Science.gov

Sample records for linking dwarf galaxies

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

    PubMed

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

    2010-03-01

    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. PMID:20203604

  2. Dwarf galaxies around the Milky Way: linking ages, kinematics and chemistry

    NASA Astrophysics Data System (ADS)

    Hill, Vanessa

    2015-08-01

    I will review recent observations of stellar populations in dwarf galaxies around the Milky Way, with special emphasis on the picture emerging for the evolution of these systems when combining kinematics, metallicities, detailed abundances and ages for large samples of stars. Very recently, the conditions in these systems at the earliest epochs have been the subject of dedicated investigations, in the form of observing extremely-metal poor stars in dwarf galaxies, and I will review with special care this area, including the apparent carbon paucity in some dwarf galaxies (eg. Sculptor). Finally, I will highlight the expected return from the Gaia space astrometric mission for our understanding of nearby dwarf galaxies.

  3. The Evolution of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Dunn, Jacqueline M.

    2016-01-01

    Dwarf galaxies are the most numerous galaxies in the Universe, yet the driving forces in their evolution remain elusive. The proposed evolutionary link between dwarf irregular and dwarf elliptical/spheroidal galaxies is investigated using broad-band UBVR photometry obtained for a sample of 29 dwarf galaxies. The galaxies span a range of absolute B-band magnitude from -13.67 to -19.86 mag. Broad-band colors and Sérsic surface brightness profile fits are compared for the two morphological types. All optical parameters are statistically different between the two subsamples, as evidenced by the significance level of the Kolmogorov-Smirnov statistic.Others have noted that dwarf ellipticals might have looked much like the currently observed dwarf irregulars in the past based on optical colors. An overlap between in the range of colors observed is noted for these targets, implying the possibility of an evolutionary link. A difference is noted between the two samples in the value of n (the power-law exponent determined from the Sérsic profile fitting), suggesting that the two main types of dwarf galaxy are structurally distinct. The differences in the structure of the stellar components would imply that dwarf irregulars do not evolve to become dwarf ellipticals in isolation, meaning that some sort of external interaction is required if the transformation is to occur. However, when the brightest dwarf elliptical targets are eliminated from the comparison, the two dwarf samples are much more similar in their values and range for the power-law exponent, which again suggests a possible evolutionary link. The environments of the galaxies are initially classified as either field or group/cluster, though no definitive environmental comparison is presented here.

  4. 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.

  5. 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.

  6. 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

  7. 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.

  8. Blue compact dwarfs - Extreme dwarf irregular galaxies

    NASA Technical Reports Server (NTRS)

    Thuan, Trinh X.

    1987-01-01

    Observational data on the most extreme members of the irregular dwarf (dI) galaxy class, the blue compact dwarfs (BCDs), are characterized, reviewing the results of recent investigations. The properties of the young stellar population, the ionized gas, the older star population, and the gas and dust of BCDs are contrasted with those of other dIs; BCD morphology is illustrated with sample images; and the value of BCDs (as nearby 'young' chemically unevolved galaxies) for studies of galaxy formation, galactic evolution, and starburst triggering mechanisms is indicated.

  9. Chemical Signatures in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.; Hill, Vanessa M.

    2008-12-01

    Chemical signatures in dwarf galaxies describe the examination of specific elemental abundance ratios to investigate the formation and evolution of dwarf galaxies, particularly when compared with the variety of stellar populations in the Galaxy. Abundance ratios can come from HII region emission lines, planetary nebulae, or supernova remnants, but mostly they come from stars. Since stars can live a very long time, for example, a 0.8 MSun star born at the time of the Big Bang would only now be ascending the red giant branch, and, if, for the most part, its quiescent main sequence lifetime had been uneventful, then it is possible that the surface chemistry of stars actually still resembles their natal chemistry. Detailed abundances of stars in dwarf galaxies can be used to reconstruct their chemical evolution, which we now find to be distinct from any other component of the Galaxy, questioning the assertion that dwarf galaxies like these built up the Galaxy. Potential solutions to reconciling dwarf galaxy abundances and Galaxy formation models include the timescale for significant merging and the possibility for uncovering different stellar populations in the new ultra-faint dwarfs.

  10. 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.

  11. Abundances in dwarf irregular galaxies

    NASA Technical Reports Server (NTRS)

    Dufour, Reginald J.

    1986-01-01

    The results of abundance studies of dwarf irregular galaxies and similar objects are reviewed with special attention to variations in the CNO element group. Observations of the forbidden N II and semiforbidden C III lines in the most metal-poor galaxy known, IZw 18, are presented for the first time and CNO abundances are derived via a photoionization model and discussed in the context of the abundances found in other metal-poor H II regions and galaxies.

  12. From tidal dwarf galaxies to satellite galaxies

    NASA Astrophysics Data System (ADS)

    Bournaud, F.; Duc, P.-A.

    2006-09-01

    The current popular cosmological models have granted the population of dwarf satellite galaxies a key role: their number, location, and masses constrain both the distribution of dark matter and the physical evolution of their hosts. In the past years, there has been increasing observational evidence that objects with masses of dwarf galaxies can form in the tidal tails of colliding galaxies, as well as speculations that they could become satellite-like galaxies orbiting around their progenitors and thus be cosmologically important. Yet, whether the so-called "Tidal Dwarf Galaxy" (TDG) candidates are really long-lived objects and not transient features only present in young interacting systems is still largely an open question to which numerical simulations may give precise answers. We present here a set of 96 N-body simulations of colliding galaxies with various mass ratios and encounter geometries, including gas dynamics and star formation. We study the formation and long-term evolution of their TDG candidates. Among the 593 substructures initially identified in tidal tails, about 75% fall back onto their progenitor or are disrupted in a few 108 years. The remaining 25% become long-lived bound objects that typically survive more than 2 Gyr with masses above 108 M⊙. These long-lived, satellite-like objects, are found to form in massive gaseous accumulations originally located in the outermost regions of the tidal tails. Studying the statistical properties of the simulated TDGs, we infer several basic properties that dwarf galaxies should meet to have a possible tidal origin and apply these criteria to the Local Group dwarfs. We further found that the presence of TDGs would foster the anisotropy observed in the distribution of classical satellite galaxies around their host. Identifying the conditions fulfilled by interacting systems that were able to form long-lived tidal dwarfs - a spiral merging with a galaxy between 1/4 and 8 times its mass, on a prograde orbit

  13. 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.

  14. Uncovering Blue Diffuse Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    James, Bethan; Koposov, Sergey; Stark, Daniel; Belokurov, Vasily; Pettini, Max; Olszewski, Edward W.

    2015-01-01

    Extremely metal-poor galaxies (XMPs) and the star-formation within their chemically pristine environments are fundamental to our understanding of the galaxy formation process at early times. However, traditional emission-line surveys detect only the brightest metal-poor galaxies where star-formation occurs in compact, starbursting environments, and thereby give us only a partial view of the dwarf galaxy population. To avoid such biases, we have developed a new search algorithm based on the morphological, rather then spectral, properties of XMPs and have applied to the Sloan Digital Sky Survey database of images. Using this novel approach, we have discovered ~100 previously undetected, faint blue galaxies, each with isolated HII regions embedded in a diffuse continuum. In this talk I will present the first results from follow-up optical spectroscopy of this sample, which reveals these blue diffuse dwarfs (BDDs) to be young, very metal-poor and actively forming stars despite their intrinsically low luminosities. I will present evidence showing that BDDs appear to bridge the gap between quiescent dwarf irregular (dIrr) galaxies and blue compact galaxies (BCDs) and as such offer an ideal opportunity to assess how star-formation occurs in more `normal' metal-poor systems.

  15. Uncovering blue diffuse dwarf galaxies

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    Extremely metal poor (XMP) galaxies are known to be very rare, despite the large numbers of low-mass galaxies predicted by the local galaxy luminosity function. This paper presents a subsample of galaxies that were selected via a morphology-based search on Sloan Digital Sky Survey images with the aim of finding these elusive XMP galaxies. By using the recently discovered XMP galaxy, Leo P, as a guide, we obtained a collection of faint, blue systems, each with isolated H II regions embedded in a diffuse continuum, that have remained optically undetected until now. Here we show the first results from optical spectroscopic follow-up observations of 12 of ˜100 of these blue diffuse dwarf (BDD) galaxies yielded by our search algorithm. Oxygen abundances were obtained via the direct method for eight galaxies, and found to be in the range 7.45 < 12 + log (O/H) < 8.0, with two galaxies being classified as XMPs. All BDDs were found to currently have a young star-forming population (<10 Myr) and relatively high ionization parameters of their H II regions. Despite their low luminosities (-11 ≲ MB ≲ -18) and low surface brightnesses (˜23-25 mag arcsec-2), the galaxies were found to be actively star forming, with current star formation rates between 0.0003 and 0.078 M⊙ yr-1. From our current subsample, BDD galaxies appear to be a population of non-quiescent dwarf irregular galaxies, or the diffuse counterparts to blue compact galaxies and as such may bridge the gap between these two populations. Our search algorithm demonstrates that morphology-based searches are successful in uncovering more diffuse metal-poor star-forming galaxies, which traditional emission-line-based searches overlook.

  16. Stellar Rotation Curves of Starbursting Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    van Zee, Liese; Skillman, Evan D.; Salzer, John J.

    2001-02-01

    A year ago, we successfully completed a pilot project to obtain stellar rotation curves of starbursting dwarf galaxies. These observations provided the first spatially resolved stellar rotation curves of gas-rich dwarf galaxies. We now propose to expand our sample (by a factor of 2) by observing 4 additional dwarf galaxies with the CTIO 4m. The fundamental question to be addressed is whether the gas and stars are kinematically coupled in these small galaxies. These observations will place the first kinematic constraints on evolutionary models for dwarf galaxies.

  17. Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Corbelli, E.; Bizzocchi, L.; Giovanardi, C.; Bomans, D.; Coelho, B.; De Looze, I.; Gonçalves, T. S.; Hunt, L. K.; Leonardo, E.; Madden, S.; Menéndez-Delmestre, K.; Pappalardo, C.; Riguccini, L.

    2016-05-01

    We present 12CO(1-0) and 12CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log (O / H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 μm emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses ≲ 109 M⊙, contrary to the atomic hydrogen fraction, MHI/M∗, which increases inversely with M∗. The flattening of the MH2/M∗ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both Hi-deficient and Hi-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, τdep, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between Hi deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity. Based on observations carried out with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany

  18. 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.

  19. Near-infrared photometry and stellar populations in dwarf elliptical and irregular galaxies

    NASA Technical Reports Server (NTRS)

    Thuan, T. X.

    1985-01-01

    Studies of three different types of dwarf galaxies have been conducted, taking into account low surface brightness (LSB) dwarf irregular (dI) galaxies, dwarf elliptical (dE) galaxies, and blue compact dwarf galaxies (BCDGs). Near-infrared observations for a large sample of BCDGs have been reported by Thuan (1983). The present paper is concerned with complementary near-infrared observations for a large sample of LSB dI and dE galaxies, and, in addition, with a few additional BCDG observations. In a discussion of dwarf elliptical galaxies, attention is given to an infrared color-color diagram, optical-infrared colors and burst ages, and the UVK color plane. Low surface brightness dwarf irregular galaxies and blue compact dwarf galaxies are considered along with the possible evolutionary scenarios which may link LSB dIs, BCDGs, and dEs.

