordinary supernovae to be a source of these `dark photons'. Dark matter halos can expand and contract in response to the heating and cooling processes, but for a sufficiently isolated halo could have evolved to a steady state or `equilibrium' configuration where heating and cooling rates locally balance. This dynamics allows the dark matter density profile to be related to the distribution of ordinary supernovae in the disk of a given galaxy. In a previous paper a simple and predictive formula was derived encoding this relation. Here we improve on previous work by modelling the supernovae distribution via the measured UV and Hα fluxes, and compare the resulting dark matter halo profiles with the rotation curve data for each dwarf galaxy in the LITTLE THINGS sample. The dissipative dark matter concept is further developed and some conclusions drawn.

  16. Structural parameters and blue stragglers in Sagittarius dwarf spheroidal galaxy globular clusters

    NASA Astrophysics Data System (ADS)

    Salinas, Ricardo; Jílková, Lucie; Carraro, Giovanni; Catelan, Márcio; Amigo, Pía.

    2012-04-01

    We present BV photometry of four Sagittarius dwarf spheroidal galaxy globular clusters: Arp 2, NGC 5634, Palomar 12 and Terzan 8, obtained with the Danish Telescope at ESO La Silla. We measure the structural parameters of the clusters using a King profile fitting, obtaining the first reliable measurements of the tidal radius of Arp 2 and Terzan 8. These two clusters are remarkably extended and with low concentrations; with a concentration of only c= 0.41 ± 0.02, Terzan 8 is less concentrated than any cluster in our Galaxy. Blue stragglers are identified in the four clusters, and their spatial distribution is compared to those of horizontal branch and red giant branch stars. The blue straggler properties do not provide evidence of mass segregation in Terzan 8, while Arp 2 probably shares the same status, although with less confidence. In the case of NGC 5634 and Palomar 12, blue stragglers are significantly less populous, and their analysis suggests that the two clusters have probably undergone mass segregation. References: (1) <link href="#b56">Peterson (1976)link>; (2) <link href="#b37">Kron, Hewitt & Wasserman (1984)link>; (3) <link href="#b14">Chernoff & Djorgovski (1989)link>; (4) <link href="#b70">Trager, Djorgovski & King (1993)link>; (5) <link href="#b71">Trager et al. (1995)link>; (6) <link href="#b58">Rosenberg et al. (1998)link>; (7) <link href="#b45">Mackey & Gilmore (2003b)link>; (8) <link href="#b46">McLaughlin & van der Marel (2005)link> and (9) <link href="#b10">Carballo-Bello et al. (2012)link>.

  17. STELLAR KINEMATICS OF THE ANDROMEDA II DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Ho, Nhung; Geha, M.; Tollerud, E.; Munoz, R. R.; Guhathakurta, P.; Gilbert, K. M.; Bullock, J.; Beaton, R. L.; Majewski, S. R. E-mail: marla.geha@yale.edu

    2012-10-20

    We present kinematical profiles and metallicity for the M31 dwarf spheroidal (dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy of 531 red giant branch stars. Our kinematical sample is among the largest for any M31 satellite and extends out to two effective radii (r {sub eff} = 5.'3 = 1.1 kpc). We find a mean systemic velocity of -192.4 {+-} 0.5 km s{sup -1} and an average velocity dispersion of {sigma} {sub v} = 7.8 {+-} 1.1 km s{sup -1}. While the rotation velocity along the major axis of And II is nearly zero (<1 km s{sup -1}), the rotation along the minor axis is significant with a maximum rotational velocity of v {sub max} = 8.6 {+-} 1.8 km s{sup -1}. We find a kinematical major axis, with a maximum rotational velocity of v {sub max} = 10.9 {+-} 2.4 km s{sup -1}, misaligned by 67 Degree-Sign to the isophotal major axis. And II is thus the first dwarf galaxy with evidence for nearly prolate rotation with a v {sub max}/{sigma} {sub v} = 1.1, although given its ellipticity of {epsilon} = 0.10, this object may be triaxial. We measured metallicities for a subsample of our data, finding a mean metallicity of [Fe/H] = -1.39 {+-} 0.03 dex and an internal metallicity dispersion of 0.72 {+-} 0.03 dex. We find a radial metallicity gradient with metal-rich stars more centrally concentrated, but do not observe a significant difference in the dynamics of the two metallicity populations. And II is the only known dwarf galaxy to show minor axis rotation, making it a unique system whose existence offers important clues on the processes responsible for the formation of dSphs.

  18. CALIBRATING UV STAR FORMATION RATES FOR DWARF GALAXIES FROM STARBIRDS

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Mitchell, Noah P.; Dolphin, Andrew E.

    2015-08-01

    Integrating our knowledge of star formation (SF) traced by observations at different wavelengths is essential for correctly interpreting and comparing SF activity in a variety of systems and environments. This study compares extinction corrected integrated ultraviolet (UV) emission from resolved galaxies with color–magnitude diagram (CMD) based star formation rates (SFRs) derived from resolved stellar populations and CMD fitting techniques in 19 nearby starburst and post-starburst dwarf galaxies. The data sets are from the panchromatic Starburst Irregular Dwarf Survey and include deep legacy GALEX UV imaging, Hubble Space Telescope optical imaging, and Spitzer MIPS imaging. For the majority of the sample, the integrated near-UV fluxes predicted from the CMD-based SFRs—using four different models—agree with the measured, extinction corrected, integrated near-UV fluxes from GALEX images, but the far-UV (FUV) predicted fluxes do not. Furthermore, we find a systematic deviation between the SFRs based on integrated FUV luminosities and existing scaling relations, and the SFRs based on the resolved stellar populations. This offset is not driven by different SF timescales, variations in SFRs, UV attenuation, nor stochastic effects. This first comparison between CMD-based SFRs and an integrated FUV emission SFR indicator suggests that the most likely cause of the discrepancy is the theoretical FUV–SFR calibration from stellar evolutionary libraries and/or stellar atmospheric models. We present an empirical calibration of the FUV-based SFR relation for dwarf galaxies, with uncertainties, which is ∼53% larger than previous relations.

  19. Black Holes At the Centers of Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Moran, Edward C.; Shahinyan, Karlen; Sugarman, Hannah R.; Vélez, Darik O.; Eracleous, Michael

    2014-12-01

    Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby (d≤slant 80 Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the galaxy centers are expected to be intermediate-mass black holes (IMBHs) with {{M}BH}≤slant {{10}6} {{M}⊙ }. The AGNs were selected using several optical emission-line diagnostics after careful modeling of the continuum present in the spectra. We have limited our survey to objects with spectral characteristics similar to those of Seyfert nuclei, excluding emission-line galaxies with ambiguous spectra that could be powered by stellar processes. Thus, as a set, the host galaxies in our sample are the least massive objects in the very local universe certain to contain central black holes. Our sample is dominated by narrow-line (type 2) AGNs, and it appears to have a much lower fraction of broad-line objects than that observed for luminous, optically selected Seyfert galaxies. Given our focus on the nearest objects included in the SDSS, our survey is more sensitive to low-luminosity emission than previous optical searches for AGNs in low-mass galaxies. The [O iii] λ 5007 luminosities of the Seyfert nuclei in our sample have a median value of {{L}5007}=2× {{10}5} {{L}⊙ } and extend down to ˜ {{10}4} {{L}⊙ }. Using published data for broad-line IMBH candidates, we have derived an [O iii] bolometric correction of log ({{L}bol}/{{L}5007})=3.0+/- 0.3, which is significantly lower than values obtained for high-luminosity AGNs. Applying this correction to our sample, we obtain minimum black hole mass estimates that fall mainly in the 103 {{M}⊙ }-104 {{M}⊙ } range, which is roughly where the predicted mass functions for different black hole seed formation scenarios overlap the most. In the stellar mass range that includes the bulk of the AGN host galaxies in our sample, we derive a lower limit on the AGN fraction of a few

  20. The origin of dwarf early-type galaxies

    NASA Astrophysics Data System (ADS)

    Toloba, E.

    2013-05-01

    We have conducted a spectrophotometric study of dwarf early-type galaxies (dEs) in the Virgo cluster and in regions of lower density. We have found that these galaxies show many properties in common with late-type galaxies but not with more massive early-types (E/S0). The properties of the dEs in Virgo show gradients within the cluster. dEs in the outer parts of the Virgo cluster are kinematically supported by rotation, while those in the center are supported by the random motions of their stars (i.e. pressure supported). The rotationally supported dEs have disky isophotes and faint underlying spiral/irregular substructures, they also show younger ages than those pressure supported, which have boxy isophotes and are smooth and regular, without any substructure. We compare the position of these dEs with massive early-type galaxies in the Faber-Jackson and Fundamental Plane relations, and we find that, although there is no difference between the position of rotationally and pressure supported dEs, both deviate from the relations of massive early-type galaxies in the direction of dwarf spheroidal systems (dSphs). We have used their offset with respect to the Fundamental Plane of E/S0 galaxies to estimate their dark matter fraction. All the properties studied in this work agree with a ram pressure stripping scenario, where late-type galaxies infall into the cluster, their interaction with the intergalactic medium blows away their gas and, as a result, they are quenched in a small amount of time. However, those dEs in the center of the cluster seem to have been fully transformed leaving no trace of their possible spiral origin, thus, if that is the case, they must have experienced a more violent mechanism in combination with ram pressure stripping, the open problem is that even galaxy harassment does not fully explain the observed properties for the pressure supported dEs in the center of the Virgo cluster.

  1. Variable stars in the Pegasus dwarf galaxy (DDO 216)

    SciTech Connect

    Hoessel, J.G.; Abbott, M.J.; Saha, A.; Mossman, A.E.; Danielson, G.E. Space Telescope Science Institute, Baltimore, MD Palomar Observatory, Pasadena, CA )

    1990-10-01

    Observations obtained over a period of five years of the resolved stars in the Pegasus dwarf irregular galaxy (DDO 216) have been searched for variable stars. Thirty-one variables were found, and periods established for 12. Two of these variable stars are clearly eclipsing variables, seven are very likely Cepheid variables, and the remaining three are probable Cepheids. The period-luminosity relation for the Cepheids indicates a distance modulus for Pegasus of m - M = 26.22 + or - 0.20. This places Pegasus very near the zero-velocity surface of the Local Group. 25 refs.

  2. Variable stars in the Leo A dwarf galaxy (DDO 69)

    NASA Technical Reports Server (NTRS)

    Hoessel, John G.; Saha, A.; Krist, John; Danielson, G. Edward

    1994-01-01

    Observations of the Leo A dwarf galaxy, obtained over the period from 1980 to 1991 are reported. Forty two separate Charge Coupled Devices (CCD) frames were searched for variable stars. A total of 14 suspected variables were found, 9 had sufficient coverage for period determination, and 5 had Cepheid light curves. Four of these stars fit well on a P-L relation and yield a distance modulus, after correction for Galactic foreground extinction, of m-M = 26.74. This corresponds to a distance of 2.2 Mpc, placing Leo A near the Local Group zero-velocity surface.

  3. Variable stars in the Pegasus dwarf galaxy (DDO 216)

    NASA Technical Reports Server (NTRS)

    Hoessel, J. G.; Abbott, Mark J.; Saha, A.; Mossman, Amy E.; Danielson, G. Edward

    1990-01-01

    Observations obtained over a period of five years of the resolved stars in the Pegasus dwarf irregular galaxy (DDO 216) have been searched for variable stars. Thirty-one variables were found, and periods established for 12. Two of these variable stars are clearly eclipsing variables, seven are very likely Cepheid variables, and the remaining three are probable Cepheids. The period-luminosity relation for the Cepheids indicates a distance modulus for Pegasus of m - M = 26.22 + or - 0.20. This places Pegasus very near the zero-velocity surface of the Local Group.

  4. Variable stars in the dwarf galaxy GR 8 (DDO 155)

    NASA Technical Reports Server (NTRS)

    Tolstoy, Eline; Saha, A.; Hoessel, John G.; Danielson, G. Edward

    1995-01-01

    Observations of the resolved stars in dwarf galaxy GR 8, obtained over the period 1980 February to 1994 March, are presented. Thirty-four separate epochs were searched for variable stars, and a total of six were found, of which one has Cepheid characteristics. After correction for Galactic extinction this single Cepheid yields a distance modulus of m - M = 26.75 +/- 0.35. This corresponds to a distance of 2.24 Mpc, placing GR 8 near the Local Group (LG) zero-velocity surface. The other five variable stars are very red, and possibly have long periods of order 100 days or more.

  5. Globular Clusters Indicate That Ultra-diffuse Galaxies Are Dwarfs

    NASA Astrophysics Data System (ADS)

    Beasley, Michael A.; Trujillo, Ignacio

    2016-10-01

    We present an analysis of archival HST/ACS imaging in the F475W (g 475), F606W (V 606), and F814W (I 814) bands of the globular cluster (GC) system of a large (3.4 kpc effective radius) ultra-diffuse galaxy (DF17) believed to be located in the Coma Cluster of galaxies. We detect 11 GCs down to the 5σ completeness limit of the imaging (I 814 = 27 mag). Correcting for background and our detection limits yields a total population of GCs in this galaxy of 27 ± 5 and a V-band specific frequency S N = 28 ± 5. Based on comparisons to the GC systems of local galaxies, we show that both the absolute number and the colors of the GC system of DF17 are consistent with the GC system of a dark-matter-dominated dwarf galaxy with virial mass ˜9.0 × 1010 M ⊙ and a dark-to-stellar mass ratio M vir/M star ˜ 1000. Based on the stellar mass growth of the Milky Way, we show that DF17 cannot be understood as a failed Milky-Way-like system, but is more similar to quenched Large-Magellanic-Cloud-like systems. We find that the mean color of the GC population, g 475-I 814 = 0.91 ± 0.05 mag, coincides with the peak of the color distribution of intracluster GCs and is also similar to those of the blue GCs in the outer regions of massive galaxies. We suggest that both the intracluster GC population in Coma and the blue peak in the GC populations of massive galaxies may be fed—at least in part—by the disrupted equivalents of systems such as DF17.

  6. DGSAT: Dwarf Galaxy Survey with Amateur Telescopes. I. Discovery of low surface brightness systems around nearby spiral galaxies

    NASA Astrophysics Data System (ADS)

    Javanmardi, B.; Martinez-Delgado, D.; Kroupa, P.; Henkel, C.; Crawford, K.; Teuwen, K.; Gabany, R. J.; Hanson, M.; Chonis, T. S.; Neyer, F.

    2016-04-01

    Context. We introduce the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) project and report the discovery of eleven low surface brightness (LSB) galaxies in the fields of the nearby galaxies NGC 2683, NGC 3628, NGC 4594 (M 104), NGC 4631, NGC 5457 (M 101), and NGC 7814. Aims: The DGSAT project aims to use the potential of small-sized telescopes to probe LSB features around large galaxies and to increase the sample size of the dwarf satellite galaxies in the Local Volume. Methods: Using long exposure images, fields of the target spiral galaxies are explored for extended LSB objects. After identifying dwarf galaxy candidates, their observed properties are extracted by fitting models to their light profiles. Results: We find three, one, three, one, one, and two new LSB galaxies in the fields of NGC 2683, 3628, 4594, 4631, 5457, and 7814, respectively. In addition to the newly found galaxies, we analyse the structural properties of nine already known galaxies. All of these 20 dwarf galaxy candidates have effective surface brightnesses in the range 25.3 ≲ μe ≲ 28.8 mag arcsec-2 and are fit with Sersic profiles with indices n ≲ 1. Assuming that they are in the vicinity of the above mentioned massive galaxies, their r-band absolute magnitudes, their effective radii, and their luminosities are in the ranges -15.6 ≲ Mr ≲ -7.8, 160 pc ≲ Re ≲ 4.1 kpc, and 0.1 × 106 ≲ (L/L⊙)r ≲ 127 × 106, respectively. To determine whether these LSB galaxies are indeed satellites of the above mentioned massive galaxies, their distances need to be determined via further observations. Conclusions: Using small telescopes, we are readily able to detect LSB galaxies with similar properties to the known dwarf galaxies of the Local Group.

  7. Dwarfs and Giants in the local flows of galaxies.

    NASA Astrophysics Data System (ADS)

    Chernin, A. D.; Emelyanov, N. V.; Karachentsev, I. D.

    We use recent Hubble Space Telescope data on nearby dwarf and giant galaxies to study the dynamical structure and evolutionary trends of the local expansion flows of galaxies. It is found that antigravity of dark energy dominates the force field of the flows and makes them expand with acceleration. It also cools the flows and introduces to them the nearly linear velocity-distance relation with the time-rate close to the global Hubble's factor. There are grounds to expect that this is the universal physical regularity that is common not only for the nearby flows we studied here, but also for all the expansion flows of various spatial scales from the 1 Mpc scale and up to the scale of the global cosmological expansion.

  8. The Abundance Spread in the Booetes I Dwarf Spheroidal Galaxy

    SciTech Connect

    Norris, John E.; Gilmore, Gerard; Wilkinson, Mark I.; Belokurov, V.; Evans, N. Wyn; Zucker, Daniel B.; Wyse, Rosemary F. G.

    2008-12-20

    We present medium-resolution spectra of 16 radial velocity red-giant members of the low-luminosity Booetes I dwarf spheroidal (dSph) galaxy that have sufficient S/N for abundance determination, based on the strength of the Ca II K line. Assuming [Ca/Fe] {approx} 0.3, the abundance range in the sample is {delta}[Fe/H] {approx} 1.7 dex, with one star having [Fe/H] = -3.4. The dispersion is {sigma}([Fe/H]) = 0.45 {+-} 0.08-similar to those of the Galaxy's more luminous dSph systems and {omega} Centauri. This suggests that the large mass ({approx}>10{sup 7} M{sub sun}) normally assumed to foster self-enrichment and the production of chemical abundance spreads was provided by the nonbaryonic material in Booetes I.

  9. A Study of Dwarf Galaxies in Five Rich Clusters I: Abell 1689 and Abell 1703

    NASA Astrophysics Data System (ADS)

    Bruursema, Justice; Riley, S.; Ford, H. C.; Zekser, K. C.; Infante, L.; Postman, M.

    2008-05-01

    Dwarf galaxies play an important role in understanding galactic formation, cluster dynamics, and large scale structure. Although local dwarf populations have been well studied, dwarf galaxies outside the local supercluster remain relatively unexamined. Using ACS Investigation Definition Team data, we examine the dwarf galaxy populations of A1689 (z=0.1832), A1703 (z=0.2580), A2218 (z=0.1756), CL0024+16 (z=0.395), and MS1358+62 (z=0.328). We have modeled and subtracted the light from the brighter elliptical galaxies using the XVISTA subroutine SNUC. An assumption of concentric elliptical isophotes is made and the position angle, ellipticity, and brightness are fit using a nonlinear least-squares algorithm. The subtraction of the models reveals a population of dwarf galaxies usually hidden by the light of bright ellipticals. SExtractor and Bayesian Photometric Redshifts (BPZ) are used in order to identify cluster members. With the 0.05" per pixel resolution of ACS and a completeness of mF625 = 28 we are able to identify approximately 1000 dwarf galaxies candidates, defined as MF625 > -18, in all five clusters combined. We will discuss the results of this research including, but not limited to, dwarf galaxy luminosity functions, radial distribution, and the characteristics of dwarfs compared to those in other well studied clusters. ACS was developed under NASA contract NAS5-32865, and this research was supported by NASA grant NAG5-7697.

  10. CANDIDATE TIDAL DWARF GALAXIES IN Arp 305: LESSONS ON DWARF DETACHMENT AND GLOBULAR CLUSTER FORMATION

    SciTech Connect

    Hancock, Mark; Smith, Beverly J.; Giroux, Mark L.; Hurlock, Sabrina; Struck, Curtis E-mail: smithbj@etsu.edu E-mail: zshh7@goldmail.etsu.edu

    2009-06-15

    To search for Tidal Dwarf Galaxies (TDGs) and to study star formation (SF) in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the Galaxy Evolution Explorer (GALEX) telescope. As part of that study, we present a GALEX UV and Sloan Digital Sky Survey and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and SF outside the disks. This includes a luminous star-forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star-forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages {approx}6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.

  11. Exploring the Metal Retention Fractions of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Morris, Melissa Elizabeth; McQuinn, Kristen B.; Cannon, John M.; Dalcanton, Julianne; Dolphin, Andrew E.; Skillman, Evan D.; Williams, Benjamin F.; van Zee, Liese

    2017-01-01

    Using a novel technique that combines star formation and chemical evolution histories from resolved stellar populations, nebular abundances, and gas masses, McQuinn et al. 2015 measured that only 5% of the oxygen produced by stellar nucleosynthesis was retained in the gas and stars in the very low-mass (stellar mass = 6x105 Msun) galaxy Leo P. In contrast to expectations, metal production and metal loss for spirals in the mass range ~109 - 1011 Msun show that these galaxies retain 20-25% of their metals, independent of mass (Peeples et al. 2014). This suggests there is only a factor of ~5 difference in the ability of galaxies to retain metals, despite a factor of 106 difference in mass. In this prototype study, we explore using the same technique from McQuinn et al. on a small sample of dwarfs with HST archival data, with particular attention to understanding the uncertainties in the approach. Our results will provide a measurement in the intervening mass range between Leo P and more massive spirals. This will allow us to test theoretical predictions of metal loss as a function of galaxy mass.

  12. A Spectroscopic Binary in the Hercules Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Koch, Andreas; Hansen, Terese; Feltzing, Sofia; Wilkinson, Mark I.

    2014-01-01

    We present the radial velocity curve of a single-lined spectroscopic binary in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual spectra covering more than 2 yr of observations. This is the first time that orbital elements could be derived for a binary in a dSph. The system consists of a metal-poor red giant and a low-mass companion, possibly a white dwarf, with a 135 day period in a moderately eccentric (e = 0.18) orbit. Its period and eccentricity are fully consistent with metal-poor binaries in the Galactic halo, while the projected semimajor axis is small, at ap sin i = 38 R ⊙. In fact, a very close orbit could inhibit the production of heavier elements through s-process nucleosynthesis, leading to the very low abundances of neutron-capture elements that are found in this star. We discuss the further implications for the chemical enrichment history of the Hercules dSph, but find no compelling binary scenario that could reasonably explain the full, peculiar abundance pattern of the Hercules dSph galaxy. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, and is based in part on observations made with ESO Telescopes at the Paranal Observatory under programs ID 079.B-0447(A) and 083.D-0688(A).

  13. PERSEUS I: A DISTANT SATELLITE DWARF GALAXY OF ANDROMEDA

    SciTech Connect

    Martin, Nicolas F.; Laevens, Benjamin P. M.; Schlafly, Edward F.; Rix, Hans-Walter; Slater, Colin T.; Bell, Eric F.; Bernard, Edouard J.; Ferguson, Annette M. N.; Finkbeiner, Douglas P.; Burgett, William S.; Chambers, Kenneth C.; Hodapp, Klaus W.; Kaiser, Nicholas; Kudritzki, Rolf-Peter; Magnier, Eugene A.; Morgan, Jeffrey S.; Tonry, John L.; Draper, Peter W.; Metcalfe, Nigel; Price, Paul A.; and others

    2013-12-10

    We present the discovery of a new dwarf galaxy, Perseus I/Andromeda XXXIII, found in the vicinity of Andromeda (M31) in stacked imaging data from the Pan-STARRS1 3π survey. Located 27.°9 away from M31, Perseus I has a heliocentric distance of 785 ± 65 kpc, compatible with it being a satellite of M31 at 374{sub −10}{sup +14} kpc from its host. The properties of Perseus I are typical for a reasonably bright dwarf galaxy (M{sub V} = –10.3 ± 0.7), with an exponential half-light radius of r{sub h} = 1.7 ± 0.4 arcmin or r{sub h}=400{sub −85}{sup +105} pc at this distance, and a moderate ellipticity (ϵ=0.43{sub −0.17}{sup +0.15}). The late discovery of Perseus I is due to its fairly low surface brightness (μ{sub 0}=25.7{sub −0.9}{sup +1.0} mag arcsec{sup –2}), and to the previous lack of deep, high quality photometric data in this region. If confirmed to be a companion of M31, the location of Perseus I, far east from its host, could place interesting constraints on the bulk motion of the satellite system of M31.

  14. APOGEE Chemical Abundances of the Sagittarius Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Hasselquist, Sten; Shetrone, Matthew D.; Smith, Verne V.; Cunha, Katia M. L.; McWilliam, Andrew; Holtzman, Jon A.; Majewski, Steven R.; Sobeck, Jennifer; Frinchaboy, Peter M.; Roman-Lopes, Alexandre; Ivans, Inese I.; Allende-Prieto, Carlos; Placco, Vinicius M.; Lane, Richard; Zasowski, Gail; APOGEE

    2017-01-01

    The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides elemental abundances for C, N, O, Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, and Ni. We analyze the chemical abundance patterns of these elements for ~ 350 stars belonging to the Sagittarius Dwarf Galaxy (Sgr). This is the largest sample of Sgr stars with detailed chemical abundances and the first time C, N, P, K, V, Cr, Co, and Ni have been studied in the dwarf galaxy. For Sgr stars with [Fe/H] > -0.9, we find that Sgr is deficient in all elemental abundance ratios (expressed as [X/Fe]) relative to the Milky Way, which suggests that Sgr stars observed today were formed from gas that was less enriched by both Type II and Type Ia SNe. By examining the relative deficiencies of the hydrostatic (O, Mg, and Al) and explosive (Si, K, and Mn) elements , we find support that previous generations of Sgr stars were formed with a top-light IMF, lacking the most massive stars that would normally pollute the ISM with the hydrostatic elements.

  15. Investigation of dwarf galaxies in the Virgo cluster

    SciTech Connect

    Bothun, G.D.; Mould, J.R.; Wirth, A.; Caldwell, N.

    1985-05-01

    We have obtained 21-cm H I observations of a sample of 32 dwarf irregular (dI) and 12 dwarf elliptical (dE) galaxies that are located in the Virgo cluster. Altogether, 18 of 32 DIs were detected in H I, but none of the dEs were detected at a sensitivity level of M/sub Htsi/ = 2--3 x 10/sup 6/ M/sub sun/. The detected dIs have M/sub Htsi/>3 x 10/sup 7/ M/sub sun/. This disparity in H I content between dIs and dEs effectively dispels the possibility that the dEs are presently in a stage of quiescence (hibernation), between bursts of star formation. In order to supplement the 21-cm data, we have acquired optical spectroscopy, CCD images, and infrared photometry for a limited subsample of these dwarfs. The most significant result provided by this additional data is that the dEs, although very H I poor, nevertheless have observed (J-K) colors which indicate somewhat high metallicity, implying some degree of enrichment due to multiple generations of star formation. In contrast, most of the dIs are quite H I rich (with some having fractional H I contents that exceed 30% by mass), yet they are apparently in a quiescent phase, judging by their low central surface brightnesses (<10% of sky) and lack of resolution into obvious regions of star formation. A small gas-poor contingent of dIs have been found but there is no apparent correlation between dI gas content and either their velocity with respect to the Virgo ICM or their position in the cluster. In general, the velocity distribution of the dIs is flat with only a weak peak that corresponds to the mean velocity of the brighter galaxies in Virgo.

  16. EXTREME EMISSION-LINE GALAXIES IN CANDELS: BROADBAND-SELECTED, STARBURSTING DWARF GALAXIES AT z > 1

    SciTech Connect

    Van der Wel, A.; Rix, H.-W.; Jahnke, K.; Straughn, A. N.; Finkelstein, S. L.; Salmon, B. W.; Koekemoer, A. M.; Ferguson, H. C.; Weiner, B. J.; Wuyts, S.; Bell, E. F.; Faber, S. M.; Trump, J. R.; Koo, D. C.; Hathi, N. P.; Dunlop, J. S.; Newman, J. A.; Dickinson, M.; De Mello, D. F.; and others

    2011-12-01

    We identify an abundant population of extreme emission-line galaxies (EELGs) at redshift z {approx} 1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). Sixty-nine EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broadband magnitudes. Supported by spectroscopic confirmation of strong [O III] emission lines-with rest-frame equivalent widths {approx}1000 A-in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with {approx}10{sup 8} M{sub Sun} in stellar mass, undergoing an enormous starburst phase with M{sub *}/ M-dot{sub *} of only {approx}15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the comoving number density (3.7 Multiplication-Sign 10{sup -4} Mpc{sup -3}) can produce in {approx}4 Gyr much of the stellar mass density that is presently contained in 10{sup 8}-10{sup 9} M{sub Sun} dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z > 1.

  17. Mid-infrared Colors of Dwarf Galaxies: Young Starbursts Mimicking Active Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Hainline, Kevin N.; Reines, Amy E.; Greene, Jenny E.; Stern, Daniel

    2016-12-01

    Searching for active galactic nuclei (AGNs) in dwarf galaxies is important for our understanding of the seed black holes that formed in the early universe. Here, we test infrared selection methods for AGN activity at low galaxy masses. Our parent sample consists of ˜18,000 nearby dwarf galaxies (M * < 3 × 109 M ⊙, z < 0.055) in the Sloan Digital Sky Survey with significant detections in the first three bands of the AllWISE data release from the Wide-field Infrared Survey Explorer (WISE). First, we demonstrate that the majority of optically selected AGNs in dwarf galaxies are not selected as AGNs using WISE infrared color diagnostics and that the infrared emission is dominated by the host galaxies. We then investigate the infrared properties of optically selected star-forming dwarf galaxies, finding that the galaxies with the reddest infrared colors are the most compact, with blue optical colors, young stellar ages, and large specific star formation rates. These results indicate that great care must be taken when selecting AGNs in dwarf galaxies using infrared colors, as star-forming dwarf galaxies are capable of heating dust in such a way that mimics the infrared colors of more luminous AGNs. In particular, a simple W1-W2 color cut alone should not be used to select AGNs in dwarf galaxies. With these complications in mind, we present a sample of 41 dwarf galaxies that fall in the WISE infrared color space typically occupied by more luminous AGNs and that are worthy of follow-up observations.

  18. Supernova Feedback in Smoothed Particle Hydrodynamics Simulations of Dwarf Galaxy Formation

    NASA Astrophysics Data System (ADS)

    Stinson, Gregory S.; Quinn, T.; Dalcanton, J.; Wadsley, J.; Gogarten, S.

    2007-12-01

    I will describe simulations that represent the evolution of galaxies using N-body smoothed particle hydrodynamic. The simulations present a novel recipe for star formation and the consequent supernova feedback. The recipes are employed in a number of isolated galaxies with different masses. The supernova feedback is effective at delaying star formation in low mass galaxies. The feedback also drives winds from low mass galaxies. In these winds, we find that stars can form and populate a stellar halo surrounding dwarf galaxies that compares well with the observed stellar halos surrounding Local Group Dwarfs.

  19. The dynamical and chemical evolution of dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Revaz, Y.; Jablonka, P.; Sawala, T.; Hill, V.; Letarte, B.; Irwin, M.; Battaglia, G.; Helmi, A.; Shetrone, M. D.; Tolstoy, E.; Venn, K. A.

    2009-07-01

    We present a large sample of fully self-consistent hydrodynamical Nbody/Tree-SPH simulations of isolated dwarf spheroidal galaxies (dSphs). It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. The initial total mass (gas + dark matter) of these galaxies is the main driver of their evolution. Star formation (SF) occurs in series of short bursts. In massive systems, the very short intervals between the SF peaks mimic a continuous star formation rate, while less massive systems exhibit well separated SF bursts, as identified observationally. The delay between the SF events is controlled by the gas cooling time dependence on galaxy mass. The observed global scaling relations, luminosity-mass and luminosity-metallicity, are reproduced with low scatter. We take advantage of the unprecedentedly large sample size and data homogeneity of the ESO Large Programme DART, and add to it a few independent studies, to constrain the star formation history of five Milky Way dSphs, Sextans, LeoII, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs. [Fe/H] diagrams derived from high-resolution spectroscopy of hundreds of individual stars are confronted with model predictions. We find that the diversity in dSph properties may well result from intrinsic evolution. We note, however, that the presence of gas in the final state of our simulations, of the order of what is observed in dwarf irregulars, calls for removal by external processes. Appendix A is only available in electronic form at http://www.aanda.org

  20. Chemical enrichment in Ultra-Faint Dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Romano, Donatella

    2015-08-01

    The origin and evolution of galaxies is one of the great outstanding problems of astrophysics. The elusive, basic processes that govern galaxy formation can be advantageously studied in the simplest structures found orbiting the Milky Way, namely, the newly discovered Ultra-Faint Dwarf (UFD) galaxies. These low luminosity objects are the most dark matter-dominated galaxies known and likely the relics of the complex assembly processes that shaped the Galaxy over a Hubble time. They formed stars long (~12 Gyr) ago and have been quiescent ever since. Their present-day stellar masses are comparable to those of typical Milky Way globular clusters (GCs) but, at variance with GCs, they have large [Fe/H] spreads and average metallicities lower than those of the most metal-poor GCs, which clearly indicates they followed distinct evolutionary paths. The abundance ratios [α/Fe] measured in a few UFD giant stars closely resemble those of similarly metal-poor Galactic halo stars, suggesting that chemical evolution proceeded similarly in different systems at the earliest times. Despite their simplicity, a number of questions remain unanswered with regards to these systems: did they suffer 'one-shot' or extended star formation? what is the level and significance of chemical inhomogeneity inside these systems? was stellar feedback effective in removing all the gas left over from the star formation process, or did the interaction with the environment play a major role? In this contribution we discuss these issues basing on results we got from pure chemical evolution models, as well as three-dimensional hydrodynamical simulations run at subparsec resolution.

  1. XMM-Newton study of the Draco dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Saeedi, Sara; Sasaki, Manami; Ducci, Lorenzo

    2016-02-01

    Aims: We present the results of the analysis of five XMM-Newton observations of the Draco dwarf spheroidal galaxy (dSph). The aim of the work is the study of the X-ray population in the field of the Draco dSph. Methods: We classified the sources on the basis of spectral analysis, hardness ratios, X-ray-to-optical flux ratio, X-ray variability, and cross-correlation with available catalogues in X-ray, optical, infrared, and radio wavelengths. Results: We detected 70 X-ray sources in the field of the Draco dSph in the energy range of 0.2 - 12 keV and classified 18 AGNs, 9 galaxies and galaxy candidates, 6 sources as foreground stars, 4 low-mass X-ray binary candidates, 1 symbiotic star, and 2 binary system candidates. We also identified 9 sources as hard X-ray sources in the field of the galaxy. We derived the X-ray luminosity function of X-ray sources in the Draco dSph in the 2 - 10 keV and 0.5 - 2 keV energy bands. Using the X-ray luminosity function in the energy range of 0.5 - 2 keV, we estimate that ~10 X-ray sources are objects in the Draco dSph. We have also estimated the dark matter halo mass that would be needed to keep the low-mass X-ray binaries gravitationally bound to the galaxy. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

  2. Episodic model for star formation history and chemical abundances in giant and dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Debsarma, Suma; Chattopadhyay, Tanuka; Das, Sukanta; Pfenniger, Daniel

    2016-11-01

    In search for a synthetic understanding, a scenario for the evolution of the star formation rate and the chemical abundances in galaxies is proposed, combining gas infall from galactic haloes, outflow of gas by supernova explosions, and an oscillatory star formation process. The oscillatory star formation model is a consequence of the modelling of the fractional masses changes of the hot, warm and cold components of the interstellar medium. The derived periods of oscillation vary in the range (0.1-3.0) × 107 yr depending on various parameters existing from giant to dwarf galaxies. The evolution of metallicity varies in giant and dwarf galaxies and depends on the outflow process. Observed abundances in dwarf galaxies can be reproduced under fast outflow together with slow evaporation of cold gases into hot gas whereas slow outflow and fast evaporation is preferred for giant galaxies. The variation of metallicities in dwarf galaxies supports the fact that low rate of SNII production in dwarf galaxies is responsible for variation in metallicity in dwarf galaxies of similar masses as suggested by various authors.

  3. Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups

    NASA Astrophysics Data System (ADS)

    Stierwalt, S.; Liss, S. E.; Johnson, K. E.; Patton, D. R.; Privon, G. C.; Besla, G.; Kallivayalil, N.; Putman, M.

    2017-01-01

    The demographics of dwarf galaxy populations have long been in tension with predictions from the Λ cold dark matter (ΛCDM) paradigm 1-4 . If primordial density fluctuations were scale-free as predicted, dwarf galaxies should themselves host dark-matter subhaloes 5 , the most massive of which may have undergone star formation resulting in dwarf galaxy groups. Ensembles of dwarf galaxies are observed as sate­llites of more massive galaxies 6-9 , and there is observational 10 and theoretical 11 evidence to suggest that these satellites at redshift z = 0 were captured by the massive host halo as a group. However, the evolution of dwarf galaxies is highly susceptible to environment 12-14 , making these satellite groups imperfect probes of ΛCDM in the low-mass regime. Here we report one of the clearest examples yet of hierarchical structure formation at low masses: using deep multi-wavelength data, we identify seven isolated, spectroscopically confirmed groups of only dwarf galaxies. Each group hosts three to five known members, has a baryonic mass of ~4.4 × 109 to 2 × 1010 solar masses (M ⊙), and requires a mass-to-light ratio of <100 to be gravitationally bound. Such groups are predicted to be rare theoretically and found to be rare observationally at the current epoch, and thus provide a unique window into the possible formation mechanism of more massive, isolated galaxies.

  4. A STAR FORMATION LAW FOR DWARF IRREGULAR GALAXIES

    SciTech Connect

    Elmegreen, Bruce G.; Hunter, Deidre A. E-mail: dah@lowell.edu

    2015-06-01

    The radial profiles of gas, stars, and far-ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed model of this instability involving gaseous and stellar fluids with self-consistent thicknesses and energy dissipation on a perturbation crossing time gives the unstable growth rates. We find that all locations are effectively stable to 2D perturbations, mostly because the disks are thick. We then consider the average volume densities in the midplanes, evaluated from the observed H i surface densities and calculated scale heights. The radial profiles of the star-formation rates are equal to about 1% of the H i surface densities divided by the free fall times at the average midplane densities. This 1% resembles the efficiency per unit free fall time commonly found in other cases. There is a further variation of this efficiency with radius in all of our galaxies, following the exponential disk with a scale length equal to about twice the stellar mass scale length. This additional variation is modeled by the molecular fraction in a diffuse medium using radiative transfer solutions for galaxies with the observed dimensions and properties of our sample. We conclude that star formation is activated by a combination of three-dimensional gaseous gravitational processes and molecule formation. Implications for outer disk structure and formation are discussed.

  5. Dwarf Galaxy Formation with H2-regulated Star Formation

    NASA Astrophysics Data System (ADS)

    Kuhlen, Michael; Krumholz, Mark R.; Madau, Piero; Smith, Britton D.; Wise, John

    2012-04-01

    We describe cosmological galaxy formation simulations with the adaptive mesh refinement code Enzo that incorporate a star formation prescription regulated by the local abundance of molecular hydrogen. We show that this H2-regulated prescription leads to a suppression of star formation in low-mass halos (Mh <~ 1010 M ⊙) at z > 4, alleviating some of the dwarf galaxy problems faced by theoretical galaxy formation models. H2 regulation modifies the efficiency of star formation of cold gas directly, rather than indirectly reducing the cold gas content with "supernova feedback." We determine the local H2 abundance in our most refined grid cells (76 proper parsec in size at z = 4) by applying the model of Krumholz, McKee, & Tumlinson, which is based on idealized one-dimensional radiative transfer calculations of H2 formation-dissociation balance in ~100 pc atomic-molecular complexes. Our H2-regulated simulations are able to reproduce the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low Σgas cutoff due to the transition from atomic to molecular phase and the metallicity dependence thereof, without the use of an explicit density threshold in our star formation prescription. We compare the evolution of the luminosity function, stellar mass density, and star formation rate density from our simulations to recent observational determinations of the same at z = 4-8 and find reasonable agreement between the two.

  6. Alfalfa discovery of the nearby gas-rich dwarf galaxy Leo P. IV. Distance measurement from LBT optical imaging

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Berg, Danielle; Cannon, John M.; Salzer, John J.; Rhode, Katherine L.; Adams, Elizabeth A. K.; Giovanelli, Riccardo; Haynes, Martha P.; Dolphin, Andrew E-mail: skillman@astro.umn.edu E-mail: jcannon@macalester.edu E-mail: slaz@astro.indiana.edu E-mail: riccardo@astro.cornell.edu

    2013-12-01

    Leo P is a low-luminosity dwarf galaxy discovered through the blind H I Arecibo Legacy Fast ALFA survey. The H I and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with both active star formation and an underlying older population, as well as an extremely low oxygen abundance. Here, we measure the distance to Leo P by applying the tip of the red giant branch (TRGB) distance method to photometry of the resolved stellar population from new Large Binocular Telescope V and I band imaging. We measure a distance modulus of 26.19{sub −0.50}{sup +0.17} mag corresponding to a distance of 1.72{sub −0.40}{sup +0.14} Mpc. Although our photometry reaches 3 mag below the TRGB, the sparseness of the red giant branch yields higher uncertainties on the lower limit of the distance. Leo P is outside the Local Group with a distance and velocity consistent with the local Hubble flow. While located in a very low-density environment, Leo P lies within ∼0.5 Mpc of a loose association of dwarf galaxies which include NGC 3109, Antlia, Sextans A, and Sextans B, and 1.1 Mpc away from its next nearest neighbor, Leo A. Leo P is one of the lowest metallicity star-forming galaxies known in the nearby universe, comparable in metallicity to I Zw 18 and DDO 68, but with stellar characteristics similar to dwarf spheriodals (dSphs) in the Local Volume such as Carina, Sextans, and Leo II. Given its physical properties and isolation, Leo P may provide an evolutionary link between gas-rich dwarf irregular galaxies and dSphs that have fallen into a Local Group environment and been stripped of their gas.

  7. Star Formation at Low Metallicity in Local Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Elmegreen, Bruce; Hunter, Deidre Ann; Rubio, Monica; Brinks, Elias; Cortés, Juan R.; Cigan, Phil

    2016-01-01

    The radial profiles of star formation rates and surface mass densities for gas and stars have been compiled for 20 local dwarf irregular galaxies and converted into disk scale heights and Toomre Q values. The scale heights are relatively large compared to the galaxy sizes (~0.6 times the local radii) and generally increase with radius in a flare. The gaseous Q values are high, ~4, at most radii and even higher for the stars. Star formation proceeds even with these high Q values in a normal exponential disk as viewed in the far ultraviolet. Such normal star formation suggests that Q is not relevant to star formation in dIrrs. The star formation rate per unit area always equals approximately the gas surface density divided by the midplane free fall time with an efficiency factor of about 1% that decreases systematically with radius in approximate proportion to the gas surface density. We view this efficiency variation as a result of a changing molecular fraction in a disk where atomic gas dominates both stars and molecules. In a related study, CO observations with ALMA of star-forming regions at the low metallicities of these dwarfs, which averages 13% solar, shows, in the case of the WLM galaxy, tiny CO clouds inside much larger molecular and atomic hydrogen envelopes. The CO cloud mass fraction within the molecular region is only one percent or so. Nevertheless, the CO clouds have properties that are similar to solar neighborhood clouds: they satisfy the size-linewidth relation observed in the LMC, SMC, and other local dwarfs where CO has been observed, and the same virial mass versus luminosity relation. This uniforming of CO cloud properties seems to be the result of a confining pressure from the weight of the overlying molecular and atomic shielding layers. Star formation at low metallicity therefore appears to be a three dimensional process independent of 2D instabilities involving Q, in highly atomic gas with relatively small CO cores, activated at a rate

  8. The Origin of Dwarf Galaxies in Clusters: The Faint-End Slope of Abell 85 Galaxy Luminosity Function

    NASA Astrophysics Data System (ADS)

    Agulli, I.; Aguerri, J. A. L.; Barrena, R.; Diaferio, A.; Sánchez-Janssen, R.

    2016-10-01

    Dwarf galaxies (Mb>-18) are important because of their cosmological interest as tests of hierarchical theories. The formation of these galaxies is still an open question but red dwarf galaxies are preferentially located in high density environments, indicating that they are end-products of galaxy transformations in clusters. Deep spectroscopic studies of galaxy clusters are needed to put some constraints on dwarf galaxy formation and evolution. We have observed and analyzed Abell 85, a nearby (z = 0.055) and massive cluster down to M*+6, using the MOS instruments VIMOS@VLT and AF2@WHT. The first and powerful tool to study the characteristics of galaxies and compare with different density environments is the galaxy luminosity function. The comparison of the results for Abell 85 with literature outcomes for clusters and field, allows us to conclude that, at least for this cluster, the environment plays a major role in the nature of the faint-end galaxies, transforming blue dwarfs in the field into red ones in the cluster, but not in the formation of the luminosity function slope.

  9. Constraining the Stellar Populations and Star Formation Histories of Blue Compact Dwarf Galaxies with SED Fits

    NASA Astrophysics Data System (ADS)

    Janowiecki, Steven; Salzer, John J.; van Zee, Liese; Rosenberg, Jessica L.; Skillman, Evan

    2017-02-01

    We discuss and test possible evolutionary connections between blue compact dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low-mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each galaxy with a grid of theoretical models to determine their stellar masses and star formation properties. We compare our results for BCDs with the LVL galaxies to put BCDs in the context of normal galaxy evolution. The SED fits demonstrate that the star formation events currently underway in BCDs are at the extreme of the continuum of normal dwarf galaxies, both in terms of the relative mass involved and in the relative increase over previous star formation rates. Today’s BCDs are distinctive objects in a state of extreme star formation that is rapidly transforming them. This study also suggests ways to identify former BCDs whose star formation episodes have since faded.

  10. The spatial distribution of dwarf galaxies in the CfA slice of the universe

    NASA Technical Reports Server (NTRS)

    Thuan, Trinh X.; Gott, J. Richard, III; Schneider, Stephen E.

    1987-01-01

    A complete (with the the exception of one) redshift sample of 58 galaxies in the Nilson catalog classified as dwarf, irregular, or Magellanic irregular is used to investigate the large-scale clustering properties of these low-surface brightness galaxies in the CfA slice of the universe (alpha in the range of 8-17 h, delta in the range of 26.5-32.5 deg). It is found that the low-surface brightness dwarf galaxies also lie on the structures delineated by the high-surface brightness normal galaxies and that they do not fill in the voids. This is inconsistent with a class of biased galaxy formation theories which predict that dwarf galaxies should be present everywhere, including the voids.

  11. The Origin of the Diversity of Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Revaz, Yves; Jablonka, Pascale

    2010-06-01

    We present a large sample of 166 fully self-consistent hydrodynamical N-body/Tree-SPH simulations of isolated dwarf spheroidal galaxies [1]. It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. Using the recent data of the ESO Large Programme DART, we have constrained the star formation history of four Milky Way dSphs, Sextans, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs [Fe/H] diagrams derived from high-resolution spectroscopy of hundreds of individual stars are confronted with model predictions. Global relations of dSph are successfully reproduced. Our study shows that the total initial mass of these systems is the main driver of their evolution and explains the diversity in luminosity and metallicity observed in the Local Group dSphs.

  12. X-Ray Sources in the Dwarf Spheroidal Galaxy Draco

    NASA Astrophysics Data System (ADS)

    Sonbas, E.; Rangelov, B.; Kargaltsev, O.; Dhuga, K. S.; Hare, J.; Volkov, I.

    2016-04-01

    We present the spectral analysis of an 87 ks XMM-Newton observation of Draco, a nearby dwarf spheroidal galaxy. Of the approximately 35 robust X-ray source detections, we focus our attention on the brightest of these sources, for which we report X-ray and multiwavelength parameters. While most of the sources exhibit properties consistent with active galactic nuclei, few of them possess the characteristics of low-mass X-ray binaries (LMXBs) and cataclysmic variable (CVs). Our analysis places constraints on the population of X-ray sources with LX > 3 × 1033 erg s-1 in Draco, suggesting that there are no actively accreting black hole and neutron star binaries. However, we find four sources that could be quiescent state LMXBs/CVs associated with Draco. We also place constraints on the central black hole luminosity and on a dark matter decay signal around 3.5 keV.

  13. WISE DISCOVERY OF LOW-METALLICITY BLUE COMPACT DWARF GALAXIES

    SciTech Connect

    Griffith, Roger L.; Tsai, Chao-Wei; Jarrett, Thomas H.; Wu Yanling; Yan Lin; Stern, Daniel; Eisenhardt, Peter R. M.; Wu Jingwen; Blain, Andrew; Harrison, Fiona; Madsen, Kristin; Stanford, Spencer A.; Wright, Edward L.

    2011-07-20

    We report two new low-metallicity blue compact dwarf galaxies (BCDs), WISEP J080103.93+264053.9 (hereafter W0801+26) and WISEP J170233.53+180306.4 (hereafter W1702+18), discovered using the Wide-field Infrared Survey Explorer (WISE). We identified these two BCDs from their extremely red colors at mid-infrared wavelengths and obtained follow-up optical spectroscopy using the Low Resolution Imaging Spectrometer on Keck I. The mid-infrared properties of these two sources are similar to the well-studied, extremely low metallicity galaxy SBS 0335-052E. We determine metallicities of 12 + log (O/H) = 7.75 and 7.63 for W0801+26 and W1702+18, respectively, placing them among a very small group of very metal deficient galaxies (Z {<=} 1/10 Z{sub sun}). Their >300 A H{beta} equivalent widths, similar to SBS 0335-052E, imply the existence of young (<5 Myr) star-forming regions. We measure star formation rates of 2.6 and 10.9 M{sub sun} yr{sup -1} for W0801+26 and W1702+18, respectively. These BCDs, showing recent star formation activity in extremely low metallicity environments, provide new laboratories for studying star formation in extreme conditions and are low-redshift analogs of the first generation of galaxies to form in the universe. Using the all-sky WISE survey, we discuss a new method to identify similar star-forming, low-metallicity BCDs.

  14. THE CHEMICAL EVOLUTION OF THE DRACO DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Cohen, Judith G.; Huang Wenjin E-mail: hwenjin@astro.washington.edu

    2009-08-20

    We present an abundance analysis based on high-resolution spectra of eight stars selected to span the full range in metallicity in the Draco dwarf spheroidal (dSph) galaxy. We find that [Fe/H] for the sample stars ranges from -1.5 to -3.0 dex. Combining our sample with previously published work for a total of 14 luminous Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe], and [Si/Fe] for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H] probed, but are significantly lower for the higher Fe-metallicity Draco stars. For the explosive {alpha}-elements Ca and Ti, the abundance ratios for Draco giants with [Fe/H] > - 2.4 dex are approximately constant and slightly subsolar, well below values characteristic of Galactic halo stars. The s-process contribution to the production of heavy elements begins at significantly lower Fe metallicity than in the Galactic halo. Using a toy model we compare the behavior of the abundance ratios within the sample of Draco giants with those from the literature of Galactic globular clusters, and the Carina and Sgr dSph galaxies. The differences appear to be related to the timescale for buildup of the heavy elements, with Draco having the slowest rate. We note the presence of a Draco giant with [Fe/H] <-3.0 dex in our sample, and reaffirm that the inner Galactic halo could have been formed by early accretion of Galactic satellite galaxies and dissolution of young globular clusters, while the outer halo could have formed from those satellite galaxies accreted later.

  15. The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Spekkens, Kristine; Urbancic, Natasha; Mason, Brian S.; Willman, Beth; Aguirre, James E.

    2014-11-01

    We present new upper limits on the neutral hydrogen (H I) content within the stellar half-light ellipses of 15 Galactic dwarf spheroidal galaxies (dSphs), derived from pointed observations with the Green Bank Telescope (GBT) as well as Arecibo L-band Fast ALFA survey and Galactic All-Sky Survey data. All of the limits MH \\scriptsize{I}^lim are more stringent than previously reported values, and those from the GBT improve upon constraints in the literature by a median factor of 23. Normalizing by V-band luminosity LV and dynamical mass M dyn, we find MH \\scriptsize{I}^lim/L_V˜ 10-3 {M⊙ / L⊙ } and MH \\scriptsize{I}^lim/M_dyn˜ 5× 10-5, irrespective of location in the Galactic halo. Comparing these relative H I contents to those of the Local Group and nearby neighbor dwarfs compiled by McConnachie, we find that the Galactic dSphs are extremely gas-poor. Our H I upper limits therefore provide the clearest picture yet of the environmental dependence of the H I content in Local Volume dwarfs. If ram pressure stripping explains the dearth of H I in these systems, then orbits in a relatively massive Milky Way are favored for the outer halo dSph Leo I, while Leo II and Canes Venatici I have had a pericentric passage in the past. For Draco and Ursa Minor, the interstellar medium mass that should accumulate through stellar mass loss in between pericentric passages exceeds MH \\scriptsize{I}^lim by a factor of ~30. In Ursa Minor, this implies that either this material is not in the atomic phase, or that another mechanism clears the recycled gas on shorter timescales.

  16. Ejection of Supernova-Enriched Gas From Dwarf Disk Galaxies

    SciTech Connect

    Fragile, P C; Murray, S D; Lin, D C

    2004-06-15

    We examine the efficiency with which supernova-enriched gas may be ejected from dwarf disk galaxies, using a methodology previously employed to study the self-enrichment efficiency of dwarf spheroidal systems. Unlike previous studies that focused on highly concentrated starbursts, in the current work we consider discrete supernova events spread throughout various fractions of the disk. We model disk systems having gas masses of 10{sup 8} and 10{sup 9} M{sub {circle_dot}} with supernova rates of 30, 300, and 3000 Myr{sup -1}. The supernova events are confined to the midplane of the disk, but distributed over radii of 0, 30, and 80% of the disk radius, consistent with expectations for Type II supernovae. In agreement with earlier studies, we find that the enriched material from supernovae is largely lost when the supernovae are concentrated near the nucleus, as expected for a starburst event. In contrast, we find the loss of enriched material to be much less efficient (as low as 21%) when the supernovae occur over even a relatively small fraction of the disk. The difference is due to the ability of the system to relax following supernova events that occur over more extended regions. Larger physical separations also reduce the likelihood of supernovae going off within low-density ''chimneys'' swept out by previous supernovae. We also find that, for the most distributed systems, significant metal loss is more likely to be accompanied by significant mass loss. A comparison with theoretical predications indicates that, when undergoing self-regulated star formation, galaxies in the mass range considered shall efficiently retain the products of Type II supernovae.

  17. First Stellar Abundances in the Dwarf Irregular Galaxy IC 1613

    NASA Astrophysics Data System (ADS)

    Tautvaišienė, Gražina; Geisler, Doug; Wallerstein, George; Borissova, Jura; Bizyaev, Dmitry; Pagel, Bernard E. J.; Charbonnel, Corinne; Smith, Verne

    2007-12-01

    Chemical abundances in three M supergiants in the Local Group dwarf irregular galaxy IC 1613 have been determined using high-resolution spectra obtained with the UVES spectrograph on the ESO 8.2 m Kueyen telescope. A detailed synthetic-spectrum analysis has been used to determine the atmospheric parameters and abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Fe, Co, Ni, La, and Eu. We find the overall metallicity of the stars to be [Fe/H] = -0.67 ± 0.09 and the age 9-13 Myr, which is in excellent agreement with the present-day values in the age-metallicity relationship model of IC 1613 by Skillman et al. We have found that the three supergiants investigated have a mean [α/Fe] equal to about -0.1, which is lower than seen in Galactic stars at the same metallicity and is in agreement with the results obtained in other dwarf irregular galaxies. The oxygen abundances are in agreement with the upper values of the nebular oxygen determinations in IC 1613. The abundance ratios of s- and r-process elements to iron are enhanced relative to solar by about 0.3 dex. The abundance pattern of the elements studied is similar to that of the Small Magellanic Cloud, except for Co and Ni, which are underabundant in the SMC. The observed elemental abundances are generally in very good agreement with the recent chemical evolution model of Yuk and Lee. Based on observations collected with the Very Large Telescope and the 2.2 m Telescope of the European Southern Observatory within the Observing Programs 70.B-0361(A) and 072.D-0113(D).

  18. The early days of the Sculptor dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Jablonka, P.; North, P.; Mashonkina, L.; Hill, V.; Revaz, Y.; Shetrone, M.; Starkenburg, E.; Irwin, M.; Tolstoy, E.; Battaglia, G.; Venn, K.; Helmi, A.; Primas, F.; François, P.

    2015-11-01

    We present the high-resolution spectroscopic study of five -3.9 ≤ [Fe/H] ≤ -2.5 stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the number of stars with comparable observations in this metallicity range. We carry out a detailed analysis of the chemical abundances of α, iron peak, and light and heavy elements, and draw comparisons with the Milky Way halo and the ultra-faint dwarf stellar populations. We show that the bulk of the Sculptor metal-poor stars follow the same trends in abundance ratios versus metallicity as the Milky Way stars. This suggests similar early conditions of star formation and a high degree of homogeneity of the interstellar medium. We find an outlier to this main regime, which seems to miss the products of the most massive of the Type II supernovae. In addition to its help in refining galaxy formation models, this star provides clues to the production of cobalt and zinc. Two of our sample stars have low odd-to-even barium isotope abundance ratios, suggestive of a fair proportion of s-process. We discuss the implication for the nucleosynthetic origin of the neutron capture elements. Based on ESO programs 087.D-0928(A) and 091.D-0912(A).

  19. Stellar Clusters Forming in the Blue Dwarf Galaxy NGC 5253

    NASA Astrophysics Data System (ADS)

    2004-11-01

    Star formation is one of the most basic phenomena in the Universe. Inside stars, primordial material from the Big Bang is processed into heavier elements that we observe today. In the extended atmospheres of certain types of stars, these elements combine into more complex systems like molecules and dust grains, the building blocks for new planets, stars and galaxies and, ultimately, for life. Violent star-forming processes let otherwise dull galaxies shine in the darkness of deep space and make them visible to us over large distances. Star formation begins with the collapse of the densest parts of interstellar clouds, regions that are characterized by comparatively high concentration of molecular gas and dust like the Orion complex (ESO PR Photo 20/04) and the Galactic Centre region (ESO Press Release 26/03). Since this gas and dust are products of earlier star formation, there must have been an early epoch when they did not yet exist. But how did the first stars then form? Indeed, to describe and explain "primordial star formation" - without molecular gas and dust - is a major challenge in modern Astrophysics. A particular class of relatively small galaxies, known as "Blue Dwarf Galaxies", possibly provide nearby and contemporary examples of what may have occurred in the early Universe during the formation of the first stars. These galaxies are poor in dust and heavier elements. They contain interstellar clouds which, in some cases, appear to be quite similar to those primordial clouds from which the first stars were formed. And yet, despite the relative lack of the dust and molecular gas that form the basic ingredients for star formation as we know it from the Milky Way, those Blue Dwarf Galaxies sometimes harbour very active star-forming regions. Thus, by studying those areas, we may hope to better understand the star-forming processes in the early Universe. Very active star formation in NGC 5253 NGC 5253 is one of the nearest of the known Blue Dwarf Galaxies

  20. NUCLEOSYNTHESIS AND THE INHOMOGENEOUS CHEMICAL EVOLUTION OF THE CARINA DWARF GALAXY

    SciTech Connect

    Venn, Kim A.; Divell, Mike; Shetrone, Matthew D.; Irwin, Mike J.; Hill, Vanessa; Jablonka, Pascale; Tolstoy, Eline; Lemasle, Bertrand; Starkenburg, Else; Helmi, Amina; Letarte, Bruno; Baldner, Charles; Battaglia, Giuseppina; Primas, Francesca; Kaufer, Andreas

    2012-06-01

    The detailed abundances of 23 chemical elements in nine bright red giant branch stars in the Carina dwarf spheroidal galaxy are presented based on high-resolution spectra gathered at the Very Large Telescope (VLT) and Magellan telescopes. A spherical model atmospheres analysis is applied using standard methods (local thermodynamic equilibrium and plane-parallel radiative transfer) to spectra ranging from 380 to 680 nm. Stellar parameters are found to be consistent between photometric and spectroscopic analyses, both at moderate and high resolution. The stars in this analysis range in metallicity from -2.9 < [Fe/H] <-1.3, and adopting the ages determined by Lemasle et al., we are able to examine the chemical evolution of Carina's old and intermediate-aged populations. One of the main results from this work is the evidence for inhomogeneous mixing in Carina and therefore for a poor statistical sampling of the supernova contributions when forming stars; a large dispersion in [Mg/Fe] indicates poor mixing in the old population, an offset in the [{alpha}/Fe] ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in {alpha}-enriched gas, and one star, Car-612, seems to have formed in a pocket enhanced in SN Ia/II products. This latter star provides the first direct link between the formation of stars with enhanced SN Ia/II ratios in dwarf galaxies to those found in the outer Galactic halo (Ivans et al.). Another important result is the potential evidence for SN II driven winds. We show that the very metal-poor stars in Carina have not been enhanced in asymptotic giant branch or SN Ia products, and therefore their very low ratios of [Sr/Ba] suggests the loss of contributions from the early SNe II. Low ratios of [Na/Fe], [Mn/Fe], and [Cr/Fe] in two of these stars support this scenario, with additional evidence from the low [Zn/Fe] upper limit for one star. It is

  1. The frequency and properties of young tidal dwarf galaxies in nearby groups

    NASA Astrophysics Data System (ADS)

    Lee-Waddell, K.; Spekkens, K.; Chandra, P.; Patra, N.; Cuillandre, J.-C.; Wang, J.; Haynes, M. P.; Cannon, J.; Stierwalt, S.; Sick, J.; Giovanelli, R.

    2017-03-01

    We present the results of a multi-wavelength investigation of the dwarf galaxy populations in three interacting galaxy groups: NGC 871/6/7, NGC 3166/9, NGC 4725/47. Using degree-scale Giant Metrewave Radio Telescope Hi mosaics and deep optical photometry from the Canada-France-Hawaii Telescope, we measured the Hi and stellar properties of the gas-rich low-mass group members to classify each one as a classical dwarf galaxy, a short-lived tidal knot or a tidal dwarf galaxy (TDG). Our observations detect several dwarf irregulars and various tidal knots. We identify four potentially long-lived tidal objects in the three groups, implying that TDGs are not readily produced. The tidal objects examined in this small survey also appear to have a wider variety of properties than TDGs formed in current simulations.

  2. THE CHEMICAL EVOLUTION OF THE URSA MINOR DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Cohen, Judith G.; Hung Wenjin E-mail: hwenjin@astro.washington.ed

    2010-08-10

    We present an abundance analysis based on high-resolution spectra of 10 stars selected to span the full range in metallicity in the Ursa Minor (UMi) dwarf spheroidal (dSph) galaxy. We find that [Fe/H] for the sample stars ranges from -1.35 to -3.10 dex. Combining our sample with previously published work for a total of 16 luminous UMi giants, we establish the trends of abundance ratios [X/Fe] as functions of [Fe/H] for 15 elements. In key cases, particularly for the {alpha}-elements, these trends resemble those for stars in the outer part of the Galactic halo, especially at the lowest metallicities probed. The neutron-capture elements show an r-process distribution over the full range of Fe metallicity reached in this dSph galaxy. This suggests that the duration of star formation in the UMi dSph was shorter than in other dSph galaxies. The derived ages for a larger sample of UMi stars with more uncertain metallicities also suggest a population dominated by uniformly old ({approx}13 Gyr) stars, with a hint of an age-metallicity relationship. Upon comparing our results for UMi, our earlier work in Draco, and published studies of more metal-rich dSph Galactic satellites, there appears to be a pattern of moving from a chemical inventory for dSph giants with [Fe/H] {approx}<-2 dex, which is very similar to that of stars in the outer part of the Galactic halo (enhanced {alpha}/Fe relative to the Sun, coupled with subsolar [X/Fe] for the heavy neutron-capture elements and r-process domination), switching to subsolar {alpha}-elements and super-solar s-process-dominated neutron-capture elements for the highest [Fe/H] dSph stars. The combination of low star formation rates over a varying and sometimes extended duration that produced the stellar populations in the local dSph galaxies with [Fe/H] > - 1.5 dex leads to a chemical inventory wildly discrepant from that of any component of the Milky Way. We note the presence of two UMi giants with [Fe/H] <-3.0 dex in our sample and

  3. Globular Clusters, Ultracompact Dwarfs, and Dwarf Galaxies in Abell 2744 at a Redshift of 0.308

    NASA Astrophysics Data System (ADS)

    Lee, Myung Gyoon; Jang, In Sung

    2016-11-01

    We report a photometric study of globular clusters (GCs), ultracompact dwarfs (UCDs), and dwarf galaxies in the giant merging galaxy cluster Abell 2744 at z = 0.308. Color-magnitude diagrams of the point sources derived from deep F814W (rest frame r‧) and F105W (rest frame I) images of Abell 2744 in the Hubble Space Telescope Frontier Field show a rich population of point sources, which have colors that are similar to those of typical GCs. These sources are as bright as -14.9\\lt {M}r\\prime ≤slant -11.4 (26.0 < F814W(Vega) ≤ 29.5) mag, being mostly UCDs and bright GCs in Abell 2744. The luminosity function (LF) of these sources shows a break at {M}r\\prime ≈ -12.9 (F814W ≈ 28.0) mag, indicating a boundary between UCDs and bright GCs. The numbers of GCs and UCDs are estimated to be 1,711,640+589,760 -430,500 and 147 ± 26, respectively. The clustercentric radial number density profiles of the UCDs and bright GCs show similar slopes, but these profiles are much steeper than those of the dwarf galaxies and the mass density profile based on gravitational lensing analysis. We derive an LF of the red sequence galaxies for -22.9\\lt {M}r\\prime ≤slant -13.9 mag. The faint end of this LF is fit well by a flat power law with α =-1.14+/- 0.08, showing no faint upturn. These results support the galaxy-origin scenario for bright UCDs: they are the nuclei of dwarf galaxies that are stripped when they pass close to the center of massive galaxies or a galaxy cluster, while some of the faint UCDs are at the bright end of the GCs.

  4. Gas-rich vs. gas-poor: LCID insights on the origin of different dwarf galaxy types

    NASA Astrophysics Data System (ADS)

    Gallart, C.; LCID Team

    In this paper, we discuss on the origin of different dwarf galaxy types, and the possible evolutionary links among them, based on full star formation histories (SFH) obtained through the Local Cosmology from Isolated Dwarfs (LCID) project. In the case of the dIrr and dT galaxies, these are the first time that full SFHs, describing the early episodes of star formation with fine time resolution (≃ 2 Gyr), have been obtained. The LCID results show that the early SFH of dIrr galaxies is fundamentally different from that of the studied dT and dSph, in that they haven't experienced a strong early burst, followed by a very mild or nil star formation activity. Instead, they started forming stars slowly and continued similarly over most of their lifetimes. Their SFH is similar to that of the (minority) dSph satellites of the Milky Way that have important amounts of intermediate-age population (namely, Carina, Fornax and Leo I). Based on the derived SFHs, we hypothesize that there are two fundamental types of dwarf galaxies: one type started their evolution vigorously forming stars, but their period of star formation activity has been short lived (few Gyr). Another type has had a mild star formation activity at early times, and has continued forming stars to the present time (or almost). The current availability of gas or lack thereof doesnÕt necessarily correlate with these two globally different evolutionary paths.

  5. Dwarf spheroidal galaxies as degenerate gas of free fermions

    SciTech Connect

    Domcke, Valerie; Urbano, Alfredo E-mail: alfredo.urbano@sissa.it

    2015-01-01

    In this paper we analyze a simple scenario in which Dark Matter (DM) consists of free fermions with mass m{sub f}. We assume that on galactic scales these fermions are capable of forming a degenerate Fermi gas, in which stability against gravitational collapse is ensured by the Pauli exclusion principle. The mass density of the resulting con figuration is governed by a non-relativistic Lane-Emden equation, thus leading to a universal cored profile that depends only on one free parameter in addition to m{sub f}. After reviewing the basic formalism, we test this scenario against experimental data describing the velocity dispersion of the eight classical dwarf spheroidal galaxies of the Milky Way. We find that, despite its extreme simplicity, the model exhibits a good fit to the data and realistic predictions for the size of DM halos providing that m{sub f}≅ 200 eV. Furthermore, we show that in this setup larger galaxies correspond to the non-degenerate limit of the gas. We propose a concrete realization of this model in which DM is produced non-thermally via inflaton decay. We show that imposing the correct relic abundance and the bound on the free-streaming length constrains the inflation model in terms of inflaton mass, its branching ratio into DM and the reheating temperature.

  6. Simulations of the formation of tidal dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Wetzstein, Markus

    2005-02-01

    We present results of numerical simulations of the tidal arms in interacting systems of galaxies. The formation of new dwarf galaxies inside the tidal arms has been studied with very high resolution. We find that in pure N-body simulations, no collapsed objects are formed and that gas efficiently triggers the onset of collapse in tidal tails if the gas distribution in the progenitor disk is extended enough. Using an analysis which closely matches the procedure observers use, we determine detailed kinematical properties for the most massive object formed in the tidal tails of our simulation with gas. Numerical tools for such high resolution simulations are also presented. We have developed VINE, a new N-body SPH code for astrophysical particle simulations. The code uses a binary tree structure for the computation of gravitational interactions. It can make efficient use of the special purpose hardware GRAPE and is parallelized for shared memory supercomputers. A leapfrog and a Runge-Kutta-Fehlberg integrator have been implemented together with an individual particle time step scheme. Other implemented features include periodic boundary conditions and cosmological expansion. We present detailed parallel timings of the code.

  7. Star Formation in NGC4532/DDO 137'S Tidal Dwarf Galaxies and 500 KPC HI Stream

    NASA Astrophysics Data System (ADS)

    Higdon, Sarah

    Mergers and close-passages between gas rich galaxies can result in the formation of long HI/stellar streams. The tidally induced star formation and gas concentrations can result in the creation of tidal dwarf galaxies (TDGs). TDGs may contribute significantly to the dwarf galaxy population, by far the most common galaxy type in the current epoch. We have discovered one of the longest known tidal streams (500 kpc) in the NGC 4535/DDO 137 system. We propose 3 ksec FUV/NUV images centered on the stream and its five TDGs. We will readily detect faint/low mass star forming regions (~2E-17 erg s-1 cm-2 A-1) to 5-sigma. The GALEX observations are a unique opportunity to undertake a sensitive and comprehensive study of tidally induced star formation, dwarf galaxy formation and inter-galactic enrichment in this system.

  8. THE FIRST GENERATION OF VIRGO CLUSTER DWARF ELLIPTICAL GALAXIES?

    SciTech Connect

    Lisker, Thorsten; Janz, Joachim; Hielscher, Oliver; Paudel, Sanjaya; Hensler, Gerhard; Kim, Suk; Rey, Soo-Chang; Weinmann, Simone; Mastropietro, Chiara; Kotulla, Ralf

    2009-11-20

    In the light of the question whether most early-type dwarf (dE) galaxies in clusters formed through infall and transformation of late-type progenitors, we search for an imprint of such an infall history in the oldest, most centrally concentrated dE subclass of the Virgo cluster: the nucleated dEs that show no signatures of disks or central residual star formation. We select dEs in a (projected) region around the central elliptical galaxies, and subdivide them by their line-of-sight velocity into fast-moving and slow-moving ones. These subsamples turn out to have significantly different shapes: while the fast dEs are relatively flat objects, the slow dEs are nearly round. Likewise, when subdividing the central dEs by their projected axial ratio into flat and round ones, their distributions of line-of-sight velocities differ significantly: the flat dEs have a broad, possibly two-peaked distribution, whereas the round dEs show a narrow single peak. We conclude that the round dEs probably are on circularized orbits, while the flat dEs are still on more eccentric or radial orbits typical for an infalling population. In this picture, the round dEs would have resided in the cluster already for a long time, or would even be a cluster-born species, explaining their nearly circular orbits. They would thus be the first generation of Virgo cluster dEs. Their shape could be caused by dynamical heating through repeated tidal interactions. Further investigations through stellar population measurements and studies of simulated galaxy clusters would be desirable to obtain definite conclusions on their origin.

  9. MEASURING SIZES OF ULTRA-FAINT DWARF GALAXIES

    SciTech Connect

    Munoz, Ricardo R.; Padmanabhan, Nikhil; Geha, Marla

    2012-02-01

    The discovery of ultra-faint dwarf (UFD) galaxies in the halo of the Milky Way extends the faint end of the galaxy luminosity function to a few hundred solar luminosities. This extremely low luminosity regime poses a significant challenge for the photometric characterization of these systems. We present a suite of simulations aimed at understanding how different observational choices related to the properties of a low-luminosity system impact our ability to determine its true structural parameters such as half-light radius and central surface brightness. We focus on estimating half-light radii (on which mass estimates depend linearly) and find that these numbers can have up to 100% uncertainties when relatively shallow photometric surveys, such as the Sloan Digital Sky Survey, are used. Our simulations suggest that to recover structural parameters within 10% or better of their true values: (1) the ratio of the field of view to the half-light radius of the satellite must be greater than three, (2) the total number of stars, including background objects should be larger than 1000, and (3) the central to background stellar density ratio must be higher than 20. If one or more of these criteria are not met, the accuracy of the resulting structural parameters can be significantly compromised. In the context of future surveys such as Large Synoptic Survey Telescope, the latter condition will be closely tied to our ability to remove unresolved background galaxies. Assessing the reliability of measured structural parameters will become increasingly critical as the next generation of deep wide-field surveys detects UFDs beyond the reach of current spectroscopic limits.

  10. Sulphur in the Sculptor dwarf spheroidal galaxy. Including NLTE corrections

    NASA Astrophysics Data System (ADS)

    Skúladóttir, Á.; Andrievsky, S. M.; Tolstoy, E.; Hill, V.; Salvadori, S.; Korotin, S. A.; Pettini, M.

    2015-08-01

    In Galactic halo stars, sulphur has been shown to behave like other α-elements, but until now, no comprehensive studies have been done on this element in stars of other galaxies. Here, we use high-resolution ESO VLT/FLAMES/GIRAFFE spectra to determine sulphur abundances for 85 stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range -2.5 ≤ [ Fe / H ] ≤ -0.8. The abundances are derived from the S I triplet at 9213, 9228, and 9238 Å. These lines have been shown to be sensitive to departure from local thermodynamic equilibrium, i.e. NLTE effects. Therefore, we present new NLTE corrections for a grid of stellar parameters covering those of the target stars. The NLTE-corrected sulphur abundances in Sculptor show the same behaviour as other α-elements in that galaxy (such as Mg, Si, and Ca). At lower metallicities ([ Fe / H ] ≲ -2) the abundances are consistent with a plateau at [ S / Fe ] ≈ + 0.16, similar to what is observed in the Galactic halo, [ S / Fe ] ≈ + 0.2. With increasing [Fe/H], the [S/Fe] ratio declines, reaching negative values at [ Fe / H ] ≳ -1.5. The sample also shows an increase in [S/Mg] with [Fe/H], most probably because of enrichment from Type Ia supernovae. Based on observations made with ESO/VLT/FLAMES at the La Silla Paranal observatory under program ID 089.B-0304(B).Appendix is available in electronic form at http://www.aanda.org

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

    SciTech Connect

    Tollerud, Erik J.; Geha, Marla C.; Grcevich, Jana; Putman, Mary E.; Stern, Daniel E-mail: marla.geha@yale.edu E-mail: mputman@astro.columbia.edu

    2015-01-01

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

  12. THE MOST METAL-POOR DAMPED Lyα SYSTEMS: AN INSIGHT INTO DWARF GALAXIES AT HIGH-REDSHIFT

    SciTech Connect

    Cooke, Ryan J.; Pettini, Max; Jorgenson, Regina A.

    2015-02-10

    In this paper we analyze the kinematics, chemistry, and physical properties of a sample of the most metal-poor damped Lyα systems (DLAs), to uncover their links to modern-day galaxies. We present evidence that the DLA population as a whole exhibits a ''knee'' in the relative abundances of the α-capture and Fe-peak elements when the metallicity is [Fe/H] ≅ –2.0, assuming that Zn traces the buildup of Fe-peak elements. In this respect, the chemical evolution of DLAs is clearly different from that experienced by Milky Way halo stars, but resembles that of dwarf spheroidal galaxies in the Local Group. We also find a close correspondence between the kinematics of Local Group dwarf galaxies and of high-redshift metal-poor DLAs, which further strengthens this connection. On the basis of such similarities, we propose that the most metal-poor DLAs provide us with a unique opportunity to directly study the dwarf galaxy population more than ten billion years in the past, at a time when many dwarf galaxies were forming the bulk of their stars. To this end, we have measured some of the key physical properties of the DLA gas, including their neutral gas mass, size, kinetic temperature, density, and turbulence. We find that metal-poor DLAs contain a warm neutral medium with T {sub gas} ≅ 9600 K predominantly held up by thermal pressure. Furthermore, all of the DLAs in our sample exhibit a subsonic turbulent Mach number, implying that the gas distribution is largely smooth. These results are among the first empirical descriptions of the environments where the first few generations of stars may have formed in the universe.

  13. Determining the Large-scale Environmental Dependence of Gas-phase Metallicity in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Douglass, Kelly A.; Vogeley, Michael S.

    2017-01-01

    We study how the cosmic environment affects galaxy evolution in the universe by comparing the metallicities of dwarf galaxies in voids with dwarf galaxies in more dense regions. Ratios of the fluxes of emission lines, particularly those of the forbidden [O iii] and [S ii] transitions, provide estimates of a region’s electron temperature and number density. From these two quantities and the emission line fluxes [O ii] λ3727, [O iii] λ4363, and [O iii] λλ4959, 5007, we estimate the abundance of oxygen with the direct Te method. We estimate the metallicity of 42 blue, star-forming void dwarf galaxies and 89 blue, star-forming dwarf galaxies in more dense regions using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, as reprocessed in the MPA-JHU value-added catalog. We find very little difference between the two sets of galaxies, indicating little influence from the large-scale environment on their chemical evolution. Of particular interest are a number of extremely metal-poor dwarf galaxies that are less prevalent in voids than in the denser regions.

  14. Revealing Massive Black Holes in Dwarf Galaxies with X-rays

    NASA Astrophysics Data System (ADS)

    Reines, A.

    2014-07-01

    Supermassive black holes (BHs) live at the heart of essentially all massive galaxies, power AGN, and are thought to be important agents in the evolution of their hosts. However, the origin of these monster BHs is largely unknown. While direct observations of the first ``seeds" of supermassive BHs in the infant Universe are unobtainable with current telescopes, finding and studying dwarf galaxies hosting massive BHs today can provide valuable constraints on the masses, host galaxies, and formation mechanism of supermassive BH seeds. We have recently completed the first systematic search for AGN in dwarf galaxies using optical spectroscopy, increasing the number of known dwarfs with massive BHs by more than an order of magnitude (Reines et al. 2013). However, this optical search is biased towards BHs radiating at high fractions of their Eddington limit in galaxies with little on-going star formation. Alternative search techniques and diagnostics at other wavelengths are necessary to make further progress. I will discuss our efforts to find and study massive BHs in dwarf galaxies using observations at X-ray wavelengths. These observations are more sensitive to weakly accreting massive BHs and are already beginning to reveal massive BHs hidden at optical wavelengths in star-forming dwarf galaxies.

  15. TIDAL INTERACTION AS THE ORIGIN OF EARLY-TYPE DWARF GALAXIES IN GROUP ENVIRONMENTS

    SciTech Connect

    Paudel, Sanjaya; Ree, Chang H.

    2014-11-20

    We present a sample of dwarf galaxies that suffer ongoing disruption by the tidal forces of nearby massive galaxies. By analyzing structural and stellar population properties using the archival imaging and spectroscopic data from the Sloan Digital Sky Survey (SDSS), we find that they are likely a ''smoking gun'' example of the formation through tidal stirring of early-type dwarf galaxies (dEs) in the galaxy group environment. The inner cores of these galaxies are fairly intact and the observed light profiles are well fit by the Sérsic functions while the tidally stretched stellar halos are prominent in the outer parts. They are all located within a sky-projected distance of 50 kpc from the centers of the host galaxies and no dwarf galaxies have relative line-of-sight velocities larger than 205 km s{sup –1} to their hosts. We derive the Composite Stellar Population properties of these galaxies by fitting the SDSS optical spectra to a multiple-burst composite stellar population model. We find that these galaxies accumulate a significant fraction of stellar mass within the last 1 Gyr and contain a majority stellar population with an intermediate age of 2 to 4 Gyr. Based on this evidence, we argue that tidal stirring, particularly through the galaxy-galaxy interaction, might have an important role in the formation and evolution of dEs in the group environment where the influence of other gas stripping mechanism might be limited.

  16. Mass-to-Light versus Color Relations for Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Herrmann, Kimberly A.; Hunter, Deidre Ann; Zhang, Hong-Xin; Elmegreen, Bruce; LITTLE THINGS

    2017-01-01

    We have determined new relations between UBV colors and mass-to-light (M/L) ratios for dwarf irregular galaxies, as well as for transformed g‧ - r‧. These M/L to color relations (MLCRs) are based on stellar mass density profiles determined for 34 LITTLE THINGS dwarfs from spectral energy distribution fitting to multi-wavelength surface photometry in passbands from the FUV to the NIR. These relations can be used to determine stellar masses in dwarf irregular galaxies for situations where other determinations of stellar mass are not possible. Our MLCRs are shallower than comparable MLCRs in the literature determined for spiral galaxies. We divided our dwarf data into four metallicity bins and found indications of a steepening of the MLCR with increased oxygen abundance, perhaps due to more line blanketing occurring at higher metallicity.

  17. Nature and Nurture of Early-Type Dwarf Galaxies in Low Density Environments

    NASA Astrophysics Data System (ADS)

    Grützbauch, R.; Annibali, F.; Rampazzo, R.; Bressan, A.; Zeilinger, W. W.

    We study stellar population parameters of a sample of 13 dwarf galaxies located in poor groups of galaxies using high resolution spectra observed with VIMOS at the ESO-VLT [Grützbauch et al., A&A 502, 473 (2009)]. LICK-indices were compared with Simple Stellar Population models to derive ages, metallicities and [α/Fe]-ratios. Comparing the dwarfs with a sample of giant ETGs residing in comparable environments we find that the dwarfs are on average younger, less metal-rich, and less enhanced in alpha-elements than giants. Age, Z, and [α/Fe] ratios are found to correlate both with velocity dispersion and with morphology. We also find possible evidence that low density environment (LDE) dwarfs experienced more prolonged star formation histories than Coma dwarfs, however, larger samples are needed to draw firm conclusions.

  18. ON THE EXTENDED STRUCTURE OF THE PHOENIX DWARF GALAXY

    SciTech Connect

    Hidalgo, Sebastian L.; Aparicio, Antonio; MartInez-Delgado, David; Gallart, Carme E-mail: antapaj@iac.e E-mail: carme@iac.e

    2009-11-01

    We present the star formation history (SFH) and its variations with galactocentric distance for the Local Group dwarf galaxy of Phoenix. They have been derived from a (F555W, F814W) color-magnitude diagram obtained from WFPC2-HST data, which reaches the oldest main-sequence turnoffs. The IAC-star and IAC-pop codes and the MinnIAC suite have been used to obtain the star formation rate as a function of time and metallicity, psi(t, z). We find that Phoenix has had ongoing but gradually decreasing star formation over nearly a Hubble time. The highest level of star formation occurred from the formation of the galaxy till 10.5 Gyr ago, when 50% of the total star formation had already taken place. From that moment, star formation continues at a significant level until 6 Gyr ago (an additional 35% of the stars are formed in this time interval), and at a very low level till the present time. The chemical enrichment law shows a trend of slowly increasing metallicity as a function of time until 6-8 Gyr ago, when metallicity starts to increase steeply to the current value. We have paid particular attention to the study of the variations of the SFH as a function of radius. Young stars are found in the inner region of the galaxy only, but intermediate-age and old stars can be found at all galactocentric distances. The distribution of mass density in alive stars and its evolution with time has been studied. This study shows that star formation started at all galactocentric distances in Phoenix at an early epoch. If stars form in situ in Phoenix, the star formation onset took place all over the galaxy (up to a distance of about 400 pc from the center), but preferentially out of center regions. After that, our results are compatible with a scenario in which the star formation region envelope slowly shrinks as time goes on, possibly as a natural result of pressure support reduction as gas supply diminishes. As a consequence, the star formation stopped first (about 7-8 Gyr ago) in

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

    NASA Astrophysics Data System (ADS)

    Reines, Amy Ellen

    2011-01-01

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

  20. Lack of nuclear clusters in dwarf spheroidal galaxies: implications for massive black holes formation and the cusp/core problem

    NASA Astrophysics Data System (ADS)

    Arca-Sedda, Manuel; Capuzzo-Dolcetta, Roberto

    2017-01-01

    One of the leading scenarios for the formation of nuclear star clusters in galaxies is related to the orbital decay of globular clusters (GCs) and their subsequent merging, though alternative theories are currently debated. The availability of high-quality data for structural and orbital parameters of GCs allows us to test different nuclear star cluster formation scenarios. The Fornax dwarf spheroidal (dSph) galaxy is the heaviest satellite of the Milky Way and it is the only known dSph hosting five GCs, whereas there are no clear signatures for the presence of a central massive black hole. For this reason, it represents a suited place to study the orbital decay process in dwarf galaxies. In this paper, we model the future evolution of the Fornax GCs simulating them and the host galaxy by means of direct N-body simulations. Our simulations also take into account the gravitational field generated by the Milky Way. We found that if the Fornax galaxy is embedded in a standard cold dark matter halo, the nuclear cluster formation would be significantly hampered by the high central galactic mass density. In this context, we discuss the possibility that infalling GCs drive the flattening of the galactic density profile, giving a possible alternative explanation to the so-called cusp/core problem. Moreover, we briefly discuss the link between GC infall process and the absence of massive black holes in the centre of dSphs.