  20. Tidal Dwarf Galaxies In Gas-rich Interacting Galaxy Groups

    NASA Astrophysics Data System (ADS)

    Eigenthaler, Paul

    2014-01-01

    Galaxy-galaxy interactions in gas-rich galaxy groups or pairs can form tidal bridges and tails. These tidal arms can contain kinematically decoupled structures with active star formation in the same mass range as dwarf galaxies, so-called tidal dwarf galaxies (TDGs). They differ from ordinary dwarf galaxies by their lack of dark matter and higher metallicity content. Compact groups of galaxies are an ideal environment to study the origin and evolution of TDGs since the high spatial volume density of member galaxies allows for frequent and efficient interactions between galaxies forming tidal tails. Hunsberger et al. (1996) identified 47 TDG candidates in Hickson compact groups (HCGs) and estimated that more than 50% of all dwarf galaxies in compact groups are former TDGs. Statistical considerations based on observations of interacting galaxies illustrate that a significant fraction of today's dwarf galaxies could have had a tidal origin. In their early evolution, TDGs can easily be distinguished from classical dwarf galaxies as they are still embedded in large tidal structures and show ongoing star formation, identified via strong Hα emission in these aggregates. Simulations of interacting galaxies, and of TDGs in particular, have shown that TDGs can survive their first starburst event and turn into long-lived dwarf sized objects. Preliminary results from deep Hα imaging with the SOAR telescope to detect new TDGs in a sample of 10 Hickson compact groups will be presented.

  1. 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.

  2. Splendid Isolation: Using DDO 210 to Benchmark Dwarf Galaxy Evolution

    NASA Astrophysics Data System (ADS)

    Cole, Andrew

    2012-10-01

    Dwarf galaxies are sensitive probes of galaxy formation theory because of their vulnerability to local and cosmic environmental effects. The star formation histories {SFH} of nearby galaxies can test the effects of post-reionization photoheating, supernova-driven outflows, and harassment. We are undertaking a study of the SFH, chemistry, structure, and dynamics of Local Group dwarf galaxies to inform the next generation of theory and simulations. This combines HST photometry, wide-field ground-based data, and spectra of individual stars from 8-10m class telescopes for isolated dwarf galaxies in the Local Group. Dwarf galaxies that have never interacted with either M31 or the Milky Way {MW} are a pristine environment in which to study the progress of galaxy evolution without external influences. Theory and observation suggest that star formation should be delayed in low-mass, low-metallicity galaxies that evolve in isolation; we propose to test that assertion using DDO 210. DDO 210 is the closest galaxy which can never have interacted with the MW or M31 and is thus a uniquely powerful testbed for models of dwarf galaxy evolution. We propose to photometer over 100,000 stars using ACS/WFC in order to measure its star formation rate vs. time with age resolution better than 15% over its entire lifetime. These observations will be combined with Keck/DEIMOS spectra of stars spanning the age of the galaxy to build a complete picture of its chemical and kinematic evolution. DDO 210 is the only galaxy that is both isolated and nearby enough to use as a link between deep studies of nearby galaxies and the ANGST survey of galaxies from 1.5-4 Mpc.

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. Neutral Hydrogen in Local Group Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Grcevich, Jana

    The gas content of the faintest and lowest mass dwarf galaxies provide means to study the evolution of these unique objects. The evolutionary histories of low mass dwarf galaxies are interesting in their own right, but may also provide insight into fundamental cosmological problems. These include the nature of dark matter, the disagreement between the number of observed Local Group dwarf galaxies and that predicted by lambda cold dark matter models, and the discrepancy between the observed census of baryonic matter in the Milky Way's environment and theoretical predictions. This thesis explores these questions by studying the neutral hydrogen (HI) component of dwarf galaxies. First, limits on the HI mass of the ultra-faint dwarfs are presented, and the HI content of all Local Group dwarf galaxies is examined from an environmental standpoint. We find that those Local Group dwarfs within 270 kpc of a massive host galaxy are deficient in HI as compared to those at larger galactocentric distances. Ram-pressure arguments are invoked, which suggest halo densities greater than 2-3 x 10-4 cm-3 out to distances of at least 70 kpc, values which are consistent with theoretical models and suggest the halo may harbor a large fraction of the host galaxy's baryons. We also find that accounting for the incompleteness of the dwarf galaxy count, known dwarf galaxies whose gas has been removed could have provided at most 2.1 x 108 M⊙ of HI gas to the Milky Way. Second, we examine the possibility of discovering unknown gas-rich ultra-faint galaxies in the Local Group using HI. The GALFA-HI Survey catalog is searched for compact, isolated HI clouds which are most similar to the expected HI characteristics of low mass dwarf galaxies. Fifty-one Local Group dwarf galaxy candidates are identified through column density, brightness temperature, and kinematic selection criteria, and their properties are explored. Third, we present hydrodynamic simulations of dwarf galaxies experiencing a

  8. Charting Unexplored Dwarf Galaxy Territory with RR Lyrae

    NASA Astrophysics Data System (ADS)

    Baker, Mariah; Willman, Beth

    2015-11-01

    Observational biases against finding Milky Way (MW) dwarf galaxies at low Galactic latitudes (b ≲ 20°) and at low surface brightnesses ({μ }{{V,0}} ≳ 29 mag arcsec{}-2) currently limit our understanding of the faintest limits of the galaxy luminosity function. This paper is a proof-of-concept that groups of two or more RR Lyrae stars could reveal MW dwarf galaxies at d > 50 kpc in these unmined regions of parameter space, with only modest contamination from interloper groups when large halo structures are excluded. For example, a friends-of-friends (FOF) search with a 2D linking length of 500 pc could reveal dwarf galaxies more luminous than {M}{{V}} = -3.2 mag and with surface brightnesses as faint as 31 mag arcsec{}-2 (or even fainter, depending on RR Lyrae specific frequency). Although existing public RR Lyrae catalogs are highly incomplete at d > 50 kpc and/or include <1% of the MW halo’s volume, a FOF search reveals two known dwarfs (Boötes I and Sextans) and two dwarf candidate groups possibly worthy of follow-up. PanSTARRS 1 (PS1) may catalog RR Lyrae to 100 kpc (in the absence of Galactic extinction) which would include up to ˜15% of predicted MW dwarf galaxies. Groups of PS1 RR Lyrae should therefore reveal very low surface brightness and low Galactic latitude dwarfs within its footprint, if they exist. With sensitivity to RR Lyrae to d ≳ 600 kpc, LSST is the only planned survey that will be both wide-field and deep enough to use RR Lyrae to definitively measure the MW’s dwarf galaxy census to extremely low surface brightnesses, and through the Galactic plane.

  9. 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%.

  10. 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.

  11. The Missing Baryons Around Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Bregman, Joel

    2013-10-01

    Dwarf galaxies are missing nearly all of their baryons, which have presumably flowed away as a wind. This mass loss accounts for a significant fraction of all baryons lost from galaxies, so there is great interest in determining the size and scope of the gas lost. This gas is not visible in emission but is detectable through absorption features toward background AGNs. Here we propose to observe the absorbing material around three isolated dwarfs on the periphery of the Local Group: Sextans A, Sextans B, and NGC 3109. Unlike more distant dwarfs, the star formation history and cold gaseous content of these galaxies are well-studied. The isolation of these dwarfs, far from large galaxies, means that they have not yet interacted with other systems so their mass loss history is well-preserved, making them ideal targets for study.

  12. The Hunt for Missing Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2015-11-01

    Theories of galaxy formation and evolution predict that there should be significantly more dwarf galaxies than have been observed. Are our theories wrong? Or are dwarf galaxies just difficult to detect? Recent results from a survey of a galaxy cluster 62 million light-years away suggest there may be lots of undiscovered dwarf galaxies hiding throughout the universe!Hiding in FaintnessThe missing dwarf problem has had hints of a resolution with the recent discovery of Ultra-Diffuse Galaxies (UDGs) in the Coma and Virgo galaxy clusters. UDGs have low masses and large radii, resulting in a very low surface brightness that makes them extremely difficult to detect. If many dwarfs are UDGs, this could well explain why weve been missing them!But the Coma and Virgo galaxy clusters are similar in that theyre both very massive. Are there UDGs in other galaxy clusters as well? To answer this question, an international team of scientists is running the Next Generation Fornax Survey (NGFS), a survey searching for faint dwarf galaxies in the central 30 square degrees of the Fornax galaxy cluster.The NGFS uses near-UV and optical observations from the Dark Energy Camera mounted on the 4m Blanco Telescope in Chile. The survey is still underway, but in a recent publication led by Roberto P. Muoz (Institute of Astrophysics at the Pontifical Catholic University of Chile), the team has released an overview of the first results from only the central 3 square degrees of the NGFS field.Surprising DetectionGalaxy radii vs. their absolute i-band magnitudes, for the dwarfs found in NGFS as well as other stellar systems in the nearby universe. The NGFS dwarfs are similar to the ultra-diffuse dwarfs found in the Virgo and Coma clusters, but are several orders of magnitude fainter. [Muoz et al. 2015]In just this small central field, the team has found an astounding 284 low-surface-brightness dwarf galaxy candidates 158 of them previously undetected. At the bright end of this sample are dwarf

  13. Infrared Properties of Star Forming Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Vaduvescu, Ovidiu

    2005-11-01

    Dwarf galaxies are the most common galaxies in the Universe. They are systems believed to consist of matter in a near-primordial state, from which giant galaxies probably form. As such, they are important probes for studying matter in its near-primordial state. In an effort to study the main physical and chemical properties of dwarfs, the present thesis focuses upon the main physical properties of dwarfs. Two classes of star forming dwarf galaxies are considered: dwarf irregulars (dIs), and blue compact dwarfs (BCDs). A third class, dwarf ellipticals (dEs), is studied based on its structural properties and compared with dIs. Possible evolutionary connections are addressed between dIs and BCDs. To measure the luminosity, deep imaging in the near-infrared (NIR) is considered. Compared with the visible, the NIR domain gives a better gauge of the galaxy mass contained in the old stellar populations, minimising the starburst contribution and also the effects of extinction. Two observing samples of star-forming dwarf galaxies are considered. The first includes 34 dIs in the Local Volume closer than 5 Mpc. The second sample includes 16 BCDs in the Virgo Cluster. In six observing runs between 2001 and 2004, we acquired deep NIR images (J and K_s) using the 3.6m Canada-France-Hawaii-Telescope (CFHT) in Hawaii and the 2.1m telescope at the National Astronomical Observatory ''San Pedro Martir'' (OAN-SPM) in Mexico. Deep spectrocopy was acquired in 2003 on the 8.1m Gemini-North telescope in Hawaii. We completed the observed samples with spectroscopic data from the literature, and photometry from the 2MASS survey and GOLDMine database. From a statistical study at CFHT, we derived some strategies necessary to image optimally faint extended sources in the NIR. Due to the airglow variation in the atmosphere and the thermal contribution of the dome, telescope and the instrumentation, repeated observations of the sky must be alternated every 3-4 minutes with the science images, in

  14. Constraining the subgrid physics in simulations of isolated dwarf galaxies

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    Simulating dwarf galaxy haloes in a reionizing Universe puts severe constraints on the subgrid model employed in the simulations. Using the same subgrid model that works for simulations without a UV-background (UVB) results in gas-poor galaxies that stop forming stars very early on, except for haloes with high masses. This is in strong disagreement with observed galaxies, which are gas rich and star forming down to a much lower mass range. To resolve this discrepancy, we ran a large suite of isolated dwarf galaxy simulations to explore a wide variety of subgrid models and parameters, including timing and strength of the UVB, strength of the stellar feedback and metallicity-dependent Pop III feedback. We compared these simulations to observed dwarf galaxies by means of the baryonic Tully-Fisher relation (BTFR), which links the baryonic content of a galaxy to the observationally determined strength of its gravitational potential. We found that the results are robust to changes in the UVB. The strength of the stellar feedback shifts the results on the BTFR, but does not help to form gas-rich galaxies at late redshifts. Only by including Pop III feedback are we able to produce galaxies that lie on the observational BTFR and that have neutral gas and ongoing star formation at redshift zero.

  15. Observing Dwarf Galaxies in the Local Universe

    NASA Astrophysics Data System (ADS)

    Simon, Joshua

    2016-03-01

    Dwarf galaxies in the Local Group are key probes of both dark matter and galaxy formation. They are the smallest, oldest, most dark matter-dominated, and least chemically enriched stellar systems currently known. However, despite two decades of major computational, theoretical, and observational advances in this field, we are still working toward a complete understanding of star and galaxy formation at the faint end of the galaxy luminosity function. In the last year, large sky surveys such as the Dark Energy Survey and Pan-STARRS have made an unprecedented series of discoveries, nearly doubling the population of Milky Way satellite galaxies that was known at the start of 2015. This increase in the number of nearby dwarfs may significantly improve the sensitivity of searches for dark matter annihilation radiation. Many of these new dwarfs are likely to have originated as satellites of the Magellanic Clouds, providing a unique opportunity to study the effect of galactic environment on the formation of the faintest dwarfs. I will provide an overview of recent discoveries and how they fit in to the previously known population of nearby dwarf galaxies, highlighting a few of the most interesting objects from the perspective of dark matter and stellar nucleosynthesis.

  16. Massive Star Clusters in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Larsen, Soeren

    2015-08-01

    Dwarf galaxies are often characterized by very high globular cluster specific frequencies, in some cases exceeding that of the Milky Way by a factor of 100 or more. Moreover, the GCs are typically much more metal-poor than the bulk of the field stars, so that a substantial fraction (up to 20-25% or more) of all metal-poor stars in some dwarf galaxies are associated with GCs. The metal-poor components of these galaxies thus represent an extreme case of the "specific frequency problem". In this talk I will review the current status of our understanding of GC systems in dwarf galaxies. Particular emphasis will be placed on the implications of the high GC specific frequencies for the amount of mass loss the clusters could have experienced and the constraints this provides on theories for the origin of multiple populations in globular clusters.

  17. 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.

  18. "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

  19. 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. PMID:18048653

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. 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 to examine the fraction of baryons in $\\Lambda$CDM 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 dwarfs seem to violate this constraint, having baryonic masses up to ten 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 they inhabit haloes with extreme deficits in their dark matter content. This `missing dark matter' is reminiscent of the inner mass deficits of galaxies with slowly-rising rotation curves, but extends to regions larger than the luminous galaxies themselves, disfavouring explanations based on star formation-induced `cores' in the dark matter. An alternative could be that galaxy inclination errors have been underestimated, and that these are just systems where inferred mass profiles have been compromised by systematic uncertainties in interpreting the velocity field. This should be investigated further, since it might provide a simple explanation not only for missing-dark-matter galaxies but also for other challenges to our understanding of the inner structure of cold dark matter haloes.

  5. Evolutionary paths in starbursting transition dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Dellenbusch, Kate Erika

    2008-10-01

    In this thesis we present an observational optical study of a subgroup of dwarf galaxies which have characteristics of a possible evolutionary transition between actively star-forming systems and inactive dwarf galaxies. The goal of this thesis is to assess the transition nature of these systems and gain insight into their evolutionary histories. Data for the investigation consist primarily of broad-band and narrow-band Ha images taken with the WIYN 0.9m telescope. We find that these galaxies contain central starbursts embedded in older, smooth, elliptical outer stellar envelopes. They also have small HI contents and apparently lack sufficient amounts of ISM to sustain high star formation rates over a significant cosmic timescale; gas exhaustion timescales are < 1 Gyr. We also find these objects have surprisingly high HII region oxygen abundances with values near solar. This suggests the starburst came from internal gas that was previously enriched and that a significant fraction of the synthesized metals are retained. Additionally, these systems are located in loose groups and are not currently interacting with any nearby galaxies. Thus their origins are not immediately clear. We explore possible evolutionary histories for such starburst "transition" dwarf galaxies based on this puzzling set of characteristics and results from moderately deep optical imaging. We consider mechanisms where the starbursts are tied either to interactions with other galaxies or to the state of the interstellar medium.

  6. 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.

  7. 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.

  8. Faint Blue Galaxies and the Epoch of Dwarf Galaxy Formation

    NASA Astrophysics Data System (ADS)

    Babul, Arif; Ferguson, Henry C.

    1996-02-01

    Several independent lines of reasoning, both theoretical and observational, suggest that the very faint (B ≳ 24) galaxies seen in deep images of the sky are small low-mass galaxies that experienced a short starburst at redshifts 0.5 ≲ z ≲ 1 and have since faded into low-luminosity, low surface brightness (LSB) objects. We examine this hypothesis in detail in order to determine whether a model incorporating such dwarfs can account for the observed wavelength-dependent number counts, as well as redshift, color, and size distributions. Low-mass galaxies generically arise in large numbers in hierarchical clustering scenarios with realistic initial conditions. Generally, these galaxies are expected to form at high redshifts. Babul & Rees have argued that the formation epoch of these galaxies is, in fact, delayed until z ≲ 1 due to the photoionization of the gas by the metagalactic UV radiation at high redshifts. We combine these two elements, along with simple heuristic assumptions regarding star formation histories and efficiency, to construct our bursting dwarf model. The slope and the normalization of the mass function of the dwarf galaxies are derived from the initial conditions and are not adjusted to fit the data. We further augment the model with a phenomenological prescription for the formation and evolution of the locally observed population of galaxies (E, S0, Sab, Sbc, and Sdm types). We use spectral synthesis and Monte Carlo methods to generate realistic model galaxy catalogs for comparison with observations. We find that for reasonable choices of the star formation histories for the dwarf galaxies, the model results are in very good agreement with the results of the deep galaxy surveys. Such a dwarf-dominated model is also qualitatively supported by recent studies of faint galaxy gravitational lensing and clustering, by galaxy size distributions measured with the Hubble Space Telescope, and by the evidence for very modest evolution in regular galaxy

  9. The Metamorphosis of Tidally Stirred Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Mayer, Lucio; Governato, Fabio; Colpi, Monica; Moore, Ben; Quinn, Thomas; Wadsley, James; Stadel, Joachim; Lake, George

    2001-10-01

    We present results from high-resolution N-body/SPH (smoothed particle hydrodynamic) simulations of rotationally supported dwarf irregular galaxies moving on bound orbits in the massive dark matter halo of the Milky Way. The dwarf models span a range in disk surface density and the masses and sizes of their dark halos are consistent with the predictions of cold dark matter cosmogonies. We show that the strong tidal field of the Milky Way determines severe mass loss in their halos and disks and induces bar and bending instabilities that transform low surface brightness dwarfs (LSBs) into dwarf spheroidals (dSphs) and high surface brightness dwarfs (HSBs) into dwarf ellipticals (dEs) in less than 10 Gyr. The final central velocity dispersions of the remnants are in the range 8-30 km s-1 and their final v/σ falls to values less than 0.5, matching well the kinematics of early-type dwarfs. The transformation requires the orbital time of the dwarf to be <~3-4 Gyr, which implies a halo as massive and extended as predicted by hierarchical models of galaxy formation to explain the origin of even the farthest dSph satellites of the Milky Way, Leo I, and Leo II. We show that only dwarfs with central dark matter densities as high as those of Draco and Ursa Minor can survive for 10 Gyr in the proximity of the Milky Way. A correlation between the central density and the distance of the dwarfs from the primary galaxy is indeed expected in hierarchical models, in which the densest objects should have small orbital times because of their early formation epochs. Part of the gas is stripped and part is funneled to the center because of the bar, generating one strong burst of star formation in HSBs and smaller, multiple bursts in LSBs. Therefore, the large variety of star formation histories observed in Local Group dSphs arises because different types of dIrr progenitors respond differently to the external perturbation of the Milky Way. Our evolutionary model naturally explains the

  10. Stellar Mass Distributions in Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Zhang, Hongxin; Hunter, D.; LITTLE THINGS Team

    2011-01-01

    We present the radial distributions of the stellar mass and the star formation histories for a large sample of dwarf irregular galaxies assembled by the LITTLE THINGS project (Local Irregulars That Trace Luminosity Extremes The HI Nearby Galaxy Survey, http://www.lowell.edu/users/dah/littlethings/index.html). Specifically, utilizing the multi-band data including FUV/NUV/UBV/Hα/3.6μm, and with the CB07 stellar population synthesis models, we analyze the variations of the SEDs as a function of radius. By studying the relationship between the stellar mass, star formation histories, star formation and HI gas, we will discuss the possible star formation modes and the roles played by the stellar mass and gas in determining the star formation in dwarf irregular galaxies in general. We gratefully acknowledge funding for this research from the National Science Foundation (AST-0707563).

  11. Suites of dwarfs around Nearby giant galaxies

    SciTech Connect

    Karachentsev, Igor D.; Kaisina, Elena I.; Makarov, Dmitry I. E-mail: kei@sao.ru

    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 Θ{sub 1}, 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 Θ{sub 1}. All suite members with positive Θ{sub 1} 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 {sup –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 M{sub B} = –18{sup m}. 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

  12. 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

  13. The dwarf galaxy population of nearby galaxy clusters

    NASA Astrophysics Data System (ADS)

    Lisker, Thorsten; Wittmann, Carolin; Pak, Mina; Janz, Joachim; Bialas, Daniel; Peletier, Reynier; Grebel, Eva; Falcon Barroso, Jesus; Toloba, Elisa; Smakced Collaboration, Focus Collaboration

    2015-01-01

    The Fornax, Virgo, Ursa Major and Perseus galaxy clusters all have very different characteristics, in terms of their density, mass, and large-scale environment. We can regard these clusters as laboratories for studying environmental influence on galaxy evolution, using the sensitive low-mass galaxies as probes for external mechanisms. Here we report on recent and ongoing observational studies of the said clusters with imaging and spectroscopy, as well as on the interpretation of present-day cluster galaxy populations with the aid of cosmological simulations.Multicolor imaging data allow us to identify residual star formation in otherwise red early-type dwarf galaxies, which hold clues to the strength of gas stripping processes. Major-axis spectra and 2D kinematical maps provide insight regarding the amount of rotational support and how much dynamical heating a dwarf galaxy may have experienced. To this end, dedicated N-body simulations that follow the evolution of galaxies since early epochs reveal their path through parameter space, and can be compared to observations in order to understand the time-integrated effect of environmental influence.

  14. 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.

  15. 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.

  16. 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.

  17. Chemical substructure and inhomogeneous mixing in Local Group dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Venn, K. A.

    Evidence for inhomogeneous mixing in the Carina, Draco, and Sculptor dwarf galaxies is examined from chemical abundance patterns. Inhomogeneous mixing at early times is indicated in the classical dwarf galaxies, though cannot be ascertained in ultra faint dwarfs. Mixing efficiencies can affect the early metallicity distribution function, the pre-enrichment levels in globular clusters, and also have an impact on the structure of dwarf systems at early times. Numerical models that include chemical evolution explicitly do a better job in reproducing the observations, and make interesting predictions for the nature of dwarf galaxies and their first stars at the earliest times.