  1. Variations in a Universal Density Profile for the Milky Way's Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Jardel, John; Gebhardt, K.

    2014-01-01

    On the largest scales, the Cold Dark Matter (CDM) paradigm for structure formation has enjoyed remarkable success in describing the universe we live in. The current frontier in our knowledge of galaxy formation is at the low-mass level. Here we find disagreement between theory and observations in a number of interesting cases. One such problem that has received considerable attention is the debate over the shape of the dark matter density profiles in the Milky Way's dwarf spheroidal (dSph) galaxies, known as the core/cusp problem. CDM simulations predict every halo should have a cuspy profile with an inner logarithmic slope of -1, but some observers have found that profiles with constant density inner cores are preferred. However, a major weakness of this previous work is that the dynamical models constructed to measure the mass distribution have had to assume a parameterization for the dark matter profile--exactly the thing one wishes to measure. For my thesis I introduced a new modeling technique, based on Schwarzschild's method, that instead calculates the dark matter profile non-parametrically. Applying these models to five of the Milky Way's dSphs I found a variety of profile shapes including cores, cusps, and other completely unexpected shapes. When scaled to a common normalization, however, I found the combined profile appears to follow an approximate power law with slope -1. The results of this averaging suggest that the individual formation histories of each galaxy produce differing dark matter profiles, but with a net result that is similar to CDM predictions. To better understand the role baryons play in this process, I compare my results to recent hydrodynamical simulations of the formation of dwarf galaxies. Together, my results and the simulations suggest a trend of flatter profiles in galaxies where more stars have formed. This implies that star formation and dark matter halos are linked through the effects of supernova-induced outflows which are

  2. The no-spin zone: rotation versus dispersion support in observed and simulated dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Wheeler, Coral; Pace, Andrew B.; Bullock, James S.; Boylan-Kolchin, Michael; Oñorbe, Jose; Elbert, Oliver D.; Fitts, Alex; Hopkins, Philip F.; Kereš, Dušan

    2017-02-01

    We perform a systematic Bayesian analysis of rotation versus dispersion support (vrot/σ) in 40 dwarf galaxies throughout the local volume (LV) over a stellar mass range of 10^{3.5} M_{⊙}< M_{star }< 108 M_{⊙}. We find that the stars in ∼80 per cent of the LV dwarf galaxies studied - both satellites and isolated systems - are dispersion-supported. In particular, we show that 6/10 isolated dwarfs in our sample have vrot/σ ≲ 1.0, while all have vrot/σ ≲ 2.0. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally supported stellar discs, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion-supported stars. We see no clear trend between vrot/σ and distance to the closest L⋆ galaxy, nor between vrot/σ and M⋆ within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies (10^9 M_{⊙}< M_{vir}< 10^{10} M_{⊙}) and show that the simulated isolated dIrr galaxies have stellar ellipticities and stellar vrot/σ ratios that are consistent with the observed population of dIrrs and dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-dominated systems, rather than cold, angular-momentum-supported discs. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.

  3. VIMOS Integral Field Spectroscopy of Gaseous Nebulae in Local Group Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Held, E. V.; Gullieuszik, M.; Saviane, I.; Sabbadin, F.; Momany, Y.; Rizzi, L.; Bresolin, F.

    The study of very metal-poor dwarf irregular (dIrr) galaxies is fundamental to test the cosmological scenarios of galaxy formation. Among Local Group galaxies, Leo A and SagDIG are probably the most metal-poor dwarfs, as suggested by estimates of their nebular abundances based on the empirical method [I. Saviane, L. Rizzi, E.V. Held, F. Bresolin, Y. Momany in Astron. Astrophys. 390, 59 (2002); E.D. Skillman, R. Terlevich, J. Melnick in Mon. Not. R. Astron. Soc. 240, 563 (1989); L. van Zee, E.D. Skillman, M.P. Haynes in Astrophys. J. 637, 269 (2006)].

  4. Carbon and Oxygen Abundances in Low Metallicity Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Berg, Danielle A.; Skillman, Evan D.; Henry, Richard B. C.; Erb, Dawn K.; Carigi, Leticia

    2016-08-01

    The study of carbon and oxygen abundances yields information on the time evolution and nucleosynthetic origins of these elements, yet they remain relatively unexplored. At low metallicities, (12+log(O/H) < 8.0), nebular carbon measurements are limited to rest-frame UV collisionally excited emission lines. Therefore, we present the UV spectrophotometry of 12 nearby low-metallicity high-ionization H ii regions in dwarf galaxies obtained using the Cosmic Origins Spectrograph on the Hubble Space Telescope. We present the first analysis of the C/O ratio in local galaxies based solely on simultaneous significant detections of the UV {{{O}}}+2 and {{{C}}}+2 collisionally excited lines in seven of our targets and five objects from the literature to create a final sample of 12 significant detections. Our sample is complemented by optical SDSS spectra, from which we measured the nebular physical conditions and oxygen abundances using the direct method. At low metallicity, (12+log(O/H) < 8.0), no clear trend is evident in C/O versus O/H for the present sample given the large dispersion observed. When combined with recombination line observations at higher values of O/H, a general trend of increasing C/O with increasing O/H is also viable but with some significant outliers. Additionally, we find the C/N ratio appears to be constant (but with significant scatter) over a large range in oxygen abundance, indicating that carbon is predominantly produced by similar nucleosynthetic mechanisms as nitrogen. If true, and our current understanding of nitrogen production is correct, this would indicate that primary production of carbon (a flat trend) dominates at low metallicity, but quasi-secondary production (an increasing trend) becomes prominent at higher metallicities. A larger sample will be needed to determine the true nature and dispersion of the relation.

  5. Strangers in the Night: Discovery of a Dwarf Spheroidal Galaxy on Its First Local Group Infall

    NASA Astrophysics Data System (ADS)

    Chapman, S. C.; Peñarrubia, J.; Ibata, R.; McConnachie, A.; Martin, N.; Irwin, M.; Blain, A.; Lewis, G. F.; Letarte, B.; Lo, K.; Ludlow, A.; O'neil, K.

    2007-06-01

    We present spectroscopic observations of the And XII dwarf spheroidal galaxy using DEIMOS/Keck II, showing it to be moving rapidly through the Local Group (-556 km s-1 heliocentric velocity, -281 km s-1 relative to Andromeda), falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. And XII therefore represents a dwarf galaxy plausibly falling into the Local Group for the first time and never having experienced a dense galactic environment. From Green Bank Telescope observations, a limit on the H I gas mass of <3×103 Msolar suggests that And XII's gas could have been removed prior to experiencing the tides of the Local Group galaxies. Orbit models suggest that the dwarf is close to the escape velocity of M31 for published mass models. And XII is our best direct evidence for the late infall of satellite galaxies, a prediction of cosmological simulations.

  6. THE ACS LCID PROJECT: ON THE ORIGIN OF DWARF GALAXY TYPES—A MANIFESTATION OF THE HALO ASSEMBLY BIAS?

    SciTech Connect

    Gallart, Carme; Monelli, Matteo; Aparicio, Antonio; Battaglia, Giuseppina; Drozdovsky, Igor; Hidalgo, Sebastian L.; Mayer, Lucio; Bernard, Edouard J.; Cassisi, Santi; Cole, Andrew A.; Dolphin, Andrew E.; Navarro, Julio F.; Salvadori, Stefania; Skillman, Evan D.; Stetson, Peter B.; Weisz, Daniel R.

    2015-10-01

    We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy types. We suggest that these types may be imprinted by the early conditions of formation rather than only being the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from color–magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal versus dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower-density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy primarily reflects the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation.

  7. Observations of the impact of starbursts on the interstellar medium in dwarf galaxies

    NASA Technical Reports Server (NTRS)

    Marlowe, Amanda T.; Heckman, Timothy M.; Wyse, Rosemary F. G.; Schommer, Robert

    1995-01-01

    Dwarf galaxies play a crucial role in our understanding of the formation and evolution of galaxies, and the concept of supernova-driven mass outflows is a vital ingredient in theories of the structure and evolution of dwarf galaxies. Despite the theoretical importance of these outflows, there is a very limited amount of direct observational evidence for their existence. We have therefore begun a detailed multi-wave-band search for outflows in dwarf (M(sub B) greater than or = -18) galaxies with extensive recent or ongoing centrally concentrated star formation. We report the first results of this search in the present paper. Observations of the ionized gas in dwarf amorphous galaxies with centrally concentrated populations of massive stars provide evidence for the large-scale expansion of their expansion of their ionized interstellar media. Fabry-Perot H alpha images reveal the presence of kiloparsec-scale 'superbubbles' and filaments which tend to be oriented along the galaxy minor axis. These structures are comparable in size to the chracteristic optical sizes of the galaxies, and dominate the morphology of the galaxies at low surface brightness in H alpha. Since expanding structure of this size and velocity are not observed in all low-mass galaxies with recent or ongoing star formation, we suggest that we are witnessing transient events that likely have a relatively low 'duty cycle' in such galaxies. That is, we argue that the particular galaxies in the present paper have had significantly elevated star formation rates over the past 10(exp 7)-10(exp 8) yr (i.e., these are starburst or young poststarburst systems). This interpretation is consistent with the optical colors and emission-line properties of these galaxies.

  8. Preliminary stellar photographic photometry in the Sculptor Dwarf Irregular Galaxy /SDIG/

    NASA Astrophysics Data System (ADS)

    Lequeux, J.; West, R. M.

    1981-11-01

    Preliminary electronographic V photometry is given for individual stars in the Sculptor Dwarf Irregular Galaxy (SDIG), in the Sculptor group of galaxies. These are the first such data available for a galaxy outside the Local Group. The brightest star has an absolute V magnitude of -6.3. The lack of brighter stars which are present in bigger galaxies is not statistically significant. The luminosity function is similar to that in the irregular galaxies IC 1613 and NGC 6822 and, like these objects, SDIG appears to be in a relatively quiescent stage of star formation.

  9. X-RAY SOURCES IN THE DWARF SPHEROIDAL GALAXY DRACO

    SciTech Connect

    Sonbas, E.; Rangelov, B.; Kargaltsev, O.; Dhuga, K. S.; Hare, J.; Volkov, I.

    2016-04-10

    We present the spectral analysis of an 87 ks XMM-Newton observation of Draco, a nearby dwarf spheroidal galaxy. Of the approximately 35 robust X-ray source detections, we focus our attention on the brightest of these sources, for which we report X-ray and multiwavelength parameters. While most of the sources exhibit properties consistent with active galactic nuclei, few of them possess the characteristics of low-mass X-ray binaries (LMXBs) and cataclysmic variable (CVs). Our analysis places constraints on the population of X-ray sources with L{sub X} > 3 × 10{sup 33} erg s{sup −1} in Draco, suggesting that there are no actively accreting black hole and neutron star binaries. However, we find four sources that could be quiescent state LMXBs/CVs associated with Draco. We also place constraints on the central black hole luminosity and on a dark matter decay signal around 3.5 keV.

  10. Spatially resolved kinematics of an ultracompact dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Frank, M. J.; Hilker, M.; Mieske, S.; Baumgardt, H.; Grebel, E. K.; Infante, L.

    2011-06-01

    We present the internal kinematics of UCD3, the brightest known ultracompact dwarf galaxy (UCD) in the Fornax cluster, making this the first UCD with spatially resolved spectroscopy. Our study is based on seeing-limited observations obtained with the ARGUS Integral Field Unit of the VLT/FLAMES spectrograph under excellent seeing conditions (0.5-0.67 arcsec FWHM). The velocity field of UCD3 shows the signature of weak rotation, comparable to that found in massive globular clusters. Its velocity dispersion profile is fully consistent with an isotropic velocity distribution and the assumption that mass follows the light distribution obtained from Hubble Space Telescope imaging. In particular, there is no evidence for the presence of an extended dark matter halo contributing a significant (≳33 per cent within R < 200 pc) mass fraction, nor for a central black hole more massive than ˜5 per cent of the UCD's mass. While this result does not exclude a galaxian origin for UCD3, we conclude that its internal kinematics are fully consistent with it being a massive star cluster. Based on observations obtained at the European Southern Observatory, Chile [Observing Programme 078.B-0496(B)].

  11. Detectability of Ultra Faint Dwarf Galaxies with Gaia

    NASA Astrophysics Data System (ADS)

    Mateu, C.; Antoja, T.; Aguilar, L.; Figueras, F.; Brown, A.; Antiche, E.; Hernández-Pérez, F.; Valenzuela, O.; Aparicio, A.; Hidalgo, S.; Velázquez, H.

    2014-07-01

    We present a technique to detect Ultra-Faint Dwarf Galaxies (UFDs) in the Galactic Halo, using sky and proper motion information.The method uses wavelet transforms to detect peaks in the sky and proper motion planes, and to evaluate the probability of these being stochastic fluctuations. We aim to map thoroughly the detection limits of this technique. For this, we have produced a library of 15,000 synthetic UFDs, embedded in the Gaia Universe Model Snapshot (GUMS) background (Robin et al. 2012), each at a different distance, different luminosity, half-light radius, velocity dispersion and center-of-mass velocity, varying in ranges that extend well beyond those spanned by known classical and ultra-faint dSphs. We use these synthetic UFDs as a benchmark to characterize the completeness and detection limits of our technique, and present our results as a function of different physical and observable parameters of the UFDs (see full poster for more details at https://gaia.ub.edu/Twiki/pub/GREATITNFC/ProgramFinalconference/Poster_UFGX_Bcn_C_Mateu.pdf).

  12. Particle dark matter constraints from the Draco dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Tyler, Craig Edward

    2002-10-01

    It is widely thought that neutralinos, the lightest supersymmetric particles, could comprise most of the dark matter. If so, then dark halos will emit radio and gamma ray signals initiated by neutralino annihilation. A particularly promising place to look for these indicators is at the center of the local group dwarf spheroidal galaxy Draco, and recent measurements of the motion of its stars have revealed it to be an even better target for dark matter detection than previously thought. We compute limits on WIMP properties for various models of Draco's dark matter halo. We find that if the halo is nearly isothermal, as the new measurements indicate, then current gamma ray flux limits prohibit much of the neutralino parameter space. If Draco has a moderate magnetic field, then current radio limits can rule out more of it. These results are appreciably stronger than other current constraints, and so acquiring more detailed data on Draco's density profile may become one of the most promising avenues for identifying dark matter.

  13. Delayed star formation in isolated dwarf galaxies: Hubble space telescope star formation history of the Aquarius dwarf irregular

    SciTech Connect

    Cole, Andrew A.; Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; McConnachie, Alan W.; Brooks, Alyson M.; Leaman, Ryan E-mail: drw@ucsc.edu E-mail: skillman@astro.umn.edu E-mail: abrooks@physics.rutgers.edu

    2014-11-01

    We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope Advanced Camera for Surveys. The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ≈10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ≈10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ≈ 2). The star formation rate increased dramatically ≈6-8 Gyr ago (z ≈ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar M {sub H} {sub I}/M {sub *}, dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degrees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies.

  14. Abundance ratios of red giants in low-mass ultra-faint dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    François, P.; Monaco, L.; Bonifacio, P.; Moni Bidin, C.; Geisler, D.; Sbordone, L.

    2016-04-01

    Context. Low-mass dwarf spheroidal galaxies are key objects for our understanding of the chemical evolution of the pristine Universe and the Local Group of galaxies. Abundance ratios in stars of these objects can be used to better understand their star formation and chemical evolution. Aims: We report on the analysis of a sample of 11 stars belonging to five different ultra-faint dwarf spheroidal galaxies (UfDSph) that is based on X-Shooter spectra obtained at the VLT. Methods: Medium-resolution spectra have been used to determine the detailed chemical composition of their atmosphere. We performed a standard 1D LTE analysis to compute the abundances. Results: Considering all the stars as representative of the same population of low-mass galaxies, we found that the [α/Fe] ratios vs.s [Fe/H] decreases as the metallicity of the star increases in a way similar to that which is found for the population of stars that belong to dwarf spheroidal galaxies. The main difference is that the solar [α/Fe] is reached at a much lower metallicity for the UfDSph than for the dwarf spheroidal galaxies. We report for the first time the abundance of strontium in CVn II. The star we analyzed in this galaxy has a very high [Sr/Fe] and a very low upper limit of barium which makes it a star with an exceptionally high [Sr/Ba] ratio.

  15. Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM.

    PubMed

    Rubio, Monica; Elmegreen, Bruce G; Hunter, Deidre A; Brinks, Elias; Cortés, Juan R; Cigan, Phil

    2015-09-10

    Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations they are molecular, with H2 the dominant species and carbon monoxide (CO) the best available tracer. When the abundances of carbon and oxygen are low compared with that of hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies, CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds. Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM), which has a metallicity that is 13 per cent of the solar value and 50 per cent lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.

  16. THE NUMBER OF TIDAL DWARF SATELLITE GALAXIES IN DEPENDENCE OF BULGE INDEX

    SciTech Connect

    López-Corredoira, Martín; Kroupa, Pavel E-mail: pavel@astro.uni-bonn.de

    2016-01-20

    We show that a significant correlation (up to 5σ) emerges between the bulge index, defined to be larger for a larger bulge/disk ratio, in spiral galaxies with similar luminosities in the Galaxy Zoo 2 of the Sloan Digital Sky Survey and the number of tidal-dwarf galaxies in the catalog by Kaviraj et al. In the standard cold or warm dark matter cosmological models, the number of satellite galaxies correlates with the circular velocity of the dark matter host halo. In generalized gravity models without cold or warm dark matter, such a correlation does not exist, because host galaxies cannot capture infalling dwarf galaxies due to the absence of dark-matter-induced dynamical friction. However, in such models, a correlation is expected to exist between the bulge mass and the number of satellite galaxies because bulges and tidal-dwarf satellite galaxies form in encounters between host galaxies. This is not predicted by dark matter models in which bulge mass and the number of satellites are a priori uncorrelated because higher bulge/disk ratios do not imply higher dark/luminous ratios. Hence, our correlation reproduces the prediction of scenarios without dark matter, whereas an explanation is not found readily from the a priori predictions of the standard scenario with dark matter. Further research is needed to explore whether some application of the standard theory may explain this correlation.

  17. The Number of Tidal Dwarf Satellite Galaxies in Dependence of Bulge Index

    NASA Astrophysics Data System (ADS)

    López-Corredoira, Martín; Kroupa, Pavel

    2016-01-01

    We show that a significant correlation (up to 5σ) emerges between the bulge index, defined to be larger for a larger bulge/disk ratio, in spiral galaxies with similar luminosities in the Galaxy Zoo 2 of the Sloan Digital Sky Survey and the number of tidal-dwarf galaxies in the catalog by Kaviraj et al. In the standard cold or warm dark matter cosmological models, the number of satellite galaxies correlates with the circular velocity of the dark matter host halo. In generalized gravity models without cold or warm dark matter, such a correlation does not exist, because host galaxies cannot capture infalling dwarf galaxies due to the absence of dark-matter-induced dynamical friction. However, in such models, a correlation is expected to exist between the bulge mass and the number of satellite galaxies because bulges and tidal-dwarf satellite galaxies form in encounters between host galaxies. This is not predicted by dark matter models in which bulge mass and the number of satellites are a priori uncorrelated because higher bulge/disk ratios do not imply higher dark/luminous ratios. Hence, our correlation reproduces the prediction of scenarios without dark matter, whereas an explanation is not found readily from the a priori predictions of the standard scenario with dark matter. Further research is needed to explore whether some application of the standard theory may explain this correlation.

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

    NASA Astrophysics Data System (ADS)

    Hallenbeck, Gregory L.; Koopmann, Rebecca A.

    2017-01-01

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

  19. Chemical evolution of classical and ultra-faint dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Vincenzo, F.; Matteucci, F.; Vattakunnel, S.; Lanfranchi, G. A.

    2014-07-01

    We present updated chemical evolution models of two dwarf spheroidal galaxies (Sculptor and Carina) and the first detailed chemical evolution models of two ultra-faint dwarfs (Hercules and Boötes I). Our results suggest that the dwarf spheroidals evolve with a low efficiency of star formation, confirming previous results, and the ultra-faint dwarfs with an even lower one. Under these assumptions, we can reproduce the stellar metallicity distribution function, the [α/Fe] versus [Fe/H] abundance patterns and the total stellar and gas masses observed at the present time in these objects. In particular, for the ultra-faint dwarfs we assume a strong initial burst of star formation, with the mass of the system being already in place at early times. On the other hand, for the classical dwarf spheroidals the agreement with the data is found by assuming the star formation histories suggested by the colour-magnitude diagrams and a longer time-scale of formation via gas infall. We find that all these galaxies should experience galactic winds, starting in all cases before 1 Gyr from the beginning of their evolution. From comparison with Galaxy data, we conclude that it is unlikely that the ultra-faint dwarfs have been the building blocks of the whole Galactic halo, although more data are necessary before drawing firm conclusions.

  20. Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at Z greater than 1

    NASA Technical Reports Server (NTRS)

    VanDerWel, A.; Straughn, A. N.; Rix, H.-W.; Finkelstein, S. L.; Koekemoer, A. M.; Weiner, B. J.; Wuyts, S.; Bell, E. F.; Faber, S. M.; Trump, J. R.; Koo, D.; Ferguson, H. C.; Scarlata, C.; Hathi, N. P.; Dunlop, J. S.; Newman, J. A.; Kocevski, D. D.; Lai, K.; Grogin, N. A.; Rodney, S. A.; Lee, K.-S.; Guo, Y.

    2011-01-01

    We identify an abundant population of extreme emission line galaxies at redshift z=1.6 - 1.8 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared, broad-band fluxes. Supported by spectroscopic confirmation of strong [OIII] emission lines - with equivalent widths approximately 1000A - in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are dwarf galaxies with approximately 10(exp 8) solar mass in stellar mass, undergoing an enormous star-burst phase with M*/M* of only approximately 10 Myr. The star formation activity and the co-moving number density (3.7 x 10(exp -4) Mpc(exp -3)) imply that strong, short-lived bursts play a significant, perhaps even dominant role in the formation and evolution of dwarf galaxies at z greater than 1. The observed star formation activity can produce in less than 5 Gyr the same amount of stellar mass density as is presently contained in dwarf galaxies. Therefore, our observations provide a strong indication that the stellar populations of present-day dwarf galaxies formed mainly in strong, short-lived bursts, mostly at z greater than 1.

  1. VizieR Online Data Catalog: Dwarf galaxies surface brightness profiles. II. (Herrmann+, 2016)

    NASA Astrophysics Data System (ADS)

    Herrmann, K. A.; Hunter, D. A.; Elmegreen, B. G.

    2016-07-01

    Our galaxy sample (see Table1) is derived from the survey of nearby (>30Mpc) late-type galaxies conducted by Hunter & Elmegreen 2006 (cat. J/ApJS/162/49). The full survey includes 94 dwarf Irregulars (dIms), 26 Blue Compact Dwarfs (BCDs), and 20 Magellanic-type spirals (Sms). The 141 dwarf sample presented in the first paper of the present series (Paper I; Herrmann et al. 2013, Cat. J/AJ/146/104) contains one fewer Sm galaxy and two additional dIm systems than the original survey. A multi-wavelength data set has been assembled for these galaxies. The data include Hα images (129 galaxies with detections) to trace star formation over the past 10Myr (Hunter & Elmegreen 2004, Cat. J/AJ/128/2170) and satellite UV images (61 galaxies observed) obtained with the Galaxy Evolution Explorer (GALEX) to trace star formation over the past ~200Myr. The GALEX data include images from two passbands with effective wavelengths of 1516Å (FUV) and 2267Å (NUV) and resolutions of 4'' and 5.6'', respectively. Three of the galaxies in our sample with NUV data do not have FUV data. To trace older stars we have UBV images, which are sensitive to stars formed over the past 1Gyr for on-going star formation, and images in at least one band of JHK for 40 galaxies in the sample, which integrates the star formation over the galaxy's lifetime. Note that nine dwarfs are missing UB data and three more are missing U-band data. In addition we made use of 3.6μm images (39 galaxies) obtained with the Infrared Array Camera (IRAC) in the Spitzer archives also to probe old stars. (3 data files).

  2. The Herschel Virgo Cluster Survey. XVIII. Star-forming dwarf galaxies in a cluster environment

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Hunt, L. K.; Madden, S. C.; Hughes, T. M.; Auld, R.; Baes, M.; Bendo, G. J.; Bianchi, S.; Bizzocchi, L.; Boquien, M.; Boselli, A.; Clemens, M.; Corbelli, E.; Cortese, L.; Davies, J.; De Looze, I.; di Serego Alighieri, S.; Fritz, J.; Pappalardo, C.; Pierini, D.; Rémy-Ruyer, A.; Smith, M. W. L.; Verstappen, J.; Viaene, S.; Vlahakis, C.

    2015-02-01

    To assess the effects of the cluster environment on the different components of the interstellar medium, we analyse the far-infrared (FIR) and submillimetre (submm) properties of a sample of star-forming dwarf galaxies detected by the Herschel Virgo Cluster Survey (HeViCS). We determine dust masses and dust temperatures by fitting a modified black body function to the spectral energy distributions (SEDs). Stellar and gas masses, star formation rates (SFRs), and metallicities are obtained from the analysis of a set of ancillary data. Dust is detected in 49 out of a total 140 optically identified dwarfs covered by the HeViCS field; considering only dwarfs brighter than mB = 18 mag, this gives a detection rate of 43%. After evaluating different emissivity indices, we find that the FIR-submm SEDs are best-fit by β = 1.5, with a median dust temperature Td = 22.4 K. Assuming β = 1.5, 67% of the 23 galaxies detected in all five Herschel bands show emission at 500 μm in excess of the modified black-body model. The fraction of galaxies with a submillimetre excess decreases for lower values of β, while a similarly high fraction (54%) is found if a β-free SED modelling is applied. The excess is inversely correlated with SFR and stellar masses. To study the variations in the global properties of our sample that come from environmental effects, we compare the Virgo dwarfs to other Herschel surveys,such as the Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH), the Dwarf Galaxy Survey (DGS), and the HeViCS Bright Galaxy Catalogue (BGC). We explore the relations between stellar mass and Hi fraction, specific star formation rate, dust fraction, gas-to-dust ratio over a wide range of stellar masses (from 107 to 1011 M⊙) for both dwarfs and spirals. Highly Hi-deficient Virgo dwarf galaxies are mostly characterised by quenched star formation activity and lower dust fractions giving hints for dust stripping in cluster dwarfs. However, to explain the

  3. Does the dwarf galaxy system of the Milky Way originate from Andromeda?

    NASA Astrophysics Data System (ADS)

    Fouquet, Sylvain; Hammer, François; Yang, Yanbin; Puech, Mathieu; Flores, Hector

    2012-12-01

    The Local Group is often seen to be a quiescent environment without significant merger events. However, an ancient major merger may have occurred in the most massive galaxy as suggested by the M31 classical bulge and its halo haunted by numerous stellar streams. Numerical simulations have shown that tidal tails formed during gas-rich major mergers are long-lived and could be responsible for old stellar streams and likely induce the formation of tidal dwarf galaxies (TDGs). Using several hydrodynamical simulations we have investigated the most prominent tidal tail formed during the first passage, which is gas rich and contains old and metal-poor stars. We discovered several striking coincidences after comparing its location and motion to those of the Milky Way (MW) and of the Magellanic Clouds (MCs). First, the tidal tail is sweeping a relatively small volume in which the MW precisely lies. Because the geometry of the merger is somehow fixed by the anisotropic properties of the giant stream (GS), we evaluate the chance of the MW to be at such a rendezvous with this gigantic tidal tail to be 5 per cent. Secondly, the velocity of the tidal tail matches the Large Magellanic Cloud (LMC) proper motion, and reproduces quite well the geometrical and angular momentum properties of the MW dwarfs, that is, the so-called disc of satellites, also known as the vast polar structure (VPOS). Thirdly, the simulation of the tidal tail reveals one of the formed TDGs with the mass and location almost comparable to those of the LMC. Our present modelling is, however, too limited to study the detailed interaction of gas-rich TDGs with the potential of the MW, and a complementary study is required to test whether the dwarf intrinsic properties can be accounted for by our scenario. Nevertheless this study suggests a causal link between an expected event, an ancient, gas-rich major merger at the M31 location, and several enigmas in the Local Group, namely the GS in the M31 outskirts, the

  4. AGN Absorption Linked to Host Galaxies

    NASA Astrophysics Data System (ADS)

    Juneau, Stéphanie

    2014-07-01

    Multiwavelength identification of AGN is crucial not only to obtain a more complete census, but also to learn about the physical state of the nuclear activity (obscuration, efficiency, etc.). A panchromatic strategy plays an especially important role when the host galaxies are star-forming. Selecting far-Infrared galaxies at 0.3galaxies, indicating a physical link between X-ray absorption and either the gas fraction or the gas geometry in the hosts. These findings have implications for our current understanding of both the AGN unification model and the nature of the black hole-galaxy connection.

  5. An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10.

    PubMed

    Reines, Amy E; Sivakoff, Gregory R; Johnson, Kelsey E; Brogan, Crystal L

    2011-02-03

    Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first 'seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize 2-10 (refs 5 and 6) contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize 2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize 2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.

  6. CHEMICAL ABUNDANCES OF SEVEN IRREGULAR AND THREE TIDAL DWARF GALAXIES IN THE M81 GROUP

    SciTech Connect

    Croxall, Kevin V.; Van Zee, Liese; Lee, Henry; Miller, Bryan W.; Skillman, Evan D.; Lee, Janice C.; Cote, Stephanie; Kennicutt, Robert C. E-mail: vanzee@astro.indiana.ed E-mail: skillman@astro.umn.ed E-mail: stephanie.cote@nrc-cnrc.gc.c E-mail: bmiller@gemini.ed

    2009-11-01

    We have derived nebular abundances for 10 dwarf galaxies belonging to the M81 Group, including several galaxies which do not have abundances previously reported in the literature. For each galaxy, multiple H II regions were observed with GMOS-N at the Gemini Observatory in order to determine abundances of several elements (oxygen, nitrogen, sulfur, neon, and argon). For seven galaxies, at least one H II region had a detection of the temperature sensitive [O III] lambda4363 line, allowing a 'direct' determination of the oxygen abundance. No abundance gradients were detected in the targeted galaxies, and the observed oxygen abundances are typically in agreement with the well-known metallicity-luminosity relation. However, three candidate 'tidal dwarf' galaxies lie well off this relation: UGC 5336, Garland, and KDG 61. The nature of these systems suggests that UGC 5336 and Garland are indeed recently formed systems, whereas KDG 61 is most likely a dwarf spheroidal galaxy which lies along the same line of sight as the M81 tidal debris field. We propose that these H II regions formed from previously enriched gas which was stripped from nearby massive galaxies (e.g., NGC 3077 and M81) during a recent tidal interaction.

  7. Outskirts of Local Group Dwarf Galaxies Revealed by Subaru Hyper Suprime-Cam

    NASA Astrophysics Data System (ADS)

    Komiyama, Yutaka

    2017-03-01

    Local Group galaxies are important targets since their stellar populations can be resolved, and their properties can be investigated in detail with the help of stellar evolutionary models. The newly-built instrument for the 8.2m Subaru Telescope, Hyper Suprime-Cam (HSC), which has a 1 Giga pixel CCD camera with 1.5 degrees field of view, is the best instrument for observing Local Group galaxies. We have carried out a survey for Local Group dwarf galaxies using HSC aiming to shed light on the outskirts of these galaxies. The survey covers target galaxies out beyond the tidal radii down to a depth unexplored by previous surveys. Thanks to the high spatial resolution and high sensitivity provided by the Subaru Telescope, we are able to investigate properties such as spatial distribution and stellar population from the very center of galaxies to the outskirts. In this article, I will show results for the dwarf irregular galaxy NGC 6822 and the dwarf spheroidal galaxy Ursa Minor.

  8. An actively accreting massive black hole in the dwarf starburst galaxy Henize2-10

    NASA Astrophysics Data System (ADS)

    Reines, Amy E.; Sivakoff, Gregory R.; Johnson, Kelsey E.; Brogan, Crystal L.

    2011-02-01

    Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first `seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize2-10 (refs 5 and 6) contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.

  9. Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows.

    PubMed

    Governato, F; Brook, C; Mayer, L; Brooks, A; Rhee, G; Wadsley, J; Jonsson, P; Willman, B; Stinson, G; Quinn, T; Madau, P

    2010-01-14

    For almost two decades the properties of 'dwarf' galaxies have challenged the cold dark matter (CDM) model of galaxy formation. Most observed dwarf galaxies consist of a rotating stellar disk embedded in a massive dark-matter halo with a near-constant-density core. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate. Here we report hydrodynamical simulations (in a framework assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies-bulgeless and with shallow central dark-matter profiles-arise naturally in these simulations.

  10. Dark matter in dwarf irregular galaxies, DDO 210 and DDO 169

    NASA Astrophysics Data System (ADS)

    Iyer, Mansie G.

    It has been proposed that dwarf irregular galaxies can be separated into two classes based on their formation mechanism; that they are the result of the collapse of a primordial gas cloud or that they are the product of condensation of gas in the tidal tails of interacting galaxies. Simulations of galaxy interactions indicate that one can differentiate between these two scenarios by the dark matter content, with a low dark matter content indicating a fossil tidal dwarf. The purpose of this dissertation was to explore the dark matter distribution of two dwarf irregular galaxies using optical and neutral atomic hydrogen data. For DDO 210, the method of mass-modelling was used to determine its dark matter. About 64% of the galaxy mass was calculated to be in the form of dark matter and hence it is unlikely to be a fossil tidal dwarf. The method of mass-modelling could not be used for DDO 169 as the galaxy shows evidence of being tidally disrupted and hence, has a disturbed velocity field. Instead, the suggestion that dark matter might be responsible for a pressure anomaly in DDO 169 was tested to determine its dark matter content. According to this method, a pressure anomaly does exist but without a concrete value for the scale- height, it is unclear whether the anomaly is due to the presence of dark matter. Hence one cannot say how much dark matter might actually be present in DDO 169. A rotation curve would be required to do this.

  11. THE STELLAR STRUCTURE AND KINEMATICS OF DWARF SPHEROIDAL GALAXIES FORMED BY TIDAL STIRRING

    SciTech Connect

    Lokas, Ewa L.; Klimentowski, Jaroslaw; Kazantzidis, Stelios; Mayer, Lucio; Callegari, Simone E-mail: stelios@mps.ohio-state.ed

    2010-01-10

    Using high-resolution N-body simulations, we study the stellar properties of dwarf spheroidal (dSph) galaxies resulting from the tidally induced morphological transformation of disky dwarfs on a cosmologically motivated eccentric orbit around the Milky Way. The dwarf galaxy models initially consist of an exponential stellar disk embedded in an extended spherical dark matter halo. Depending on the initial orientation of the disk with respect to the orbital plane, different final configurations are obtained. The least evolved dwarf is triaxial and retains a significant amount of rotation. The more evolved dwarfs are prolate spheroids with little rotation. We show that in this scenario the final density distribution of stars can be approximated by a simple modification of the Plummer law. The kinematics of the dwarfs is significantly different depending on the line of sight which has important implications for mapping the observed stellar velocity dispersions of dwarfs to subhalo circular velocities. When the dwarfs are observed along the long axis, the measured velocity dispersion is higher and decreases faster with radius. In the case where rotation is significant, when viewed perpendicular to the long axis, the effect of minor axis rotation is detected, as expected for triaxial systems. We model the velocity dispersion profiles and rotation curves of the dwarfs under the assumption of constant mass-to-light ratio by solving the Jeans equations for spherical and axisymmetric systems and adjusting different sets of free parameters, including the total mass. We find that the mass is typically overestimated when the dwarf is seen along the long axis and underestimated when the observation is along the short or intermediate axis. For the studied cases, the effect of non-sphericity cannot, however, bias the inferred mass by more than 60% in either direction, even for the most strongly stripped dwarf which is close to disruption.

  12. Extended stellar substructure surrounding the Boötes I dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Roderick, T. A.; Mackey, A. D.; Jerjen, H.; Da Costa, G. S.

    2016-10-01

    We present deep stellar photometry of the Boötes I dwarf spheroidal galaxy in g- and i-band filters, taken with the Dark Energy Camera at Cerro Tololo in Chile. Our analysis reveals a large, extended region of stellar substructure surrounding the dwarf, as well as a distinct overdensity encroaching on its tidal radius. A radial profile of the Boötes I stellar distribution shows a break radius indicating the presence of extra-tidal stars. These observations strongly suggest that Boötes I is experiencing tidal disruption, although not as extreme as that exhibited by the Hercules dwarf spheroidal. Combined with revised velocity dispersion measurements from the literature, we see evidence suggesting the need to review previous theoretical models of the Boötes I dwarf spheroidal galaxy.

  13. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback.

    PubMed

    Forbes, John C; Krumholz, Mark R; Goldbaum, Nathan J; Dekel, Avishai

    2016-07-28

    Photoelectric heating--heating of dust grains by far-ultraviolet photons--has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity--as is expected with photoelectric heating,but not with supernovae--reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time,suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.

  14. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback

    NASA Astrophysics Data System (ADS)

    Forbes, John C.; Krumholz, Mark R.; Goldbaum, Nathan J.; Dekel, Avishai

    2016-07-01

    Photoelectric heating—heating of dust grains by far-ultraviolet photons—has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity—as is expected with photoelectric heating, but not with supernovae—reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space- and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time, suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.

  15. OBSERVATIONAL CONSTRAINTS ON THE MOLECULAR GAS CONTENT IN NEARBY STARBURST DWARF GALAXIES

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Dalcanton, Julianne J.; Weisz, Daniel R.; Williams, Benjamin F.; Dolphin, Andrew E.; Cannon, John M.; Holtzman, Jon

    2012-06-01

    Using star formation histories derived from optically resolved stellar populations in 19 nearby starburst dwarf galaxies observed with the Hubble Space Telescope, we measure the stellar mass surface densities of stars newly formed in the bursts. By assuming a star formation efficiency (SFE), we then calculate the inferred gas surface densities present at the onset of the starbursts. Assuming an SFE of 1%, as is often assumed in normal star-forming galaxies, and assuming that the gas was purely atomic, translates to very high H I surface densities ({approx}10{sup 2}-10{sup 3} M{sub Sun} pc{sup -2}), which are much higher than have been observed in dwarf galaxies. This implies either higher values of SFE in these dwarf starburst galaxies or the presence of significant amounts of H{sub 2} in dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with observations of more massive starbursts associated with merging galaxies), still results in H I surface densities higher than observed in 10 galaxies. Thus, these observations appear to require that a significant fraction of the gas in these dwarf starbursts galaxies was in the molecular form at the onset of the bursts. Our results imply molecular gas column densities in the range 10{sup 19}-10{sup 21} cm{sup -2} for the sample. In the galaxies where CO observations have been made, these densities correspond to values of the CO-H{sub 2} conversion factor (X{sub CO}) in the range >(3-80) Multiplication-Sign 10{sup 20} cm{sup -2} (K km s{sup -1}){sup -1}, or up to 40 Multiplication-Sign greater than Galactic X{sub CO} values.

  16. DRIVERS OF H I TURBULENCE IN DWARF GALAXIES

    SciTech Connect

    Stilp, Adrienne M.; Dalcanton, Julianne J.; Skillman, Evan; Warren, Steven R.; Ott, Juergen; Koribalski, Baerbel

    2013-08-20

    Neutral hydrogen (H I) velocity dispersions are believed to be set by turbulence in the interstellar medium (ISM). Although turbulence is widely believed to be driven by star formation, recent studies have shown that this driving mechanism may not be dominant in regions of low star formation surface density ({Sigma}{sub SFR}), such those as found in dwarf galaxies or the outer regions of spirals. We have generated average H I line profiles in a number of nearby dwarfs and low-mass spirals by co-adding H I spectra in subregions with either a common radius or {Sigma}{sub SFR}. We find that the individual spatially resolved ''superprofiles'' are composed of a central narrow peak ({approx}5-15 km s{sup -1}) with higher velocity wings to either side, similar to their global counterparts as calculated for the galaxy as a whole. Under the assumption that the central peak reflects the H I turbulent velocity dispersion, we compare measures of H I kinematics determined from the superprofiles to local ISM properties, including surface mass densities and measures of star formation. The shape of the wings of the superprofiles do not show any correlation with local ISM properties, which indicates that they may be an intrinsic feature of H I line-of-sight spectra. On the other hand, the H I velocity dispersion is correlated most strongly with baryonic and H I surface mass density, which points toward a gravitational origin for turbulence, but it is unclear which, if any, gravitational instabilities are able to operate efficiently in these systems. Star formation energy is typically produced at a level sufficient to drive H I turbulent motions at realistic coupling efficiencies in regimes where {Sigma}{sub SFR} {approx}> 10{sup -4} M{sub Sun} yr{sup -1} kpc{sup -2}, as is typically found in inner spiral disks. At low star formation intensities, on the other hand, star formation cannot supply enough energy to drive the observed turbulence, nor does it uniquely determine the

  17. Mass-to-light versus Color Relations for Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Herrmann, Kimberly A.; Hunter, Deidre A.; Zhang, Hong-Xin; Elmegreen, Bruce G.