  18. The Unexpected Past of a Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    1996-08-01

    New Light on Cannibalism in the Local Group of Galaxies The Local Group of Galaxies consists of a few large spiral galaxies - for instance the Milky Way galaxy in which we live, and the Andromeda galaxy that is visible to the unaided eye in the northern constellation of the same name - as well as two dozen much smaller galaxies of mostly irregular shape. Whereas the larger galaxies have extended halos of very old stars, no such halos have ever been seen around the smaller ones. Now, however, Dante Minniti and Albert Zijlstra [1], working at the ESO 3.5-metre New Technology Telescope (NTT), have found a large halo of old and metal-poor stars around one of the dwarf galaxies in the Local Group. This finding is quite unexpected. It revises our understanding of star formation in these galaxies and provides important information about the past evolution of galaxies [2]. Galaxy halos The Milky Way galaxy is surrounded by a large, roughly spherical halo of old stars. The diameter is about 100,000 light years and the stars therein, known as Population II stars, are among the oldest known, with ages of 10 billion years or even more. They also differ from the younger stars nearer to the main plane of the Milky Way (in which our 4.7 billion year old Sun is located) by being very metal-poor. Many of the halo stars consist almost solely of hydrogen and helium, reflecting the composition of matter in the young Universe. This halo is important for our understanding of the processes that led to the formation of the Milky Way galaxy. It is believed that many of the halo stars and those of the same type found in globular clusters existed already before the Milky Way had fully formed. Galaxy cannibalism Many astronomers suspect that galaxies evolve and gradually grow larger and heavier by practising cannibalism on their own kind. In this picture, when two galaxies collide in space, the stars and nebulae in the smaller one will disperse and soon be taken over by the larger one, which

  19. Chemical Abundances of Metal-poor stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.; Jablonka, Pascale; Hill, Vanessa; Starkenburg, Else; Lemasle, Bertrand; Shetrone, Matthew; Irwin, Mike; Norris, John; Yong, David; Gilmore, Gerry; Salvadori, Stephania; Skuladottir, Asa; Tolstoy, Eline

    2016-08-01

    Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. The slower star formation efficiency make dwarf galaxies ideal systems for testing nucleosynthetic yields. Not only are alpha-poor stars found at lower metallicities, and a higher fraction of carbon-enhanced stars, but we are also finding stars in dwarf galaxies that appear to be iron-rich. These are compared with yields from a variety of supernova predictions.

  20. 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.

  1. Dwarf Galaxies In The Near-Infrared

    NASA Astrophysics Data System (ADS)

    Juette, M.; Chini, R.

    We present first results of a dwarf galaxy survey at J, K' and Br \\gamma using IRAC2b at La Silla/Chile and MAGIC at Calar Alto/Spain at the corresponding 2.2 m telescopes. We achieved a field size of 129 x 129 arcsec with an applied scale of 0.507 arcsec/pix for IRAC2 and a field of 415 x 415 arcsec with an applied scale of 1.61 arcsec/pix for MAGIC. So far we have imaged a group of 32 objects consisting of 20 dwarf irregulars (dI), 6 dwarf ellipticals (dE) and 6 blue compact dwarf galaxies (BCD). The targets were taken from three different samples: The optical catalogue of Feitzinger & Galinski (1985) for dE and dI (selection criterion: m_b = 12 - 14 mag), the optical Calan-Tololo-Survey of HII galaxies for BCD's, and the FIR-sample of Melisse & Israel (1994) for dI's (selection criterion: IRAS-fluxes S100 > 1000 mJy). Our NIR data and the B and R photometry of Lauberts & Valentijn (1989) are used to construct colour maps. These maps indicate distinctly different colour--gradients between NIR and optical isophotes originating from different populations and/or from reddening by dust. In addition, some global properties like the total luminosity, the HI content and the star formation activity as traced by our Br \\gamma observations are investigated and compared among the various morphological types.

  2. Abundance Patterns and the Chemical Enrichment of Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hill, V.; DART Collaboration

    2012-08-01

    I review here the chemical abundances of individual stars in the nearest classical dwarf spheroidal galaxies, that have become available in increasing numbers (sample size and galaxies probed) in the last decade.

  3. The dwarf galaxy UGC 5272 and its small companion galaxy

    NASA Technical Reports Server (NTRS)

    Hopp, U.; Schulte-Ladbeck, R. E.

    1991-01-01

    The present study of optical images and spectroscopy of the dwarf irregular galaxy UGC 5272 notes the presence, at 3.6 kpc, of a small neighboring galaxy which is also of irregular type and has a Holmberg diameter of 0.6 kpc. Attention is given to the possibility that the two galaxies, which are resolved into single stars, may form a physical pair. It is suggested that the blue-to-red supergiant ratio of UGC 5272 is high due to its low metallicity. While its extremely blue colors are suggestive of a recent starburst, the structural parameters of the galaxy are surprisingly normal. The gas contribution to total mass is high.

  4. 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

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

    NASA Astrophysics Data System (ADS)

    Vader, J. Patricia; Chaboyer, Brian

    1994-10-01

    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 r1/4 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 r1/4 law profile, in bright galaxies to a lens-like structure in dwarf galaxies. We prove detailed surface photometry to be a helpful if not always sufficient tool in investigating the structure of dwarf galaxies. In many cases kinematic information is needed to complete the picture. We find the shapes of the surface brightness profiles to be loosely associated with morphological type. Our sample contains several new galaxies with properties intermediate between those of giant and dwarf ellipticals (but no M32-like objects). This shows that such intermediate galaxies exist so that at least a fraction of early-type dwarf ellipticals is structurally related to early-type giants instead of belonging to a totally unrelated, disjunct family. This supports an origin of early-type dwarf galaxies as originally more massive systems that acquired their current morphology as a result of substantial, presumable supernova-driven, mass loss. On the other hand, several early-type dwarfs in our sample are merger

  6. A Bayesian Approach to Constraining Dwarf Galaxy Evolution

    NASA Astrophysics Data System (ADS)

    Lotz, J. M.; Ferguson, H. C.

    2001-12-01

    We use a Bayesian - maximum likelihood analysis of the Hubble Deep Field to constrain the epoch of dwarf galaxy formation. Late formation of dwarf galaxies arises as a natural consequence of proposed solutions to the "over-cooling" problem in hierarchical structure formation. Although dwarf-sized halos are among the first objects to collapse out of a cold dark matter dominated universe, photo-ionization from the inter-galactic UV background and stellar feedback at early epochs may suppress or delay significant star formation in dwarf galaxies until redshifts ~ 1. Such late-forming dwarf galaxies may make up a portion of the population of the faint blue galaxies observed at intermediate redshifts. Previous attempts to understand the nature of the faint blue galaxy population have fit the binned number counts, luminosity functions, color and size distributions and compared the results to a handful of possible scenarios. Our approach sums the likelihood of observing each object in the HDF catalog given a dwarf galaxy formation scenario and computes the total likelihood of the given dwarf formation scenario. The parameters of the input model are then varied, and the model with the maximum total likelihood is determined. This technique does not bin the data in any way, tests a wide range of input model parameters, and allows us to quantify the goodness-of-fit and constraints on dwarf galaxy evolution.

  7. FINDING DWARF GALAXIES FROM THEIR TIDAL IMPRINTS

    SciTech Connect

    Chakrabarti, Sukanya; Bigiel, Frank; Chang, Philip; Blitz, Leo E-mail: chang65@uwm.edu

    2011-12-10

    We describe ongoing work on a new method that allows one to approximately determine the mass and relative position (in galactocentric radius and azimuth) of galactic companions purely from analysis of observed disturbances in gas disks. We demonstrate the validity of this method, which we call Tidal Analysis, by applying it to local spirals with known optical companions, namely M51 and NGC 1512. These galaxies span the range from having a very low mass companion ({approx}one-hundredth the mass of the primary galaxy) to a fairly massive companion ({approx}one-third the mass of the primary galaxy). This approach has broad implications for many areas of astrophysics-for the indirect detection of dark matter (or dark-matter-dominated dwarf galaxies), and for galaxy evolution in its use to decipher the dynamical impact of satellites on galactic disks. Here, we provide a proof of principle of the method by applying it to infer and quantitatively characterize optically visible galactic companions of local spirals, from the analysis of observed disturbances in outer gas disks.

  8. Identifying old Tidal Dwarf Galaxies in Simulations and in the Nearby Universe

    NASA Astrophysics Data System (ADS)

    Duc, P.-A.; Bournaud, F.; Masset, F. S.

    2004-06-01

    Most Tidal Dwarf Galaxies (TDGs) so-far discussed in the literature may be considered as young ones or even newborns, as they are still physically linked to their parent galaxies by an umbilical cord: the tidal tail at the tip of which they are usually observed. Old Tidal Dwarf Galaxies, completely detached from their progenitors, are still to be found. Using N-body numerical simulations, we have shown that tidal objects as massive as 109 solar masses may be formed in interacting systems and survive for more than one Gyr. Old TDGs should hence exist in the Universe. They may be identified looking at a peculiarity of their ``genetic identity card": a relatively high abundance in heavy elements, inherited from their parent galaxies. Finally, using this technique, we revisit the dwarf galaxies in the local Universe trying to find arguments pro and con a tidal origin.

  9. Revised sizes and positions for the Mailyan dwarf galaxy catalog

    NASA Technical Reports Server (NTRS)

    Madore, B. F.; Sun, H.; Bennett, J.; Corwin, H. G., Jr.; Helou, G.; Lague, C.; Schmitz, M.; Wu, X.

    1994-01-01

    New positions (good to +/- 15 arcsec), revised sizes, and sample cross-identifications are presented for dwarf galaxies discovered and published by Mailyan (1973, Astrofizika, 9, 33). Of the 104 originally cataloged galaxies we were able to recover only 100, despite an extensive search of the red and blue Palomar Observatory Sky Survey (POSS) prints. Over half of the recovered objects were found to correspond to previously cataloged galaxies; few if any of these are dwarf spheriodal galaxies, but rather mostly dwarf irregulars of low surface brightness.

  10. Mass and Substructure in Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Walker, Matthew G.

    2006-12-01

    I present results from a large spectroscopic survey of individual stars in dwarf spheroidal (dSph) galaxies, conducted using the Michigan/MIKE Fiber System (MMFS) at the Magellan Telescopes. dSph galaxies have come under intense scrutiny because they represent the lower extreme of the galaxy mass function, and thereby provide important constraints on models of structure formation. The proximity of the Milky Way's (MW's) dSph satellites allows us to study the resolved stellar populations of these systems in detail. Toward this end I have acquired MMFS spectra (5140-5180 Angstroms at resolution 20000) for more than 5000 stars in the MW dSphs Carina, Fornax, Sculptor, and Sextans. The spectra yield measurements of both radial velocity (median precision ± 1.8 km/s) and [Fe/H] metallicity (± 0.2 dex). I present radial velocity dispersion profiles for each dSph, as well as halo mass profiles derived using a variety of models and nonparametric estimation techniques. In some cases, the bulk stellar component is separable into populations following distinct distributions in position, kinematics, and chemistry, indicating a surprising level of complexity in these diminutive galaxies. Taking advantage of the fine spatial sampling of the MMFS data, I identify regions showing tentative evidence of localized chemo-dynamical substructure. This work is supported by grants from the National Science Foundation and the University of Michigan.

  11. Formation and Evolution of Dwarf Elliptical Galaxies

    NASA Astrophysics Data System (ADS)

    Chilingarian, Igor

    2006-11-01

    This thesis presents observational studies of evolution of dwarf elliptical galaxies. dE's are numerically dominant population in clusters of galaxies, but their origin and evolution is a matter of debate. Several scenarios of gas removal from dE's exist: galactic winds, ram pressure stripping, gravitaional harassment. We present new method to estimate stellar population parameters and internal kinematics, based on fitting observed spectra in the pixel space by PEGASE.HR synthetic populations. We apply this technique to 3D-spectroscopic observations of dE galaxies in the Virgo cluster and nearby groups and multiobject spectroscopy of several dozens of dE's in the Abell 496 cluster. We present discovery of young nuclei in bright dE galaxies in the Virgo cluster. Based on the analysis of observational data we conclude that: (1) there is an evolutionary connection between dE's and dIrr's, (2) the most probable scenario of gas removal is ram pressure stripping by the intergalactic medium.

  12. Uncovering AGN Fueling and Feedback in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Mushotzky, Richard

    2014-09-01

    While massive black holes (BHs) are ubiquitous in massive galaxies, their presence in dwarf galaxies is less certain. This constitutes a major gap in our understanding of BH-galaxy co-evolution. Identifying low-mass BHs in local dwarfs could also constrain BH seed formation at high z. Active BHs in dwarfs can reveal BH fueling and feedback in a vastly different regime than the well-studied AGN in massive hosts. Most AGN feedback models focus on the latter, as star formation is thought to suffice to drive feedback in dwarfs. However, AGN are increasingly found in dwarfs, where their effect on hosts may be most dramatic. We propose an unprecedented program to detect and characterize AGN fueling and feedback in the low-mass regime, using a unique sample of Swift-BAT AGN in dwarf

  13. 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.

  14. Chemical evolution of dwarf irregular galaxies

    NASA Astrophysics Data System (ADS)

    Pantelaki, Irini Andreas

    A historical scenario was proposed for galaxy formation that may find application to the problems of dwarf irregular galaxies in general and of I Zw 18 in particular. Calculations of the C:N:O ratios were presented within this simple model to explain the apparent paradox, that characterizes I Zw 18, of having very low concentrations of C, N and O in H II regions of a current starburst (some forty times smaller than solar) and yet having nearly solar ratios for C/O and N/O. Concentrations were calculated in a hot ambient medium that suffers a Hubble-like flow wind in a galaxy that has experienced several bursts of star formation. This hot matrix contains H I clouds whose collisions initiate the starbursts. The ambient gas which is found to have large and variable XO,XC,and XN concentrations, is mixed very slowly into the clouds, so that today a few percent of the cloud mass has been gathered from the hot medium by admixing, resulting in cloud concentrations comparable to those found in I Zw 18. Different assumptions were tried for the details of the galactic history. The resulting ratios of the abundances in the clouds were found to be intensive to these details.

  15. Testing Modified Gravity with Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Haghi, Hosein; Amiri, Vahid

    2016-08-01

    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 (SPS) models. Using Jeans analysis, we calculate the line-of-sight velocity dispersion (σ _ph {los}) 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 THINGS catalog 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.

  16. 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.

  17. CONFIRMATION OF FAINT DWARF GALAXIES IN THE M81 GROUP

    SciTech Connect

    Chiboucas, Kristin; Jacobs, Bradley A.; Tully, R. Brent; Karachentsev, Igor D. E-mail: bjacobs@ifa.hawaii.edu E-mail: ikar@luna.sao.ru

    2013-11-01

    We have followed up on the results of a 65 deg{sup 2} CFHT/MegaCam imaging survey of the nearby M81 Group searching for faint and ultra-faint dwarf galaxies. The original survey turned up 22 faint candidate dwarf members. Based on two-color HST ACS/WFC and WFPC2 photometry, we now confirm 14 of these as dwarf galaxy members of the group. Distances and stellar population characteristics are discussed for each. To a completeness limit of M{sub r{sup '}}= -10, we find a galaxy luminosity function slope of –1.27 ± 0.04 for the M81 Group. In this region, there are now 36 M81 Group members known, including 4 blue compact dwarfs; 8 other late types including the interacting giants M81, NGC 3077, and M82; 19 early type dwarfs; and at least 5 potential tidal dwarf galaxies. We find that the dSph galaxies in M81 appear to lie in a flattened distribution, similar to that found for the Milky Way and M31. One of the newly discovered dSph galaxies has properties similar to the ultra-faint dwarfs being found in the Local Group with a size R{sub e} ∼ 100 pc and total magnitude estimates M{sub r{sup '}}= -6.8 and M{sub I} ∼ –9.1.

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

    NASA Astrophysics Data System (ADS)

    Hammer, Derek M.

    The infall regions of galaxy clusters are unique laboratories for studying the impact of environment on galaxy evolution. This intermediate region links the low-density field environment and the dense core of the cluster, and is thought to host recently accreted galaxies whose star formation is being quenched by external processes associated with the cluster. In this dissertation, we measure the star formation properties of galaxies at the infall region of the nearby rich cluster of galaxies, Coma. We rely primarily on Ultraviolet (UV) data owing to its sensitivity to recent star formation and we place more emphasis on the properties of dwarf galaxies. Dwarf galaxies are good tracers of external processes in clusters but their evolution is poorly constrained as they are intrinsically faint and hence more challenging to detect. We make use of deep GALEX far-UV and near-UV observations at the infall region of the Coma cluster. This area of the cluster has supporting photometric coverage at optical and IR wavelengths in addition to optical spectroscopic data that includes deep redshift coverage of dwarf galaxies in Coma. Our GALEX observations were the deepest exposures taken for a local galaxy cluster. The depth of these images required alternative data analysis techniques to overcome systematic effects that limit the default GALEX pipeline analysis. Specifically, we used a deblending method that improved detection efficiency by a factor of ˜2 and allowed reliable photometry a few magnitudes deeper than the pipeline catalog. We performed deep measurements of the total UV galaxy counts in our field that were used to measure the source confusion limit for crowded GALEX fields. The star formation properties of Coma members were studied for galaxies that span from starbursts to passive galaxies. Star-forming galaxies in Coma tend to have lower specific star formation rates, on average, as compared to field galaxies. We show that the majority of these galaxies are likely

  19. Dust Production in Local Group Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Zijlstra, Albert; Sloan, Greg; Bernard-Salas, Jeronimo; Blommaert, Joris A. D. L.; Cioni, Maria-Rosa; Devost, Daniel; Feast, Michael W.; Groenewegen, Martin A. T.; Habing, Harm; Hony, Sacha; Lagadec, Eric; Loup, Cecile; Matsuura, Mikako; Menzies, John W.; Sloan, Greg C.; Waters, L. B. F. M.; Whitelock, Patricia A.; Wood, Peter R.; van Loon, Jacco Th.

    2006-05-01

    The superwind phase on the Asymptotic Giant Branch is a crucial ingredient of stellar and galactic evolution. The superwind ejecta are responsible for much of the interstellar hydrogen of evolved galaxies, and are a dominant contributor to the dust input into the ISM. The superwind determines the final mass of stellar remnants, and therefore affects, e.g., the type-I supernova rate. The characteristics of the superwind are still very poorly known, especially at non-solar metallicities. Spitzer has contributed a large and invaluable dataset on Magellanic Cloud stars, measuring dust, molecular bands and allowing accurate mass-loss measurements. We now propose to extend the (age, metallicity) parameter range by observing a number of other Milky Way satellites. The carbon stars in these galaxies trace an older population than the Magellanic Clouds, and extend to much lower metallicities. They are therefore crucial to allow us to extrapolate the Magellanic Cloud measurements to metal-poor environments. We propose to acquire low-resolution spectra of stars in the Sagittarius dwarf galaxy, Carina, Sculptor and Fornax. The selected stars range in metallicity from -0.55 to -2.0, and in age from 5-8 Gyr. Two low-metallicity planetary nebulae in these galaxies are also included. We will study the dust continuum, dust minerals (SiC, MgS) and gas-phase molecular bands (especially acetylene). Mass loss rates will be determined using our dust models, and we will measure the fractional abundances of amorphous carbon dust and SiC grains. Only Spitzer can provide these crucial measurements of extra-galactic AGB stars. The result will be our first knowledge of mass loss efficiency, dust formation, and dust abundances, at low to very low metallicities. These data are necessary to obtain reliable models of mass loss and of stellar evolution.

  20. Mass Stripping in Dwarf Spheroidal Galaxies and ω Cen

    NASA Astrophysics Data System (ADS)

    Tsujimoto, T.; Shigeyama, T.

    2004-06-01

    The stellar abundance pattern of neutron-capture elements such as Ba is used as a powerful tool to infer how star formation proceeded in dwarf spheroidal (dSph) galaxies. It is found that the abundance correlation of Ba with Fe in stars belonging to dSph galaxies have a feature similar to the Ba-Fe correlation in Galactic metal-poor stars. The common feature of these two correlations implies that dSph stars formed from gas with a velocity dispersion of ˜26 km s-1. This velocity dispersion together with the stellar luminosities strongly suggests that dark matter dominated dSph galaxies. The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≤ 10 km s-1 by stripping the dark matter in dSph galaxies. We also investigate the ram pressure exerted on the gas in ω Cen. It is found that the ram pressure is not strong enough to strip the gas but is expected to induce a bulk motion of the gas during the star formation epoch which is compatible with the recent observed finding and then the remaining gas after the star formation is likely to be stripped due to the gradual increase in the gas density in the forming Galactic disk.

  1. 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.

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. Diffuse Ionized Gas in the Dwarf Galaxy DDO 53

    NASA Astrophysics Data System (ADS)

    Flores-Fajardo, N.; Hidalgo-Gámez, A. M.

    We study the diffuse ionized gas (DIG) in the M81 group dwarf irregular galaxy DDO 53. We use long-slit spectroscopy in order to determine the most interesting line ratios. We compare these ratios with classical and leaking photoionization, shocks and turbulent layer models. As other dwarf irregular galaxies, the spectral characteristics are very diferent to those of the DIG in spiral galaxies: the excitation is higher and the [SII/Hα] much lower. A combination of leakage photoionization models plus shocks will be able to explain these characteristics.

  9. Dark matter and dark energy in dwarf galaxy systems

    NASA Astrophysics Data System (ADS)

    Chernin, A. D.; Teerikorpi, P.

    2014-01-01

    Quantitative estimates of themaximumallowed totalmasses and sizes of the dark-matter halos in groups and associations of dwarf galaxies—special types of metagalactic populations identified in recent astronomical observations with the Hubble Space Telescope—are presented. Dwarf-galaxy systems are formed of isolated dark-matter halos with a small number of dark galaxies embedded in them. Data on the sizes of these systems and the velocity dispersions of the embedded galaxies can be used to determine lower limits on the total dark-halo masses using the virial theorem. Upper limits follow from the conditions that the systems immersed in the cosmic dark-energy background be gravitationally bound. The median maximum masses are close to 1012 M ⊙ for both groups and associations of dwarf galaxies, although the median virial masses for these two types of systems differ by approximately a factor of ten.

  10. 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.

  11. 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

  12. Conformal Gravity Rotation Curves in Tidal Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    O'Brien, James

    2013-04-01

    We extend the application of the conformal gravity theory to tidal dwarf galaxies (TDGs). These dwarf galaxies are formed in the tidal tails of collisions of disk galaxies, and are thought to be predominantly composed of material expelled from the galactic disk of a parent galaxy. With any dark matter present in the parent galaxies expected to predominantly be in spherical haloes, tidal galaxies should thus have a very low dark matter content, and thus should not themselves be expected to possess the substantial spherical dark matter haloes that are ordinarily required to accompany and stabilize disk galaxies in standard gravity. In consequence, in the standard dark matter picture TDG rotation curves should not be expected to display any substantial mass discrepancies. Tidal dwarf galaxies thus provide a quite unusual laboratory for exploring the missing mass problem. Rotation curve data have become available for three TDGs associated with the parent galaxy NGC 5291, and it has been shown that there are in fact mass discrepancies, and that a good accounting of the data can be provided by MOND. Here we show that conformal gravity can also provide a good accounting of the data.

  13. 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.

  14. 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

  15. Dissecting early-type dwarf galaxies into their multiple components

    NASA Astrophysics Data System (ADS)

    Janz, Joachim; Laurikainen, Eija; Lisker, Thorsten; Salo, Heikki

    2015-03-01

    Early-type dwarf galaxies are often thought to be either more diffuse versions of giant ellipticals or to be low-mass disk galaxies that were quenched and heated by the environment. In both cases, the picture that most astronomers have in mind probably is that of a dynamically hot, regular shaped galaxy, in which any previous substructure has either been smeared out, or has never been there. However, the early-type dwarfs are not that simple. We analyzed ~100 such objects in the Virgo cluster using deep near-infrared images and found that the majority has a multi-component structure, sometimes even with bars or lenses. The study was done by applying GALFIT to images from the SMAKCED collaboration (Stellar content, MAss and K inematics of Cluster Early-type Dwarfs, http://www.smakced.net). The sample comprises early-type galaxies in the Virgo cluster in a brightness range of -19 <= M r <= -16 mag, and the data is complete down to M r =-16.73 mag. The images typically reach a signal-to-noise ratio of 1 per pixel of ~0.25'' at a surface brightness of ~22.5 mag/arcsec2 in the H-band. The galaxies were fitted with two-dimensional models, either with a simple Sérsic model or inner and outer components, as well as bars and lenses. Only a fraction of 31% of the galaxies can be fitted with a single Sérsic function. This fraction of ``simple`` galaxies turns out to be a strong function of luminosity, with a smaller fraction for brighter objects. The bar fraction is 14% and also in 14% of the galaxies lenses were fitted. When comparing the flattening distribution, the early-type dwarfs are more similar to spiral galaxies than to elliptical or lenticular galaxies. It is disputable whether or not the dwarfs follow a common relation with the bright elliptical galaxies, e.g. in the brightness versus size diagram. At the same time, they appear as smooth continuation of bright late-type galaxies in this diagram. The inner and outer components, as well as the simple galaxies have

  16. New low surface brightness dwarf galaxies detected around nearby spirals

    NASA Astrophysics Data System (ADS)

    Karachentsev, I. D.; Riepe, P.; Zilch, T.; Blauensteiner, M.; Elvov, M.; Hochleitner, P.; Hubl, B.; Kerschhuber, G.; Küppers, S.; Neyer, F.; Pölzl, R.; Remmel, P.; Schneider, O.; Sparenberg, R.; Trulson, U.; Willems, G.; Ziegler, H.