    2016-12-01

    We have determined new relations between UBV colors and mass-to-light ratios (M/L) for dwarf irregular (dIrr) galaxies, as well as for transformed g‧ - r‧. These M/L to color relations (MLCRs) are based on stellar mass density profiles determined for 34 LITTLE THINGS dwarfs from spectral energy distribution fitting to multi-wavelength surface photometry in passbands from the FUV to the NIR. These relations can be used to determine stellar masses in dIrr galaxies for situations where other determinations of stellar mass are not possible. Our MLCRs are shallower than comparable MLCRs in the literature determined for spiral galaxies. We divided our dwarf data into four metallicity bins and found indications of a steepening of the MLCR with increased oxygen abundance, perhaps due to more line blanketing occurring at higher metallicity.

  18. Prolate rotation and metallicity gradient in the transforming dwarf galaxy Phoenix

    NASA Astrophysics Data System (ADS)

    Kacharov, Nikolay; Battaglia, Giuseppina; Rejkuba, Marina; Cole, Andrew A.; Carrera, Ricardo; Fraternali, Filippo; Wilkinson, Mark I.; Gallart, Carme G.; Irwin, Mike; Tolstoy, Eline

    2017-04-01

    Transition type dwarf galaxies are thought to be systems undergoing the process of transformation from a star-forming into a passively evolving dwarf, which makes them particularly suitable to study evolutionary processes driving the existence of different dwarf morphological types. Here we present results from a spectroscopic survey of ∼200 individual red giant branch stars in the Phoenix dwarf, the closest transition type with a comparable luminosity to 'classical' dwarf galaxies. We measure a systemic heliocentric velocity Vhelio = -21.2 ± 1.0 km s-1. Our survey reveals the clear presence of prolate rotation that is aligned with the peculiar spatial distribution of the youngest stars in Phoenix. We speculate that both features might have arisen from the same event, possibly an accretion of a smaller system. The evolved stellar population of Phoenix is relatively metal-poor (<[Fe/H] > = -1.49 ± 0.04 dex) and shows a large metallicity spread (σ[Fe/H] = 0.51 ± 0.04 dex), with a pronounced metallicity gradient of -0.13 ± 0.01 dex arcmin-1 similar to luminous, passive dwarf galaxies. We also report a discovery of an extremely metal-poor star candidate in Phoenix and discuss the importance of correcting for spatial sampling when interpreting the chemical properties of galaxies with metallicity gradients. This study presents a major leap forward in our knowledge of the internal kinematics of the Phoenix transition type dwarf galaxy and the first wide area spectroscopic survey of its metallicity properties. A table containing the measured velocities, metallicities, and CaT equivalent widths of all spectroscopic targets is available online at the CDS.

  19. Resolving the Tip of the Red Giant Branch of Two New Candidate Local Group Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Tollerud, Erik

    2014-10-01

    We propose to use ACS/WFC to observe two faint dwarf galaxies recently discovered via their HI emission. Based on a blind HI search of 40 HI clumps from 7500 square degrees of the GALFA-HI survey, these two candidates are the only objects with optical counterparts. They show HI and Halpha emission consistent with nearby galaxies, and have blue stars that are barely resolved in ground-based optical imaging with good seeing. These resolved stars are consistent with the galaxies being at Local Group distances. If they are in the Local Group, these galaxies are both less luminous and more compact than the recently-discovered Leo P, also found first with HI observations. They may then also be the faintest known star-forming galaxies. The ground-based imaging leaves large distance uncertainty, however, because the tip of the red giant branch cannot be resolved. We propose one orbit per galaxy of ACS/WFC imaging in F606W and F814W to measure accurate TRGB distances and determine if they truly are Local Group galaxies. If so, these galaxies provide tests on both the efficacy of Lambda CDM in predicting the properties of dwarf galaxies in low density environments, and the lowest-luminosity data points on models of galaxy star formation.

  20. A new investigation on the Antlia Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Piersimoni, A. M.; Bono, G.; Castellani, M.; Marconi, G.; Cassisi, S.; Buonanno, R.; Nonino, M.

    1999-12-01

    We present deep (I,V-I) and (I,B-I) color-magnitude diagrams (CMDs) of the Antlia dwarf galaxy, based on Science Verification (SV) data collected with the FORS I camera on the ESO Very Large Telescope (VLT). The CMDs present two key features: a well-defined Red Giant Branch (RGB), and a sample of bright blue stars, belonging to a young stellar component. The comparison between theory and observations confirms that the bright stars are consistent with the occurrence of a star formation episode ~0.1 Gyr ago. In agreement with Sarajedini et al. (1997) and Aparicio et al. (1997), we find that the young stars are more centrally concentrated than the old ones. By adopting the new calibration of the Tip of the RGB (TRGB) provided by Salaris & Cassisi (1998) we estimated that the Antlia distance modulus is (m-M)_0=25.89+/-0.10 mag, and therefore a distance D=1.51+/- 0.07 Mpc. This distance determination is ~13% larger than the values suggested in previous investigations. We estimated that the mean metallicity of Antlia is of the order of [Fe/H]~-1.3. This estimate is at least 0.3 dex more metal-rich than similar evaluations available in the literature. The disagreement both in the distance and in the metallicity determinations are mainly due to difference in the calibration of the TRGB method and in the (V-I) color index vs. metallicity relation. The differential RGB luminosity function shows an excess in the observed counts -at the 2sigma level- when compared with theoretical predictions. This discrepancy might be due to a stellar component with an age approximately equal to 0.7 Gyr. Based on observations collected with VLT-UT1 telescope of ESO in Paranal, during the Science Verification Program

  1. Is the Large Magellanic Cloud Microlensing Due to an Intervening Dwarf Galaxy?

    NASA Astrophysics Data System (ADS)

    Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A. C.; Bennett, D. P.; Cook, K. H.; Freeman, K. C.; Griest, K.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Peterson, B. A.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Rorabeck, A.; Stubbs, C. W.; Sutherland, W.; Tomaney, A. B.; Vandehei, T.; Welch, D. L.

    1997-11-01

    The recent suggestion that the microlensing events observed toward the Large Magellanic Cloud are due to an intervening Sgr-like dwarf galaxy is examined. A search for foreground RR Lyrae in the MACHO photometry database yields 20 stars whose distance distribution follow the expected halo density profile. Cepheid and red giant branch clump stars in the MACHO database are consistent with membership in the LMC. There is also no evidence in the literature for a distinct kinematic population, for intervening gas, or for the turnoff of such a hypothetical galaxy. We conclude that if the lenses are in a foreground galaxy, it must be a particularly dark galaxy.

  2. Cores in dwarf galaxies from dark matter with a Yukawa potential.

    PubMed

    Loeb, Abraham; Weiner, Neal

    2011-04-29

    We show that cold dark matter particles interacting through a Yukawa potential could naturally explain the recently observed cores in dwarf galaxies without affecting the dynamics of objects with a much larger velocity dispersion, such as clusters of galaxies. The velocity dependence of the associated cross section as well as the possible exothermic nature of the interaction alleviates earlier concerns about strongly interacting dark matter. Dark matter evaporation in low-mass objects might explain the observed deficit of satellite galaxies in the Milky Way halo and have important implications for the first galaxies and reionization.

  3. A VLT/FORS2 spectroscopic survey of individual stars in a transforming dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Battaglia, G.; Kacharov, N.; Rejkuba, M.

    2017-03-01

    Understanding the properties of dwarf galaxies is important not only to put them in their proper cosmological context, but also to understand the formation and evolution of the most common type of galaxies. Dwarf galaxies are divided into two main classes, dwarf irregulars (dIrrs) and dwarf spheroidals (dSphs), which differ from each other mainly because the former are gas-rich objects currently forming stars, while the latter are gas-deficient with no on-going star formation. Transition types (dT) are thought to represent dIs in the process of losing their gas, and can therefore shed light into the possible process of dwarf irregulars (dIrrs) becoming gas-deficient, passively evolving galaxies. Here we present preliminary results from our wide-area VLT/FORS2 MXU spectroscopic survey of the Phoenix dT, from which we obtained line-of-sight velocities and metallicities from the nIR Ca II triplet lines for a large sample of individual Red Giant Branch stars.

  4. Globular Cluster Luminosity Functions and Specific Frequencies in Dwarf Elliptical Galaxies

    NASA Astrophysics Data System (ADS)

    Miller, B. W.; Lotz, J. M.

    2005-12-01

    We present the final results on the globular cluster luminosity functions (GCLFs) and specific frequencies (SN) from 69 dwarf elliptical galaxies in the HST Dwarf Elliptical Galaxy Snapshot Survey (Lotz et al. 2004). The GCLFs for the Virgo and Fornax clusters are well fit by a t5 function with a peak at MV0=-7.25 ± 0.2 and an equivalent Gaussian sigma of 1.2 magnitudes. These values are very similar to those of globular clusters systems in giant elliptical galaxies. We also confirm our previous results (Miller et al. 1998) that SN in nucleated dwarfs is about a factor of two higher than in non-nucleated dwarfs. We also discuss the fraction of the stellar mass in dwarf elliptical galaxies that is currently found in globular clusters. Supported by the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., on behalf of the international Gemini partnership of Argentina, Australia, Brazil, Canada, Chile, the United Kingdom, and the United States of America.

  5. Failures no More: The Radical Consequences of Realistic Stellar Feedback for Dwarf Galaxies, the Milky Way, and Reionization

    NASA Astrophysics Data System (ADS)

    Hopkins, Philip F.

    2016-06-01

    Many of the most fundamental unsolved questions in star and galaxy formation revolve around star formation and "feedback" from massive stars, in-extricably linking galaxy formation and stellar evolution. I'll present simulations with un-precedented resolution of Milky-Way (MW) mass galaxies, followed cosmologically to redshift zero. For the first time, these simulations resolve the internal structure of small dwarf satellites around a MW-like host, with detailed models for stellar evolution including radiation pressure, supernovae, stellar winds, and photo-heating. I'll show that, without fine-tuning, these feedback processes naturally resolve the "missing satellites," "too big to fail," and "cusp-core" problems, and produce realistic galaxy populations. At high redshifts however, the realistic ISM structure predicted, coupled to standard stellar population models, naively leads to the prediction that only ~1-2% of ionizing photons can ever escape galaxies, insufficient to ionize the Universe. But these models assume all stars are single: if we account for binary evolution, the escape fraction increases dramatically to ~20% for the small, low-metallicity galaxies believed to ionize the Universe.

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

    SciTech Connect

    Chiboucas, Kristin; Tully, R. Brent; Marzke, Ronald O.; Trentham, Neil; Ferguson, Henry C.; Hammer, Derek; Carter, David; Khosroshahi, Habib E-mail: tully@ifa.hawaii.ed E-mail: trentham@ast.cam.ac.u E-mail: hammerd@pha.jhu.ed E-mail: habib@ipm.i

    2010-11-01

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

  7. Probing the low surface brightness dwarf galaxy population of the virgo cluster

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    We have used data from the Next Generation Virgo Survey to investigate the dwarf galaxy population of the Virgo cluster. We mask and smooth the data, and then use the object detection algorithm SEXTRACTOR to make our initial dwarf galaxy selection. All candidates are then visually inspected to remove artefacts and duplicates. We derive parameters to best select low surface brightness galaxies using central surface brightness values of 22.5 ≤ μ g0 ≤ 26.0 μg and exponential scale lengths of 3.0 ≤ h ≤ 10.0 arcsec to identify 443 cluster dwarf galaxies - 303 of which are new detections, with a surface density that decreases with radius from the cluster centre. We also apply our selection algorithm to `background', non-cluster, fields and find zero detections. In combination, this leads us to believe that we have isolated a cluster dwarf galaxy population. The range of objects we detect is limited because smaller scale sized galaxies are confused with the background, while larger galaxies are split into numerous smaller objects by the detection algorithm. Combining our data with that from other surveys, we find a faint-end slope to the luminosity function of -1.35 ± 0.03, which is not significantly different to what has previously been found, but is a little steeper than the slope for field galaxies. There is no evidence for a faint-end slope steep enough to correspond with galaxy formation models, unless those models invoke either strong feedback processes or use warm dark matter.

  8. Midlife Crises in Dwarf Galaxies in the NGC 5353/4 Group

    NASA Astrophysics Data System (ADS)

    Tully, R. Brent; Trentham, Neil

    2008-04-01

    This third paper in a series about the dwarf galaxy populations in groups within the Local Supercluster concerns the intermediate mass (2.1 × 1013 M sun) NGC 5353/4 Group with a core dominated by S0 systems and a periphery of mostly spiral systems. Dwarf galaxies are strongly concentrated toward the core. The mass-to-light ratio M/LR = 105 M sun/L sun is a factor of 3 lower than for the two groups studied earlier in the series. The properties of the group suggest it is much less dynamically evolved than those two groups of early-type galaxies. By comparison, the NGC 5353/4 Group lacks superluminous systems but has a large fraction of intermediate-luminosity galaxies; or equivalently, a luminosity function with a flatter faint-end slope. The luminosity function for the NGC 5353/4 Group should steepen as the intermediate-luminosity galaxies merge. Evidence for the ongoing collapse of the group is provided by the unusually large incidence of star-formation activity in small galaxies with early morphological types. The pattern in the distribution of galaxies with activity suggests a succession of infall events. Residual gas in dwarfs that enter the group is used up in sputtering events. The resolution of midlife crises is exhaustion.

  9. The influence of the merger history of dwarf galaxies in a reionized universe

    NASA Astrophysics Data System (ADS)

    Verbeke, Robbert; Vandenbroucke, Bert; De Rijcke, Sven; Koleva, Mina

    2015-08-01

    In the ΛCDM model, cosmic structure forms in a hierarchical fashion. According to this paradigm, even low-mass dwarf galaxies grow via smooth accretion and mergers. Given the low masses of dwarf galaxies and their even smaller progenitors, the UV background is expected to have a significant influence on their gas content and, consequently, their star formation histories. Generally, cosmological simulations predict that most dwarf systems with circular velocities below ~30 km/s should not be able to form significant amounts of stars or contain gas and be, in effect, "dark" galaxies (Sawala et al. 2013, 2014; Hopkins et al. 2014; Shen et al. 2014). This is in contradiction with the recent discovery of low-mass yet gas-rich dwarf galaxies, such as Leo P (Skillman et al. 2013), Pisces A (Tollerud et al. 2014), and SECCO 1 (Bellazzini et al. 2015). Moreover, Tollerud et al. (2014) point out that most isolated dark-matter halos down to circular velocities of ~15 km/s contain neutral gas, in contradiction with the predictions of current simulations.Based on a suite of simulations of the formation and evolution of dwarf galaxies we show that, by reducing the first peak of star formation by including Pop-III stars in the simulations, the resulting dwarf galaxies have severely suppressed SFRs and can hold on to their gas reservoirs. Moreover, we show that the majority of the zero-metallicity stars are ejected during mergers, resulting in an extended, low-metallicity stellar halo. This results in a marked difference between a galaxy's "total" star-formation history and the one read from the stars in the center of the galaxy at z=0. This mechanism leads to the formation of realistic low-mass, gas-rich dwarfs with a broad range of SFHs and which adhere to the observed scaling relations, such as the baryonic Tully-Fisher relation.In short, the simulations presented here are for the first time able to reproduce the observed properties of low-mass, gas-rich dwarfs such as DDO 210

  10. R-process enrichment from a single event in an ancient dwarf galaxy.

    PubMed

    Ji, Alexander P; Frebel, Anna; Chiti, Anirudh; Simon, Joshua D

    2016-03-31

    Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae. But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process. Reticulum II is one such galaxy. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this 'r-process galaxy' is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy. This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae, but consistent with other possible sources, such as neutron star mergers.

  11. Surface Brightness Profiles of Dwarf Galaxies. II. Color Trends and Mass Profiles

    NASA Astrophysics Data System (ADS)

    Herrmann, Kimberly A.; Hunter, Deidre A.; Elmegreen, Bruce G.

    2016-06-01

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

  12. ALFALFA DISCOVERY OF THE NEARBY GAS-RICH DWARF GALAXY LEO P. III. AN EXTREMELY METAL DEFICIENT GALAXY

    SciTech Connect

    Skillman, Evan D.; Berg, Danielle A.; Olive, Keith A.; McQuinn, Kristen B. W. E-mail: berg@astro.umn.edu E-mail: kmcquinn@astro.umn.edu; and others

    2013-07-01

    We present KPNO 4 m and LBT/MODS spectroscopic observations of an H II region in the nearby dwarf irregular galaxy Leo P discovered recently in the Arecibo ALFALFA survey. In both observations, we are able to accurately measure the temperature sensitive [O III] {lambda}4363 line and determine a ''direct'' oxygen abundance of 12 + log(O/H) = 7.17 {+-} 0.04. Thus, Leo P is an extremely metal deficient (XMD) galaxy, and, indeed, one of the most metal deficient star-forming galaxies ever observed. For its estimated luminosity, Leo P is consistent with the relationship between luminosity and oxygen abundance seen in nearby dwarf galaxies. Leo P shows normal {alpha} element abundance ratios (Ne/O, S/O, and Ar/O) when compared to other XMD galaxies, but elevated N/O, consistent with the ''delayed release'' hypothesis for N/O abundances. We derive a helium mass fraction of 0.2509{sup +0.0184}{sub -0.0123}, which compares well with the WMAP + BBN prediction of 0.2483 {+-} 0.0002 for the primordial helium abundance. We suggest that surveys of very low mass galaxies compete well with emission line galaxy surveys for finding XMD galaxies. It is possible that XMD galaxies may be divided into two classes: the relatively rare XMD emission line galaxies which are associated with starbursts triggered by infall of low-metallicity gas and the more common, relatively quiescent XMD galaxies like Leo P, with very low chemical abundances due to their intrinsically small masses.

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

    SciTech Connect

    Verbeke, R.; Vandenbroucke, B.; Rijcke, S. De

    2015-12-20

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

  14. COMPARING THE OBSERVABLE PROPERTIES OF DWARF GALAXIES ON AND OFF THE ANDROMEDA PLANE

    SciTech Connect

    Collins, Michelle L. M.; Martin, Nicolas F.; Rich, R. M.; Ibata, Rodrigo A.; Chapman, Scott C.; McConnachie, Alan W.; Ferguson, Annette M.; Irwin, Michael J.; Lewis, Geraint F.

    2015-01-20

    The thin, extended planes of satellite galaxies detected around both the Milky Way and Andromeda are not a natural prediction of the Λ-cold dark matter paradigm. Galaxies in these distinct planes may have formed and evolved in a different way (e.g., tidally) from their off-plane neighbors. If this were the case, one would expect the on- and off-plane dwarf galaxies in Andromeda to have experienced different evolutionary histories, which should be reflected by the chemistries, dynamics, and star formation histories of the two populations. In this work, we present new, robust kinematic observations for two on-plane M31 dwarf spheroidal galaxies (And XVI and XVII) and compile and compare all available observational metrics for the on- and off-plane dwarfs to search for a signal that would corroborate such a hypothesis. We find that, barring their spatial alignment, the on- and off-plane Andromeda dwarf galaxies are indistinguishable from one another, arguing against vastly different formative and evolutionary histories for these two populations.

  15. The remnant of a merger between two dwarf galaxies in Andromeda II.

    PubMed

    Amorisco, N C; Evans, N W; van de Ven, G

    2014-03-20

    Driven by gravity, massive structures like galaxies and clusters of galaxies are believed to grow continuously through hierarchical merging and accretion of smaller systems. Observational evidence of accretion events is provided by the coherent stellar streams crossing the outer haloes of massive galaxies, such as the Milky Way or Andromeda. At similar mass scales, around 10(11) solar masses in stars, further evidence of merging activity is also ample. Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation, but have hitherto not been seen for galaxies with less than about 10(9) solar masses in stars. Here we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal Andromeda II, which has a mass of only 10(7) solar masses in stars. The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a drastic influence on the dynamics of the remnant, which is now rotating around its projected major axis. The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.

  16. Made-to-measure dark matter haloes, elliptical galaxies and dwarf galaxies in action coordinates

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    We provide a family of action-based distribution functions (DFs) for the double power-law family of densities often used to model galaxies. The DF itself is a double power law in combinations of the actions, and reduces to the pure power-law case at small and large radii. Our method enables the velocity anisotropy of the model to be tuned, and so the anisotropy in the inner and outer parts can be specified for the application in hand. We provide self-consistent DFs for the Hernquist and Jaffe models - both with everywhere isotropic velocity dispersions, and with kinematics that gradually become more radially anisotropic on moving outwards. We also carry out this exercise for a cored dark matter model. These are tailored to represent dark haloes and elliptical galaxies, respectively, with kinematic properties inferred from simulations or observational data. Finally, we relax a cored luminous component within a dark matter halo to provide a self-consistent model of a dwarf spheroidal embedded in dark matter. The DFs provide us with non-rotating spherical stellar systems, but one of the virtues of working with actions is the relative ease with which such models can be converted into axisymmetry and triaxiality.

  17. Environmental effects on stellar populations of star clusters and dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Pasetto, Stefano; Cropper, Mark; Fujita, Yutaka; Chiosi, Cesare; Grebel, Eva K.

    2017-03-01

    We investigate the competitive role of the different dissipative phenomena acting on the onset of star formation of gravitationally bound systems in an external environment. Ram pressure, Kelvin-Helmholtz and Rayleigh-Taylor instabilities, and tidal forces are accounted for separately in an analytical framework and compared in their role in influencing the star forming regions. We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system on its surrounding environment. We consider the different signatures of these phenomena in synthetically realized colour-magnitude diagrams (CMDs) of the orbiting system thus investigating the detectability limits of these different effects for future observational projects and their relevance. The developed theoretical framework has direct applications to the cases of massive star clusters, dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy.

  18. Galaxy Populations and Evolution in Clusters IV Deep H 1 Observations of Dwarf Elliptical Galaxies in the Virgo Cluster

    NASA Technical Reports Server (NTRS)

    Conselice, Christopher J.; ONeil, Karen; Gallagher, John S.; Wyse, Rosemary F. G.

    2003-01-01

    In this paper we present deep Arecibo H I and WIYN optical observations of Virgo Cluster dwarf elliptical galaxies. Based on this data we argue that a significant fraction of low-mass galaxies in the Virgo Cluster recently underwent evolution. Our new observations consist of H I 21 cm line observations for 22 classified dE galaxies with optical radial velocities consistent with membership in the Virgo Cluster. Cluster members VCC 390 and VCC 1713 are detected with H 1 masses M H1= 6 x 10 sup 7 and 8 x 10 sup 7 M , respectively, while MH I values in the remaining 20 dE galaxies have upper limits as low as about 5 x 1O sup 5 M. We combine our results with those for 26 other Virgo Cluster dE galaxies with H 1 observations in the literature, seven of which have H I detection claims.

  19. The frequency and properties of young tidal dwarf galaxies in nearby gas-rich groups

    NASA Astrophysics Data System (ADS)

    Lee-Waddell, K.; Spekkens, K.; Chandra, P.; Patra, N.; Cuillandre, J.-C.; Wang, J.; Haynes, M. P.; Cannon, J.; Stierwalt, S.; Sick, J.; Giovanelli, R.

    2016-08-01

    We present high-resolution Giant Metrewave Radio Telescope (GMRT) H I observations and deep Canada-France-Hawaii Telescope (CFHT) optical imaging of two galaxy groups: NGC 4725/47 and NGC 3166/9. These data are part of a multi-wavelength unbiased survey of the gas-rich dwarf galaxy populations in three nearby interacting galaxy groups. The NGC 4725/47 group hosts two tidal knots and one dwarf irregular galaxy (dIrr). Both tidal knots are located within a prominent H I tidal tail, appear to have sufficient mass (Mgas ≈ 108 M⊙) to evolve into long-lived tidal dwarf galaxies (TDGs) and are fairly young in age. The NGC 3166/9 group contains a TDG candidate, AGC 208457, at least three dIrrs and four H I knots. Deep CFHT imaging confirms that the optical component of AGC 208457 is bluer - with a 0.28 mag g - r colour - and a few Gyr younger than its purported parent galaxies. Combining the results for these groups with those from the NGC 871/6/7 group reported earlier, we find that the H I properties, estimated stellar ages and baryonic content of the gas-rich dwarfs clearly distinguish tidal features from their classical counterparts. We optimistically identify four potentially long-lived tidal objects associated with three separate pairs of interacting galaxies, implying that TDGs are not readily produced during interaction events as suggested by some recent simulations. The tidal objects examined in this survey also appear to have a wider variety of properties than TDGs of similar mass formed in current simulations of interacting galaxies, which could be the result of pre- or post-formation environmental influences.

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

    SciTech Connect

    Monachesi, Antonela; Bell, Eric F.; Radburn-Smith, David J.; Dalcanton, Julianne J.; De Jong, Roelof S.; Streich, David; Vlajić, Marija; Bailin, Jeremy; Holwerda, Benne W.; Alyson Ford, H.; Zucker, Daniel B.

    2014-01-10

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

  1. HOW TO MAKE AN ULTRA-FAINT DWARF SPHEROIDAL GALAXY: TIDAL STIRRING OF DISKY DWARFS WITH SHALLOW DARK MATTER DENSITY PROFILES

    SciTech Connect

    Lokas, Ewa L.; Kazantzidis, Stelios; Mayer, Lucio E-mail: stelios@astronomy.ohio-state.edu

    2012-05-20

    In recent years the Sloan Digital Sky Survey has unraveled a new population of ultra-faint dwarf galaxies (UFDs) whose origin remains a puzzle in the vicinity of the Milky Way (MW). Using a suite of collisionless N-body simulations, we investigate the formation of UFDs in the context of the tidal stirring model for the formation of dwarf spheroidal galaxies in the Local Group (LG). Our simulations are designed to reproduce the tidal interactions between MW-sized host galaxies and rotationally supported dwarfs embedded in 10{sup 9} M{sub Sun} dark matter (DM) halos. We explore a variety of inner density slopes {rho}{proportional_to}r{sup -{alpha}} for the dwarf DM halos, ranging from core-like ({alpha} = 0.2) to cuspy ({alpha} = 1), and different dwarf orbital configurations. Our experiments demonstrate that UFDs can be produced via tidal stirring of disky dwarfs on relatively tight orbits, consistent with a redshift of accretion by the host galaxy of z {approx} 1, and with intermediate values for the halo inner density slopes ({rho}{proportional_to}r{sup -0.6}). The inferred slopes are in excellent agreement with those resulting from both the modeling of the rotation curves of dwarf galaxies and recent cosmological simulations of dwarf galaxy formation. Comparing the properties of observed UFDs with those of their simulated counterparts, we find remarkable similarities in terms of basic observational parameters. We conclude that tidal stirring of rotationally supported dwarfs represents a viable mechanism for the formation of UFDs in the LG environment.

  2. Semi-analytic models for HI gas in disk and local dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Fu, Jian

    2015-08-01

    We construct the radially-resolved semi-analytic models of galaxy formation based on the L-Galaxies model framework, which include both atomic and molecular gas phase in ISM. The models adopt the ΛCDM cosmology simulation Millennium, Millennium II and Aquarius. Our models can reproduce varies properties of HI gas in nearby galaxies, e.g. the HI mass function, the HI-to-star ratio vs stellar mass and stellar surface density, universal HI radial surface density profile in outer disks etc. We can also give some physical origins of HI size mass relation in many observations.Based on our model results for local dwarf galaxies, we show that the "missing satellite problem" also exists in the HI component, i.e., the models over predict dwarf galaxies with low HI mass. That is a shortcoming of current ΛCDM cosmology framework. Future survey for HI gas in dwarf galaxies (e.g. SKA or FAST) in local group can help to verify the correctness of cold dark matter.

  3. Photometry of the globular cluster system of the Sagittarius dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Ibata, Rodrigo

    1996-07-01

    We propose to use the WFPC2 to obtain deep V {F555W} and I {F814W} band images of the globular clusters of the Sagittarius dwarf galaxy: M54 {NGC 6715}, Ter 7, Ter 8 and Arp 2. Isochrone fits to these data will yield the accurate ages of M54, Ter 8 and the Sagittarius dwarf galaxy {Sgr}. These age estimates are needed to model the dynamics and evolution of the Sgr system and its interaction with the Milky Way. Sgr is in the final stages of tidal disruption {Velazquez & White 1995}, so its globular clusters will drift away from their parent galaxy and soon disperse into the Galactic halo. Therefore at least some of the globular clusters of giant spiral galaxies like the Milky Way will have been captured from tidally destroyed dwarf galaxies. This statement has fundamental and wide ranging implications for the formation and evolution of globular clusters and spiral galaxies. It is therefore essential to conduct a detailed and accurate comparative study of the stellar populations of the Sgr and Galactic globular clusters, as we propose here. This, in turn, will allow detailed modeling of the chemical and dynamical evolution of Sgr and its globular clusters.

  4. Improving the sensitivity of gamma-ray telescopes to dark matter annihilation in dwarf spheroidal galaxies

    SciTech Connect

    Carlson, Eric; Hooper, Dan; Linden, Tim

    2015-03-01

    The Fermi-LAT Collaboration has studied the gamma-ray emission from a stacked population of dwarf spheroidal galaxies and used this information to set constraints on the dark matter annihilation cross section. Interestingly, their analysis uncovered an excess with a test statistic (TS) of 8.7. If interpreted naively, this constitutes a 2.95σ local excess (p-value=0.003), relative to the expectations of their background model. In order to further test this interpretation, the Fermi-LAT team studied a large number of blank sky locations and found TS>8.7 excesses to be more common than predicted by their background model, decreasing the significance of their dwarf excess to 2.2σ(p-value=0.027). We argue that these TS>8.7 blank sky locations are largely the result of unresolved blazars, radio galaxies, and star-forming galaxies, and show that multiwavelength information can be used to reduce the degree to which such sources contaminate the otherwise blank sky. In particular, we show that masking regions of the sky that lie within 1° of sources contained in the BZCAT or CRATES catalogs reduce the fraction of blank sky locations with TS>8.7 by more than a factor of 2. Taking such multiwavelength information into account can enable experiments such as Fermi to better characterize their backgrounds and increase their sensitivity to dark matter in dwarf galaxies, the most important of which remain largely uncontaminated by unresolved point sources. We also note that for the range of dark matter masses and annihilation cross sections currently being tested by studies of dwarf spheroidal galaxies, simulations predict that Fermi should be able to detect a significant number of dark matter subhalos. These subhalos constitute a population of subthreshold gamma-ray point sources and represent an irreducible background for searches for dark matter annihilation in dwarf galaxies.

  5. r-process Production Sites as Inferred from Eu Abundances in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Beniamini, Paz; Hotokezaka, Kenta; Piran, Tsvi

    2016-12-01

    Recent observations of r-process material in ultrafaint dwarf galaxies (UFDs) shed light on the sources of these elements. Strong upper limits on the Eu mass in some UFDs, combined with detections of much larger masses in a UFD, Reticulum II, and other dwarf galaxies, imply that Eu production is dominated by rare events, and that the minimal Eu mass observed in any UFD is approximately the amount of Eu mass produced per event. This is consistent with other independent observations in the Galaxy. We estimate, using a model-independent likelihood analysis, the rate and Eu (Fe) mass produced per r-process (Fe production) event in dwarf galaxies, including classical dwarfs and UFDs. The mass and rate of the Fe production events are consistent with the normal core-collapse supernova (CCSN) scenario. The Eu mass per event is 3× {10}-5 {M}⊙ \\lt {\\tilde{m}}{Eu}\\lt 2× {10}-4 {M}⊙ , corresponding to a total r-process mass per event of 6× {10}-3 {M}⊙ \\lt {\\tilde{m}}r {- {process}}\\lt 4× {10}-2 {M}⊙ . The rate of r-process events is 2.5 × 10-4 < R rp/SN < 1.4 × 10-3 as compared with the CCSN rate. These values are consistent with the total Eu mass observed in our own Galaxy, suggesting that the same mechanism is behind the production of r-process events in both dwarf galaxies and the Milky Way, and that it may be the dominant mechanism for production of r-process elements in the universe. The results are consistent with neutron star merger estimates but cannot rule out other rare core-collapse scenarios, provided that they produce significant amounts of r-process material per event.

  6. The G-dwarf problem in the Galaxy

    NASA Astrophysics Data System (ADS)

    Caimmi, R.

    2008-07-01

    This paper has two parts: one about observational constraints, and the other about chemical evolution models. In the first part, the empirical differential metallicity distribution (EDMD) is deduced from three different samples involving (i) local thick disk stars derived from Gliese and scaled in situ samples within the range, -1.20⩽[Fe/H]⩽-0.20 [Wyse, R.F.G., Gilmore, G., 1995. AJ 110, 2771]; (ii) 46 likely metal-weak thick disk stars within the range, -2.20⩽[Fe/H]⩽-1.00 [Chiba, M., Beers, T.C., 2000. AJ 119, 2843]; (iii) 287 chemically selected G dwarfs within 25 pc from the Sun, with the corrections performed in order to take into account the stellar scale height [Rocha-Pinto, H.J., Maciel, W.J., 1996. MNRAS 279, 447]; in addition to previous results [Caimmi, R., 2001b. AN 322, 241; Caimmi, R., 2007. NewA 12, 289] related to (iv) 372 solar neighbourhood halo subdwarfs [Ryan, S.G., Norris, J.E., 1991. AJ 101, 1865]; and (v) 268 K-giant bulge stars [Sadler, E.M., Rich, R.M., Terndrup, D.M., 1996. AJ 112, 171]. The metal-poor and metal-rich EDMD related to the thick disk shows similarities with their halo and bulge counterparts, respectively. Then the thick disk is conceived as made of two distinct regions: the halo-like and the bulge-like thick disk, and the related EDMD is deduced. Under the assumption that each distribution is typical for the corresponding subsystem, the EDMD of the thick disk, the thick + thin disk, and the Galaxy, is determined by weighting the mass. In the second part, models of chemical evolution for the halo-like thick disk, the bulge-like thick disk, and the thin disk, are computed assuming the instantaneous recycling approximation. The EDMD data are fitted, to an acceptable extent, by simple models of chemical evolution implying both homogeneous and inhomogeneous star formation, provided that star formation is inhibited during thick disk formation, with respect to the thin disk. The initial mass function (IMF) is assumed to be a

  7. Stellar hydrodynamical modeling of dwarf galaxies: simulation methodology, tests, and first results

    NASA Astrophysics Data System (ADS)

    Vorobyov, Eduard I.; Recchi, Simone; Hensler, Gerhard

    2015-07-01

    Context. In spite of enormous progress and brilliant achievements in cosmological simulations, they still lack numerical resolution or physical processes to simulate dwarf galaxies in sufficient detail. Accurate numerical simulations of individual dwarf galaxies are thus still in demand. Aims: We aim to improve available numerical techniques to simulate individual dwarf galaxies. In particular, we aim to (i) study in detail the coupling between stars and gas in a galaxy, exploiting the so-called stellar hydrodynamical approach; and (ii) study for the first time the chemodynamical evolution of individual galaxies starting from self-consistently calculated initial gas distributions. Methods: We present a novel chemodynamical code for studying the evolution of individual dwarf galaxies. In this code, the dynamics of gas is computed using the usual hydrodynamics equations, while the dynamics of stars is described by the stellar hydrodynamics approach, which solves for the first three moments of the collisionless Boltzmann equation. The feedback from stellar winds and dying stars is followed in detail. In particular, a novel and detailed approach has been developed to trace the aging of various stellar populations, which facilitates an accurate calculation of the stellar feedback depending on the stellar age. The code has been accurately benchmarked, allowing us to provide a recipe for improving the code performance on the Sedov test problem. Results: We build initial equilibrium models of dwarf galaxies that take gas self-gravity into account and present different levels of rotational support. Models with high rotational support (and hence high degrees of flattening) develop prominent bipolar outflows; a newly-born stellar population in these models is preferentially concentrated to the galactic midplane. Models with little rotational support blow away a large fraction of the gas and the resulting stellar distribution is extended and diffuse. Models that start from non

  8. THE ACS NEARBY GALAXY SURVEY TREASURY. X. QUANTIFYING THE STAR CLUSTER FORMATION EFFICIENCY OF NEARBY DWARF GALAXIES

    SciTech Connect

    Cook, David O.; Dale, Daniel A.; Seth, Anil C.; Johnson, L. Clifton; Weisz, Daniel R.; Fouesneau, Morgan; Dalcanton, Julianne J.; Olsen, Knut A. G.; Engelbracht, Charles W.

    2012-06-01

    We study the relationship between the field star formation and cluster formation properties in a large sample of nearby dwarf galaxies. We use optical data from the Hubble Space Telescope and from ground-based telescopes to derive the ages and masses of the young (t{sub age} {approx}< 100 Myr) cluster sample. Our data provide the first constraints on two proposed relationships between the star formation rate (SFR) of galaxies and the properties of their cluster systems in the low SFR regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data, suggesting that there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models.

  9. The Herschel Virgo Cluster Survey . VII. Dust in cluster dwarf elliptical galaxies

    NASA Astrophysics Data System (ADS)

    De Looze, I.; Baes, M.; Zibetti, S.; Fritz, J.; Cortese, L.; Davies, J. I.; Verstappen, J.; Bendo, G. J.; Bianchi, S.; Clemens, M.; Bomans, D. J.; Boselli, A.; Corbelli, E.; Dariush, A.; di Serego Alighieri, S.; Fadda, D.; Garcia-Appadoo, D. A.; Gavazzi, G.; Giovanardi, C.; Grossi, M.; Hughes, T. M.; Hunt, L. K.; Jones, A. P.; Madden, S.; Pierini, D.; Pohlen, M.; Sabatini, S.; Smith, M. W. L.; Vlahakis, C.; Xilouris, E. M.

    2010-07-01

    We use the science demonstration phase data of the Herschel Virgo Cluster Survey to search for dust emission of early-type dwarf galaxies in the central regions of the Virgo cluster as an alternative way of identifying the interstellar medium. We present the first possible far-infrared detection of cluster early-type dwarf galaxies: VCC 781 and VCC 951 are detected at the 10σ level in the SPIRE 250 μm image. Both detected galaxies have dust masses of the order of 105 M_⊙ and average dust temperatures ≈20 K. The detection rate (less than 1%) is quite high compared to the 1.7% detection rate for Hi emission, considering that dwarfs in the central regions are more Hi deficient. We conclude that the removal of interstellar dust from dwarf galaxies resulting from ram pressure stripping, harassment, or tidal effects must be as efficient as the removal of interstellar gas. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

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

    PubMed

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

    2014-09-18

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

  11. Constraints on MACHO Dark Matter from Compact Stellar Systems in Ultra-Faint Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Brandt, Timothy

    2017-01-01

    I show that a recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive compact halo objects (MACHOs) of >5 M⊙ as the main component of dark matter. MACHO dark matter will dynamically heat the cluster, driving it to larger sizes and higher velocity dispersions until it dissolves into its host galaxy. The stars in compact ultra-faint dwarf galaxies themselves will be subject to the same dynamical heating; the survival of at least ten such galaxies places independent limits on MACHO dark matter of masses >10 M⊙. Both Eri II's cluster and the compact ultra-faint dwarfs are characterized by stellar masses of just a few thousand M⊙ and half-light radii of 13 pc (for the cluster) and 30 pc (for the ultra-faint dwarfs). These systems close the 20 -100 M⊙ window of allowed MACHO dark matter and combine with existing constraints from microlensing, wide binaries, and disk kinematics to rule out dark matter composed entirely of MACHOs from 10-7 M⊙ up to arbitrarily high masses. NASA Sagan Fellow.

  12. A Model for Gas Dynamics and Chemical Evolution of the Fornax Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Yuan, Zhen

    We present an empirical model for the halo evolution, global gas dynamics and chemical evolution of Fornax, the brightest Milky Way (MW) dwarf spheroidal galaxy (dSph). Assuming a global star formation rate psi(t) = lambda*(t)[Mg( t)/M[solar masses

  13. Dwarf galaxies in the Local Group: cornerstones for stellar astrophysics and cosmology

    NASA Astrophysics Data System (ADS)

    Bono, G.; Stetson, P. B.; Monelli, M.; Fabrizio, M.; Sanna, N.; Nonino, M.; Walker, A. R.; Bresolin, F.; Buonanno, R.; Caputo, F.; Castellani, M.; Corsi, C. E.; Dall'Ora, M.; Ferraro, I.; François, P.; Iannicola, G.; Matsunaga, M.; Pulone, L.; Romaniello, M.; Storm, J.; Thévenin, F.

    2010-12-01

    Dwarf galaxies have been the crossroad of significant theoretical and observational efforts, but we still lack firm constraints concerning their formation and evolution. They are also fundamental laboratories to investigate the impact of the environment on star formation and on chemical evolution in stellar systems that are order of magnitudes smaller than giant galaxies. We present some recent results concerning the dwarf spheroidal Carina and the dwarf irregular IC10. In particular, we focus our attention on the evolutionary properties of their stellar populations using accurate and deep color-magnitude diagrams. We also briefly discuss the impact that the transition from old, low-mass (horizontal branch) to intermediate-age (red clump) helium burning stars has in constraining the star formation history of complex stellar systems.

  14. A CANDIDATE MASSIVE BLACK HOLE IN THE LOW-METALLICITY DWARF GALAXY PAIR MRK 709

    SciTech Connect

    Reines, Amy E.; Condon, James J.; Plotkin, Richard M.; Russell, Thomas D.; Mezcua, Mar; Sivakoff, Gregory R.; Johnson, Kelsey E.

    2014-06-01

    The incidence and properties of present-day dwarf galaxies hosting massive black holes (BHs) can provide important constraints on the origin of high-redshift BH seeds. Here we present high-resolution X-ray and radio observations of the low-metallicity, star-forming, dwarf-galaxy system Mrk 709 with the Chandra X-ray Observatory and the Karl G. Jansky Very Large Array. These data reveal spatially coincident hard X-ray and radio point sources with luminosities suggesting the presence of an accreting massive BH (M {sub BH} ∼ 10{sup 5-7} M {sub ☉}). Based on imaging from the Sloan Digital Sky Survey (SDSS), we find that Mrk 709 consists of a pair of compact dwarf galaxies that appear to be interacting with one another. The position of the candidate massive BH is consistent with the optical center of the southern galaxy (Mrk 709 S), while no evidence for an active BH is seen in the northern galaxy (Mrk 709 N). We derive stellar masses of M {sub *} ∼ 2.5 × 10{sup 9} M {sub ☉} and M {sub *} ∼ 1.1 × 10{sup 9} M {sub ☉} for Mrk 709 S and Mrk 709 N, respectively, and present an analysis of the SDSS spectrum of the BH host Mrk 709 S. At a metallicity of just ∼10% solar, Mrk 709 is among the most metal-poor galaxies with evidence for an active galactic nucleus. Moreover, this discovery adds to the growing body of evidence that massive BHs can form in dwarf galaxies and that deep, high-resolution X-ray and radio observations are ideally suited to reveal accreting massive BHs hidden at optical wavelengths.

  15. TIDAL INTERACTIONS AT THE EDGE OF THE LOCAL GROUP: NEW EVIDENCE FOR TIDAL FEATURES IN THE ANTLIA DWARF GALAXY

    SciTech Connect

    Penny, Samantha J.; Pimbblet, Kevin A.; Brown, Michael J. I.; Floyd, David J. E.; Conselice, Christopher J.; Gruetzbauch, Ruth

    2012-10-20

    Using deep B-band imaging down to {approx}{mu}{sub B} = 26 mag arcsec{sup -2}, we present evidence for tidal tails in the Antlia Dwarf galaxy, one of the most distant members of the Local Group. This elongation is in the direction of Antlia's nearest neighbor, the Magellanic-type NGC 3109. The tail is offset by <10 Degree-Sign from a vector linking the centers of the two galaxies, indicative of interactions between the pair. Combined with the warped disk previously identified in NGC 3109, Antlia and NGC 3109 must be at a small separation for tidal features to be present in Antlia. We calculate that Antlia cannot be completely disrupted by NGC 3109 in a single interaction unless its orbit pericenter is <6 kpc; however, multiple interactions could significantly alter its morphology. Therefore despite being located right at the edge of the Local Group, environmental effects are playing an important role in Antlia's evolution.

  16. THE NATURE OF STARBURSTS. II. THE DURATION OF STARBURSTS IN DWARF GALAXIES

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Stark, David; Weisz, Daniel; Cannon, John M.; Dalcanton, Julianne; Williams, Benjamin; Dolphin, Andrew; Hidalgo-RodrIguez, Sebastian

    2010-11-20

    The starburst phenomenon can shape the evolution of the host galaxy and the surrounding intergalactic medium. The extent of the evolutionary impact is partly determined by the duration of the starburst, which has a direct correlation with both the amount of stellar feedback and the development of galactic winds, particularly for smaller mass dwarf systems. We measure the duration of starbursts in twenty nearby, ongoing, and 'fossil' starbursts in dwarf galaxies based on the recent star formation histories derived from resolved stellar population data obtained with the Hubble Space Telescope. Contrary to the shorter times of 3-10 Myr often cited, the starburst durations we measure range from 450to650 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; these longer durations are comparable to or longer than the dynamical timescales for each system. The same feedback from massive stars that may quench the flickering star formation does not disrupt the overall burst event in our sample of galaxies. While five galaxies present fossil bursts, fifteen galaxies show ongoing bursts and thus the final durations may be longer than we report here for these systems. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 10{sup 53.9}-10{sup 57.2} erg of energy into the interstellar medium through stellar winds and supernovae, and produced 3%-26% of the host galaxy's mass.

  17. The ACS Nearby Galaxy Survey Treasury. VIII. The Global Star Formation Histories of 60 Dwarf Galaxies in the Local Volume

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Dalcanton, Julianne J.; Williams, Benjamin F.; Gilbert, Karoline M.; Skillman, Evan D.; Seth, Anil C.; Dolphin, Andrew E.; McQuinn, Kristen B. W.; Gogarten, Stephanie M.; Holtzman, Jon; Rosema, Keith; Cole, Andrew; Karachentsev, Igor D.; Zaritsky, Dennis

    2011-09-01

    We present uniformly measured star formation histories (SFHs) of 60 nearby (D <~ 4 Mpc) dwarf galaxies based on color-magnitude diagrams of resolved stellar populations from images taken with the Hubble Space Telescope and analyzed as part of the ACS Nearby Galaxy Survey Treasury program (ANGST). This volume-limited sample contains 12 dwarf spheroidal (dSph)/dwarf elliptical (dE), 5 dwarf spiral, 28 dwarf irregular (dI), 12 dSph/dI (transition), and 3 tidal dwarf galaxies. The sample spans a range of ~10 mag in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best-fit SFHs, we find three significant results for dwarf galaxies in the ANGST volume: (1) the majority of dwarf galaxies formed the bulk of their mass prior to z ~ 1, regardless of current morphological type; (2) the mean SFHs of dIs, transition dwarf galaxies (dTrans), and dSphs are similar over most of cosmic time, and only begin to diverge a few Gyr ago, with the clearest differences between the three appearing during the most recent 1 Gyr and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single bursts, constant star formation rates (SFRs), or smooth, exponentially declining SFRs. The mean SFHs show clear divergence from the cosmic SFH at z <~ 0.7, which could be evidence that low-mass systems have experienced delayed star formation relative to more massive galaxies. The sample shows a strong density-morphology relationship, such that the dSphs in the sample are less isolated than the dIs. We find that the transition from a gas-rich to gas-poor galaxy cannot be solely due to internal mechanisms such as stellar feedback, and instead is likely the result of external mechanisms, e.g., ram pressure and tidal stripping and tidal forces. In terms of their environments, SFHs, and gas fractions, the majority of the dTrans appear to be low-mass dIs that simply lack Hα emission, similar to Local Group (LG) dTrans DDO 210

  18. Predicting the Velocity Dispersions of the Dwarf Satellite Galaxies of Andromeda

    NASA Astrophysics Data System (ADS)

    McGaugh, Stacy S.

    2016-05-01

    Dwarf Spheroidal galaxies in the Local Group are the faintest and most diffuse stellar systems known. They exhibit large mass discrepancies, making them popular laboratories for studying the missing mass problem. The PANDAS survey of M31 revealed dozens of new examples of such dwarfs. As these systems were discovered, it was possible to use the observed photometric properties to predict their stellar velocity dispersions with the modified gravity theory MOND. These predictions, made in advance of the observations, have since been largely confirmed. A unique feature of MOND is that a structurally identical dwarf will behave differently when it is or is not subject to the external field of a massive host like Andromeda. The role of this "external field effect" is critical in correctly predicting the velocity dispersions of dwarfs that deviate from empirical scaling relations. With continued improvement in the observational data, these systems could provide a test of the strong equivalence principle.

  19. THE OBSERVED PROPERTIES OF DWARF GALAXIES IN AND AROUND THE LOCAL GROUP

    SciTech Connect

    McConnachie, Alan W.

    2012-07-15

    Positional, structural, and dynamical parameters for all dwarf galaxies in and around the Local Group are presented, and various aspects of our observational understanding of this volume-limited sample are discussed. Over 100 nearby galaxies that have distance estimates reliably placing them within 3 Mpc of the Sun are identified. This distance threshold samples dwarfs in a large range of environments, from the satellite systems of the MW and M31, to the quasi-isolated dwarfs in the outer regions of the Local Group, to the numerous isolated galaxies that are found in its surroundings. It extends to, but does not include, the galaxies associated with the next nearest groups, such as Maffei, Sculptor, and IC 342. Our basic knowledge of this important galactic subset and their resolved stellar populations will continue to improve dramatically over the coming years with existing and future observational capabilities, and they will continue to provide the most detailed information available on numerous aspects of dwarf galaxy formation and evolution. Basic observational parameters, such as distances, velocities, magnitudes, mean metallicities, as well as structural and dynamical characteristics, are collated, homogenized (as far as possible), and presented in tables that will be continually updated to provide a convenient and current online resource. As well as discussing the provenance of the tabulated values and possible uncertainties affecting their usage, the membership and spatial extent of the MW sub-group, M31 sub-group, and the Local Group are explored. The morphological diversity of the entire sample and notable sub-groups is discussed, and timescales are derived for the Local Group members in the context of their orbital/interaction histories. The scaling relations and mean stellar metallicity trends defined by the dwarfs are presented, and the origin of a possible 'floor' in central surface brightness (and, more speculatively, stellar mean metallicity) at

  20. Searching for Intermediate Mass Black Holes in Dwarf Galaxies using Jet Emission

    NASA Astrophysics Data System (ADS)

    Musaeva, Ayna; Koribalski, Baerbel; Sadler, Elaine; Farrell, Sean

    2014-10-01

    Supermassive black holes (SMBHs) are the central engines of active galactic nuclei and lie at the heart of many large galaxies, including our own Milky Way. If massive black holes exist in dwarf galaxies, these galaxies are expected to contain intermediate mass black holes (IMBHs) with masses in the range between 100 and 100,000 solar masses. Only a handful of IMBH candidates have been found in dwarf galaxies, which suggests that IMBHs may not be as common as SMBHs or they are more difficult to detect. For accreting black holes, one way to estimate their mass is by the fundamental plane relation between their X-ray and radio luminosities at 5 GHz. The radio emission comes from relativistic jets of a black hole accreting at a low fraction of the Eddington limit. We have identified several X-ray sources which are candidates for an accreting black hole in the central part of three nearby dwarf galaxies. We request 5 GHz Australia Telescope Compact Array (ATCA) observations of these X-ray sources in order to search for radio counterparts and identify IMBH candidates. The ~2’’ spatial resolution of ATCA with a 6-km array configuration allows us to make sure X-ray and radio emission originates from the same source, and a broad bandwidth of ~4 GHz makes ATCA an ideal instrument for detecting the radio counterparts of the X-ray sources, confirming the radio emission is consistent with jets, and estimating the black hole masses.

  1. Dynamics of starbursting dwarf galaxies. III. A H I study of 18 nearby objects

    NASA Astrophysics Data System (ADS)

    Lelli, Federico; Verheijen, Marc; Fraternali, Filippo

    2014-06-01

    We investigate the dynamics of starbursting dwarf galaxies, using both new and archival H I observations. We consider 18 nearby galaxies that have been resolved into single stars by HST observations, providing their star formation history and total stellar mass. We find that 9 objects have a regularly rotating H I disk, 7 have a kinematically disturbed H I disk, and 2 show unsettled H I distributions. Two galaxies (NGC 5253 and UGC 6456) show a velocity gradient along the minor axis of the H I disk, which we interpret as strong radial motions. For galaxies with a regularly rotating disk we derive rotation curves, while for galaxies with a kinematically disturbed disk, we estimate the rotation velocities in their outer parts. We derive baryonic fractions within about 3 optical scale lengths and find that, on average, baryons constitute at least 30% of the total mass. Despite the star formation having injected ~1056 ergs in the ISM in the past ~500 Myr, these starbursting dwarfs have both baryonic and gas fractions similar to those of typical dwarf irregulars, suggesting that they did not eject a large amount of gas out of their potential wells. Appendices are available in electronic form at http://www.aanda.orgH I datacubes (FITS files) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/566/A71

  2. Astrophysical tests of modified gravity: the morphology and kinematics of dwarf galaxies

    SciTech Connect

    Vikram, Vinu; Cabré, Anna; Jain, Bhuvnesh; VanderPlas, J.T. E-mail: annanusca@gmail.com E-mail: jakevdp@cs.washington.edu

    2013-08-01

    This paper is the third in a series on tests of gravity using observations of stars and nearby dwarf galaxies. We carry out four distinct tests using published data on the kinematics and morphology of dwarf galaxies, motivated by the theoretical work of Hui et al. (2009) and Jain and Vanderplas (2011). In a wide class of gravity theories a scalar field couples to matter and provides an attractive fifth force. Due to their different self-gravity, stars and gas may respond differently to the scalar force leading to several observable deviations from standard gravity. HI gas, red giant stars and main sequence stars can be displaced relative to each other, and the stellar disk can display warps or asymmetric rotation curves aligned with external potential gradients. To distinguish the effects of modified gravity from standard astrophysical phenomena, we use a control sample of galaxies that are expected to be screened from the fifth force. In all cases we find no significant deviation from the null hypothesis of general relativity. The limits obtained from dwarf galaxies are not yet competitive with the limits from cepheids obtained in our first paper, but can be improved to probe regions of parameter space that are inaccessible using other tests. We discuss how our methodology can be applied to new radio and optical observations of nearby galaxies.

  3. Dense Cloud Cores revealed by ALMA CO observations in the low metallicity dwarf galaxy WLM

    NASA Astrophysics Data System (ADS)

    Rubio, M.; Elmegreen, B.; Hunter, D.; Cortes, J.; Brinks, E.; Cigan, P.

    2017-03-01

    Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations, they are molecular with H2 the dominant species and CO the best available. When the abundances of carbon and oxygen are low compared to hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies CO forms slowly and is easily destroyed, so it cannot accumulate inside dense clouds. Then we lose our ability to trace the gas in regions of star formation and we lose critical information on the temperatures, densities, and velocities of the material that collapses. I will report on high resolution observations with ALMA of CO clouds in the local group dwarf irregular galaxy WLM, which has a metallicity that is 13% of the solar value and 50% lower than the previous CO detection threshold and the properties derived of very small dense CO clouds mapped..

  4. A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling

    PubMed Central

    Ural, Uğur; Wilkinson, Mark I.; Read, Justin I.; Walker, Matthew G.

    2015-01-01

    Dark matter-only simulations of galaxy formation predict many more subhalos around a Milky Way-like galaxy than the number of observed satellites. Proposed solutions require the satellites to inhabit dark matter halos with masses 109–1010 Msun at the time they fell into the Milky Way. Here we use a modelling approach, independent of cosmological simulations, to obtain a pre-infall mass of Msun for one of the Milky Way's satellites: Carina. This determination of a low halo mass for Carina can be accommodated within the standard model only if galaxy formation becomes stochastic in halos below ∼1010 Msun. Otherwise Carina, the eighth most luminous Milky Way dwarf, would be expected to inhabit a significantly more massive halo. The implication of this is that a population of ‘dark dwarfs' should orbit the Milky Way: halos devoid of stars and yet more massive than many of their visible counterparts. PMID:26133650

  5. How the first stars shaped the faintest gas-dominated dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Verbeke, Robbert; Vandenbroucke, Bert; de Rijcke, Sven

    2016-08-01

    Cosmological simulations predict that dark matter halos with circular velocities lower than 30 km/s should have lost most of their neutral gas by heating of the ultra-violet background. This is in stark contrast with gas-rich galaxies such as e.g. Leo T, Leo P and Pisces A, which all have circular velocities of ~15 km/s (Ryan-Weber et al. 2008, Bernstein-Cooper et al. 2014, Tollerud et al. 2015). We show that when we include feedback from the first stars into our models, simulated dwarfs have very different properties at redshift 0 than when this form of feedback is not included. Including this Population-III feedback leads to galaxies that lie on the baryonic Tully-Fisher relation over the entire mass range of star forming dwarf galaxies, as well as reproducing a broad range of other observational properties.

  6. Metallicity Distribution Functions of Dwarf Galaxies: A Probe of Star Formation History and Baryonic Physics

    NASA Astrophysics Data System (ADS)

    Escala, Ivanna; Kirby, Evan N.; Wetzel, Andrew R.; Hopkins, Philip F.

    2016-06-01

    We examine the metallicity distribution functions (MDFs) of simulated, isolated dwarf galaxies (M_{star} = 4 × 10^{4} - 3 × 10^{8} M_{⊙}) from the Feedback in Realistic Environments (FIRE) project to quantify the impact of star formation history (SFH) and baryonic physics. These high-resolution cosmological simulations include realistic treatments of stellar evolution and complex gas dynamics and do not require the usual approximations (e.g., instantaneous recycling and instantaneous mixing) of analytic chemical evolution models. The evolution of the MDF with redshift informs which processes drive the dominant contributions to the distribution at z = 0, thus enabling a reconstruction of the SFH and gas loss/accretion history. We then compare the theoretical MDFs to the observed MDFs of Local Group dwarf galaxies to infer plausible SFHs for each matched galaxy.

  7. CCD photometry of Andromeda IV - Dwarf irregular galaxy or M31 open cluster?

    NASA Technical Reports Server (NTRS)

    Jones, Joseph H.

    1993-01-01

    CCD photometry of Andromeda IV was obtained during discretionary time in August of 1989 at the Canada-France-Hawaii Telescope on Mauna Kea and the data were reduced at CFHT during the summer of 1991. And IV has been catalogued both as a dwarf galaxy and as an open star cluster in M31. The color-magnitude diagrams presented indicate that this object has a young population of stars with a narrow age range, consistent with the characteristics of an open star cluster or stellar association. A radial velocity measurement taken from the literature and analyzed with respect to the rotation curve of M31 indicates this object resides in the disk of the Andromeda Galaxy, strengthening the conclusion that it is indeed a very large open star cluster or a densely populated stellar association rather than a dwarf irregular galaxy.

  8. Large-scale environmental dependence of chemical abundances in dwarf galaxies and implications for connecting star formation history and halo mass

    NASA Astrophysics Data System (ADS)

    Douglass, Kelly; Vogeley, Michael S.

    2017-01-01

    We study how the cosmic environment affects galaxy evolution in the Universe by comparing the gas-phase metallicities and abundance ratios of dwarf galaxies in voids with dwarf galaxies in more dense regions. Using spectroscopic observations from Sloan Digital Sky Survey Data Release 7, we estimate the oxygen and nitrogen abundances of 1014 void dwarf galaxies and 787 dwarf galaxies in more dense regions. We develop an alternate calculation for the oxygen abundance that does not use the [OII] λ3727 doublet, permitting oxygen abundance estimates of SDSS dwarf galaxies with the Direct Te method at all redshifts. We find that void dwarf galaxies (Mr > -17) have 9% higher average oxygen abundances and 9% lower average nitrogen abundances than dwarf galaxies in more dense regions. There is a 23% difference in the relative abundances of nitrogen and oxygen in the dwarf galaxies between the two environments. We also find similar N/O abundance ratio shifts in a larger sample (2050 void galaxies and 3883 galaxies in dense regions) of somewhat brighter galaxies (-17 > Mr > -20). These abundance shifts in galaxies fainter than L* may indicate retarded star formation and larger dark matter halo mass to stellar mass ratios in void galaxies, as seen in high-resolution hydrodynamic simulations.

  9. Escape fraction of ionizing photons from a dwarf galaxy NGC 4214

    NASA Astrophysics Data System (ADS)

    Choi, Yumi; Fouesneau, Morgan; Gordon, Karl D.; Williams, Benjamin F.; Dalcanton, Julianne; Weisz, Daniel R.; Arab, Heddy; Sandstrom, Karin; Dolphin, Andrew E.

    2015-01-01

    Recent studies suggest that starburst dwarf galaxies played an important role in the early universe. Because these galaxies dominate by number, their leaked ionizing photons are likely main contributors to the reionization of the intergalactic medium (IGM). However, the complex structure of the interstellar medium (ISM) even at the pc scale makes it hard to predict the escape fraction of ionizing photons from high-redshift galaxies accurately. Analogues to their high-redshift counterparts, nearby starburst dwarf galaxies provide excellent laboratories to study the impact of star formation on the surrounding ISM and IGM in detail. Thanks to its proximity, the dwarf galaxy, NGC 4214, has been imaged with the high-resolution of WFC3 on HST from the near-UV to the near-IR (F225W, F336W, F438W, F814W, F110W, and F160W). These observations yielded measurements of the broad spectral energy distributions (SEDs) for ˜36,000 resolved stars within this galaxy. We developed a probabilistic tool (Bayesian Extinction and Stellar Tool, a.k.a. BEAST) to simultaneously infer from their SEDs the stellar properties of individual stars and the intervening dust properties along the line of sight to each star. With the aid of BEAST, we are able to infer the intrinsic ionizing flux produced by individual stars. By comparing this intrinsic ionizing flux with the flux that is used to ionize the ISM in the galaxy, derived based on the extinction-corrected Hα emission, we can estimate the escape fraction and its local variation within the galaxy. Our preliminary results show that the global UV leakage of NGC 4214 is ˜10%.

  10. Chemistry and Kinematics of the Late-forming Dwarf Irregular Galaxies Leo A, Aquarius, and Sagittarius DIG

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Rizzi, Luca; Held, Enrico V.; Cohen, Judith G.; Cole, Andrew A.; Manning, Ellen M.; Skillman, Evan D.; Weisz, Daniel R.

    2017-01-01

    We present Keck/DEIMOS spectroscopy of individual stars in the relatively isolated Local Group dwarf galaxies Leo A, Aquarius, and the Sagittarius dwarf irregular galaxy. The three galaxies—but especially Leo A and Aquarius—share in common delayed star formation histories (SFHs) relative to many other isolated dwarf galaxies. The stars in all three galaxies are supported by dispersion. We found no evidence of stellar velocity structure, even for Aquarius, which has rotating H i gas. The velocity dispersions indicate that all three galaxies are dark-matter-dominated, with dark-to-baryonic mass ratios ranging from {4.4}-0.8+1.0 (SagDIG) to {9.6}-1.8+2.5 (Aquarius). Leo A and SagDIG have lower stellar metallicities than Aquarius, and they also have higher gas fractions, both of which would be expected if Aquarius were further along in its chemical evolution. The metallicity distribution of Leo A is inconsistent with a closed or leaky box model of chemical evolution, suggesting that the galaxy was pre-enriched or acquired external gas during star formation. The metallicities of stars increased steadily for all three galaxies, but possibly at different rates. The [α/Fe] ratios at a given [Fe/H] are lower than that of the Sculptor dwarf spheroidal galaxy, which indicates more extended SFHs than Sculptor, consistent with photometrically derived SFHs. Overall, the bulk kinematic and chemical properties for the late-forming dwarf galaxies do not diverge significantly from those of less delayed dwarf galaxies, including dwarf spheroidal galaxies. 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.

  11. Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at Z greater than 1

    NASA Technical Reports Server (NTRS)

    vanderWel, A.; Straughn, A. N.; Rix, H.-W.; Finkelstein, S. L.; Koekemoer, A. M.; Weiner, B. J.; Wuyts, S.; Bell, E. F.; Faber, S. M.; Trump, J. R.; Koo, D. C.; Ferguson, H. C.; Scarlata, C.; Hathi, N. P.; Dunlop, J. S.; Newman, J. A.; Dickinson, M.; Jahnke, K.; Salmon, B. W.; deMello, D. F.; Kkocevski, D. D.; Lai, K.; Grogin, N. A.; Rodney, S. A.; Guo, Yicheng

    2012-01-01

    We identify an abundant population of extreme emission line galaxies (EELGs) at redshift z approx. 1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broad-band magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines . with rest-frame equivalent widths approx. 1000A in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with approx.10(exp 8) Solar Mass in stellar mass, undergoing an enormous starburst phase with M*/M* of only approx. 15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the co-moving number density (3.7x10(exp -4) Mpc(sup -3) can produce in approx.4 Gyr much of the stellar mass density that is presently contained in 10(exp 8) - 10(exp 9) Solar Mass dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z > 1.

  12. Understanding the shape and diversity of dwarf galaxy rotation curves in ΛCDM

    NASA Astrophysics Data System (ADS)

    Read, J. I.; Iorio, G.; Agertz, O.; Fraternali, F.

    2016-11-01

    The shape and diversity of dwarf galaxy rotation curves is at apparent odds with dark matter halos in a Λ Cold Dark Matter (ΛCDM) cosmology. We use mock data from isolated dwarf galaxy simulations to show that this owes to three main effects. Firstly, stellar feedback heats dark matter, leading to a `CORENFW' dark matter density profile with a slowly rising rotation curve. Secondly, if close to a recent starburst, large H I bubbles push the rotation curve out of equilibrium, deforming the rotation curve shape. Thirdly, when galaxies are viewed near face-on, their best fit inclination is biased high. This can lead to a very shallow rotation curve that falsely implies a large dark matter core. All three problems can be avoided, however, by a combination of improved mass models and a careful selection of target galaxies. Fitting our CORENFW model to mock rotation curve data, we show that we can recover the rotation curve shape, dark matter halo mass M200 and concentration parameter c within our quoted uncertainties. We fit our CORENFW model to real data for four isolated dwarf irregulars, chosen to span a wide range of rotation curve shapes. We obtain an excellent fit for NGC 6822 and WLM, with tight constraints on M200, and c consistent with ΛCDM. However, IC 1613 and DDO 101 give a poor fit. For IC 1613, we show that this owes to disequilibria and its uncertain inclination i; for DDO 101, it owes to its uncertain distance D. If we assume iIC1613 ˜ 15° and DDDO101 ˜ 12 Mpc, consistent with current uncertainties, we are able to fit both galaxies very well. We conclude that ΛCDM appears to give an excellent match to dwarf galaxy rotation curves.

  13. The role of dwarf galaxy interactions in shaping the Magellanic System and implications for Magellanic Irregulars

    NASA Astrophysics Data System (ADS)

    Besla, Gurtina; Kallivayalil, Nitya; Hernquist, Lars; van der Marel, Roeland P.; Cox, T. J.; Kereš, Dušan

    2012-04-01

    We present a novel pair of numerical models of the interaction history between the Large and Small Magellanic Clouds (LMC and SMC, respectively) and our Milky Way (MW) in light of recent high-precision proper motions from the Hubble Space Telescope. Given the updated velocities, cosmological simulations of hierarchical structure formation favour a scenario where the Magellanic Clouds (MCs) are currently on their first infall towards ourGalaxy. We illustrate here that the observed irregular morphology and internal kinematics of the Magellanic System (in gas and stars) are naturally explained by interactions between the LMC and SMC, rather than gravitational interactions with the MW. These conclusions provide further support that the MCs are completing their first infall to our system. In particular, we demonstrate that the Magellanic Stream, a band of H I gas trailing behind the Clouds 150° across the sky, can be accounted for by the action of LMC tides on the SMC before the system was accreted by the MW. We further demonstrate that the off-centre, warped stellar bar of the LMC, and its one-armed spiral can be naturally explained by a recent direct collision with its lower mass companion, the SMC. Such structures are key morphological characteristics of a class of galaxies referred to as Magellanic Irregulars, the majority of which are not associated with massive spiral galaxies. We infer that dwarf-dwarf galaxy interactions are important drivers for the morphological evolution of Magellanic Irregulars and can dramatically affect the efficiency of baryon removal from dwarf galaxies via the formation of extended tidal bridges and tails. Such interactions are not only important for the evolution of dwarf galaxies but also have direct consequences for the build-up of baryons in our own MW, as LMC-mass systems are believed to be the dominant building blocks of MW-type haloes.

  14. On the Stellar Content of the Carina Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Bono, G.; Stetson, P. B.; Walker, A. R.; Monelli, M.; Fabrizio, M.; Pietrinferni, A.; Brocato, E.; Buonanno, R.; Caputo, F.; Cassisi, S.; Castellani, M.; Cignoni, M.; Corsi, C. E.; Dall’Ora, M.; Degl’Innocenti, S.; François, P.; Ferraro, I.; Iannicola, G.; Nonino, M.; Prada Moroni, P. G.; Pulone, L.; Smith, H. A.; Thevenin, F.

    2010-06-01

    We present deep, accurate, and homogeneous multiband optical (U, B, V, I) photometry of the Carina dwarf spheroidal galaxy, based on more than 4000 individual CCD images from three different ground-based telescopes. Special attention was given to the photometric calibration, and the precision for the B, V, and I bands is generally better than 0.01 mag. We have performed detailed comparisons in the V, B - V, and V, B - I color-magnitude diagrams (CMDs) between Carina and three old, metal-poor Galactic globular clusters (GGCs, M53, M55, M79). We find that only the more metal-poor GCs (M55, [Fe/H] = -1.85 M53, [Fe/H] = -2.02 dex) provide a good match with the Carina giant branch. We have performed a similar comparison in the V, V - I CMD with three intermediate-age clusters (IACs) of the Small Magellanic Cloud (Kron 3, NGC 339, Lindsay 38). We find that the color extent of the subgiant branch (SGB) of the two more metal-rich IACs (Kron 3, [Fe/H] = -1.08 NGC339, [Fe/H] = -1.36 dex) is smaller than the range among Carina’s intermediate-age stars. Moreover, the slope of the RGB of these two IACs is shallower than the slope of the Carina RGB. However, the ridge line of the more metal-poor IAC (Lindsay 38, [Fe/H] = -1.59 dex) agrees quite well with the Carina intermediate-age stars. These findings indicate that Carina’s old stellar population is metal-poor and appears to have a limited spread in metallicity (Δ[Fe/H] = 0.2–0.3 dex). The Carina’s intermediate-age stellar population can hardly be more metal-rich than Lindsay 38, and its spread in metallicity also appears modest. We also find that the synthetic CMD constructed assuming a metallicity spread of 0.5 dex for the intermediate-age stellar component predicts evolutionary features not supported by observations. In particular, red clump stars should attain colors that are redder than red giant stars, but this is not seen. These results are at odds with recent spectroscopic investigations suggesting that Carina

  15. SCALING LAWS FOR DARK MATTER HALOS IN LATE-TYPE AND DWARF SPHEROIDAL GALAXIES

    SciTech Connect

    Kormendy, John; Freeman, K. C. E-mail: kenneth.freeman@anu.edu.au

    2016-02-01

    Dark matter (DM) halos of Sc–Im and dwarf spheroidal (dSph) galaxies satisfy scaling laws: halos in lower-luminosity galaxies have smaller core radii, higher central densities, and smaller velocity dispersions. These results are based on maximum-disk rotation curve decompositions for giant galaxies and Jeans equation analysis for dwarfs. (1) We show that spiral, Im, and Sph galaxies with absolute magnitudes M{sub V} > −18 form a sequence of decreasing baryon-to-DM surface density with decreasing luminosity. We suggest that this is a sequence of decreasing baryon retention versus supernova-driven losses or decreasing baryon capture after cosmological reionization. (2) The structural differences between S+Im and Sph galaxies are small. Both are affected mostly by the physics that controls baryon depletion. (3) There is a linear correlation between the maximum rotation velocities of baryonic disks and the outer circular velocities V{sub circ} of test particles in their DM halos. Baryons become unimportant at V{sub circ} = 42 ± 4 km s{sup −1}. Smaller galaxies are dim or dark. (4) We find that, absent baryon “depletion” and with all baryons converted into stars, dSph galaxies would be brighter by ∼4.6 mag and dIm galaxies would be brighter by ∼3.5 mag. Both have DM halos that are massive enough to help to solve the “too big to fail” problem with DM galaxy formation. (5) We suggest that there exist many galaxies that are too dark to be discovered by current techniques, as required by cold DM theory. (6) Central surface densities of DM halos are constant from M{sub B} ∼ −5 to −22. This implies a Faber–Jackson law with halo mass M ∝ (halo dispersion){sup 4}.

  16. REVERSAL OF FORTUNE: INCREASED STAR FORMATION EFFICIENCIES IN THE EARLY HISTORIES OF DWARF GALAXIES?

    SciTech Connect

    Madau, Piero; Weisz, Daniel R.; Conroy, Charlie

    2014-08-01

    On dwarf galaxy scales, the different shapes of the galaxy stellar mass function and the dark halo mass function require a star-formation efficiency (SFE) in these systems that is currently more than 1 dex lower than that of Milky Way-size halos. Here, we argue that this trend may actually be reversed at high redshift. Specifically, by combining the resolved star-formation histories of nearby isolated dwarfs with the simulated mass-growth rates of dark matter halos, we show that the assembly of these systems occurs in two phases: (1) an early, fast halo accretion phase with a rapidly deepening potential well, characterized by a high SFE; and (2) a late, slow halo accretion phase where, perhaps as a consequence of reionization, the SFE is low. Nearby dwarfs have more old stars than predicted by assuming a constant or decreasing SFE with redshift, a behavior that appears to deviate qualitatively from the trends seen among more massive systems. Taken at face value, the data suggest that at sufficiently early epochs, dwarf galaxy halos above the atomic cooling mass limit can be among the most efficient sites of star formation in the universe.

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

    NASA Astrophysics Data System (ADS)

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

    2017-02-01

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

  18. On The gamma-ray emission from Reticulum II and other dwarf galaxies

    SciTech Connect

    Hooper, Dan; Linden, Tim E-mail: trlinden@uchicago.edu

    2015-09-01

    The recent discovery of ten new dwarf galaxy candidates by the Dark Energy Survey (DES) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) could increase the Fermi Gamma-Ray Space Telescope's sensitivity to annihilating dark matter particles, potentially enabling a definitive test of the dark matter interpretation of the long-standing Galactic Center gamma-ray excess. In this paper, we compare the previous analyses of Fermi data from the directions of the new dwarf candidates (including the relatively nearby Reticulum II) and perform our own analysis, with the goal of establishing the statistical significance of any gamma-ray signal from these sources. We confirm the presence of an excess from Reticulum II, with a spectral shape that is compatible with the Galactic Center signal. The significance of this emission is greater than that observed from 99.84% of randomly chosen high-latitude blank-sky locations, corresponding to a local detection significance of 3.2σ. We caution that any dark matter interpretation of this excess must be validated through observations of additional dwarf spheroidal galaxies, and improved calculations of the relative J-factor of dwarf spheroidal galaxies. We improve upon the standard blank-sky calibration approach through the use of multi-wavelength catalogs, which allow us to avoid regions that are likely to contain unresolved gamma-ray sources.

  19. A single prolific r-process event preserved in an ultra-faint dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Ji, Alexander; Frebel, Anna; Chiti, Anirudh; Simon, Joshua

    2016-03-01

    The heaviest elements in the periodic table are synthesized through the r-process, but the astrophysical site for r-process nucleosynthesis is still unknown. Ultra-faint dwarf galaxies contain a simple fossil record of early chemical enrichment that may determine this site. Previous measurements found very low levels of neutron-capture elements in ultra-faint dwarfs, preferring supernovae as the r-process site. I present high-resolution chemical abundances of nine stars in the recently discovered ultra-faint dwarf Reticulum II, which display extremely enhanced r-process abundances 2-3 orders of magnitude higher than the other ultra-faint dwarfs. Stars with such extreme r-process enhancements are only rarely found in the Milky Way halo. The r-process abundances imply that the neutron-capture material in Reticulum II was synthesized in a single prolific event that is incompatible with r-process yields from ordinary core-collapse supernovae. Reticulum II provides an opportunity to discriminate whether the source of this pure r-process signature is a neutron star merger or magnetorotationally driven supernova. The single event is also a uniquely stringent constraint on the metal mixing and star formation history of this ultra-faint dwarf galaxy.

  20. The evolution of brightest cluster and dwarf galaxies using integral field unit spectroscopy

    NASA Astrophysics Data System (ADS)

    Jimmy

    The study of galaxy formation and evolution utilizes empirical scaling relations as a tool to better understand complex physical processes that occur below the resolution of observations. New observations of galaxy populations can provide tests of the range of validity for known scaling relations. We test the following scaling relations using IFU spectroscopy of galaxies at opposite ends of the stellar-mass spectrum. Brightest cluster galaxies undergo, on average, more mergers than a typical early-type galaxy. They also lie at the center of their host cluster's gravitational potential well, meaning there is no preferred alignment for mergers. As such, we would expect to find that BCGs should have preferentially lower angular momentum when compared to a population of early-type galaxies. We have spatially mapped the kinematic properties of 10 nearby brightest cluster galaxies (BCGs) in the mass range 1010.5 solar masses < Mdyn < 1011.9 solar masses. We find that 30% (3/10) of the BCGs are fast rotators as de?ned by the ATLAS 3D criteria. This is significantly lower than the ATLAS3D population as a whole, however our sample is biased towards higher dynamical-mass galaxies. When controlling for dynamical-mass, we find that above Mdyn ˜ 1011.5 solar masses, both samples show the same ratio of slow rotators. This suggests that the relation be-tween galaxy angular momentum and dynamical mass is independent of the number of mergers a galaxy has undergone. Dwarf irregular galaxies are the local universe analogs of the young high-redshift galaxies that eventually merge to form BCGs. The mass-metallicity scaling relation (MZR) says that galaxies with higher stellar-mass have preferentially higher gas metallicity. Mergers have been suggested as a potential cause of the scatter in the mass-metallicity relation as pristine gas is driven into a galaxy, diluting the metal content. Two different three-dimensional extensions of MZR, as a function of either HI-gas mass (FMRHI) or

  1. The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey. A ground-based follow-up survey of CO(1-0), CO(2-1), and CO(3-2)

    NASA Astrophysics Data System (ADS)

    Cormier, D.; Madden, S. C.; Lebouteiller, V.; Hony, S.; Aalto, S.; Costagliola, F.; Hughes, A.; Rémy-Ruyer, A.; Abel, N.; Bayet, E.; Bigiel, F.; Cannon, J. M.; Cumming, R. J.; Galametz, M.; Galliano, F.; Viti, S.; Wu, R.

    2014-04-01

    Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments. Aims: We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity. Methods: We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [C ii] 157μm and [O i] 63μm observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC 4861, NGC 625, and UM 311. We derived their molecular gas masses from several methods, including using the CO-to-H2 conversion factor XCO (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Results: We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z ~ 0.4 Z⊙), Mrk 930 (0.2 Z⊙), and UM 311 (0.5 Z⊙), but CO remains undetected in NGC 4861 (0.2 Z⊙). The CO luminosities are low, while [C ii] is bright in these galaxies, resulting in [C ii]/CO(1-0) ≥ 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled XCO factors. Those galaxies are dominated by their H i gas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in

  2. Kinematic properties and dark matter fraction of Virgo dwarf early-type galaxies

    NASA Astrophysics Data System (ADS)

    Toloba, E.; Boselli, A.; Peletier, R.; Gorgas, J.

    2015-03-01

    What happens to dwarf galaxies as they enter the cluster potential well is one of the main unknowns in studies of galaxy evolution. Several evidence suggests that late-type galaxies enter the cluster and are transformed to dwarf early-type galaxies (dEs). We study the Virgo cluster to understand which mechanisms are involved in this transformation. We find that the dEs in the outer parts of Virgo have rotation curves with shapes and amplitudes similar to late-type galaxies of the same luminosity (Fig. 1). These dEs are rotationally supported, have disky isophotes, and younger ages than those dEs in the center of Virgo, which are pressure supported, often have boxy isophotes and are older (Fig. 1). Ram pressure stripping, thus, explains the properties of the dEs located in the outskirts of Virgo. However, the dEs in the central cluster regions, which have lost their angular momentum, must have suffered a more violent transformation. A combination of ram pressure stripping and harassment is not enough to remove the rotation and the spiral/disky structures of these galaxies. We find that on the the Faber-Jackson and the Fundamental Plane relations dEs deviate from the trends of massive elliptical galaxies towards the position of dark matter dominated systems such as the dwarf spheroidal satellites of the Milky Way and M31. Both, rotationally and pressure supported dEs, however, populate the same region in these diagrams. This indicates that dEs have a non-negligible dark matter fraction within their half light radius.

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

    NASA Technical Reports Server (NTRS)

    Babul, Arif; Rees, Martin J.

    1993-01-01

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

  4. Relative abundances in the low-metallicity dwarf irregular galaxy UGC 4483

    NASA Technical Reports Server (NTRS)

    Skillman, Evan D.

    1991-01-01

    UGC 4483 is a dwarf irregular galaxy in the M 81 group. Narrow-band optical imaging has revealed an H II region in UGC 4483 with an H-alpha flux of about 2 x 10 to the -13th erg/sq cm per A per s. Optical, UV and NIR spectroscopy of this H II region yield He, C, N, O, Ne, and S abundances for the ISM in this galaxy. With an oxygen abundance of 0.000021 12 + log(O/H) = 7.3, this galaxy is among the most metal-poor dwarf irregulars known to date. A comparison of the S/O abundance ratio in this galaxy with that in I Zw 18 supports the claim by Garnett (1990) that the S/O ratio is consistent with the solar ratio for low-metallicity dwarf irregulars. The C/O ratio in UGC 4483 is lower than that derived for I Zw 18 and, therefore, more in line with the trend seen in higher metallicity H II regions. The derived helium abundance is He/H = 0.075, which converts to a He mass fraction of 0.23, consistent with earlier determinations of the primordial He abundance.

  5. Feedback and the formation of dwarf galaxy stellar haloes

    NASA Astrophysics Data System (ADS)

    Stinson, G. S.; Dalcanton, J. J.; Quinn, T.; Gogarten, S. M.; Kaufmann, T.; Wadsley, J.

    2009-05-01

    Stellar population studies show that low-mass galaxies in all environments exhibit stellar haloes that are older and more spherically distributed than the main body of the galaxy. In some cases, there is a significant intermediate age component that extends beyond the young disc. We examine a suite of Smoothed Particle Hydrodynamic simulations and find that elevated early star formation activity combined with supernova feedback can produce an extended stellar distribution that resembles these haloes for model galaxies ranging from v200 = 15 to 35 km s-1, without the need for accretion of subhaloes.

  6. The Promise of First Spectroscopy of Normal and Dwarf Galaxies

    DTIC Science & Technology

    2001-07-01

    The [C II]158µm line flux versus FIR flux for 12 ULIGs observed with the LWS ( Luhman et al. 1998; 2001) com- pared with a sample of normal and...starburst galaxies ( Luhman et al. 1998; Lord et al. 1996; Colbert et al. 1999; Stacey et al. 1999; Bradford et al. 1999). In the symbol key, the galaxies are...defined as in Sanders & Mirabel (1996). The dashed lines mark the regime typical of normal and star- burst galaxies. From Luhman et al. (2001). feature

  7. The Herschel Virgo Cluster Survey - XIV. Transition-type dwarf galaxies in the Virgo cluster

    NASA Astrophysics Data System (ADS)

    De Looze, Ilse; Baes, Maarten; Boselli, Alessandro; Cortese, Luca; Fritz, Jacopo; Auld, Robbie; Bendo, George J.; Bianchi, Simone; Boquien, Médéric; Clemens, Marcel; Ciesla, Laure; Davies, Jonathan; di Serego Alighieri, Sperello; Grossi, Marco; Jones, Anthony; Madden, Suzanne C.; Pappalardo, Ciro; Pierini, Daniele; Smith, Matthew W. L.; Verstappen, Joris; Vlahakis, Catherine; Zibetti, Stefano

    2013-12-01

    We use dust scaling relations to investigate the hypothesis that Virgo cluster transition-type dwarfs are infalling star-forming field galaxies, which is argued based on their optical features (e.g. discs, spiral arms and bars) and kinematic properties similar to late-type galaxies. After their infall, environmental effects gradually transform them into early-type galaxies through the removal of their interstellar medium and quenching of all star formation activity. In this paper, we aim to verify whether this hypothesis holds using far-infrared diagnostics based on Herschel observations of the Virgo cluster taken as part of the Herschel Virgo Cluster Survey. We select transition-type objects in the nearest cluster, Virgo, based on spectral diagnostics indicative for their residual or ongoing star formation. We detect dust (Md ˜ 105-6 M⊙) in 36 per cent of the transition-type dwarfs located on the high end of the stellar mass distribution. This suggests that the dust reservoirs present in non-detections fall just below the Herschel detection limit (≲1.1 × 105 M⊙). Dust scaling relations support the hypothesis of a transformation between infalling late-type galaxies to quiescent low-mass spheroids governed by environmental effects, with dust-to-stellar mass fractions for transition-type dwarfs in between values characteristic for late-type objects and the lower dust fractions observed in early-type galaxies. Several transition-type dwarfs demonstrate blue central cores, hinting at the radially outside-in removal of gas and quenching of star formation activity. The fact that dust is also confined to the inner regions suggests that metals are stripped in the outer regions along with the gas. In the scenario of most dust being stripped from the galaxy along with the gas, we argue that the ejected metals by transition-type dwarfs significantly contribute to the enrichment of the intracluster medium over the lifetime of the Virgo cluster. The accretion of gas

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

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

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

  9. Northern dwarf and low surface brightness galaxies. II - The Green Bank neutral hydrogen survey

    NASA Technical Reports Server (NTRS)

    Schneider, Stephen E.; Thuan, Trinh X.; Mangum, Jeffrey G.; Miller, John

    1992-01-01

    The paper reports neutral hydrogen observations of a large sample of dwarf and other low surface brightness galaxies. A detailed discussion and error analysis of the observations are presented, and spectra are displayed for 329 galaxies detected for the first time, or detected with substantially better signal-to-noise ratios than achieved previously. The positions on the sky of 667 galaxies meeting the present selection criteria north of delta = 38 deg are shown. The distribution of the redshifts of galaxies detected at Green Bank is illustrated. The Green Bank detections tapered off strongly below the median H I flux of 3.7 Jy km/s detected at Arecibo: only 12 percent of the Green Bank sample was detected with smaller fluxes.