    2015-10-01

    We conduct a survey of low surface brightness (LSB) satellite galaxies around the Local Volume massive spirals using long exposures with small amateur telescopes. We identified 27 low and very low surface brightness objects around the galaxies NGC672, 891, 1156, 2683, 3344, 4258, 4618, 4631, and 5457 situated within 10 Mpc from us, and found nothing new around NGC2903, 3239, 4214, and 5585. Assuming that the dwarf candidates are the satellites of the neighboring luminous galaxies, their absolute magnitudes are in the range of -8.6 > M B > -13.3, their effective diameters are 0.4-4.7 kpc, and the average surface brightness is 26ṃ1/□″. The mean linear projected separation of the satellite candidates from the host galaxies is 73 kpc. Our spectroscopic observations of two LSB dwarfs with the Russian 6-meter telescope confirm their physical connection to the host galaxies NGC891 and NGC2683.

  17. THE FORNAX DWARF GALAXY AS A REMNANT OF RECENT DWARF-DWARF MERGING IN THE LOCAL GROUP

    SciTech Connect

    Yozin, C.; Bekki, K.

    2012-09-01

    We present results from the first numerical analysis to support the hypothesis, first proposed in Coleman et al., that the Fornax dwarf galaxy was formed from the minor merging of two dwarfs about 2 Gyr ago. Using orbits for the Fornax dwarf that are consistent with the latest proper motion measurements, our dynamical evolution models show that the observed asymmetric shell-like substructures can be formed from the remnant of a smaller dwarf during minor merging. These models also predict the formation of diffuse stellar streams. We discuss how these stellar substructures depend on model parameters of dwarf-dwarf merging, and how the intermediate-age subpopulations found in the vicinity of these substructures may be formed from gas accretion in past merger events. We also suggest that one of Fornax's globular clusters originates from a merged dwarf companion, and demonstrate where as yet undetected tidal streams or H I gas formed from the dwarf merging may be found in the outer halo of the Galaxy.

  18. Surveying for Dwarf Galaxies Within Void FN8

    NASA Astrophysics Data System (ADS)

    McNeil, Stephen R.

    2016-06-01

    The dwarf galaxy population in low density volumes, or voids, is a test of galaxy formation models and how they treat dark matter; some models say dwarf galaxies cannot be in void centers while others say they can. Since it appears many dwarf galaxies are H-alpha emitters, a well-designed deep survey through a nearby void center will either find nothing, and thus constrain the population there to be at some percentage below the mean, or it will find H-alpha emitters and significantly challenge several otherwise successful theories. Either result is a significant step in better understanding galaxy formation and large-scale structure. In 2013, a redshifted H-alpha imaging survey was begun for dwarf galaxies with ‑14.0 ≤ Mr ≤ ‑12.0 in the heart and back of the void FN8. Our first results have been surprising, furnishing significantly more candidate objects than anticipated. Through the Gemini Fast Turnaround Program, seven spectrum have been obtained, with one spectrum being a strong candidate for habitation within the center of the void.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. The morphology-density relation for dwarf galaxies

    NASA Technical Reports Server (NTRS)

    Ferguson, H. C.; Sandage, A.

    1990-01-01

    The morphology-density relation is examined for dwarf galaxies with absolute magnitudes -18 less than or equal to M sub B sub T less than or equal to -12.5, based on a deep photographic survey of nearby groups and clusters of galaxies. Results are given. Compared to dwarf ellipticals, dwarf irregulars form a more extended population in nearby clusters, and may in fact be entirely absent from the cluster cores. The spatial distribution of dwarf ellipticals in clusters depends on luminosity and the presence or absence of nucleation. Nucleated dE's and non-nucleated dE's fainter than M sub B sub T approx. -13.5 are concentrated toward the centers of clusters like the giant E and S0 galaxies. In contrast, non-nucleated dE's brighter than M sub B sub T approx. -14.5 are distributed like the spirals and irregulars. The intrinsic shapes of the bright non-nucleated dE's are similar to those of the dwarf irregulars, suggesting a possible evolutionary connection between these two classes of galaxies.

  4. Stellar structure of the dwarf irregular galaxy DDO 216

    NASA Astrophysics Data System (ADS)

    Tikhonov, N. A.

    2006-03-01

    Observations with the 6-mBTA telescope and archival Hubble Space Telescope data were used for the photometry of stars in the dwarf edge-on irregular galaxy DDO 216 (Peg DIG). We determined the change in the number density of stars of various ages along the major and minor axes of the galaxy. We found that the young stars of the galaxy concentrate toward the center, while its old stars, red giants, form an extended thick disk 5 kpc in diameter and 2 kpc in thickness around the galaxy.

  5. 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).

  6. Connections between MWG Star Clusters and Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.

    2015-03-01

    It seems that in the past decade, there have been two paradigm shifts regarding star clusters. Firstly, the observational evidence for multiple stellar populations requires more extended and often complex star formation histories in star clusters. Secondly, theoretical models that form globular clusters in dwarf galaxies that are accreted at very early epochs (z > 5) are able to reproduce the age-metallicity relations observed. For the accretion scenario to be viable, globular clusters should also resemble the chemistry of at least some dwarf galaxies.

  7. The Violent Interstellar Medium in Dwarf Galaxies: Atomic Gas

    NASA Astrophysics Data System (ADS)

    Brinks, E.; Walter, F.

    1998-12-01

    We review the morphology of the warm, neutral ISM as observed in the 21 cm line of neutral hydrogen (H I) of several nearby, gas-rich (dIrr) galaxies. The H I emission is dominated by shell-like structures, most likely superbubbles produced by the combined effects of strong stellar winds followed by supernova explosions of stars more massive than M > 8 M-sun within a region of massive star formation, an OB association. Somewhat counter-intuitively, H I superbubbles grow to larger dimensions in dwarf galaxies than in large spiral galaxies like our own or M 31. This can be explained as follows. In dwarf galaxies, the gravitational potential is lower than in spirals. Hence, for similar H I velocity dispersions, the scaleheight of the H I layer will be larger. Then, for comparable H I surface densities, the volume density will be lower. Both effects facilitate the growth to large dimensions of shells in dwarf galaxies and explains why such shells are much less likely to break through the H I layer into the halo. Moreover, dwarf galaxies lack density waves and tend to be dominated by solid-body rotation. As a result, shells will persist much longer than in spirals. A comparison with other galaxies shows that the energies needed to create H I supershells are the same for all types of galaxies, the energy output of a typical star-forming region therefore not being related to its galactic environment, at least to first order. The overall statistical properties of the H I holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities and ages.

  8. 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.

  9. On the formation of polar ring galaxies and tidal dwarf galaxies in gas-rich galaxy groups

    NASA Astrophysics Data System (ADS)

    Kilborn, Virginia; Sweet, Sarah; Meurer, Gerhardt; Drinkwater, Michael

    2015-08-01

    We are conducting a study of the properties of galaxies and dwarfs in 16 gas-rich galaxy groups identified in the Survey for Ionization in Neutral Gas Galaxies (SINGG; Meurer et al. 2006). We have found a young gas-rich coalescing galaxy group, J1051-17. Key features of this system are gas-rich tidal tails, studded with dwarf galaxies extending 200 kpc which merge in to a low surface brightness polar disk orbiting a very red edge-on host hosting a central AGN. Accretion from the polar disk may be feeding the AGN and powering a galactic wind. The example of this system suggests that tidal interactions with gas rich satellites may be a key process that aligns satellites in to polar planes while fuelling accretion down to the very centres of the host. We discuss the formation scenario of this polar ring galaxy, and investigate the formation of tidal dwarf galaxies in the wider group sample.

  10. 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).

  11. Spectroscopic Metallicities for Fornax Ultracompact Dwarf Galaxies, Globular Clusters, and Nucleated Dwarf Elliptical Galaxies

    NASA Astrophysics Data System (ADS)

    Mieske, S.; Hilker, M.; Infante, L.; Jordán, A.

    2006-05-01

    Various formation channels for the puzzling ultracompact dwarf galaxies (UCDs) have been proposed in the last few years. To better judge some of the competing scenarios, we present spectroscopic [Fe/H] estimates for a sample of 26 compact objects in the central region of the Fornax Cluster, covering the magnitude range of UCDs and bright globular clusters (18 mag-11 mag. This metallicity break is accompanied by a change in the size-luminosity relation for compact objects, as deduced from Hubble Space Telescope imaging: for MV<-11 mag, rh scales with luminosity, while for MV>-11 mag, rh is almost luminosity-independent. In our study we therefore assume a limiting absolute magnitude of MV=-11 mag between UCDs and globular clusters. The mean metallicity of five Fornax nucleated dwarf elliptical galaxy (dE,N) nuclei included in our study is about 0.8 dex lower than that of the UCDs, a difference significant at the 4.5 σ level. This difference is marginally higher than expected from a comparison of their (V-I) colors, indicating that UCDs are younger than or at most coeval to dE,N nuclei. Because of the large metallicity discrepancy between UCDs and nuclei, we disfavor the hypothesis that most of the Fornax UCDs are the remnant nuclei of tidally stripped dE,Ns. Our metallicity estimates for UCDs are closer to but slightly below those derived for young massive clusters (YMCs) of comparable masses. We therefore favor a scenario in which most UCDs in Fornax are successors of merged YMCs produced in the course of violent galaxy-galaxy mergers. It is noted that, in contrast, the properties of Virgo UCDs are more consistent with the stripping scenario, suggesting that different UCD formation channels may

  12. Discovery of a tidal dwarf galaxy in the Leo Triplet

    SciTech Connect

    Nikiel-Wroczyński, B.; Soida, M.; Urbanik, M.; Bomans, D. J. E-mail: soida@oa.uj.edu.pl E-mail: bomans@astro.rub.de

    2014-05-10

    We report the discovery of a dwarf galaxy in the Leo Triplet. Analysis of the neutral hydrogen distribution shows that it rotates independently of the tidal tail of NGC 3628, with a radial velocity gradient of 35-40 km s{sup –1} over approximately 13 kpc. The galaxy has an extremely high neutral gas content, accounting for a large amount of its total dynamic mass and suggesting a low amount of dark matter. It is located at the tip of the gaseous tail, which strongly suggests a tidal origin. If this is the case, it would be one of the most confident and nearest (to the Milky Way) detections of a tidal dwarf galaxy and, at the same time, the object most detached from its parent galaxy (≈140 kpc) of this type.

  13. Comprehensive search for dark matter annihilation in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Geringer-Sameth, Alex; Koushiappas, Savvas M.; Walker, Matthew G.

    2015-04-01

    We present a new formalism designed to discover dark matter annihilation occurring in the Milky Way's dwarf galaxies. The statistical framework extracts all available information in the data by simultaneously combining observations of all the dwarf galaxies and incorporating the impact of particle physics properties, the distribution of dark matter in the dwarfs, and the detector response. The method performs maximally powerful frequentist searches and produces confidence limits on particle physics parameters. Probability distributions of test statistics under various hypotheses are constructed exactly, without relying on large sample approximations. The derived limits have proper coverage by construction and claims of detection are not biased by imperfect background modeling. We implement this formalism using data from the Fermi Gamma-ray Space Telescope to search for an annihilation signal in the complete sample of Milky Way dwarfs whose dark matter distributions can be reliably determined. We find that the observed data are consistent with background for each of the dwarf galaxies individually as well as in a joint analysis. The strongest constraints are at small dark matter particle masses. Taking the median of the systematic uncertainty in dwarf density profiles, the cross section upper limits are below the pure s -wave weak scale relic abundance value (2.2 ×1 0-26 cm3 s-1 ) for dark matter masses below 26 GeV (for annihilation into b b ¯), 29 GeV (τ+τ-), 35 GeV (u u ¯,d d ¯,s s ¯,c c ¯, and g g ), 6 GeV (e+e-), and 114 GeV (γ γ ). For dark matter particle masses less than 1 TeV, these represent the strongest limits obtained to date using dwarf galaxies.

  14. 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.

  15. 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

    2016-08-01

    We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U- and I-band images obtained with VLT/VIMOS. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies. 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 color range of (U - I)0 ≃ 1.1 - 2.2 mag and surface brightness levels of μU ≃ 28.1 mag/arcsec2 and μI ≃ 27.4 mag/arcsec2. 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_odot } ˜eq 10^{5.7-6.3} M_{odot } and M_*|_{0.02 Z_odot } ˜eq 10^{6.3-8} M_{odot }. 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⊙.

  16. Dwarf Galaxies in the Halo of NGC 891

    NASA Astrophysics Data System (ADS)

    Schulz, Earl

    2014-07-01

    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.

  17. Nebular metallicities in two isolated local void dwarf galaxies

    SciTech Connect

    Nicholls, David C.; Jerjen, Helmut; Dopita, Michael A.; Basurah, Hassan

    2014-01-01

    Isolated dwarf galaxies, especially those situated in voids, may provide insight into primordial conditions in the universe and the physical processes that govern star formation in undisturbed stellar systems. The metallicity of H II regions in such galaxies is key to investigating this possibility. From the SIGRID sample of isolated dwarf galaxies, we have identified two exceptionally isolated objects, the Local Void galaxy [KK98]246 (ESO 461-G036) and another somewhat larger dwarf irregular on the edge of the Local Void, MCG-01-41-006 (HIPASS J1609-04). We report our measurements of the nebular metallicities in these objects. The first object has a single low luminosity H II region, while the second is in a more vigorous star forming phase with several bright H II regions. We find that the metallicities in both galaxies are typical for galaxies of this size, and do not indicate the presence of any primordial gas, despite (for [KK98]246) the known surrounding large reservoir of neutral hydrogen.

  18. 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.

  19. The Star Formation & Chemical Evolution Timescales of Two Nearby Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    de Boer, Thomas; Tolstoy, E.; Hill, V.; Saha, A.; Olsen, K.; Starkenburg, E.; Irwin, M.; Battaglia, G.

    2012-01-01

    We present wide-field photometry of resolved stars in the nearby Sculptor and Fornax dwarf spheroidal galaxies, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide-field Colour-Magnitude Diagrams are used directly in combination with spectroscopic metallicities of individual RGB stars to constrain the ages of different stellar populations, and derive the Star Formation History with particular accuracy. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old), which can easily be resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. The Fornax dSph is dominated by stellar populations of intermediate and young ages, which can be used to study the processes of galaxy formation in a more complex mix of stellar populations We find that the known metallicity gradients are well matched to an age gradient. This is the first time that this link with age has been directly quantified. The detailed Star Formation History shows the distribution of age with regards to the metallicity for different radii out from the centre of the galaxy. By linking the obtained SFH to observed spectroscopic abundances (alpha-elements, r- and s-process elements) of RGB stars it is possible to put ages on the chemical evolution patterns observed in this galaxy. In this way we can study the timescale of chemical evolution in these two dwarf galaxies. By comparing both dwarfs we determine whether the chemical abundance patterns seen in galaxies with recent episodes of star formation are a direct continuation of those with only old populations.

  20. 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

  1. 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.

  2. Dwarf galaxies in multistate scalar field dark matter halos

    NASA Astrophysics Data System (ADS)

    Martinez-Medina, L. A.; Robles, V. H.; Matos, T.

    2015-01-01

    We analyze the velocity dispersion for eight of the Milky Way dwarf spheroidal satellites in the context of finite temperature scalar field dark matter. In this model the finite temperature allows the scalar field to be in configurations that possess excited states, a feature that has proved to be necessary in order to explain the asymptotic rotational velocities found in low surface brightness (LSB) galaxies. In this work we show that excited states are not only important in large galaxies but also have visible effects in dwarf spheroidals. Additionally, we stress that contrary to previous works where the scalar field dark matter halos are consider to be purely Bose-Einstein condensates, the inclusion of excited states in these halo configurations provides a consistent framework capable of describing LSB and dwarf galaxies of different sizes without arriving to contradictions within the scalar field dark matter model. Using this new framework we find that the addition of excited states accounts very well for the raise in the velocity dispersion in Milky Way dwarf spheroidal galaxies improving the fit compared to the one obtained assuming all the dark matter to be in the form of a Bose-Einstein condensate.

  3. The unexpected diversity of dwarf galaxy rotation curves

    NASA Astrophysics Data System (ADS)

    Oman, Kyle A.; Navarro, Julio F.; Fattahi, Azadeh; Frenk, Carlos S.; Sawala, Till; White, Simon D. M.; Bower, Richard; Crain, Robert A.; Furlong, Michelle; Schaller, Matthieu; Schaye, Joop; Theuns, Tom

    2015-10-01

    We examine the circular velocity profiles of galaxies in Λ cold dark matter (CDM) cosmological hydrodynamical simulations from the EAGLE and LOCAL GROUPS projects and compare them with a compilation of observed rotation curves of galaxies spanning a wide range in mass. The shape of the circular velocity profiles of simulated galaxies varies systematically as a function of galaxy mass, but shows remarkably little variation at fixed maximum circular velocity. This is especially true for low-mass dark-matter-dominated systems, reflecting the expected similarity of the underlying CDM haloes. This is at odds with observed dwarf galaxies, which show a large diversity of rotation curve shapes, even at fixed maximum rotation speed. Some dwarfs have rotation curves that agree well with simulations, others do not. The latter are systems where the inferred mass enclosed in the inner regions is much lower than expected for CDM haloes and include many galaxies where previous work claims the presence of a constant density `core'. The `cusp versus core' issue is thus better characterized as an `inner mass deficit' problem than as a density slope mismatch. For several galaxies, the magnitude of this inner mass deficit is well in excess of that reported in recent simulations where cores result from baryon-induced fluctuations in the gravitational potential. We conclude that one or more of the following statements must be true: (i) the dark matter is more complex than envisaged by any current model; (ii) current simulations fail to reproduce the diversity in the effects of baryons on the inner regions of dwarf galaxies; and/or (iii) the mass profiles of `inner mass deficit' galaxies inferred from kinematic data are incorrect.

  4. The HST Snapshot Survey of Nearby Dwarf Galaxy Candidates. III. Resolved Dwarf Galaxies In and Beyond the Local Group

    NASA Astrophysics Data System (ADS)

    Grebel, E. K.; Seitzer, P.; Dolphin, A. E.; Geisler, D.; Guhathakurta, P.; Hodge, P. W.; Karachentsev, I. D.; Karachentseva, V. E.; Sarajedini, A.; Sharina, M. E.

    1999-12-01

    We present results for several nearby, resolved dwarf galaxies imaged with WFPC2 in the framework of our HST snapshot survey of nearby dwarf galaxy candidates (Seitzer et al., paper I in this series). All data presented here were analyzed with the automated photometry package HSTPHOT (Dolphin et al., paper IV in this series). Our closest target is the recently discovered Cassiopeia dwarf spheroidal (dSph) galaxy (Karachentsev & Karachentseva 1999, A&A, 341, 355), a new Local Group member and companion of M31 (Grebel & Guhathakurta 1999, ApJ, 511, 101). Our WFPC2 snapshot data reveal a pronounced red horizontal branch in Cas dSph. IC 5152 is a dwarf irregular (dIrr) just beyond the Local Group. Our data show a significant intermediate-age population with a strongly tilted asymptotic giant branch (AGB), a substantial young population, and a wide giant branch. Other nearby galaxies to be discussed include NGC 1560, ESO 471-G006, ESO 470-G018, and KK 035. Most of these galaxies are being resolved into stars for the first time. We describe their properties in detail and derive distances for all dwarfs with a well-defined tip of the red giant branch. Membership of these galaxies in nearby groups is discussed. Support for this work was provided by NASA through grant GO-08192.97A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. EKG acknowledges support by NASA through grant HF-01108.01-98A from the Space Telescope Science Institute. EKG and IDK are supported by the Henri Chrétien International Research Grant administered by the American Astronomical Society. PG is an Alfred P. Sloan Research Fellow.

  5. Analysis of the Sagittarius Dwarf Galaxy Tidal Tails

    NASA Astrophysics Data System (ADS)

    Snyder, Bart; Myers, Jeannette; Rusthoven, Mary; The, Lih-Sin; Hartmann, Dieter

    2007-11-01

    The Sagittarius Dwarf Galaxy is one of the satellite galaxies interacting with the Milky Way. Discovered to be located just below the galactic center; this galaxy is currently being tidally disrupted as it approaches the Milky Way disk. We performed a series of N-body simulations of the interaction between Sagittarius and the Milky Way over a 1 Gyr time period leading up to today's position. Here we present our analysis of the tidal tails and compare them to the known tidal structures we observe today.

  6. Do Tidal Interactions Trigger Starbursts in Dwarf Galaxies?

    NASA Astrophysics Data System (ADS)

    Martinkus, Charlotte; Cannon, John M.; McQuinn, Kristen B.; Johnson, Megan C.; Skillman, Evan D.; Bailin, Jeremy; Ford, Alyson; Koribalski, Baerbel

    2015-01-01

    Starburst dwarf galaxies are extensively studied systems, though the mechanism that triggers starbursts is poorly understood. Tidal interactions and gas accretion are thought to be potential starburst trigger mechanisms, although internal, secular drivers have not been ruled out. If starbursts are a result of external perturbations, then one would expect to see signatures of interaction in the gaseous disk of the galaxy. To examine this hypothesis, we analyze both archival and newly-obtained deep, wide-field HI maps from the Green Bank Telescope (GBT) of a sample of nineteen well-studied nearby starburst dwarf galaxies to search for such signs of interactions. Our sample is unique in that we have previously derived the star formation histories from Hubble Space Telescope imaging of the resolved stellar populations for all galaxies. In this work we focus on NGC 784 and NGC 672, which both may lie on a filament of dark matter isolated in space. We evaluate methods to determine the presence and properties of low surface-brightness neutral gas in the outer disk regions. This work serves as a prototype for forthcoming analysis of the full sample. With our results we hope to not only establish an effective data analysis procedure, but to also confirm or rule-out tidal interactions as a triggering mechanism of starbursts in this sample of dwarf galaxies.

  7. The Violent Interstellar Medium of Nearby Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Walter, Fabian

    1999-04-01

    High resolution HI observations of nearby dwarf galaxies (most of which are situated in the M81 group at a distance of about 3.2 Mpc) reveal that their neutral interstellar medium (ISM) is dominated by hole-like features most of which are expanding. A comparison of the physical properties of these holes with the ones found in more massive spiral galaxies (such as M31 and M33) shows that they tend to reach much larger sizes in dwarf galaxies. This can be understood in terms of the galaxy's gravitational potential. The origin of these features is still a matter of debate. In general, young star forming regions (OB-associations) are held responsible for their formation. This picture, however, is not without its critics and other mechanisms such as the infall of high velocity clouds, turbulent motions or even gamma ray bursters have been recently proposed. Here I will present one example of a supergiant shell in IC 2574 which corroborates the picture that OB associations are indeed creating these structures. This particular supergiant shell is currently the most promising case to study the effects of the combined effects of stellar winds and supernova explosions which shape the neutral interstellar medium of (dwarf) galaxies.