  10. VizieR Online Data Catalog: Dwarf galaxy planes in Local Group (Pawlowski+, 2013)

    NASA Astrophysics Data System (ADS)

    Pawlowski, M. S.; Kroupa, P.; Jerjen, H.

    2014-09-01

    The analysis presented in the following is based on the catalogue of nearby galaxies as compiled by McConnachie (2012AJ....144....4M, Cat. J/AJ/144/4) (see also Mateo, 1998ARA&A..36..435M). It includes information on all known galaxies within 3Mpc from the Sun, which have distance estimates based on resolved stellar populations. We use the galaxy positions, radial distances and line-of-sight velocities of the LG galaxies as provided by the most recent online version of the tables by McConnachie (2012AJ....144....4M, https://www.astrosci.ca/users/alan/NearbyDwarfsDatabase.html, Version 2013/Jun/17). To this we add the recently published line-of-sight velocity for Andromeda XXIX (Tollerud et al., 2013ApJ...768...50T) for which no velocities are provided in the catalogue yet. (1 data file).

  11. A Study of the Neutral Hydrogen Content of Blue Compact Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    MacHattie, Jeremy A.

    The results of a study of the neutral hydrogen (HI) content and distribution within a sample of 18 blue compact dwarf galaxies (BCDs) are presented. An investigation of the behaviour of the gas-to-dust ratio (GDR) as a function of metallicity (Z) is also presented. Observations of these BCDs was performed using the Very Large Array (VLA) in 2009, a year in which the array was undergoing a technological upgrade to the the Karl G. Jansky VLA (JVLA). The observations were reduced and images processed using the Astronomical Image Processing Software (AIPS), and data cubes for each galaxy were produced. The results include detections of eleven HI lines (two new detections) and ten background continuum sources (two new discoveries). All detections are at a higher resolution and/or sensitivity than previous measurements. These detections spanned a large range of line widths and H I masses; some masses comparable to those in normal galaxies. Of particular interest was the discovery of a compact absorption feature in the dwarf galaxy Haro 11. A paper submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS) Letters on this discovery is presented. Another significant result was the discovery of a highly extended and massive HI region in the dwarf galaxy CGCG 297-017. For those galaxies with no detected HI line or radio continuum, an upper limit to the flux density was computed, which was used to derive upper limits to the HI mass or star-formation rate (SFR) respectively. Three HI flux density upper limits are new results, and seven continuum flux density upper limits are also new. The GDR-Z relation at low metallicities shows a potential power law or broken power law relation with a turning point at Z=7.96 or Z=8.05. To within error, these turning points and power law indices of the broken power law fits are consistent with other work.

  12. A chemical confirmation of the faint Boötes II dwarf spheroidal galaxy

    SciTech Connect

    Koch, Andreas; Rich, R. Michael

    2014-10-10

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

  13. X-ray and Ultraviolet Properties of AGNs in Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Baldassare, Vivienne F.; Reines, Amy E.; Gallo, Elena; Greene, Jenny E.

    2017-02-01

    We present new Chandra X-ray Observatory and Hubble Space Telescope observations of eight optically selected broad-line active galactic nucleus (AGN) candidates in nearby dwarf galaxies (z < 0.055). Including archival Chandra observations of three additional sources, our sample contains all 10 galaxies from Reines et al. (2013) with both broad Hα emission and narrow-line AGN ratios (six AGNs, four composites), as well as one low-metallicity dwarf galaxy with broad Hα and narrow-line ratios characteristic of star formation. All 11 galaxies are detected in X-rays. Nuclear X-ray luminosities range from L 0.5–7keV ≈ 5 × 1039 to 1 × 1042 ergs‑1. In all cases except for the star-forming galaxy, the nuclear X-ray luminosities are significantly higher than would be expected from X-ray binaries, providing strong confirmation that AGNs and composite dwarf galaxies do indeed host actively accreting black holes (BHs). Using our estimated BH masses (which range from ∼7 × 104 to 1 × 106 M ⊙), we find inferred Eddington fractions ranging from ∼0.1% to 50%, i.e., comparable to massive broad-line quasars at higher redshift. We use the HST imaging to determine the ratio of UV to X-ray emission for these AGNs, finding that they appear to be less X-ray luminous with respect to their UV emission than more massive quasars (i.e., α OX values an average of 0.36 lower than expected based on the relation between α OX and 2500 Å luminosity). Finally, we discuss our results in the context of different accretion models onto nuclear BHs.

  14. ASSESSING THE MILKY WAY SATELLITES ASSOCIATED WITH THE SAGITTARIUS DWARF SPHEROIDAL GALAXY

    SciTech Connect

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

    2010-08-01

    Numerical models of the tidal disruption of the Sagittarius (Sgr) dwarf galaxy have recently been developed that for the first time simultaneously satisfy most observational constraints on the angular position, distance, and radial velocity trends of both leading and trailing tidal streams emanating from the dwarf. We use these dynamical models in combination with extant three-dimensional position and velocity data for Galactic globular clusters and dSph galaxies to identify those Milky Way satellites that are likely to have originally formed in the gravitational potential well of the Sgr dwarf, and have been stripped from Sgr during its extended interaction with the Milky Way. We conclude that the globular clusters Arp 2, M 54, NGC 5634, Terzan 8, and Whiting 1 are almost certainly associated with the Sgr dwarf, and that Berkeley 29, NGC 5053, Pal 12, and Terzan 7 are likely to be as well (albeit at lower confidence). The initial Sgr system therefore may have contained five to nine globular clusters, corresponding to a specific frequency S{sub N} = 5-9 for an initial Sgr luminosity M{sub V} = -15.0. Our result is consistent with the 8 {+-} 2 genuine Sgr globular clusters expected on the basis of statistical modeling of the Galactic globular cluster distribution and the corresponding false-association rate due to chance alignments with the Sgr streams. The globular clusters identified as most likely to be associated with Sgr are consistent with previous reconstructions of the Sgr age-metallicity relation, and show no evidence for a second-parameter effect shaping their horizontal branch morphologies. We find no statistically significant evidence to suggest that any of the recently discovered population of ultrafaint dwarf galaxies are associated with the Sgr tidal streams, but are unable to rule out this possibility conclusively for all systems.

  15. Limits on the H I content of the dwarf galaxy Hydra II

    NASA Astrophysics Data System (ADS)

    Janzen, Andrew; Klopf, Eve M.; Lockman, Felix J.; Montez, Rodolfo, Jr.; Plarre, Kurt; Pokhrel, Nau Raj; Selina, Robert J.; Togi, Aditya; Zomederis, Mehrnoush

    2015-12-01

    Sensitive 21 cm H I observations have been made with the Green Bank Telescope toward the newly-discovered Local Group dwarf galaxy Hydra II, which may lie within the leading arm of the Magellanic Stream. No neutral hydrogen was detected. Our 5σ limit of MHI ≤ 210 M⊙ for a 15 km s-1 linewidth gives a gas to luminosity ratio MHI/ LV ≤ 2.6 × 10-2M⊙L⊙-1. The limits on H I mass and MHI/LV are typical of dwarf galaxies found within a few hundred kpc of the Milky Way. Whatever the origin of Hydra II, its neutral gas properties are not unusual.

  16. Chemical evolution of r-process elements in the Draco dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Ishigaki, M. N.; Tsujimoto, T.; Shigeyama, T.; Aoki, W.

    2016-08-01

    A dominant astrophysical site for r-process, which is responsible for producing heavy neutron-capture elements, is unknown. Dwarf spheroidal galaxies around the Milky Way halo provide ideal laboratories to investigate the origin and evolution of r-process elements. We carried out high-resolution spectroscopic observations of three giant stars in the Draco dwarf spheroidal galaxy to estimate their europium abundances. We found that the upper-limits of [Eu/H] are very low in the range [Fe/H] < -2, while this ratio is nearly constant at higher metallicities. This trend is not well reproduced with models which assume that Eu is produced together with Fe by SNe, and may suggest the contribution from other objects such as neutron-star mergers.

  17. Quark nugget dark matter: no contradiction with 511 keV line emission from dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Lawson, Kyle; Zhitnitsky, Ariel

    2017-02-01

    The observed galactic 511 keV line has been interpreted in a number of papers as a possible signal of dark matter annihilation within the galactic bulge. If this is the case then it is possible that a similar spectral feature may be observed in association with nearby dwarf galaxies. These objects are believed to be strongly dark matter dominated and present a relatively clean observational target. Recently INTEGRAL observations have provided new constraints on the 511 keV flux from nearby dwarf galaxies [1] motivating further investigation into the mechanism by which this radiation may arise. In the model presented here dark matter in the form of heavy quark nuggets produces the galactic 511 keV emission line through interactions with the visible matter. It is argued that this type of interaction is not strongly constrained by the flux limits reported in [2].

  18. Probing the Histories of Local Group Dwarf Galaxies with Pulsating Variable Stars

    NASA Astrophysics Data System (ADS)

    Ordoñez, Antonio J.; Sarajedini, Ata

    2017-01-01

    I have identified and characterized the Cepheid and RR Lyrae variables in several Local Group dwarf galaxies using archival Hubble Space Telescope imaging. Template light curve fitting routines have been applied to the observations in order to accurately characterize the properties of these variable stars. The pulsation properties of these stars help to constrain their masses and ages, which in turn shed light on the evolution of their respective host systems. I will summarize what this work has yielded in the context of dwarf galaxy evolution and the accretion history of the Milky Way halo. I will also discuss simulated observations on artificial light curves which we have used to characterize different observing strategies and analysis techniques for studies of pulsating variable stars.

  19. Observations of dwarfs in nearby voids: implications for galaxy formation and evolution

    NASA Astrophysics Data System (ADS)

    Pustilnik, Simon A.

    2016-10-01

    The intermediate results of the ongoing study of deep samples of ~200 galaxies residing in nearby voids, are presented. Their properties are probed via optical spectroscopy, ugri surface photometry, and HI 21-cm line measurements, with emphasis on their evolutionary status. We derive directly the hydrogen mass M(HI), the ratio M(HI)/LB and the evolutionary parameter gas-phase O/H. Their luminosities and integrated colours are used to derive stellar mass M* and the second evolutionary parameter - gas mass-fraction f g). The colours of the outer parts, typically representative of the galaxy oldest stellar population, are used to estimate the upper limits on time since the beginning of the main SF episode. We compare properties of void galaxies with those of the similar late-type galaxies in denser environments. Most of void galaxies show smaller O/H for their luminosity, in average by ~30\\%, indicating slower evolution. Besides, the fraction of ~10\\% of the whole void sample or ~30\\% of the least luminous void LSB dwarfs show the oxygen deficiency by a factor of 2-5. The majority of this group appear very gas-rich, with f g ~(95-99)%, while their outer parts appear rather blue, indicating the time of onset of the main star-formation episode of less than 1-4 Gyr. Such unevolved LSBD galaxies appear not rare among the smallest void objects, but turned out practically missed to date due to the strong observational selection effects. Our results evidense for unusual evolutionary properties of the sizable fraction of void galaxies, and thus, pose the task of better modelling of dwarf galaxy formation and evolution in voids.

  20. TESTING MODIFIED NEWTONIAN DYNAMICS WITH ROTATION CURVES OF DWARF AND LOW SURFACE BRIGHTNESS GALAXIES

    SciTech Connect

    Swaters, R. A.; McGaugh, S. S.; Sanders, R. H.

    2010-07-20

    Dwarf and low surface brightness (LSB) galaxies are ideal objects to test modified Newtonian dynamics (MOND), because in most of these galaxies the accelerations fall below the threshold where MOND supposedly applies. We have selected from the literature a sample of 27 dwarf and LSB galaxies. MOND is successful in explaining the general shape of the observed rotation curves for roughly three quarters of the galaxies in the sample presented here. However, for the remaining quarter, MOND does not adequately explain the observed rotation curves. Considering the uncertainties in distances and inclinations for the galaxies in our sample, a small fraction of poor MOND predictions is expected and is not necessarily a problem for MOND. We have also made fits taking the MOND acceleration constant, a {sub 0}, as a free parameter in order to identify any systematic trends. We find that there appears to be a correlation between central surface brightness and the best-fit value of a{sub 0}, in the sense that lower surface brightness galaxies tend to have lower a{sub 0}. However, this correlation depends strongly on a small number of galaxies whose rotation curves might be uncertain due to either bars or warps. Without these galaxies, there is less evidence of a trend, but the average value we find for a{sub 0} {approx} 0.7 x 10{sup -8} cm s{sup -2} is somewhat lower than derived from previous studies. Such lower fitted values of a{sub 0} could occur if external gravitational fields are important.

  1. STAR FORMATION MODELS FOR THE DWARF GALAXIES NGC 2915 AND NGC 1705

    SciTech Connect

    Elson, E. C.; De Blok, W. J. G.; Kraan-Korteweg, R. C.

    2012-01-15

    Crucial to a quantitative understanding of galaxy evolution are the properties of the interstellar medium that regulate galactic-scale star formation activity. We present here the results of a suite of star formation models applied to the nearby blue compact dwarf galaxies NGC 2915 and NGC 1705. Each of these galaxies has a stellar disk embedded in a much larger, essentially starless H I disk. These atypical stellar morphologies allow for rigorous tests of star formation models that examine the effects on star formation of the H I, stellar, and dark matter mass components, as well as the kinematics of the gaseous and stellar disks. We use far-ultraviolet and 24 {mu}m images from the Galaxy Evolution Explorer and the Spitzer Infrared Nearby Galaxies Survey, respectively, to map the spatial distribution of the total star formation rate surface density within each galaxy. New high-resolution H I line observations obtained with the Australia Telescope Compact Array are used to study the distribution and dynamics of each galaxy's neutral interstellar medium. The standard Toomre Q parameter is unable to distinguish between active and non-active star-forming regions, predicting the H I disks of the dwarfs to be sub-critical. Two-fluid instability models incorporating the stellar and dark matter components of each galaxy, in addition to the gaseous component, yield unstable portions of the inner disk. Finally, a formalization in which the H I kinematics are characterized by the rotational shear of the gas produces models that very accurately match the observations. This suggests the time available for perturbations to collapse in the presence of rotational shear to be an important factor governing galactic-scale star formation.

  2. VLA-ANGST: A High-resolution H I Survey of Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Ott, Jürgen; Stilp, Adrienne M.; Warren, Steven R.; Skillman, Evan D.; Dalcanton, Julianne J.; Walter, Fabian; de Blok, W. J. G.; Koribalski, Bärbel; West, Andrew A.

    2012-10-01

    We present the "Very Large Array survey of Advanced Camera for Surveys Nearby Galaxy Survey Treasury galaxies (VLA-ANGST)." VLA-ANGST is a National Radio Astronomy Observatory Large Program consisting of high spectral (0.6-2.6 km s-1) and spatial (~6'') resolution observations of neutral, atomic hydrogen (H I) emission toward 35 nearby dwarf galaxies from the ANGST survey. ANGST is a systematic Hubble Space Telescope survey to establish a legacy of uniform multi-color photometry of resolved stars for a volume-limited sample of nearby galaxies (D <~ 4 Mpc). VLA-ANGST provides VLA H I observations of the sub-sample of ANGST galaxies with recent star formation that are observable from the northern hemisphere and that were not observed in the "The H I Nearby Galaxy Survey" (THINGS). The overarching scientific goal of VLA-ANGST is to investigate fundamental characteristics of the neutral interstellar medium (ISM) of dwarf galaxies. Here we describe the VLA observations, the data reduction, and the final VLA-ANGST data products. We present an atlas of the integrated H I maps, the intensity-weighted velocity fields, the second moment maps as a measure for the velocity dispersion of the H I, individual channel maps, and integrated H I spectra for each VLA-ANGST galaxy. We closely follow the observational setup and data reduction of THINGS to achieve comparable sensitivity and angular resolution. A major difference between VLA-ANGST and THINGS, however, is the high velocity resolution of the VLA-ANGST observations (0.65 and 1.3 km s-1 for the majority of the galaxies). The VLA-ANGST data products are made publicly available through a dedicated Web site (https://science.nrao.edu/science/surveys/vla-angst). With available star formation histories from resolved stellar populations and lower resolution ancillary observations from the far-infrared to the ultraviolet, VLA-ANGST will enable detailed studies of the relationship between the ISM and star formation in dwarf galaxies

  3. Self-consistent photometric and spectroscopic Star Formation Histories in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    García-Benito, R.; Pérez, E.; Pérez-Montero, E.; González Delgado, R.; Vílchez, J. M.

    2016-06-01

    This project aims to unify the spectroscopic and stellar photometric views by performing a comprehensive study of a sample of the nearest Blue Compact Dwarf Galaxies (BCDs). We plan to derive Star Formation Histories (SFH) both by means of Color-Magnitude Diagrams (CMDs) from extant Hubble Space Telescope (HST) optical imaging and with spectral fitting methods techniques using MUSE, allowing us to obtain state-of-the-art 2D stellar properties and abundances of the gas in BCDs.

  4. A new galactic chemical evolution model with dust: results for dwarf irregular galaxies and DLA systems

    NASA Astrophysics Data System (ADS)

    Gioannini, L.; Matteucci, F.; Vladilo, G.; Calura, F.

    2017-01-01

    We present a galactic chemical evolution model which adopts updated prescriptions for all the main processes governing the dust cycle. We follow in detail the evolution of the abundances of several chemical species (C, O, S, Si, Fe and Zn) in the gas and dust of a typical dwarf irregular galaxy. The dwarf irregular galaxy is assumed to evolve with a low but continuous level of star formation and experience galactic winds triggered by supernova (SN) explosions. We predict the evolution of the gas to dust ratio in such a galaxy and discuss critically the main processes involving dust, such as dust production by asymptotic giant branch stars and Type II SNe, destruction and accretion (gas condensation in clouds). We then apply our model to damped Lyman α (DLA) systems which are believed to be dwarf irregulars, as witnessed by their abundance patterns. Our main conclusions are the following. (i) We can reproduce the observed gas to dust ratio in dwarf galaxies. (ii) We find that the process of dust accretion plays a fundamental role in the evolution of dust and in certain cases it becomes the dominant process in the dust cycle. On the other hand, dust destruction seems to be a negligible process in irregulars. (iii) Concerning DLA systems, we show that the observed gas-phase abundances of silicon, normalized to volatile elements (zinc and sulfur), are in agreement with our model. (iv) The abundances of iron and silicon in DLA systems suggest that the two elements undergo a different history of dust formation and evolution. Our work casts light on the nature of iron-rich dust: the observed depletion pattern of iron is well reproduced only when an additional source of iron dust is considered. Here we explore the possibility of a contribution from Type Ia SNe as well as an efficient accretion of iron nanoparticles.

  5. Dark Matter Searches in Milky Way Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Wang, Mei-Yu

    2017-01-01

    Gamma-ray observations of Milky Way satellite provide one of the most sensitive and robust ways to probe WIMP dark matter. Satellite galaxies are compelling targets for dark matter searches due to their proximity, high dark matter content, and low astrophysical backgrounds. Detailed studies of the stellar kinematics of satellite galaxies precisely determine their dark matter content. In this talk, I will discuss the systematic uncertainties in determining the dark matter content of satellite galaxies from stellar kinematics, and the impacts on WIMP dark matter annihilation/decay cross section limits. I will discuss the modeling of new satellites recently discovered by the Dark Energy Survey (DES), and their impact on indirect dark matter searches. Finally I will discuss how current and future optical imaging surveys will improve our understanding of satellites dark matter distributions.

  6. Dwarf

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Alfalfa dwarf occurs rarely in alfalfa fields. Dwarf has been identified only in California, where it is found at a low frequency. Plants with symptoms of dwarf were reported in the 1950s in Mississippi, Georgia, and Rhode Island, but experimental confirmation of the disease in those States was no...

  7. CORE-COLLAPSE SUPERNOVAE FROM THE PALOMAR TRANSIENT FACTORY: INDICATIONS FOR A DIFFERENT POPULATION IN DWARF GALAXIES

    SciTech Connect

    Arcavi, Iair; Gal-Yam, Avishay; Kasliwal, Mansi M.; Quimby, Robert M.; Ofek, Eran O.; Kulkarni, Shrinivas R.; Law, Nicholas; Cooke, Jeff; Nugent, Peter E.; Poznanski, Dovi; Cenko, S. Bradley; Bloom, Joshua S.; Filippenko, Alexei V.; Sullivan, Mark; Hook, Isobel; Joensson, Jakob; Blake, Sarah; Howell, D. Andrew; Dekany, Richard; Rahmer, Gustavo

    2010-09-20

    We use the first compilation of 72 core-collapse supernovae (SNe) from the Palomar Transient Factory (PTF) to study their observed subtype distribution in dwarf galaxies compared to giant galaxies. Our sample is the largest single-survey, untargeted, spectroscopically classified, homogeneous collection of core-collapse events ever assembled, spanning a wide host-galaxy luminosity range (down to M{sub r} {approx} -14 mag) and including a substantial fraction (>20%) of dwarf (M{sub r} {>=} -18 mag) hosts. We find more core-collapse SNe in dwarf galaxies than expected and several interesting trends emerge. We use detailed subclassifications of stripped-envelope core-collapse SNe and find that all Type I core-collapse events occurring in dwarf galaxies are either SNe Ib or broad-lined SNe Ic (SNe Ic-BL), while 'normal' SNe Ic dominate in giant galaxies. We also see a significant excess of SNe IIb in dwarf hosts. We hypothesize that in lower metallicity hosts, metallicity-driven mass loss is reduced, allowing massive stars that would have appeared as 'normal' SNe Ic in metal-rich galaxies to retain some He and H, exploding as Ib/IIb events. At the same time, another mechanism allows some stars to undergo extensive stripping and explode as SNe Ic-BL (and presumably also as long-duration gamma-ray bursts). Our results are still limited by small-number statistics, and our measurements of the observed N(Ib/c)/N(II) number ratio in dwarf and giant hosts (0.25{sup +0.3}{sub -0.15} and 0.23{sup +0.11}{sub -0.08}, respectively; 1{sigma} uncertainties) are consistent with previous studies and theoretical predictions. As additional PTF data accumulate, more robust statistical analyses will be possible, allowing the evolution of massive stars to be probed via the dwarf-galaxy SN population.

  8. Automated Detection of Dwarf Galaxies and Star Clusters in SMASH through the NOAO Data Lab

    NASA Astrophysics Data System (ADS)

    Olsen, Knut A.; Nidever, David L.; Fitzpatrick, Michael J.; Mighell, Kenneth J.; SMASH Collaboration; NOAO Data Lab Team

    2017-01-01

    We present an automated method, using the NOAO Data Lab environment, for the detection of dwarf galaxy-scale objects in catalog data from the Survey of the Magellanic Stellar History (SMASH). SMASH has imaged ~480 square degrees of the southern sky, over a partially filled area of 2400 square degrees, to 24th mag in gri (uz~23) using the Dark Energy Camera (DECam). The NOAO Data Lab (http://datalab.noao.edu) is being developed to support community research of the massive data sets now being derived from NOAO’s wide-field telescopes, in particular DECam. A key feature of the Data Lab is the ability to perform efficient automated analysis of catalog and imaging data. Our method, which is an example of this feature, allows for the rapid search of candidate dwarf galaxies and stellar clusters in deep catalog data. Using SMASH as the catalog data source, we easily recover the previously discovered Hydra II dwarf galaxy and SMASH-I LMC globular cluster, as well as a number of other potentially interesting candidate stellar systems.

  9. An r-process enhanced star in the dwarf galaxy Tucana III

    DOE PAGES

    Hansen, T. T.; Simon, J. D.; Marshall, J. L.; ...

    2017-03-20

    Chemically peculiar stars in dwarf galaxies provide a window for exploring the birth environment of stars with varying chemical enrichment. We present a chemical abundance analysis of the brightest star in the newly discovered ultra-faint dwarf galaxy candidate Tucana III. Because it is particularly bright for a star in an ultra-faint Milky Way (MW) satellite, we are able to measure the abundance of 28 elements, including 13 neutron-capture species. This star, DES J235532.66–593114.9 (DES J235532), shows a mild enhancement in neutron-capture elements associated with the r-process and can be classified as an r-I star. DES J235532 is the first r-Imore » star to be discovered in an ultra-faint satellite, and Tuc III is the second extremely low-luminosity system found to contain r-process enriched material, after Reticulum II. Comparison of the abundance pattern of DES J235532 with r-I and r-II stars found in other dwarf galaxies and in the MW halo suggests a common astrophysical origin for the neutron-capture elements seen in all r-process enhanced stars. Furthermore, we explore both internal and external scenarios for the r-process enrichment of Tuc III and show that with abundance patterns for additional stars, it should be possible to distinguish between them.« less

  10. An r-process Enhanced Star in the Dwarf Galaxy Tucana III

    NASA Astrophysics Data System (ADS)

    Hansen, T. T.; Simon, J. D.; Marshall, J. L.; Li, T. S.; Carollo, D.; DePoy, D. L.; Nagasawa, D. Q.; Bernstein, R. A.; Drlica-Wagner, A.; Abdalla, F. B.; Allam, S.; Annis, J.; Bechtol, K.; Benoit-Lévy, A.; Brooks, D.; Buckley-Geer, E.; Carnero Rosell, A.; Carrasco Kind, M.; Carretero, J.; Cunha, C. E.; da Costa, L. N.; Desai, S.; Eifler, T. F.; Fausti Neto, A.; Flaugher, B.; Frieman, J.; García-Bellido, J.; Gaztanaga, E.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutierrez, G.; James, D. J.; Krause, E.; Kuehn, K.; Kuropatkin, N.; Lahav, O.; Miquel, R.; Plazas, A. A.; Romer, A. K.; Sanchez, E.; Santiago, B.; Scarpine, V.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Suchyta, E.; Swanson, M. E. C.; Tarle, G.; Walker, A. R.; DES Collaboration

    2017-03-01

    Chemically peculiar stars in dwarf galaxies provide a window for exploring the birth environment of stars with varying chemical enrichment. We present a chemical abundance analysis of the brightest star in the newly discovered ultra-faint dwarf galaxy candidate Tucana III. Because it is particularly bright for a star in an ultra-faint Milky Way (MW) satellite, we are able to measure the abundance of 28 elements, including 13 neutron-capture species. This star, DES J235532.66‑593114.9 (DES J235532), shows a mild enhancement in neutron-capture elements associated with the r-process and can be classified as an r-I star. DES J235532 is the first r-I star to be discovered in an ultra-faint satellite, and Tuc III is the second extremely low-luminosity system found to contain r-process enriched material, after Reticulum II. Comparison of the abundance pattern of DES J235532 with r-I and r-II stars found in other dwarf galaxies and in the MW halo suggests a common astrophysical origin for the neutron-capture elements seen in all r-process enhanced stars. We explore both internal and external scenarios for the r-process enrichment of Tuc III and show that with abundance patterns for additional stars, it should be possible to distinguish between them. This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

  11. The Star Formation Histories of Local Group Dwarf Galaxies. III. Characterizing Quenching in Low-mass Galaxies

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; Holtzman, Jon; Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F.

    2015-05-01

    We explore the quenching of low-mass galaxies (104 ≲ {{M}\\star } ≲ 108 {{M}⊙ }) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived by analyzing color-magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) lower-mass galaxies quench earlier than higher-mass galaxies; (2) inside of Rvirial there is no correlation between a satellite’s current proximity to a massive host and its quenching epoch; and (3) there are hints of systematic differences in the quenching times of M31 and Milky Way (MW) satellites, although the sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with results from the literature, we qualitatively consider the redshift evolution (z = 0-1) of the quenched galaxy fraction over ˜7 dex in stellar mass (104 ≲ {{M}\\star } ≲ 1011.5 {{M}⊙ }). The quenched fraction of all galaxies generally increases toward the present, with both the lowest and highest-mass systems exhibiting the largest quenched fractions at all redshifts. In contrast, galaxies between {{M}\\star } ˜ 108-1010 {{M}⊙ } have the lowest quenched fractions. We suggest that such intermediate-mass galaxies are the least efficient at quenching. Finally, we compare our quenching times with predictions for infall times for low-mass galaxies associated with the MW. We find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have been quenched before infall, while higher-mass satellites (e.g., Leo I, Fornax) typically quench ˜1-4 Gyr after infall. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA constract NAS 5-26555.

  12. A near-infrared census of the multicomponent stellar structure of early-type dwarf galaxies in the Virgo cluster

    SciTech Connect

    Janz, J.; Laurikainen, E.; Salo, H.; Lisker, T.; Hansson, K. S. A.; Meyer, H. T.; Paudel, S.; Peletier, R. F.; Den Brok, M.; Niemi, S.-M.; Toloba, E.; Falcón-Barroso, J.; Ryś, A.

    2014-05-10

    The fraction of star-forming to quiescent dwarf galaxies varies from almost infinity in the field to zero in the centers of rich galaxy clusters. What is causing this pronounced morphology-density relation? What do quiescent dwarf galaxies look like when studied in detail, and what conclusions can be drawn about their formation mechanism? Here we study a nearly magnitude-complete sample (–19 < M{sub r} < –16 mag) of 121 Virgo cluster early types with deep near-infrared images from the SMAKCED project. We fit two-dimensional models with optional inner and outer components, as well as bar and lens components (in ∼15% of the galaxies), to the galaxy images. While a single Sérsic function may approximate the overall galaxy structure, it does not entirely capture the light distribution of two-thirds of our galaxies, for which multicomponent models provide a better fit. This fraction of complex galaxies shows a strong dependence on luminosity, being larger for brighter objects. We analyze the global and component-specific photometric scaling relations of early-type dwarf galaxies and discuss similarities with bright early and late types. The dwarfs' global galaxy parameters show scaling relations that are similar to those of bright disk galaxies. The inner components are mostly fitted with Sérsic n values close to 1. At a given magnitude, they are systematically larger than the bulges of spirals, suggesting that they are not ordinary bulges. We argue that the multicomponent structures in early-type dwarfs are mostly a phenomenon inherent to the disks and may indeed stem from environmental processing.

  13. AN IONIZATION CONE IN THE DWARF STARBURST GALAXY NGC 5253

    SciTech Connect

    Zastrow, Jordan; Oey, M. S.; Veilleux, Sylvain; McDonald, Michael; Martin, Crystal L.

    2011-11-01

    There are few observational constraints on how the escape of ionizing photons from starburst galaxies depends on galactic parameters. Here we report on the first major detection of an ionization cone in NGC 5253, a nearby starburst galaxy. This high-excitation feature is identified by mapping the emission-line ratios in the galaxy using [S III] {lambda}9069, [S II] {lambda}6716, and H{alpha} narrowband images from the Maryland-Magellan Tunable Filter at Las Campanas Observatory. The ionization cone appears optically thin, which suggests the escape of ionizing photons. The cone morphology is narrow with an estimated solid angle covering just 3% of 4{pi} steradians, and the young, massive clusters of the nuclear starburst can easily generate the radiation required to ionize the cone. Although less likely, we cannot rule out the possibility of an obscured active galactic nucleus source. An echelle spectrum along the minor axis shows complex kinematics that are consistent with outflow activity. The narrow morphology of the ionization cone supports the scenario that an orientation bias contributes to the difficulty in detecting Lyman continuum emission from starbursts and Lyman break galaxies.

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

    NASA Technical Reports Server (NTRS)

    Konishi, Mihoko; Shibai, Hiroshi; Sumi, Takahiro; Fukagawa, Misato; Matsuo, Taro; Samland, Matthias S.; Yamamoto, Kodai; Sudo, Jun; Itoh, Yoichi; Arimoto, Nobuo; Kajisawa, Masaru; Abe, Lyu; Brandner, Wolfgang; Brandt, Timothy D.; Carson, Joseph; Currie, Thayne; Egner, Sebastian E.; Feldt, Marcus; Goto, Miwa; Grady, Carol A.; Guyon, Oliver; Hashimoto, Jun; Hayano, Yutaka; Hayashi, Masahiko; McElwain, Michael W.

    2014-01-01

    We compared the number of faint stars detected in deep survey fields with the current stellar distribution model of the Galaxy and found that the detected number in the H band is significantly smaller than the predicted number. This indicates that M-dwarfs, the major component, are fewer in the halo and the thick disk. We used archived data of several surveys in both the north and south field of GOODS (Great Observatories Origins Deep Survey), MODS in GOODS-N, and ERS and CANDELS in GOODS-S. The number density of M-dwarfs in the halo has to be 20+/-13% relative to that in the solar vicinity, in order for the detected number of stars fainter than 20.5 mag in the H band to match with the predicted value from the model. In the thick disk, the number density of M-dwarfs must be reduced (52+/-13%) or the scale height must be decreased ( approx. 600 pc). Alternatively, overall fractions of the halo and thick disks can be significantly reduced to achieve the same effect, because our sample mainly consists of faint M-dwarfs. Our results imply that the M-dwarf population in regions distant from the Galactic plane is significantly smaller than previously thought. We then discussed the implications this has on the suitability of the model predictions for the prediction of non-companion faint stars in direct imaging extrasolar planet surveys by using the best-fit number densities.

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

    NASA Technical Reports Server (NTRS)

    Konishi, Mihoko; Shibai, Hiroshi; Sumi, Takahiro; Fukagawa, Misato; Matsuo, Taro; Samland, Matthias S.; Yamamoto, Kodai; Sudo, Jun; Itoh, Yoichi; Arimoto, Nubuo; Kajisawa, Masaru; Lyu, Abe; Brandner, Wolfgang; Brandt, Timothy D.; Carson, Joseph; Currie, Thayne; Egner, Sebastian E.; Feldt, Markus; Goto, Miwa; Grady, Carol A.; Oliver, Guyon; Hashimoto, Jun; Hayano, Yutaka; Hayashi, Masahiko; Hayashi, Saeko S.; Henning, Thomas; Hodapp, Klaus W.; Ishii, Miki; Iye, Masanori; Janson, Markus; Kandori, Ryo; Knapp, Gillian R.; Kudo,Tomoyuki; Kusakabe, Nobuhiko; Kwon, Jungmi; McElwain, Michael W.

    2014-01-01

    We compared the number of faint stars detected in deep survey fields with the current stellar distribution model of the Galaxy and found that the detected number in the H band is significantly smaller than the predicted number. This indicates that M-dwarfs, the major component, are fewer in the halo and the thick disk. We used archived data of several surveys in both the north and south field of GOODS (Great Observatories Origins Deep Survey), MODS in GOODS-N, and ERS and CANDELS in GOODS-S. The number density of M-dwarfs in the halo has to be 20 +/- 13% relative to that in the solar vicinity, in order for the detected number of stars fainter than 20.5 mag in the H band to match with the predicted value from the model. In the thick disk, the number density of M-dwarfs must be reduced (52 +/- 13%) or the scale height must be decreased (approximately 600 pc). Alternatively, overall fractions of the halo and thick disks can be significantly reduced to achieve the same effect, because our sample mainly consists of faint M-dwarfs. Our results imply that the M-dwarf population in regions distant from the Galactic plane is significantly smaller than previously thought. We then discussed the implications this has on the suitability of the model predictions for the prediction of non-companion faint stars in direct imaging extrasolar planet surveys by using the best-fit number densities.

  16. The Lyman Continuum Escape Fraction of Dwarf, Star-Forming Galaxies at z~1

    NASA Astrophysics Data System (ADS)

    Rutkowski, Michael J.; Scarlata, Claudia; Teplitz, Harry I.; Hayes, Matthew; Salvato, Mara; Beck, Melanie; Mehta, Vihang; Pahl, Anthony

    2015-01-01

    The dominant astrophysical source(s) of Lyman Continuum (LyC, λ<912Å) photons which reionized neutral Hydrogen in the IGM at high (z > 6) redshift remains uncertain. Unfortunately, the direct detection of LyC photons escaping from the analogs of such sources --- i.e., star-forming galaxies --- in the low redshift (z<~1) universe has proven to be remarkably difficult with space-based observatories. Motivated by the few (~2) direct detections of LyC in the local Universe and the results of recent cosmological simulations of galaxy evolution which demonstrate that low-mass (M<~109M⊙) star-forming galaxies may be uniquely suited to contribute to the ionizing, UV background, we present results of recent work to study the LyC escape fraction in z~1 dwarf, star-forming galaxies. We present an independent re-reduction of the WFC3 IR grism data obtained as part of the 3DHST and AGHAST surveys, and identify and select star-forming galaxies at z=0.9-1.4 by their Hα emission. At this redshift range, GALEX FUV and NUV images can be used to cleanly measure the ratio of LyC to UV non-ionizing continuum (i.e., LyCesc,rel) photons. We join our line and redshift identifications with public photometric-redshift catalogs made available by the 3DHST team in order to select an ideal sample of star-forming galaxies which excludes likely contaminants (e.g., AGN, low-redshift interlopers, etc.). Stacking archival GALEX images of ~500 UV non-detected star-forming (SFR<~5M⊙yr-1 ) galaxies, we measure an upper limit to fLyCesc,relequal to ~5%. With these data we are also able to directly constrain fLyCesc,rel for a population of isolated, high equivalent width (EW>200Å), dwarf (M<109M⊙) star-forming galaxies, measuring an upper limit of fLyCesc,rel< ~20% from an analysis of stacked data. We will discuss the implications for reionization of these escape fractions measured from the stacking analysis, as well as possible UV detections from individual dwarf galaxies.

  17. A Deeper Look at Faint Hα Emission in Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Janice C.; Veilleux, Sylvain; McDonald, Michael; Hilbert, Bryan

    2016-02-01

    We present deep Hα imaging of three nearby dwarf galaxies, carefully selected to optimize observations with the Maryland-Magellan Tunable Filter (MMTF) on the Magellan 6.5 m telescope. An effective bandpass of ˜13 Å is used, and the images reach 3σ flux limits of ˜8 × 10-18 erg s-1 cm-2, which is about an order of magnitude lower than standard narrowband observations obtained by the most recent generation of local Hα galaxy surveys. The observations were originally motivated by the finding that the Hα/FUV flux ratio of galaxies systematically declines as global galactic properties such as the star formation rate (SFR) and stellar mass decrease. The three dwarf galaxies selected for study have SFRs that, when calculated from their Hα luminosities using standard conversion recipes, are ˜50% of those based on the FUV. Follow-up studies of many of the potential causes for the trends in the Hα/FUV flux ratio have been performed, but the possibility that previous observations have missed a non-negligible fraction of faint ionized emission in dwarf galaxies has not been investigated. The MMTF observations reveal both diffuse and structured Hα emission (filaments, shells, possible single-star H ii regions) spanning extents up to 2.5 times larger relative to previous observations. However, only up to an additional ˜5% of Hα flux is captured, which does not account for the trends in the Hα/FUV ratio. Beyond investigation of the Hα/FUV ratio, the impact of the newly detected extended flux on our understanding of star formation, the properties of H ii regions, and the propagation of ionizing photons warrant further investigation.

  18. A DEEPER LOOK AT FAINT Hα EMISSION IN NEARBY DWARF GALAXIES

    SciTech Connect

    Lee, Janice C.; Hilbert, Bryan; Veilleux, Sylvain; McDonald, Michael

    2016-02-01

    We present deep Hα imaging of three nearby dwarf galaxies, carefully selected to optimize observations with the Maryland-Magellan Tunable Filter (MMTF) on the Magellan 6.5 m telescope. An effective bandpass of ∼13 Å is used, and the images reach 3σ flux limits of ∼8 × 10{sup −18} erg s{sup −1} cm{sup −2}, which is about an order of magnitude lower than standard narrowband observations obtained by the most recent generation of local Hα galaxy surveys. The observations were originally motivated by the finding that the Hα/FUV flux ratio of galaxies systematically declines as global galactic properties such as the star formation rate (SFR) and stellar mass decrease. The three dwarf galaxies selected for study have SFRs that, when calculated from their Hα luminosities using standard conversion recipes, are ∼50% of those based on the FUV. Follow-up studies of many of the potential causes for the trends in the Hα/FUV flux ratio have been performed, but the possibility that previous observations have missed a non-negligible fraction of faint ionized emission in dwarf galaxies has not been investigated. The MMTF observations reveal both diffuse and structured Hα emission (filaments, shells, possible single-star H ii regions) spanning extents up to 2.5 times larger relative to previous observations. However, only up to an additional ∼5% of Hα flux is captured, which does not account for the trends in the Hα/FUV ratio. Beyond investigation of the Hα/FUV ratio, the impact of the newly detected extended flux on our understanding of star formation, the properties of H ii regions, and the propagation of ionizing photons warrant further investigation.

  19. Abundance ratios and IMF slopes in the dwarf elliptical galaxy NGC 1396 with MUSE

    NASA Astrophysics Data System (ADS)

    Mentz, J. J.; La Barbera, F.; Peletier, R. F.; Falcón-Barroso, J.; Lisker, T.; van de Ven, G.; Loubser, S. I.; Hilker, M.; Sánchez-Janssen, R.; Napolitano, N.; Cantiello, M.; Capaccioli, M.; Norris, M.; Paolillo, M.; Smith, R.; Beasley, M. A.; Lyubenova, M.; Munoz, R.; Puzia, T.

    2016-12-01

    Deep observations of the dwarf elliptical (dE) galaxy NGC 1396 (MV = -16.60, Mass ˜4 × 108 M⊙), located in the Fornax cluster, have been performed with the Very Large Telescope/Multi Unit Spectroscopic Explorer spectrograph in the wavelength region from 4750 to 9350 Å. In this paper, we present a stellar population analysis studying chemical abundances, the star formation history (SFH) and the stellar initial mass function (IMF) as a function of galactocentric distance. Different, independent ways to analyse the stellar populations result in a luminosity-weighted age of ˜6 Gyr and a metallicity [Fe/H]˜ -0.4, similar to other dEs of similar mass. We find unusually overabundant values of [Ca/Fe] ˜+ 0.1, and underabundant Sodium, with [Na/Fe] values around -0.1, while [Mg/Fe] is overabundant at all radii, increasing from ˜+ 0.1 in the centre to ˜+ 0.2 dex. We notice a significant metallicity and age gradient within this dwarf galaxy. To constrain the stellar IMF of NGC 1396, we find that the IMF of NGC 1396 is consistent with either a Kroupa-like or a top-heavy distribution, while a bottom-heavy IMF is firmly ruled out. An analysis of the abundance ratios, and a comparison with galaxies in the Local Group, shows that the chemical enrichment history of NGC 1396 is similar to the Galactic disc, with an extended SFH. This would be the case if the galaxy originated from a Large Magellanic Cloud-sized dwarf galaxy progenitor, which would lose its gas while falling into the Fornax cluster.

  20. Feedback in nearby dwarf starburst galaxies and giant extragalactic H II regions

    NASA Astrophysics Data System (ADS)

    Schwartz, Colleen

    Giant extragalactic H II regions in nearby normal galaxies are similar to dwarf starburst galaxies in luminosity/star formation rate, physical size, and stellar population, although they differ in gravitational potential, star formation rate per unit area, and surrounding environment. This dissertation compares feedback processes in these two types of star-forming regions. Feedback, the cycle which regulates the, relationship between star formation, the interstellar medium, and the intergalactic medium, is empirically measured via observations of interstellar gas in star-forming regions, where stellar winds and supernovae create galactic-scale outflows of interstellar gas and dust which in turn may heat and enrich the intergalactic medium. These kiloparsec-scale winds are most directly probed via the supernova-heated hot gas. However, the cold and photoionized warm interstellar gas from the disk is entrained in the flow and also traces large-scale motions of outflowing matter. I investigate this via optical and ultraviolet absorption-line spectroscopy of the cold neutral medium, as well as emission-line studies of the Ha recombination in the warm ionized gas. The H II regions in disks initially seem physically similar to dwarf starburst galaxies; shell fragments of ionized gas are found in both environments via high-resolution Ha emission spectroscopy. However, while complex Na D absorption profiles trace outflowing cold, neutral gas accelerated to near the escape velocity, no such counterpart is detected in the H II regions in outer disks. The lack of large-scale outflows of cold, neutral gas from the H II regions indicates that while dwarf starburst galaxies and the giant H II regions in normal galaxies may have similar properties, yet the feedback cycle in these regions is different.

  1. First confirmed ultra-compact dwarf galaxy in the NGC 5044 group

    NASA Astrophysics Data System (ADS)

    Faifer, Favio R.; Escudero, Carlos G.; Scalia, María C.; Smith Castelli, Analía V.; Norris, Mark; De Rossi, María E.; Forte, Juan C.; Cellone, Sergio A.

    2017-03-01

    Context. Ultra-compact dwarfs (UCDs) are stellar systems displaying colours and metallicities between those of globular clusters (GCs) and early-type dwarf galaxies, as well as sizes of Reff ≲ 100 pc and luminosities in the range -13.5 dwarf galaxies. Aims: NGC 5044 is the central massive elliptical galaxy of the NGC 5044 group. Its GC/UCD system is completely unexplored. Methods: In Gemini+GMOS deep images of several fields around NGC 5044 and in spectroscopic multi-object data of one of these fields, we detected an unresolved source with g' 20.6 mag, compatible with being an UCD. Its radial velocity was obtained with FXCOR and the penalized pixel-fitting (pPXF) code. To study its stellar population content, we measured the Lick/IDS indices and compared them with predictions of single stellar population models, and we used the full spectral fitting technique. Results: The spectroscopic analysis of the UCD revealed a radial velocity that agrees with the velocity of the elliptical galaxy NGC 5044. From the Lick/IDS indices, we have obtained a luminosity-weighted age and metallicity of 11.7+ 1.4-1.2 Gyr and [Z/H] = -0.79 ± 0.04 dex, respectively, as well as [α/ Fe] = 0.30 ± 0.06. From the full spectral fitting technique, we measured a lower age (8.52 Gyr) and a similar total metallicity ([Z/H] = -0.86 dex). Conclusions: Our results indicate that NGC 5044-UCD1 is most likely an extreme GC (MV -12.5 mag) belonging to the GC system of the elliptical galaxy NGC 5044.

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

    NASA Astrophysics Data System (ADS)

    Lin, Weikang; Ishak, Mustapha

    2016-10-01

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

  3. Blue compact dwarf galaxies. II - Near-infrared studies and stellar populations

    NASA Technical Reports Server (NTRS)

    Thuan, T. X.

    1983-01-01

    An IR photometric survey was performed of 36 blue compact dwarf galaxies (BCDG) where intense bursts of star formation have been observed. The survey covered the J, H, and K lines, with all readings taken at the level of a few mJy. Although the near-IR fluxes observed in the galaxies are due to K and M giants, the bursts have calculated ages of less than 50 million yr. However, the BCDG galaxies surveyed are not young, with the least chemically evolved galaxy observed, I Zw 18, featuring 50 pct of its stars formed prior to its last burst, but with a missing mass that is not accounted for by H I interferometric observations. It is concluded that the old stars must be more spatially extended than the young stars, and a mixture of OB stars with the K and M giants is projected as capable of displaying the colors observed. The star formation processes in the BCDG galaxies is defined as dependent on the total mass of the galaxies, with low mass galaxies having a high ratio of star formation, compared to their previous rates.

  4. Investigating environmental trends in the outer visible edges of dwarf irregular galaxies

    NASA Astrophysics Data System (ADS)

    Dunn, Jacqueline M.

    2017-03-01

    The star formation histories and evolution of 28 dwarf irregular galaxies (dIs) that reside in differing local and global environments are investigated. The shallow gravitational potentials of dwarf galaxies make these objects highly susceptible to changes in morphology or dynamics by external perturbations. Additionally, the lack of more complicated structures such as spiral arms makes environmental effects more easily discernible. Therefore, dIs are ideal candidates for a study of the role of environment in galaxy evolution. The local environment is defined by the local galaxy number density, where high indicates at least one neighbor within 200 kpc and low indicates no neighbors within 1 Mpc. The global environment is classified as either the field or a galaxy group / cluster. Absolute magnitudes, colors, central surface brightnesses, star formation rates and color profiles were compared using photometry from UBVR and Hα imaging. While some environmental trends are noted (galaxies in local high density environments have brighter central and effective surface brightnesses, while those in global high density environments have brighter absolute magnitudes, central and effective surface brightnesses, and higher star formation rates), no systematic environmental trends are seen in the shape of the color profiles or spatial distribution of recent star formation. A lack of environmental trend in star formation and galaxy color indicates that either internal processes dominate the formation and subsequent evolution of the outskirts of dIs, or there is no systematic environmental effect on such. The appearance of environmental effects should be more noticeable at the outermost edges of these systems. The lack of a trend in such implies that the exact nature of the environmental influence varies greatly depending on the exact nature of the interaction occurring.

  5. Early chemo-dynamical evolution of dwarf galaxies deduced from enrichment of r-process elements

    NASA Astrophysics Data System (ADS)

    Hirai, Yutaka; Ishimaru, Yuhri; Saitoh, Takayuki R.; Fujii, Michiko S.; Hidaka, Jun; Kajino, Toshitaka

    2017-04-01

    The abundance of elements synthesized by the rapid neutron-capture process (r-process elements) of extremely metal-poor (EMP) stars in the Local Group galaxies gives us clues to clarify the early evolutionary history of the Milky Way halo. The Local Group dwarf galaxies would have similarly evolved with building blocks of the Milky Way halo. However, how the chemo-dynamical evolution of the building blocks affects the abundance of r-process elements is not yet clear. In this paper, we perform a series of simulations using dwarf galaxy models with various dynamical times and total mass, which determine star formation histories. We find that galaxies with dynamical times longer than 100 Myr have star formation rates less than 10-3 M⊙ yr-1 and slowly enrich metals in their early phase. These galaxies can explain the observed large scatters of r-process abundance in EMP stars in the Milky Way halo regardless of their total mass. On the other hand, the first neutron star merger appears at a higher metallicity in galaxies with a dynamical time shorter than typical neutron star merger times. The scatters of r-process elements mainly come from the inhomogeneity of the metals in the interstellar medium whereas the scatters of α-elements are mostly due to the difference in the yield of each supernova. Our results demonstrate that the future observations of r-process elements in EMP stars will be able to constrain the early chemo-dynamical evolution of the Local Group galaxies.

  6. Local Group Dwarf Galaxies in the LCDM Cosmology: Theory Meets Observations

    NASA Astrophysics Data System (ADS)

    Tollerud, Erik Jon

    2012-05-01

    Dwarf galaxies include some of the most extreme low-luminosity objects in the universe, and provide important windows into a wide variety of processes in galaxy formation and evolution. In this thesis, I describe a series of comparisons between observations of dwarf galaxies and predictions of the ΛCDM concordance cosmology, with a focus on Local Group satellites. I first correct the Milky Way satellite luminosity function for luminosity bias under the assumption of a typical ΛCDM satellite distribution, finding consistency with the observations and a prediction of possibly hundreds of faint Milky Way satellites. I also describe a new technique to connect the luminous properties of these satellites (as well as brighter galaxies) to their expected dark matter halo properties. I further consider the brightest Milky Way satellite, the Large Magellanic cloud (LMC), in a cosmological context by comparing it to similar galaxies in the Sloan Digital Sky Survey (SDSS). This shows that ΛCDM n-body simulations provide a good match to observations of such satellites. I also show that, while LMC-like satellites are not uncommon, the LMC is unusual in how blue it is, especially given that the SDSS satellites are significantly redder than typical galaxies of their size. Finally, I present a large new data for faint satellites of M31, the nearest galaxy similar to the Milky Way, providing a second data point for detailed studies of faint satellite systems. I also shows that its satellites are very similar in their general properties to that of the Milky Way satellites.

  7. THE NATURE OF STARBURSTS. I. THE STAR FORMATION HISTORIES OF EIGHTEEN NEARBY STARBURST DWARF GALAXIES

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Stark, David; Weisz, Daniel; Cannon, John M.; Dalcanton, Julianne; Williams, Benjamin; Dolphin, Andrew; Hidalgo-RodrIguez, Sebastian

    2010-09-20

    We use archival Hubble Space Telescope observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper, we present the observations, color-magnitude diagrams (CMDs), and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only the currently bursting galaxies but also 'fossil' bursts increasing the sample size of starburst galaxies in the nearby (D < 8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long >100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid 'self-quenching' of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find that the strength of the H{alpha} emission usually correlates with the CMD-based SFR during the last 4-10 Myr. However, in four cases, the H{alpha} emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the H{alpha} emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy.

  8. Globular cluster systems in nearby dwarf galaxies - III. Formation efficiencies of old globular clusters

    NASA Astrophysics Data System (ADS)

    Georgiev, Iskren Y.; Puzia, Thomas H.; Goudfrooij, Paul; Hilker, Michael

    2010-08-01

    We investigate the origin of the shape of the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (SN), specific luminosity (SL), specific mass (SM) and specific number () of GCs. In the low-mass galaxy regime (MV >~ -16 mag), our analysis is based on Hubble Space Telescope/Advanced Camera for Surveys observations of GC populations of faint, mainly late-type dwarf galaxies in low-density environments. In order to sample the entire range in galaxy mass (MV = -11 to -23mag =106- 1011Lsolar), environment and morphology we augment our sample with data of spiral and elliptical galaxies from the literature, in which old GCs are reliably detected. This large data set confirms (irrespective of the galaxy type) the increase in the specific frequencies of GCs above and below a galaxy magnitude of MV ~= -20mag. Over the full mass range, the SL value of early-type galaxies is, on average, twice that of late types. To investigate the observed trends, we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of Dekel and Birnboim in which star formation processes are regulated by stellar and supernova feedback below a stellar mass of 3 × 1010Msolar and by virial shocks above it. We find that the analytical model describes remarkably well the shape of the GC system scaling parameter distributions with a universal specific GC formation efficiency, η, which relates the total mass in GCs to the total galaxy halo mass. Early-type and late-type galaxies show a similar mean value of η = 5.5 × 10-5, with an increasing scatter towards lower galaxy masses. This can be due to the enhanced stochastic nature of the star and star-cluster formation processes for such systems. Some massive galaxies have excess η values compared to what is expected from the mean model prediction for galaxies more luminous than MV ~= -20mag (LV >~ 1010Lsolar). This may be attributed to a very

  9. OUTSIDE-IN SHRINKING OF THE STAR-FORMING DISK OF DWARF IRREGULAR GALAXIES

    SciTech Connect

    Zhang Hongxin; Hunter, Deidre A.; Elmegreen, Bruce G.; Gao Yu; Schruba, Andreas E-mail: dah@lowell.edu E-mail: bge@us.ibm.com

    2012-02-15

    We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies. Our data include Galaxy Evolution Explorer (GALEX) FUV/NUV, UBV, and H{alpha} and Spitzer 3.6 {mu}m images. These galaxies constitute the majority of the LITTLE THINGS survey (Local Irregulars That Trace Luminosity Extremes-The H I Nearby Galaxy Survey). By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation histories, we derived the stellar mass surface density distributions and the star formation rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr, and a Hubble time. We find that, for {approx}80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths, corresponding to younger stellar populations, have shorter disk scale lengths than those at longer wavelengths, corresponding to older stellar populations. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and {approx}80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of the star formation rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent star formation rate to stellar mass with radius. Nevertheless, {approx}35% (12 galaxies, among which 7 have baryonic mass {approx}<10{sup 8} M{sub Sun} ) of the sample exhibit negative slopes across the observed disk, which is in contrast with the so-called inside-out disk growth scenario suggested for luminous spiral galaxies. The tendency of star formation to become concentrated toward the inner disks in low-mass dwarf irregular galaxies is

  10. Satellite accretion in action: a tidally disrupting dwarf spheroidal around the nearby spiral galaxy NGC 253

    NASA Astrophysics Data System (ADS)

    Romanowsky, Aaron J.; Martínez-Delgado, David; Martin, Nicolas F.; Morales, Gustavo; Jennings, Zachary G.; GaBany, R. Jay; Brodie, Jean P.; Grebel, Eva K.; Schedler, Johannes; Sidonio, Michael

    2016-03-01

    We report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy candidate undergoing tidal disruption around a nearby spiral galaxy, NGC 253 in the Sculptor group: the first such event identified beyond the Local Group. The dwarf was found using small-aperture amateur telescopes, and followed up with Suprime-Cam on the 8 m Subaru Telescope in order to resolve its brightest stars. Using g- and Rc-band photometry, we detect a red giant branch consistent with an old, metal-poor stellar population at a distance of ˜3.5 Mpc. From the distribution of likely member stars, we infer a highly elongated shape with a semimajor axis half-light radius of (2 ± 0.4) kpc. Star counts also yield a luminosity estimate of ˜2 × 106 L⊙,V (MV ˜ -10.7). The morphological properties of NGC 253-dw2 mark it as distinct from normal dSphs and imply ongoing disruption at a projected distance of ˜50 kpc from the main galaxy. Our observations support the hierarchical paradigm wherein massive galaxies continuously accrete less massive ones, and provide a new case study for dSph infall and dissolution dynamics. We also note the continued efficacy of small telescopes for making big discoveries.

  11. Antlia Dwarf Galaxy: distance, quantitative morphology and recent formation history via statistical field correction

    NASA Astrophysics Data System (ADS)

    Pimbblet, Kevin A.; Couch, Warrick J.

    2012-01-01

    We apply a statistical field correction technique originally designed to determine membership of high redshift galaxy clusters to Hubble Space Telescope (HST) imaging of the Antlia Dwarf Galaxy; a galaxy at the very edge of the Local Group. Using the tip of the red giant branch standard candle method coupled with a simple Sobel edge detection filter we find a new distance to Antlia of 1.31 ± 0.03 Mpc. For the first time for a Local Group member, we compute the concentration, asymmetry and clumpiness quantitative morphology parameters for Antlia from the distribution of resolved stars in the HST/Advanced Camera for Surveys (ACS) field, corrected with a new method for contaminants and complement these parameters with the Gini coefficient (G) and the second-order moment of the brightest 20 per cent of the flux (M20). We show that it is a classic dwarf elliptical (C = 2.0, A = 0.063, S = 0.077, G = 0.39 and M20=-1.17 in the F814W band), but has an appreciable blue stellar population at its core, confirming on-going star formation. The values of asymmetry and clumpiness, as well as Gini and M20 are consistent with an undisturbed galaxy. Although our analysis suggests that Antlia may not be tidally influenced by NGC 3109, it does not necessarily preclude such interaction.

  12. Mapping the Properties of Blue Compact Dwarf Galaxies by Means of Integral Field Spectroscopy

    NASA Astrophysics Data System (ADS)

    Cairós, L. M.; Caon, N.; Weilbacher, P.; Papaderos, P.; García-Lorenzo, B.

    Blue Compact Dwarf (BCD) galaxies are metal-poor and gas-rich systems undergoing intense, spatially extended star-forming activity. These galaxies offer a unique opportunity to investigate dwarf galaxy formation and evolution, and probe violent star formation and its implications on the chemical, dynamical and structural properties of low-mass extragalactic systems near and far. Several fundamental questions in BCD research, such as their star formation histories and the mechanisms that control their cyclic starburst activity, are still far from well understood. In order to improve our understanding on BCD evolution, we are carrying out a comprehensive Integral Field Spectroscopic (IFS) survey of a large sample of BCDs. Integral Field Unit (IFU) spectroscopy provides simultaneously spectral and spatial information, allowing, in just one shot, to study the morphology and evolutionary status of the stellar component, and the physical properties of the warm interstellar medium (e.g., extinction, chemical abundances, kinematics). This ongoing IFS survey will supply much needed local templates that will ease the interpretation of IFS data for intermediate and high-redshift star-forming galaxies.

  13. A COMPREHENSIVE, WIDE-FIELD STUDY OF PULSATING STARS IN THE CARINA DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Vivas, A. Katherina; Mateo, Mario E-mail: mmateo@umich.edu

    2013-12-01

    We report the detection of 388 pulsating variable stars (and some additional miscellaneous variables) in the Carina dwarf spheroidal galaxy over an area covering the full visible extent of the galaxy and extending a few times beyond its photometric (King) tidal radius along the direction of its major axis. Included in this total are 340 newly discovered dwarf Cepheids (DCs), which are mostly located ∼2.5 mag below the horizontal branch and have very short periods (<0.1 days), typical of their class and consistent with their location on the upper part of the extended main sequence of the younger populations of the galaxy. Several extra-tidal DCs were found in our survey up to a distance of ∼1° from the center of Carina. Our sample also includes RR Lyrae stars and anomalous Cepheids, some of which were found outside the galaxy's tidal radius as well. This supports past works that suggest that Carina is undergoing tidal disruption. We use the period-luminosity relationship for DCs to estimate a distance modulus of μ{sub 0} = 20.17 ± 0.10 mag, in very good agreement with the estimate from RR Lyrae stars. We find some important differences in the properties of the DCs of Carina and those in Fornax and the LMC, the only extragalactic samples of DCs currently known. These differences may reflect a metallicity spread, depth along the line of sight, and/or different evolutionary paths of the DC stars.

  14. Bursts of star formation in computer simulations of dwarf galaxies

    SciTech Connect

    Comins, N.F.

    1984-09-01

    A three-dimensional Stochastic Self-Propagating Star Formation (SSPSF) model of compact galacies is presented. Two phases of gas, active and inactive, are present, and permanent depletion of gas in the form of long lived, low mass stars and remnants occurs. Similarly, global infall of gas from a galactic halo or through galactic cannibalism is permitted. We base our parameters on the observed properties of the compact blue galaxy I Zw 36. Our results are that bursts of star formation occur much more frequently in these runs than continuous nonbursting star formation, suggesting that the blue compact galaxies are probably undergoing bursts rather than continuous, nonbursting low-level star formation activity.

  15. Stellar populations in dwarf galaxies: A review of the contribution of HST to our understanding of the nearby universe

    NASA Astrophysics Data System (ADS)

    Tolstoy, Eline

    This review aims to give an overview of the contribution of the Hubble Space Telescope to our understanding of the detailed properties of Local Group dwarf galaxies and their older stellar populations. The exquisite stable high spatial resolution combined with photometric accuracy of images from the Hubble Space Telescope have allowed us to probe further back into the history of star formation of a large variety of different galaxy types with widely differing star formation properties. It has allowed us to extend our studies out to the edges of the Local Group and beyond with greater accuracy than ever before. We have learned several important things about dwarf galaxy evolution from these studies. Firstly we have found that no two galaxies have identical star formation histories; some galaxies may superficially look the same today, but they have invariably followed different paths to this point. Now that we have managed to probe deep into the star formation history of dwarf irregular galaxies in the Local Group it is obvious that there are a number of similarities with the global properties of dwarf elliptical/spheroidal type galaxies, which were previously thought to be quite distinct. The elliptical/spheroidals tend to have one or more discrete episodes of star formation through-out their history and dwarf irregulars are characterized by quasi-continuous star-formation. The previous strong dichotomy between these two classes has been weakened by these new results and may stem from the differences in the environment in which these similar mass galaxies were born into or have inhabited for most of their lives. The more detailed is our understanding of star formation processes and their effect on galaxy evolution in the nearby Universe the better we will understand the results from studies of the integrated light of galaxies in the high-redshift Universe.

  16. VizieR Online Data Catalog: Dwarf galaxies surface brightness profiles. I. (Herrmann+, 2013)

    NASA Astrophysics Data System (ADS)

    Herrmann, K. A.; Hunter, D. A.; Elmegreen, B. G.

    2014-07-01

    Our galaxy sample (see Table 1) is from the survey of Hunter & Elmegreen (2004, Cat. J/AJ/128/2170), which includes 94 dwarf Irregulars (dIms), 26 Blue Compact Dwarfs (BCDs), and 20 Magellanic-type spirals (Sms).4 (The Sm F567-2 is missing from this study and two dIms, F473-V1 and F620-V3, have been added.) UBV images from 27 observing runs are given by Hunter & Elmegreen (2006, Cat. J/ApJS/162/49) for 136 dwarfs and images from at least one band of JHK are presented for 41 (26 dIm, 12 BCD, and 3 Sm) from an additional 9 observing runs. The remaining 5 out of 141 galaxies (DDO 125, Mrk 67, NGC 1705, NGC 2101, and NGC 3109) were observed earlier with a different detector and so were not included in Hunter & Elmegreen (2006). Most of the UBV and JHK images were obtained with the Lowell Observatory 1.1m or 1.8m telescopes. Satellite UV images were also obtained with the Galaxy Evolution Explorer (GALEX). GALEX produced images in two passbands: FUV, with a bandpass of 1350-1750Å, an effective wavelength of 1516Å, and a resolution of 4" and NUV with a bandpass of 1750-2800Å, an effective wavelength of 2267Å, and a resolution of 5.6". The FUV and NUV data are on the AB magnitude system whereas the data in all other broad bands are on the Johnson/Cousin system. Hunter et al. (2010AJ....139..447H) analyzed surface photometry and derived SF rates from archival GALEX images of 44 galaxies. (Note that NUV data were available for DDO 88, DDO 165, and DDO 180 but FUV images were not.) Hunter et al. (2011AJ....142..121H) obtained deep GALEX UV images of an additional four galaxies and a deeper image of DDO 53. The LITTLE THINGS team obtained data on another 12 galaxies (Zhang et al., 2012AJ....143...47Z). We obtained mid-infrared (3.6 and 4.5{micro}m) images with the Infrared Array Camera (IRAC) from the Spitzer archives. Hunter et al. (2006, Cat. J/ApJS/162/49) analyzed data on 21 galaxies in our sample. Other data were taken by several Spitzer Legacy projects: The Local

  17. Simulations of isolated dwarf galaxies formed in dark matter halos with different mass assembly histories

    SciTech Connect

    González-Samaniego, A.; Avila-Reese, V.; Rodríguez-Puebla, A.; Valenzuela, O.; Colín, P.

    2014-04-10

    We present zoom-in N-body/hydrodynamics resimulations of dwarf galaxies formed in isolated cold dark matter (CDM) halos with the same virial mass (M{sub v} ≈ 2.5 × 10{sup 10} M {sub ☉}) at redshift z = 0. Our goals are to (1) study the mass assembly histories (MAHs) of the halo, stellar, and gaseous components; and (2) explore the effects of the halo MAHs on the stellar/baryonic assembly of simulated dwarfs. Overall, the dwarfs are roughly consistent with observations. More specific results include: (1) the stellar-to-halo mass ratio remains roughly constant since z ∼ 1, i.e., the stellar MAHs closely follow halo MAHs. (2) The evolution of the galaxy gas fractions, f{sub g} , are episodic, showing that the supernova-driven outflows play an important role in regulating f{sub g} —and hence, the star formation rate (SFR)—however, in most cases, a large fraction of the gas is ejected from the halo. (3) The star formation histories are episodic with changes in the SFRs, measured every 100 Myr, of factors of 2-10 on average. (4) Although the dwarfs formed in late assembled halos show more extended SF histories, their z = 0 specific SFRs are still below observations. (5) The inclusion of baryons most of the time reduces the virial mass by 10%-20% with respect to pure N-body simulations. Our results suggest that rather than increasing the strength of the supernova-driven outflows, processes that reduce the star formation efficiency could help to solve the potential issues faced by CDM-based simulations of dwarfs, such as low values of the specific SFR and high stellar masses.

  18. Linking dust emission to fundamental properties in galaxies: the low-metallicity picture

    NASA Astrophysics Data System (ADS)

    Rémy-Ruyer, A.; Madden, S. C.; Galliano, F.; Lebouteiller, V.; Baes, M.; Bendo, G. J.; Boselli, A.; Ciesla, L.; Cormier, D.; Cooray, A.; Cortese, L.; De Looze, I.; Doublier-Pritchard, V.; Galametz, M.; Jones, A. P.; Karczewski, O. Ł.; Lu, N.; Spinoglio, L.

    2015-10-01

    Aims: In this work, we aim to provide a consistent analysis of the dust properties from metal-poor to metal-rich environments by linking them to fundamental galactic parameters. Methods: We consider two samples of galaxies: the Dwarf Galaxy Survey (DGS) and the Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH), totalling 109 galaxies, spanning almost 2 dex in metallicity. We collect infrared (IR) to submillimetre (submm) data for both samples and present the complete data set for the DGS sample. We model the observed spectral energy distributions (SED) with a physically-motivated dust model to access the dust properties: dust mass, total-IR luminosity, polycyclic aromatic hydrocarbon (PAH) mass fraction, dust temperature distribution, and dust-to-stellar mass ratio. Results: Using a different SED model (modified black body), different dust composition (amorphous carbon in lieu of graphite), or a different wavelength coverage at submm wavelengths results in differences in the dust mass estimate of a factor two to three, showing that this parameter is subject to non-negligible systematic modelling uncertainties. We find half as much dust with the amorphous carbon dust composition. For eight galaxies in our sample, we find a rather small excess at 500 μm (≤1.5σ). We find that the dust SED of low-metallicity galaxies is broader and peaks at shorter wavelengths compared to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The PAH mass fraction and dust temperature distribution are found to be driven mostly by the specific star formation rate, sSFR, with secondary effects from metallicity. The correlations between metallicity and dust mass or total-IR luminosity are direct consequences of the stellar mass-metallicity relation. The dust-to-stellar mass ratios of metal-rich sources follow the well-studied trend of decreasing ratio for decreasing sSFR. The relation is more complex for low-metallicity galaxies with high

  19. The Environment of X-Ray Binaries in the Dwarf Starburst Galaxy NGC 1569

    NASA Astrophysics Data System (ADS)

    Clark, David M.; Eikenberry, Stephen S.; Raines, Steven N.

    2008-05-01

    We use deep, J and Ks observations of NGC 1569 acquired with FLAMINGOS on the KPNO 4-m to search for star cluster counterparts to X-ray binaries identified in archived Chandra images of this dwarf starburst galaxy. Performing near-IR photometry on the star cluster counterparts, we determine their colors, luminosities and masses. Comparing these results to the properties for all clusters in this galaxy, we search for trends in clusters associated with X-ray sources. Combining this study with FISICA, near-IR spectral observations, we further characterize the surroundings to X-ray binaries in NGC 1569. Contrasting this work with findings from a similar study performed on the Antennae galaxies, a large, merging system, we investigate the differences in X-ray binary environments.

  20. DISTANCE TO THE SAGITTARIUS DWARF GALAXY USING MACHO PROJECT RR LYRAE STARS

    SciTech Connect

    Kunder, Andrea; Chaboyer, Brian E-mail: brian.chaboyer@dartmouth.edu

    2009-05-15

    We derive the distance to the northern extension of the Sagittarius (Sgr) dwarf spheroidal galaxy from 203 Sgr RR0 Lyrae stars found in the MACHO database. Their distances are determined differentially with respect to 288 Galactic bulge RR0 Lyrae stars also found in the MACHO data. We find a distance modulus difference of 2.41 mag at l = 5{sup 0} and b = -8{sup 0} and that the extension of the Sgr galaxy toward the galactic plane is inclined toward us. Assuming R {sub GC} = 8 kpc, this implies the distance to these stars is (m - M){sub 0} = 16.97 {+-} 0.07 mag, which corresponds to D = 24.8 {+-} 0.8 kpc. Although this estimate is smaller than previous determinations for this galaxy and agrees with previous suggestions that Sgr's body is truly closer to us, this estimate is larger than studies at comparable galactic latitudes.

  1. Spatial Correlation between Dust and Hα Emission in Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Jimmy; Tran, Kim-Vy; Saintonge, Amélie; Accurso, Gioacchino; Brough, Sarah; Oliva-Altamirano, Paola; Salmon, Brett; Forrest, Ben

    2016-07-01

    Using a sample of dwarf irregular galaxies selected from the ALFALFA blind H i-survey and observed using the VIMOS IFU, we investigate the relationship between Hα emission and Balmer optical depth ({τ }{{b}}). We find a positive correlation between Hα luminosity surface density and Balmer optical depth in 8 of 11 at ≥0.8σ significance (6 of 11 at ≥1.0σ) galaxies. Our spaxels have physical scales ranging from 30 to 80 pc, demonstrating that the correlation between these two variables continues to hold down to spatial scales as low as 30 pc. Using the Spearman’s rank correlation coefficient to test for correlation between {{{Σ }}}{{H}α } and {τ }{{b}} in all the galaxies combined, we find ρ =0.39, indicating a positive correlation at 4σ significance. Our low stellar-mass galaxy results are in agreement with observations of emission line regions in larger spiral galaxies, indicating that this relationship is independent of the size of the galaxy hosting the emission line region. The positive correlation between Hα luminosity and Balmer optical depth within spaxels is consistent with the hypothesis that young star-forming regions are surrounded by dusty birth-clouds. Based on VLT service mode observations (Programs 081.B-0649 and 083.B-0662) gathered at the European Southern Observatory, Chile.

  2. The Nature of Turbulence in the LITTLE THINGS Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Maier, Erin; Elmegreen, Bruce G.; Hunter, Deidre A.; Chien, Li-Hsin; Hollyday, Gigja; Simpson, Caroline E.

    2017-04-01

    We present probability density functions and higher order (skewness and kurtosis) analyses of the galaxy-wide and spatially resolved distributions of H i column density in the LITTLE THINGS sample of dwarf irregular galaxies. This analysis follows that of Burkhart et al. for the Small Magellanic Cloud (SMC). About 60% of our sample have galaxy-wide values of kurtosis that are similar to that found for the SMC, with a range up to much higher values, and kurtosis increases with integrated star formation rate. Kurtosis and skewness were calculated for radial annuli and for a grid of 32 pixel × 32 pixel kernels across each galaxy. For most galaxies, kurtosis correlates with skewness. For about half of the galaxies, there is a trend of increasing kurtosis with radius. The range of kurtosis and skewness values is modeled by small variations in the Mach number close to the sonic limit and by conversion of H i to molecules at high column density. The maximum H i column densities decrease with increasing radius in a way that suggests molecules are forming in the weak-field limit, where H2 formation balances photodissociation in optically thin gas at the edges of clouds.

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

    SciTech Connect

    Simon, J. D.

    2015-07-23

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

  4. Determining the Nature of the Extended H I Structure around LITTLE THINGS Dwarf Galaxy NGC 1569

    NASA Astrophysics Data System (ADS)

    Johnson, Megan

    2013-06-01

    This work presents an extended, neutral hydrogen emission map around Magellanic-type dwarf irregular galaxy (dIm) NGC 1569. In the spring of 2010, the Robert C. Byrd Green Bank Telescope was used to map a 9° × 2° region in H I line emission that includes NGC 1569 and IC 342 as well as two other dwarf galaxies. The primary objective for these observations was to search for structures potentially connecting NGC 1569 with IC 342 group members in order to trace previous interactions and thus, provide an explanation for the starburst and peculiar kinematics prevalent in NGC 1569. A large, half-degree diameter H I cloud was detected that shares the same position and velocity as NGC 1569. Also, two long structures were discovered that are reminiscent of intergalactic filaments extending out in a V-shaped manner from NGC 1569 toward UGCA 92, a nearby dwarf galaxy. These filamentary structures extend for about 1.°5, which is 77 kpc at NGC 1569. There is a continuous velocity succession with the 0.°5 H I cloud, filaments, and main body of the galaxy. The 0.°5 H I cloud and filamentary structures may be foreground Milky Way, but are suggestive as possible remnants of an interaction between NGC 1569 and UGCA 92. The data also show two tidal tails extending from UGCA 86 and IC 342, respectively. These structures may be part of a continuous H I bridge but more data are needed to determine if this is the case.

  5. VERITAS SEARCH FOR VHE GAMMA-RAY EMISSION FROM DWARF SPHEROIDAL GALAXIES

    SciTech Connect

    Acciari, V. A.; Benbow, W.; Arlen, T.; Aune, T.; Furniss, A.; Beilicke, M.; Buckley, J. H.; Bugaev, V.; Dickherber, R.; Boltuch, D.; Bradbury, S. M.; Byrum, K.; Cannon, A.; Cesarini, A.; Christiansen, J. L.; Ciupik, L.; Cui, W.; Finley, J. P.; Duke, C.; Finnegan, G.

    2010-09-10

    Indirect dark matter searches with ground-based gamma-ray observatories provide an alternative for identifying the particle nature of dark matter that is complementary to that of direct search or accelerator production experiments. We present the results of observations of the dwarf spheroidal galaxies Draco, Ursa Minor, Booetes 1, and Willman 1 conducted by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). These galaxies are nearby dark matter dominated objects located at a typical distance of several tens of kiloparsecs for which there are good measurements of the dark matter density profile from stellar velocity measurements. Since the conventional astrophysical background of very high energy gamma rays from these objects appears to be negligible, they are good targets to search for the secondary gamma-ray photons produced by interacting or decaying dark matter particles. No significant gamma-ray flux above 200 GeV was detected from these four dwarf galaxies for a typical exposure of {approx}20 hr. The 95% confidence upper limits on the integral gamma-ray flux are in the range (0.4-2.2) x 10{sup -12}photonscm{sup -2} s{sup -1}. We interpret this limiting flux in the context of pair annihilation of weakly interacting massive particles (WIMPs) and derive constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the WIMPs (({sigma}v) {approx}< 10{sup -23} cm{sup 3} s{sup -1} for m {sub {chi} {approx}}> 300 GeV c {sup -2}). This limit is obtained under conservative assumptions regarding the dark matter distribution in dwarf galaxies and is approximately 3 orders of magnitude above the generic theoretical prediction for WIMPs in the minimal supersymmetric standard model framework. However, significant uncertainty exists in the dark matter distribution as well as the neutralino cross sections which under favorable assumptions could further lower this limit.

  6. DETERMINING THE NATURE OF THE EXTENDED H I STRUCTURE AROUND LITTLE THINGS DWARF GALAXY NGC 1569

    SciTech Connect

    Johnson, Megan

    2013-06-15

    This work presents an extended, neutral hydrogen emission map around Magellanic-type dwarf irregular galaxy (dIm) NGC 1569. In the spring of 2010, the Robert C. Byrd Green Bank Telescope was used to map a 9 Degree-Sign Multiplication-Sign 2 Degree-Sign region in H I line emission that includes NGC 1569 and IC 342 as well as two other dwarf galaxies. The primary objective for these observations was to search for structures potentially connecting NGC 1569 with IC 342 group members in order to trace previous interactions and thus, provide an explanation for the starburst and peculiar kinematics prevalent in NGC 1569. A large, half-degree diameter H I cloud was detected that shares the same position and velocity as NGC 1569. Also, two long structures were discovered that are reminiscent of intergalactic filaments extending out in a V-shaped manner from NGC 1569 toward UGCA 92, a nearby dwarf galaxy. These filamentary structures extend for about 1. Degree-Sign 5, which is 77 kpc at NGC 1569. There is a continuous velocity succession with the 0. Degree-Sign 5 H I cloud, filaments, and main body of the galaxy. The 0. Degree-Sign 5 H I cloud and filamentary structures may be foreground Milky Way, but are suggestive as possible remnants of an interaction between NGC 1569 and UGCA 92. The data also show two tidal tails extending from UGCA 86 and IC 342, respectively. These structures may be part of a continuous H I bridge but more data are needed to determine if this is the case.

  7. THRESHING IN ACTION: THE TIDAL DISRUPTION OF A DWARF GALAXY BY THE HYDRA I CLUSTER

    SciTech Connect

    Koch, Andreas; Burkert, Andreas; Rich, R. Michael; Black, Christine S.; Collins, Michelle L. M.; Hilker, Michael; Benson, Andrew J.

    2012-08-10

    We report on the discovery of strong tidal features around a dwarf spheroidal galaxy in the Hydra I galaxy cluster, indicating its ongoing tidal disruption. This very low surface brightness object, HCC-087, was originally classified as an early-type dwarf in the Hydra Cluster Catalogue (HCC), but our re-analysis of the ESO-VLT/FORS images of the HCC unearthed a clear indication of an S-shaped morphology and a large spatial extent. Its shape, luminosity (M{sub V} = -11.6 mag), and physical size (at a half-light radius of 3.1 kpc and a full length of {approx}5.9 kpc) are comparable to the recently discovered NGC 4449B and the Sagittarius dwarf spheroidal, all of which are undergoing clear tidal disruption. Aided by N-body simulations we argue that HCC-087 is currently at its first apocenter, at 150 kpc, around the cluster center and that it is being tidally disrupted by the galaxy cluster's potential itself. An interaction with the nearby (50 kpc) S0 cluster galaxy HCC-005, at M{sub *} {approx} 3 Multiplication-Sign 10{sup 10} M{sub Sun} is rather unlikely, as this constellation requires a significant amount of dynamical friction and thus low relative velocities. The S-shaped morphology and large spatial extent of the satellite would, however, also appear if HCC-087 would orbit the cluster center. These features appear to be characteristic properties of satellites that are seen in the process of being tidally disrupted, independent of the environment of the destruction. An important finding of our simulations is an orientation of the tidal tails perpendicular to the orbit.

  8. Stellar Kinematics and Metallicities in the Ultra-faint Dwarf Galaxy Reticulum II

    NASA Astrophysics Data System (ADS)

    Simon, J. D.; Drlica-Wagner, A.; Li, T. S.; Nord, B.; Geha, M.; Bechtol, K.; Balbinot, E.; Buckley-Geer, E.; Lin, H.; Marshall, J.; Santiago, B.; Strigari, L.; Wang, M.; Wechsler, R. H.; Yanny, B.; Abbott, T.; Bauer, A. H.; Bernstein, G. M.; Bertin, E.; Brooks, D.; Burke, D. L.; Capozzi, D.; Carnero Rosell, A.; Carrasco Kind, M.; D'Andrea, C. B.; da Costa, L. N.; DePoy, D. L.; Desai, S.; Diehl, H. T.; Dodelson, S.; Cunha, C. E.; Estrada, J.; Evrard, A. E.; Fausti Neto, A.; Fernandez, E.; Finley, D. A.; Flaugher, B.; Frieman, J.; Gaztanaga, E.; Gerdes, D.; Gruen, D.; Gruendl, R. A.; Honscheid, K.; James, D.; Kent, S.; Kuehn, K.; Kuropatkin, N.; Lahav, O.; Maia, M. A. G.; March, M.; Martini, P.; Miller, C. J.; Miquel, R.; Ogando, R.; Romer, A. K.; Roodman, A.; Rykoff, E. S.; Sako, M.; Sanchez, E.; Schubnell, M.; Sevilla, I.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Suchyta, E.; Swanson, M. E. C.; Tarle, G.; Thaler, J.; Tucker, D.; Vikram, V.; Walker, A. R.; Wester, W.; DES Collaboration

    2015-07-01

    We present Magellan/M2FS, Very Large Telescope/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity of 62.8+/- 0.5 {km} {{{s}}}-1 and a velocity dispersion of 3.3+/- 0.7 {km} {{{s}}}-1. The mass-to-light ratio of Ret II within its half-light radius is 470+/- 210 {M}⊙ /{L}⊙ , demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 {km} {{{s}}}-1, respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span 1.3 dex in metallicity, with a dispersion of 0.28 ± 0.09 dex, and we identify several extremely metal-poor stars with {{[Fe/H]}}\\lt -3. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of {{[Fe/H]}}=-2.65+/- 0.07, Ret II matches Segue 1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of the dark matter density squared is {{log}}10(J)=18.8+/- 0.6 {GeV}{ }2 {{cm}}-5 within 0.°2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies. Based on data obtained from the ESO Science Archive Facility under request number 157689.

  9. Threshing in Action: The Tidal Disruption of a Dwarf Galaxy by the Hydra I Cluster

    NASA Astrophysics Data System (ADS)

    Koch, Andreas; Burkert, Andreas; Rich, R. Michael; Collins, Michelle L. M.; Black, Christine S.; Hilker, Michael; Benson, Andrew J.

    2012-08-01

    We report on the discovery of strong tidal features around a dwarf spheroidal galaxy in the Hydra I galaxy cluster, indicating its ongoing tidal disruption. This very low surface brightness object, HCC-087, was originally classified as an early-type dwarf in the Hydra Cluster Catalogue (HCC), but our re-analysis of the ESO-VLT/FORS images of the HCC unearthed a clear indication of an S-shaped morphology and a large spatial extent. Its shape, luminosity (MV = -11.6 mag), and physical size (at a half-light radius of 3.1 kpc and a full length of ~5.9 kpc) are comparable to the recently discovered NGC 4449B and the Sagittarius dwarf spheroidal, all of which are undergoing clear tidal disruption. Aided by N-body simulations we argue that HCC-087 is currently at its first apocenter, at 150 kpc, around the cluster center and that it is being tidally disrupted by the galaxy cluster's potential itself. An interaction with the nearby (50 kpc) S0 cluster galaxy HCC-005, at M * ~ 3 × 1010 M ⊙ is rather unlikely, as this constellation requires a significant amount of dynamical friction and thus low relative velocities. The S-shaped morphology and large spatial extent of the satellite would, however, also appear if HCC-087 would orbit the cluster center. These features appear to be characteristic properties of satellites that are seen in the process of being tidally disrupted, independent of the environment of the destruction. An important finding of our simulations is an orientation of the tidal tails perpendicular to the orbit. Based on observations carried out at the European Southern Observatory under proposal number 65.N-0459(A).

  10. VERITAS Search for VHE Gamma-ray Emission from Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Acciari, V. A.; Arlen, T.; Aune, T.; Beilicke, M.; Benbow, W.; Boltuch, D.; Bradbury, S. M.; Buckley, J. H.; Bugaev, V.; Byrum, K.; Cannon, A.; Cesarini, A.; Christiansen, J. L.; Ciupik, L.; Cui, W.; Dickherber, R.; Duke, C.; Finley, J. P.; Finnegan, G.; Furniss, A.; Galante, N.; Godambe, S.; Grube, J.; Guenette, R.; Gyuk, G.; Hanna, D.; Holder, J.; Hui, C. M.; Humensky, T. B.; Imran, A.; Kaaret, P.; Karlsson, N.; Kertzman, M.; Kieda, D.; Konopelko, A.; Krawczynski, H.; Krennrich, F.; Maier, G.; McArthur, S.; McCann, A.; McCutcheon, M.; Moriarty, P.; Ong, R. A.; Otte, A. N.; Pandel, D.; Perkins, J. S.; Pohl, M.; Quinn, J.; Ragan, K.; Reyes, L. C.; Reynolds, P. T.; Roache, E.; Rose, H. J.; Schroedter, M.; Sembroski, G. H.; Senturk, G. Demet; Smith, A. W.; Steele, D.; Swordy, S. P.; Tešić, G.; Theiling, M.; Thibadeau, S.; Varlotta, A.; Vassiliev, V. V.; Vincent, S.; Wagner, R. G.; Wakely, S. P.; Ward, J. E.; Weekes, T. C.; Weinstein, A.; Weisgarber, T.; Williams, D. A.; Wissel, S.; Zitzer, B.; VERITAS Collaboration

    2010-09-01

    Indirect dark matter searches with ground-based gamma-ray observatories provide an alternative for identifying the particle nature of dark matter that is complementary to that of direct search or accelerator production experiments. We present the results of observations of the dwarf spheroidal galaxies Draco, Ursa Minor, Boötes 1, and Willman 1 conducted by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). These galaxies are nearby dark matter dominated objects located at a typical distance of several tens of kiloparsecs for which there are good measurements of the dark matter density profile from stellar velocity measurements. Since the conventional astrophysical background of very high energy gamma rays from these objects appears to be negligible, they are good targets to search for the secondary gamma-ray photons produced by interacting or decaying dark matter particles. No significant gamma-ray flux above 200 GeV was detected from these four dwarf galaxies for a typical exposure of ~20 hr. The 95% confidence upper limits on the integral gamma-ray flux are in the range (0.4-2.2) × 10-12photonscm-2 s-1. We interpret this limiting flux in the context of pair annihilation of weakly interacting massive particles (WIMPs) and derive constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the WIMPs (langσvrang <~ 10-23 cm3 s-1 for m χ >~ 300 GeV c -2). This limit is obtained under conservative assumptions regarding the dark matter distribution in dwarf galaxies and is approximately 3 orders of magnitude above the generic theoretical prediction for WIMPs in the minimal supersymmetric standard model framework. However, significant uncertainty exists in the dark matter distribution as well as the neutralino cross sections which under favorable assumptions could further lower this limit.

  11. Effect of black holes in local dwarf spheroidal galaxies on gamma-ray constraints on dark matter annihilation

    NASA Astrophysics Data System (ADS)

    Gonzalez-Morales, Alma X.; Profumo, Stefano; Queiroz, Farinaldo S.

    2014-11-01

    Recent discoveries of optical signatures of black holes in dwarf galaxies indicates that low-mass galaxies can indeed host intermediate massive black holes. This motivates the assessment of the resulting effect on the host dark matter density profile, and the consequences for the constraints on the plane of the dark matter annihilation cross section versus mass, stemming from the nonobservation of gamma rays from local dwarf spheroidals with the Fermi Large Area Telescope. We compute the density profile using three different prescriptions for the black hole mass associated with a given spheroidal galaxy, and taking into account the cutoff to the density from dark matter pair-annihilation. We find that the limits on the dark matter annihilation rate from observations of individual dwarfs are enhanced by factors of a few up to 1 06 , depending on the specific galaxy, on the black hole mass prescription, and on the dark matter particle mass. We estimate limits from combined observations of a sample of 15 dwarfs, for a variety of assumptions on the dwarf black hole mass and on the dark matter density profile prior to adiabatic contraction. We find that if black holes are indeed present in local dwarf spheroidals, then, independent of assumptions, (i) the dark matter interpretation of the Galactic center gamma-ray excess would be conclusively ruled out, (ii) wino dark matter would be excluded up to masses of about 3 TeV, and (iii) vanilla thermal relic weakly interacting massive particles must be heavier than 100 GeV.

  12. Detailed Abundances of Two Very Metal-poor Stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Cohen, Judith G.

    2012-12-01

    The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae (SNe). These SNe could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way (MW) halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two MW dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously discovered low-metallicity, high-[α/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of SN yields is the cause. The earliest SNe in low-mass dSphs have less gas to pollute than the earliest SNe in massive halo progenitors. As a result, dSph stars at -3 < [Fe/H] < -2 sample SNe with [Fe/H] Lt -3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] ~ -3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo. Data 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 NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  13. Dark matter annihilation and decay in dwarf spheroidal galaxies: the classical and ultrafaint dSphs

    NASA Astrophysics Data System (ADS)

    Bonnivard, V.; Combet, C.; Daniel, M.; Funk, S.; Geringer-Sameth, A.; Hinton, J. A.; Maurin, D.; Read, J. I.; Sarkar, S.; Walker, M. G.; Wilkinson, M. I.

    2015-10-01

    Dwarf spheroidal (dSph) galaxies are prime targets for present and future γ-ray telescopes hunting for indirect signals of particle dark matter. The interpretation of the data requires careful assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimized spherical Jeans analysis to reconstruct the `astrophysical factor' for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimize biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 `classical' and 13 `ultrafaint' dSphs, with the quoted uncertainties directly linked to the sample size; the more flexible parametrization we use results in changes compared to previous calculations. This translates into our ranking of potentially-brightest and most robust targets - namely Ursa Minor, Draco, Sculptor - and of the more promising, but uncertain targets - namely Ursa Major 2, Coma - for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new `ultrafaint' satellites that are beginning to be discovered in southern sky surveys.

  14. Leo I - The youngest Milky Way dwarf spheroidal galaxy?

    NASA Technical Reports Server (NTRS)

    Lee, Myung G.; Freedman, Wendy; Mateo, Mario; Thompson, Ian; Roth, Miguel; Ruiz, Maria-Teresa

    1993-01-01

    Deep CCD photometry of about 16,000 stars in the Milky Way's Leo I spheroidal galaxy satellite is reported. An account is given of the features observed in the color-magnitude diagrams (CMDs) derived therefrom. A very blue and well-defined red giant branch (RGB) is noted. The CMDs of Leo I shows about 50 anomalous Cepheid candidates; there are another 50 or so asymptotic giant branch stars above the tip of the RGB, including 15 known carbon stars. The mean color of the RGB is estimated at M sub I = -3.5 mag.

  15. White Dwarfs and the Age of our Galaxy: A Professional Development Workshop for Teachers

    NASA Astrophysics Data System (ADS)

    Hemenway, M. K.; Armosky, B. J.; Kilic, M.; Shipman, H.; von Hippel, T.

    2005-12-01

    The first of a planned series of workshops for secondary school science teachers was conducted at McDonald Observatory in 2005. In preparation for the workshop, new instructional materials on white dwarfs and the age of our Galaxy were prepared and pilot-tested. Thirteen teachers from Arizona, Delaware, and Texas performed these standards-aligned activities at the workshop in preparation for using them within their own classrooms. During four nights of observing, the participants made CCD-images at the 0.8-m telescope and reduced the data with personal computers using easily available image reduction software. Working in teams, they created a color-magnitude diagram for an open cluster and found several white dwarf candidates within the cluster. With guidance, they used the work of the White Dwarf Luminosity Function Collaboration, which fostered this workshop, to estimate the age of the Galaxy. The workshop included tours of other observatory facilities, practical exercises with small telescopes, and time to reflect on their own teaching practices. Evaluation on the workshop and the use of the instructional materials is continuing. Support from the National Science Foundation (AST 0307315), National Science FoundationsDistinguished Teaching Scholars Program (DUE-0306557), and The National Aeronautics and Space Administration under an Education and Public Outreach supplement to Grant/Contract/Agreement No. NAG5-13070 issued through the Office of Space Science is gratefully acknowledged.

  16. USING M DWARF SPECTRA TO MAP EXTINCTION IN THE LOCAL GALAXY

    SciTech Connect

    Jones, David O.; West, Andrew A.; Foster, Jonathan B.

    2011-08-15

    We use spectra of more than 56,000 M dwarfs from the Sloan Digital Sky Survey (SDSS) to create a high-latitude extinction map of the local Galaxy. Our technique compares spectra from the stars in the SDSS Data Release 7 M dwarf sample in low-extinction lines of sight, as determined by Schlegel et al., to other SDSS M dwarf spectra in order to derive improved distance estimates and accurate line-of-sight extinctions. Unlike most previous studies, which have used a two-color method to determine extinction, we fit extinction curves to fluxes across the spectral range from 5700 to 9200 A for every star in our sample. Our result is an A{sub V} map that extends from a few tens of pc to approximately 2 kpc away from the Sun. We also use a similar technique to create a map of R{sub V} values within approximately 1 kpc of the Sun and find that they are consistent with the widely accepted diffuse interstellar medium value of 3.1. Using our extinction data, we derive a dust scale height for the local Galaxy of 119 {+-} 15 pc and find evidence for a local dust cavity.

  17. Indication of Gamma-Ray Emission from the Newly Discovered Dwarf Galaxy Reticulum II.

    PubMed

    Geringer-Sameth, Alex; Walker, Matthew G; Koushiappas, Savvas M; Koposov, Sergey E; Belokurov, Vasily; Torrealba, Gabriel; Evans, N Wyn

    2015-08-21

    We present a search for γ-ray emission from the direction of the newly discovered dwarf galaxy Reticulum II. Using Fermi-LAT Collaboration data, we detect a signal that exceeds expected backgrounds between ∼2-10  GeV and is consistent with annihilation of dark matter for particle masses less than a few ×10^{2}  GeV. Modeling the background as a Poisson process based on Fermi-LAT diffuse models, and taking into account trial factors, we detect emission with p value less than 9.8×10^{-5} (>3.7σ). An alternative, model-independent treatment of the background reduces the significance, raising the p value to 9.7×10^{-3} (2.3σ). Even in this case, however, Reticulum II has the most significant γ-ray signal of any known dwarf galaxy. If Reticulum II has a dark-matter halo that is similar to those inferred for other nearby dwarfs, the signal is consistent with the s-wave relic abundance cross section for annihilation.

  18. Search for dark matter annihilation signatures in H.E.S.S. observations of dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Abramowski, A.; Aharonian, F.; Ait Benkhali, F.; Akhperjanian, A. G.; Angüner, E.; Backes, M.; Balenderan, S.; Balzer, A.; Barnacka, A.; Becherini, Y.; Becker Tjus, J.; Berge, D.; Bernhard, S.; Bernlöhr, K.; Birsin, E.; Biteau, J.; Böttcher, M.; Boisson, C.; Bolmont, J.; Bordas, P.; Bregeon, J.; Brun, F.; Brun, P.; Bryan, M.; Bulik, T.; Carrigan, S.; Casanova, S.; Chadwick, P. M.; Chakraborty, N.; Chalme-Calvet, R.; Chaves, R. C. G.; Chrétien, M.; Colafrancesco, S.; Cologna, G.; Conrad, J.; Couturier, C.; Cui, Y.; Dalton, M.; Davids, I. D.; Degrange, B.; Deil, C.; deWilt, P.; Djannati-Ataï, A.; Domainko, W.; Donath, A.; Drury, L. O'C.; Dubus, G.; Dutson, K.; Dyks, J.; Dyrda, M.; Edwards, T.; Egberts, K.; Eger, P.; Espigat, P.; Farnier, C.; Fegan, S.; Feinstein, F.; Fernandes, M. V.; Fernandez, D.; Fiasson, A.; Fontaine, G.; Förster, A.; Füßling, M.; Gabici, S.; Gajdus, M.; Gallant, Y. A.; Garrigoux, T.; Giavitto, G.; Giebels, B.; Glicenstein, J. F.; Gottschall, D.; Goudelis, A.; Grondin, M.-H.; Grudzińska, M.; Hadsch, D.; Häffner, S.; Hahn, J.; Harris, J.; Heinzelmann, G.; Henri, G.; Hermann, G.; Hervet, O.; Hillert, A.; Hinton, J. A.; Hofmann, W.; Hofverberg, P.; Holler, M.; Horns, D.; Ivascenko, A.; Jacholkowska, A.; Jahn, C.; Jamrozy, M.; Janiak, M.; Jankowsky, F.; Jung, I.; Kastendieck, M. A.; Katarzyński, K.; Katz, U.; Kaufmann, S.; Khélifi, B.; Kieffer, M.; Klepser, S.; Klochkov, D.; Kluźniak, W.; Kolitzus, D.; Komin, Nu.; Kosack, K.; Krakau, S.; Krayzel, F.; Krüger, P. P.; Laffon, H.; Lamanna, G.; Lefaucheur, J.; Lefranc, V.; Lemière, A.; Lemoine-Goumard, M.; Lenain, J.-P.; Lohse, T.; Lopatin, A.; Lu, C.-C.; Marandon, V.; Marcowith, A.; Marx, R.; Maurin, G.; Maxted, N.; Mayer, M.; McComb, T. J. L.; Méhault, J.; Meintjes, P. J.; Menzler, U.; Meyer, M.; Mitchell, A. M. W.; Moderski, R.; Mohamed, M.; Morâ, K.; Moulin, E.; Murach, T.; de Naurois, M.; Niemiec, J.; Nolan, S. J.; Oakes, L.; Odaka, H.; Ohm, S.; Opitz, B.; Ostrowski, M.; Oya, I.; Panter, M.; Parsons, R. D.; Paz Arribas, M.; Pekeur, N. W.; Pelletier, G.; Perez, J.; Petrucci, P.-O.; Peyaud, B.; Pita, S.; Poon, H.; Pühlhofer, G.; Punch, M.; Quirrenbach, A.; Raab, S.; Reichardt, I.; Reimer, A.; Reimer, O.; Renaud, M.; de los Reyes, R.; Rieger, F.; Rob, L.; Romoli, C.; Rosier-Lees, S.; Rowell, G.; Rudak, B.; Rulten, C. B.; Sahakian, V.; Salek, D.; Sanchez, D. A.; Santangelo, A.; Schlickeiser, R.; Schüssler, F.; Schulz, A.; Schwanke, U.; Schwarzburg, S.; Schwemmer, S.; Serpico, P.; Sol, H.; Spanier, F.; Spengler, G.; Spieß, F.; Stawarz, L.; Steenkamp, R.; Stegmann, C.; Stinzing, F.; Stycz, K.; Sushch, I.; Tavernet, J.-P.; Tavernier, T.; Taylor, A. M.; Terrier, R.; Tluczykont, M.; Trichard, C.; Valerius, K.; van Eldik, C.; van Soelen, B.; Vasileiadis, G.; Veh, J.; Venter, C.; Viana, A.; Vincent, P.; Vink, J.; Völk, H. J.; Volpe, F.; Vorster, M.; Vuillaume, T.; Wagner, S. J.; Wagner, P.; Wagner, R. M.; Ward, M.; Weidinger, M.; Weitzel, Q.; White, R.; Wierzcholska, A.; Willmann, P.; Wörnlein, A.; Wouters, D.; Yang, R.; Zabalza, V.; Zaborov, D.; Zacharias, M.; Zdziarski, A. A.; Zech, A.; Zechlin, H.-S.; H. E. S. S. Collaboration

    2014-12-01

    Dwarf spheroidal galaxies of the Local Group are close satellites of the Milky Way characterized by a large mass-to-light ratio and are not expected to be the site of nonthermal high-energy gamma-ray emission or intense star formation. Therefore they are among the most promising candidates for indirect dark matter searches. During the last years the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes observed five of these dwarf galaxies for more than 140 hours in total, searching for TeV gamma-ray emission from annihilation of dark matter particles. The new results of the deep exposure of the Sagittarius dwarf spheroidal galaxy, the first observations of the Coma Berenices and Fornax dwarves and the reanalysis of two more dwarf spheroidal galaxies already published by the H.E.S.S. Collaboration, Carina and Sculptor, are presented. In the absence of a significant signal new constraints on the annihilation cross section applicable to weakly interacting massive particles (WIMPs) are derived by combining the observations of the five dwarf galaxies. The combined exclusion limit depends on the WIMP mass and the best constraint is reached at 1-2 TeV masses with a cross-section upper bound of ˜ 3.9 ×10-24 cm3 s-1 at a 95% confidence level.

  19. The White Dwarf Luminosity Function and the Age of the Galaxy

    NASA Astrophysics Data System (ADS)

    Smith, J. A.; Oswalt, T. D.

    1996-12-01

    We present the results for the White Dwarf Luminosity Function (WDLF) derived from our entire sample of common proper motion binaries (CPMBs) containing white dwarfs (WDs). Approximately 170 WDs are contained in our study which makes it one of the largest samples ever used in the determination of the WDLF. We will discuss our sample selection and completion techniques and the constraint this sample places on the debate about the age of the Galaxy and ultimately the Universe. The WDs in this study were identified spectroscopically [Oswalt et al. in White Dwarfs, ed. G. Wegner, (Springer: Berlin), (1990)] and satisfy the limits [V < 18.50, mu > 0.10 "/yr] over which the sample's incompleteness is well-behaved (and hence correctible). The size of this sample allows us to set firm limits on the age of Galactic disk, making use of the latest interior [Wood, in White Dwarfs, eds. D. Koester & K. Werner (Kluwer: Dordrecht), 1995] and atmosphere [Bergeron, P., et al. Astrophys. J. 449, 258-279, 1995] models. Our new results, which strengthen the >= 9.5(+1.1}_{-0.8) Gyr(1 sigma ) age for the Galactic disk [Oswalt et al. Nature 382, 692 (1996)], will be presented. JAS gratefully acknowledges support from the NASA Graduate Student Researchers Program, grant NGT-51086 and TDO gratefully acknowledges support from NSF grant AST 90-16284.

  20. Theoretical lower limits on sizes of ultrafaint dwarf galaxies from dynamical friction

    NASA Astrophysics Data System (ADS)

    Hernandez, X.

    2016-11-01

    Dwarf spheroidal galaxies (dSphs) are the smallest known stellar systems where under Newtonian interpretations, a significant amount of dark matter is required to explain observed kinematics. In fact, they are, in this sense, the most heavily dark matter-dominated objects known. That, plus the increasingly small sizes of the newly discovered ultrafaint dwarfs, puts these systems in the regime where dynamical friction on individual stars starts to become relevant. We calculate the dynamical friction time-scales for pressure-supported isotropic spherical dark matter-dominated stellar systems, yielding τDF = 0.93(rh/10 pc)2(σ/km s-1) Gyr, where rh is the half-light radius. For a stellar velocity dispersion value of 3 km s-1, as typical for the smallest of the recently detected ultrafaint dwarf spheroidals, dynamical friction time-scales become smaller than the 10 Gyr typical of the stellar ages for these systems, for rh < 19 pc. Thus, this becomes a theoretical lower limit below which dark matter-dominated stellar systems become unstable to dynamical friction. We present a comparison with structural parameters of the smallest ultrafaint dwarf spheroidals known, showing that these are already close to the stability limit derived, any future detection of yet smaller such systems would be inconsistent with a particle dark matter hypothesis.

  1. DWARF GALAXY ANNIHILATION AND DECAY EMISSION PROFILES FOR DARK MATTER EXPERIMENTS

    SciTech Connect

    Geringer-Sameth, Alex; Koushiappas, Savvas M.; Walker, Matthew E-mail: koushiappas@brown.edu

    2015-03-10

    Gamma-ray searches for dark matter annihilation and decay in dwarf galaxies rely on an understanding of the dark matter density profiles of these systems. Conversely, uncertainties in these density profiles propagate into the derived particle physics limits as systematic errors. In this paper we quantify the expected dark matter signal from 20 Milky Way dwarfs using a uniform analysis of the most recent stellar-kinematic data available. Assuming that the observed stellar populations are equilibrium tracers of spherically symmetric gravitational potentials that are dominated by dark matter, we find that current stellar-kinematic data can predict the amplitudes of annihilation signals to within a factor of a few for the ultra-faint dwarfs of greatest interest. On the other hand, the expected signal from several classical dwarfs (with high-quality observations of large numbers of member stars) can be localized to the ∼20% level. These results are important for designing maximally sensitive searches in current and future experiments using space and ground-based instruments.

  2. The Merger History, AGN and Dwarf Galaxies of Hickson Compact Group 59

    NASA Technical Reports Server (NTRS)

    Konstantopoulos, I. S.; Gallagher, S. C.; Fedotov, K.; Durrell, P. R.; Tzanavaris, P.; Hill, A. R.; Zabludoff, A. I.; Maier, M. L.; Elmegreen, D. M.; Charlton, J. C.; Johnson, K. E.; Brandt, W. N.; Walker, L. M.; Eracleous, M.; Maybhate, A.; Gronwall, C.; English, J.; Hornschemeier, A. E.; Mulchaey, J. S.

    2011-01-01

    Compact group galaxies often appear unaffected by their unusually dense environment. Closer examination can, however, reveal the subtle, cumulative effects of multiple galaxy interactions. Hickson Compact Group (HCG) 59 is an excellent example of this situation. We present a photometric study of this group in the optical (HST), infrared (Spitzer) and X-ray (Chandra) regimes aimed at characterizing the star formation and nuclear activity in its constituent galaxies and intra-group medium. We associate five dwarf galaxies with the group and update the velocity dispersion, leading to an increase in the dynamical mass of the group of up to a factor of 10 (to 2.8 x 10(exp 13) Stellar Mass), and a subsequent revision of its evolutionary stage. Star formation is proceeding at a level consistent with the morphological types of the four main galaxies, of which two are star-forming and the other two quiescent. Unlike in some other compact groups, star-forming complexes across HCG 59 closely follow mass-radius scaling relations typical of nearby galaxies. In contrast, the ancient globular cluster populations in galaxies HCG 59A and B show intriguing irregularities, and two extragalactic HII regions are found just west of B. We age-date a faint stellar stream in the intra-group medium at approx. 1 Gyr to examine recent interactions. We detect a likely low-luminosity AGN in HCG 59A by its approx. 10(exp 40) erg/s X-ray emission; the active nucleus rather than star formation can account for the UV+IR SED. We discuss the implications of our findings in the context of galaxy evolution in dense environments.

  3. The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations

    NASA Technical Reports Server (NTRS)

    Garnett, D. R.; Skillman, E. D.; Dufour, R. J.; Peimbert, M.; Torres-Peimbert, S.; Terlevich, R.; Terlevich, E.; Shields, G. A.

    1995-01-01

    We present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III 1666 A and C III 1909 A in our spectra, enabling us to determine C(+2)/O(+2) with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of carbon from intermediate-mass stars. Our results also suggest that it may not be appropiate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver & Woosley's best estimate for the 12C(alpha, gamma) 16O nuclear reaction rate, assuming negligible contanmination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III 1750 A multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma uppr limits on the 1750 A feature indicate that the N(+2)/O(+2) ratios in these objects are not significantly larger than the N(+)/O(+) ratios measured from optical spectra. This behavior is consistent with predictions of photionization models, although better detections of N III are needed to confirm the results.

  4. Evidence for a Dusty Dark Dwarf Galaxy in the Quadruple Lens MG 0414+0534

    NASA Astrophysics Data System (ADS)

    Inoue, Kaiki Taro; Matsushita, Satoki; Minezaki, Takeo; Chiba, Masashi

    2017-02-01

    We report the 4σ detection of a faint object with a flux of ∼ 0.3 {mJy}, in the vicinity of the quadruply lensed QSO MG 0414+0534 using the Atacama Large Millimeter/submillimeter array (ALMA) Band 7. The object is most probably a dusty dark dwarf galaxy, which has not been detected in either the optical, near-infrared, or radio (centimeter) bands. An anomaly in the flux ratio of the lensed images observed in Band 7 and the mid-infrared band and the reddening of the QSO light color can be simultaneously explained if we consider the object as a lensing substructure with an ellipticity of ∼0.7 at a redshift of 0.5≲ z≲ 1. Using the best-fit lens models with three lenses, we find that the dark matter plus baryon mass associated with the object is ∼ {10}9 {M}ȯ , the dust mass is ∼ {10}7 {M}ȯ , and the linear size is ≳ 5 kpc. Thus, our findings suggest that the object is a dusty dark dwarf galaxy. A substantial portion of faint submillimeter galaxies (SMGs) in the universe may be attributed to such dark objects.

  5. Ultra-light dark matter in ultra-faint dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Calabrese, Erminia; Spergel, David N.

    2016-08-01

    Cold Dark Matter (CDM) models struggle to match the observations at galactic scales. The tension can be reduced either by dramatic baryonic feedback effects or by modifying the particle physics of CDM. Here, we consider an ultra-light scalar field DM particle manifesting a wave nature below a DM particle mass-dependent Jeans scale. For DM mass m ˜ 10-22 eV, this scenario delays galaxy formation and avoids cusps in the centre of the dark matter haloes. We use new measurements of half-light mass in ultra-faint dwarf galaxies Draco II and Triangulum II to estimate the mass of the DM particle in this model. We find that if the stellar populations are within the core of the density profile then the data are in agreement with a Wave Dark Matter model having a DM particle with m ˜ 3.7-5.6 × 10-22 eV. The presence of this extremely light particle will contribute to the formation of a central solitonic core replacing the cusp of a Navarro-Frenk-White profile and bringing predictions closer to observations of cored central density in dwarf galaxies.

  6. Chemical evolution of r-process elements in Draco dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Nitta Ishigaki, Miho; Tsujimoto, Takuji; Shigeyama, Toshikazu; Aoki, Wako

    2015-08-01

    Dwarf galaxies around the Milky Way halo are ideal laboratory of nucleosynthesis and chemical enrichments in the early universe. We studied chemical compositions including r-process elements for giant stars in Draco dwarf spheroidal galaxy based on high-resolution spectra obtained with the Subaru/HDS. Draco is known to mainly consist of old (age > 10 Gyr) and metal-poor ([Fe/H]<-1.5) stellar populations, which provides us an important insights about nucleosynthesis responsible for producing heavy elements in this galaxy. As reported in previous studies, we found that the Draco stars show enhanced [α/Fe] ratios at [Fe/H]<-2, decreasing at higher metallicity. This is consistent with an expectation that the chemical evolution is proceeded in a homogeneous manner initially by Type II and later by Type Ia supernovae. On the other hand, the [Eu/H] are constant over the metallicity range -2<[Fe/H]<-1 and low upper limits have been obtained at the lower [Fe/H]. The lack of increase in Eu abundance, despite the significant increase in Fe abundance by supernovae, implies that r-process elements were produced through much rarer events such as neutron-star mergers.

  7. A search for CO in the Local Group dwarf irregular galaxy WLM

    NASA Astrophysics Data System (ADS)

    Taylor, C. L.; Klein, U.

    2001-02-01

    We present 12CO J = 1-> 0 and J = 2-> 1 observations of the low metallicity (12 + log(O/H) = 7.74) Local Group dwarf irregular galaxy WLM made with the 15 m SEST and 14 m FCRAO telescopes. Despite the presence a number of HII regions, we find no CO emission. We obtain low upper limits on the integrated intensity (I_CO<= 0.18 K km s-1 for CO (1->0)). The non-detection is consistent with the result of Taylor, Kobulnicky & Skillman (\\cite{TKS}), that dwarf galaxies below a metallicity of ~ 7.9 are not detected in CO emission. WLM shows that this trend continues for low metallicity galaxies even as their metallicities approach 7.9. These results are consistent with the models of the metal poor ISM by Norman & Spaans (\\cite{NS}). By comparing our CO data with observations of star formation in WLM, we find evidence for a high CO to H_2 conversion factor.

  8. A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling.

    PubMed

    Ural, Uğur; Wilkinson, Mark I; Read, Justin I; Walker, Matthew G

    2015-07-02

    Dark matter-only simulations of galaxy formation predict many more subhalos around a Milky Way-like galaxy than the number of observed satellites. Proposed solutions require the satellites to inhabit dark matter halos with masses 10(9)-10(10 )Msun at the time they fell into the Milky Way. Here we use a modelling approach, independent of cosmological simulations, to obtain a pre-infall mass of 3.6(-2.3)(+3.8) × 10(8) Msun for one of the Milky Way's satellites: Carina. This determination of a low halo mass for Carina can be accommodated within the standard model only if galaxy formation becomes stochastic in halos below ∼10(10 )Msun. Otherwise Carina, the eighth most luminous Milky Way dwarf, would be expected to inhabit a significantly more massive halo. The implication of this is that a population of 'dark dwarfs' should orbit the Milky Way: halos devoid of stars and yet more massive than many of their visible counterparts.

  9. NO EVIDENCE FOR CLASSICAL CEPHEIDS AND A NEW DWARF GALAXY BEHIND THE GALACTIC DISK

    SciTech Connect

    Pietrukowicz, P.; Udalski, A.; Szymański, M. K.; Soszyński, I.; Pietrzyński, G.; Wyrzykowski, Ł.; Poleski, R.; Ulaczyk, K.; Skowron, J.; Mróz, P.; Pawlak, M.; Kozłowski, S.

    2015-11-10

    Based on data from the ongoing OGLE Galaxy Variability Survey (OGLE GVS), we have verified observed properties of stars detected by the near-infrared VVV survey in a direction near the Galactic plane at longitude l ≈ −27° and recently tentatively classified as classical Cepheids belonging to, hence claimed, a dwarf galaxy at a distance of about 90 kpc from the Galactic Center. Three of four stars are detected in the OGLE GVS I-band images. We show that two of the objects are not variable at all, and the third one with a period of 5.695 days and a nearly sinusoidal light curve of an amplitude of 0.5 mag cannot be a classical Cepheid and is very likely a spotted object. These results together with a very unusual shape of the K{sub s}-band light curve of the fourth star indicate that it is very likely that none of them is a Cepheid and, thus there is no evidence for a background dwarf galaxy. Our observations show that great care must be taken when classifying objects by their low-amplitude close-to-sinusoidal near-infrared light curves, especially with a small number of measurements. We also provide a sample of high-amplitude spotted stars with periods of a few days that can mimic pulsations and even eclipses.

  10. The RR Lyrae variable population in the Phoenix dwarf galaxy

    SciTech Connect

    Ordoñez, Antonio J.; Sarajedini, Ata; Yang, Soung-Chul E-mail: ata@astro.ufl.edu

    2014-05-10

    We present the first detailed study of the RR Lyrae variable population in the Local Group dSph/dIrr transition galaxy, Phoenix, using previously obtained HST/WFPC2 observations of the galaxy. We utilize template light curve fitting routines to obtain best fit light curves for RR Lyrae variables in Phoenix. Our technique has identified 78 highly probable RR Lyrae stars (54 ab-type; 24 c-type) with about 40 additional candidates. We find mean periods for the two populations of (P {sub ab}) = 0.60 ± 0.03 days and (P{sub c} ) = 0.353 ± 0.002 days. We use the properties of these light curves to extract, among other things, a metallicity distribution function for ab-type RR Lyrae. Our analysis yields a mean metallicity of ([Fe/H]) = –1.68 ± 0.06 dex for the RRab stars. From the mean period and metallicity calculated from the ab-type RR Lyrae, we conclude that Phoenix is more likely of intermediate Oosterhoff type; however the morphology of the Bailey diagram for Phoenix RR Lyraes appears similar to that of an Oosterhoff type I system. Using the RRab stars, we also study the chemical enrichment law for Phoenix. We find that our metallicity distribution is reasonably well fitted by a closed-box model. The parameters of this model are compatible with the findings of Hidalgo et al., further supporting the idea that Phoenix appears to have been chemically enriched as a closed-box-like system during the early stage of its formation and evolution.

  11. The radial velocity, velocity dispersion, and mass-to-light ratio of the Sculptor dwarf galaxy

    NASA Technical Reports Server (NTRS)

    Armandroff, T. E.; Da Costa, G. S.

    1986-01-01

    The radial velocity, velocity dispersion, and mass-to-light ratio for 16 K giants in the Sculptor dwarf galaxy are calculated. Spectra at the Ca II triplet are analyzed using cross-correlation techniques in order to obtain the mean velocity of + 107.4 + or - 2.0 km/s. The dimensional velocity dispersion estimated as 6.3 (+1.1, -1.3) km/s is combined with the calculated core radius and observed central surface brightness to produce a mass-to-light ratio of 6.0 in solar units. It is noted that the data indicate that the Sculptor contains a large amount of mass not found in globular clusters, and the mass is either in the form of remnant stars or low-mass dwarfs.

  12. Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hunt, Leslie K.; Madden, Suzanne C.; Schneider, Raffaella

    2008-12-01

    'Shea and Michael L. Norman; 16. Damped Lyα systems as probes of chemical evolution over cosmological timescales Miroslava Dessauges-Zavadsky; 17. Connecting high-redshift galaxy populations through observations of local damped Lyman alpha dwarf galaxies Regina E. Schulte-Ladbeck; 18. Chemical enrichment and feedback in low metallicity environments: constraints on galaxy formation Francesca Matteucci; 19. Effects of reionization on dwarf galaxy formation Massimo Ricotti; 20. The importance of following the evolution of the dust in galaxies on their SEDs A. Schurer, F. Calura, L. Silva, A. Pipino, G. L. Granato, F. Matteucci and R. Maiolino; 21. About the chemical evolution of dSphs (and the peculiar globular cluster ωCen) Andrea Marcolini and Annibale D'Ercole; 22. Young star clusters in the small Magellanic cloud: impact of local and global conditions on star formation Elena Sabbi, Linda J. Smith, Lynn R. Carlson, Antonella Nota, Monca Tosi, Michele Cignoni, Jay S. Gallagher III, Marco Sirianni and Margaret Meixner; 23. Modeling the ISM properties of metal-poor galaxies and gamma-ray burst hosts Emily M. Levesque, Lisa J. Kewley, Kirsten Larson and Leonie Snijders; 24. Dwarf galaxies and the magnetisation of the IGM Uli Klein; Session III. Explosive Events in Low-Metallicity Environments: 25. Supernovae and their evolution in a low metallicity ISM Roger A. Chevalier; 26. First stars - type Ib supernovae connection Ken'ichi Nomoto, Masaomi Tanaka, Yasuomi Kamiya, Nozomu Tominaga and Keiichi Maeda; 27. Supernova nucleosynthesis in the early universe Nozomu Tominaga, Hideyuki Umeda, Keiichi Maeda, Ken'ichi Nomoto and Nobuyuki Iwamoto; 28. Powerful explosions at Z = 0? Sylvia Ekström, Georges Meynet, Raphael Hirschi and André Maeder; 29. Wind anisotropy and stellar evolution Cyril Georgy, Georges Meynet and André Maeder; 30. Low-mass and metal-poor gamma-ray burst

  13. The interstellar medium in Andromeda's dwarf spheroidal galaxies - I. Content and origin of the interstellar dust

    NASA Astrophysics Data System (ADS)

    De Looze, Ilse; Baes, Maarten; Bendo, George J.; Fritz, Jacopo; Boquien, Médéric; Cormier, Diane; Gentile, Gianfranco; Kennicutt, Robert C.; Madden, Suzanne C.; Smith, Matthew W. L.; Young, Lisa

    2016-07-01

    Dwarf spheroidal galaxies are among the most numerous galaxy population in the Universe, but their main formation and evolution channels are still not well understood. The three dwarf spheroidal satellites (NGC 147, NGC 185, and NGC 205) of the Andromeda galaxy are characterized by very different interstellar medium properties, which might suggest them being at different galaxy evolutionary stages. While the dust content of NGC 205 has been studied in detail in an earlier work, we present new Herschel dust continuum observations of NGC 147 and NGC 185. The non-detection of NGC 147 in Herschel SPIRE maps puts a strong constraint on its dust mass (≤128^{+124}_{-68} M⊙). For NGC 185, we derive a total dust mass Md = 5.1±1.0 × 103 M⊙, which is a factor of ˜2-3 higher than that derived from ISO and Spitzer observations and confirms the need for longer wavelength observations to trace more massive cold dust reservoirs. We, furthermore, estimate the dust production by asymptotic giant branch (AGB) stars and supernovae (SNe). For NGC 147, the upper limit on the dust mass is consistent with expectations of the material injected by the evolved stellar population. In NGC 185 and NGC 205, the observed dust content is one order of magnitude higher compared to the estimated dust production by AGBs and SNe. Efficient grain growth, and potentially longer dust survival times (3-6 Gyr) are required to account for their current dust content. Our study confirms the importance of grain growth in the gas phase to account for the current dust reservoir in galaxies.

  14. Inverse Compton Gamma Rays from Dark Matter Annihilation in the Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Medhi, Jayashri; Duorah, H. L.; Barua, A. G.; Duorah, K.

    2016-09-01

    Dwarf spheroidal (dSph) galaxies are thought to be good candidates for dark matter search due to their high mass-to-light (M/L) ratio. One of the most favored dark matter candidates is the lightest neutralino (neutral χ particle) as predicted in the Minimal Supersymmetric Standard Model (MSSM). In this study, we model the gamma ray emission from dark matter annihilation coming from the nearby dSph galaxies Draco, Segue 1, Ursa Minor and Willman 1, taking into account the contribution from prompt photons and photons produced from inverse Compton scattering off starlight and Cosmic Microwave Background (CMB) photons by the energetic electrons and positrons from dark matter annihilation. We also compute the energy spectra of electrons and positrons from the decay of dark matter annihilation products. Gamma ray spectra and fluxes for both prompt and inverse Compton emission have been calculated for neutralino annihilation over a range of masses and found to be in agreement with the observed data. It has been found that the ultra faint dSph galaxy Segue 1 gives the largest gamma ray flux limits while the lowest gamma ray flux limits has been obtained from Ursa Minor. It is seen that for larger M/L ratio of dwarf galaxies the intensity pattern originating from e + e - pairs scattering off CMB photons is separated by larger amount from that off the starlight photons for the same neutralino mass. As the e + e - energy spectra have an exponential cut off at high energies, this may allow to discriminate some dark matter scenarios from other astrophysical sources. Finally, some more detailed study about the effect of inverse Compton scattering may help constrain the dark matter signature in the dSph galaxies.

  15. H I observations of two new dwarf galaxies: Pisces A and B with the SKA Pathfinder KAT-7

    NASA Astrophysics Data System (ADS)

    Carignan, C.; Libert, Y.; Lucero, D. M.; Randriamampandry, T. H.; Jarrett, T. H.; Oosterloo, T. A.; Tollerud, E. J.

    2016-03-01

    Context. Pisces A and Pisces B are the only two galaxies found via optical imaging and spectroscopy out of 22 Hi clouds identified in the GALFAHI survey as dwarf galaxy candidates. Aims: We derive the Hi content and kinematics of Pisces A and B. Methods: Our aperture synthesis Hi observations used the seven-dish Karoo Array Telescope (KAT-7), which is a pathfinder instrument for MeerKAT, the South African precursor to the mid-frequency Square Kilometre Array (SKA-MID). Results: The low rotation velocities of ~5 km s-1 and ~10 km s-1 in Pisces A and B, respectively, and their Hi content show that they are really dwarf irregular galaxies (dIrr). Despite that small rotation component, it is more the random motions ~9-11 km s-1 that provide most of the gravitational support, especially in the outer parts. The study of their kinematics, especially the strong gradients of random motions, suggest that those two dwarf galaxies are not yet in equilibrium. Conclusions: These Hi- rich galaxies may be indicative of a large population of dwarfs at the limit of detectability. However, such gas-rich dwarf galaxies will most likely never be within the virial radius of MW-type galaxies and become subhalo candidates. Systems such as Pisces A and B are more likely to be found at a few Mpc s from MW-type galaxies. The final FITS cube is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/587/L3

  16. Scaling relations and the fundamental line of the local group dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Woo, Joanna; Courteau, Stéphane; Dekel, Avishai

    2008-11-01

    We study the scaling relations between global properties of dwarf galaxies in the local group. In addition to quantifying the correlations between pairs of variables, we explore the `shape' of the distribution of galaxies in log parameter space using standardized principal component analysis, the analysis is performed first in the 3D structural parameter space of stellar mass M*, internal velocity V and characteristic radius R* (or surface brightness μ*). It is then extended to a 4D space that includes a stellar population parameter such as metallicity Z or star formation rate . We find that the local group dwarfs basically define a one-parameter `fundamental line' (FL), primarily driven by stellar mass, M*