  8. 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.

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

    NASA Technical Reports Server (NTRS)

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

    1992-01-01

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

  10. 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 generated by supernovae, including the effect of diffusion. We first look at the effect of varying the uncertain cosmic ray parameters by repeatedly simulating a single galaxy. Then we fix the comic ray model and simulate five dwarf systems with virial masses range from 8-30 $\\times 10^{10}$ Msun. We find that including cosmic ray feedback (with diffusion) consistently leads to disk 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.

  11. 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.

  12. 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. PMID:16710415

  13. Abundance patterns and the chemical enrichment of nearby dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Hill, Vanessa

    2010-03-01

    As the least massive galaxies we know, dwarf spheroidal galaxies (dSph) allow to probe chemical enrichement on the smallest scales, and perhaps in its simplest expression. Particularly interesting are the issues concerning the efficency with which metals are retained or lost in these shallow potential wells (supernovae feedback), and the effect of this on star formation itself. Another fundamental issue concerns the earliest epochs of star formation: are first stars formed in similar ways and proportions in all halos ? Finally, as the smallest galaxies know, dSph have been suggested to be the surviving cousins of galaxy building blocs that (in λ-CDM) assemble to make larger galaxies. This parenthood would not necessarily hold at all late times, when survivors have lived their own differentiated life, but is expected at least at the earliest epochs. I review here the chemical abundances of individual stars in the nearest dwarf spheroidal galaxies, that have become available in increasing numbers (sample size and galaxies probed) in the last decade. Special emphasis is given to: a) recent results obtain with FLAMES on VLT, highlighting the power of detailed chemical abundance patterns of large samples of stars to unravel the various evolutionnary paths followed by dSph; b) the oldest and most metal-poor populations in dSph.

  14. 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.

  15. 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.

  16. Active Galactic Nuclei in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hein, Megan; Secrest, N.; Satyapal, S.

    2014-01-01

    Supermassive black holes (SMBHs) one million to a few billion times the mass of our sun are thought to reside in the center of most, if not all, bulge-dominated galaxies. It has been observed that the mass of these SMBHs is strongly correlated with the mass of these bulges, leading to the popular view that these central black holes are formed by galaxy mergers, which induce the growth of the galaxy's bulge and provide matter with which to feed the black hole. Although these properties and their possible consequences have been studied extensively in high mass galaxies and galaxies with large bulges, there is very little research on the possible existence and subsequent properties of SMBHs in low mass galaxies or galaxies with small or no central bulges. This is a significant weakness in the research of these objects as the study of this population of galaxies would allow us to gain valuable insight into SMBH seeds, black holes thought to have formed in the early universe. Strong X-rays are a good indicator of an accreting black hole, because they require more energy to produce and SMBHs are highly energetic, as well as being easier to see due to their ability to penetrate matter more easily than other forms of radiation. In this poster, I will present the results from an X-ray investigation using data matched from the Chandra X-ray observatory to a sample of low mass galaxies (with a mass of log(M) < 9).

  17. 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.

  18. 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

  19. 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.

  20. UVES Abundances of Stars in Nearby Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Tolstoy, Eline; Venn, Kim; Shetrone, Matt; Primas, Francesca; Hill, Vanessa; Kaufer, Andreas; Szeifert, Thomas

    2002-07-01

    It is a truth universally acknowledged, that a galaxy in possession of a good quantity of gas must want to form stars. It is the details of how and why that baffle us all. The simplest theories either would have this process a carefully self-regulated affair, or one that goes completely out of control and is capable of wrecking the galaxy which hosts it. Of course the majority of galaxies seem to amble along somewhere between these two extremes, and the mean properties tend to favour a quiescent self-regulated evolutionary scenario. But there area variety of observations which require us to invoke transitory ‘bursts’ of star-formation at one time or another in most galaxy types. Several nearby dwarf spheroidal galaxies have clearly determined star-formation histories with apparent periods of zero star formation followed by periods of fairly active star formation. If we are able to understand what separated these bursts we would understand several important phenomena in galaxy evolution. Were these galaxies able to clear out their gas reservoir in a burst of star formation? How did this gas return? or did it? Have these galaxies receieved gas from the IGM instead? Could stars from these types of galaxy contribute significantly to the halo population in our Galaxy? To answer these questions we need to combine accurate stellar photometry and Colour-Magnitude Diagram interpretation with detailed metal abundances to combine a star-formation rate versus time with a range of element abundances with time. Different elements trace different evolutionary process (e.g., relative contributions of type I and II supernovae). We often aren't even sure of the abundance spread in these galaxies. We have collected detailed high resolution UVES spectra of four nearby dwarf spheroidal galaxies (Sculptor, Fornax, Leo I & Carina) to begin to answer these questions. This is a precursor study to a more complete study with FLAMES. We presented at this meeting the initial results for

  1. Dwarf spheroidal galaxies and Bose-Einstein condensate dark matter

    NASA Astrophysics Data System (ADS)

    Diez-Tejedor, Alberto; Gonzalez-Morales, Alma X.; Profumo, Stefano

    2014-08-01

    We constrain the parameters of a self-interacting massive dark matter scalar particle in a condensate using the kinematics of the eight brightest dwarf spheroidal satellites of the Milky Way. For the case of a repulsive self-interaction, the condensate develops a mass density profile with a characteristic scale radius that is closely related to the fundamental parameters of the theory. We find that the velocity dispersion of dwarf spheroidal galaxies suggests a scale radius of the order of 1 kpc, in tension with previous results found using the rotational curve of low-surface-brightness and dwarf galaxies. The new value is however favored marginally by the constraints coming from the number of relativistic species at big bang nucleosynthesis. We discuss the implications of our findings for the particle dark matter model and argue that while a single classical coherent state can correctly describe the dark matter in dwarf spheroidal galaxies, it cannot play, in general, a relevant role for the description of dark matter in bigger objects.

  2. Dwarf Cepheids in the Sagittarius dSph Galaxy

    NASA Astrophysics Data System (ADS)

    Vivas, Kathy; Mateo, Mario; Nidever, David

    2013-08-01

    Pulsating stars below the horizontal branch (aka, dwarf cepheids) provide an interesting tool to study the stellar populations and structure of dSph galaxies. They obey a period-luminosity relationship and allow determination of distances with precisions as good as RR Lyrae stars (5-7%). They also trace a range of stellar populations since stars coming from different evolutionary paths may coexist in the region of the instability strip. We propose to use the large field of view of DECam to survey for the first time the Sagittarius galaxy for dwarf cepheids. We expect to discover several hundreds of dwarf cepheids which would allow us to investigate trends along the core region with good statistics. We will use the dwarf cepheids to study the stellar population(s) that produce them as well as any population gradient, if it exists, along the galaxy core. In addition, the large number of stars will allow us to study the 3D structure of the Sgr core which is an important constraint in any model of the disruption of Sgr. RR Lyrae and anomalous cepheids will naturally appear as well in the data, adding additional constraints in our study.

  3. Dark subhalo accretion onto dwarf galaxies in CDM

    NASA Astrophysics Data System (ADS)

    Laporte, Chervin Fabien Pierre; Penarrubia, Jorge

    2015-08-01

    Baryonic feedback at high redshifts has been proposed to explain the inference of dark matter (DM) cores in low-surface brightness and dwarf spheroidal galaxies. However, in the currently favoured cosmological model, structure grows hierarchically and CDM predicts a myriad of small substructures orbiting dwarf galaxies, some luminous (which habe been observationally identified in recent years), some dark. If such dark subhalos get close enough to the centre of cored dwarfs, they could potentially lead to cusp regrowth.In this talk, I will present the evolution of the DM profiles of dwarf galaxies driven by the accretion of DM substructures through controlled N-body experiments. The initial conditions assume that supernova feedback erases the primordial DM cusps at high redshift of halos with final masses $10^{9}-10^{10} \\rm{M_{\\odot}}$ by z=0. The orbits and masses of the infalling substructures are borrowed from the {\\it Aquarius} cosmological simulations. I will show that some halos that undergo 1:3 down to 1:30 mergers are susceptible to reform a DM cusp by $z\\approx 0$ and how this depends on the internal structure of the infalling substructures. I will show that within CDM a non-negligible level of scatter in the mass profiles of dwarfs is to be expected given their stochastic mass accretion histories and their diverse (observed) star formation histories and that this effect could possibly explain the existence of dense dwarfs like Draco, Ursa Minor or Tucana. I will argue how this process is unique to CDM and may be falsifiable. If time allows, I will show preliminary results from suites of cosmological N-body simulations designed to address the statistics of this effect.

  4. DARK AND LUMINOUS MATTER IN THINGS DWARF GALAXIES

    SciTech Connect

    Oh, Se-Heon; De Blok, W. J. G.; Brinks, Elias; Walter, Fabian; Kennicutt, Robert C. Jr. E-mail: edeblok@ast.uct.ac.za E-mail: walter@mpia.de

    2011-06-15

    We present mass models for the dark matter component of seven dwarf galaxies taken from 'The H I Nearby Galaxy Survey' (THINGS) and compare these with those taken from numerical {Lambda} cold dark matter ({Lambda}CDM) simulations. The THINGS high-resolution data significantly reduce observational uncertainties and thus allow us to derive accurate dark matter distributions in these systems. We here use the bulk velocity fields when deriving the rotation curves of the galaxies. Compared to other types of velocity fields, the bulk velocity field minimizes the effect of small-scale random motions more effectively and traces the underlying kinematics of a galaxy more properly. The 'Spitzer Infrared Nearby Galaxies Survey' 3.6 {mu}m and ancillary optical data are used for separating the baryons from their total matter content in the galaxies. The sample dwarf galaxies are found to be dark matter dominated over most radii. The relation between total baryonic (stars + gas) mass and maximum rotation velocity of the galaxies is roughly consistent with the baryonic Tully-Fisher relation calibrated from a larger sample of gas-dominated low-mass galaxies. We find discrepancies between the derived dark matter distributions of the galaxies and those of {Lambda}CDM simulations, even after corrections for non-circular motions have been applied. The observed solid body-like rotation curves of the galaxies rise too slowly to reflect the cusp-like dark matter distribution in cold dark matter halos. Instead, they are better described by core-like models such as pseudo-isothermal halo models dominated by a central constant-density core. The mean value of the logarithmic inner slopes of the mass density profiles is {alpha} = -0.29 {+-} 0.07. They are significantly different from the steep slope of {approx} - 1.0 inferred from previous dark-matter-only simulations, and are more consistent with shallower slopes found in recent {Lambda}CDM simulations of dwarf galaxies in which the effects

  5. Dwarf Elliptical Galaxies in the Coma Cluster Core

    NASA Astrophysics Data System (ADS)

    Secker, Jeff

    1995-12-01

    I have analyzed deep R- and B-band CCD images of the central ~ 700 arcmin(2) of the Coma cluster (Abell 1656, v = 7000 km/s, richness-class 2), using a statistically rigorous and automated method for the detection, photometry and classification of faint objects on digital images. The dwarf elliptical (dE) galaxies are confined to a well-defined sequence in the color range given by 0.7 <= (B-R) <= 1.9 mag; within this interval and complete to R = 22.5 mag, there are 2535 dE candidates in the cluster core, and 694 objects on the associated control field (2.57x less area). I detected a significant metallicity gradient in the radial distribution of dE galaxies, which goes as Z ~ R(-0.32) outwards from the cluster center at NGC 4874. As well, there is a strong color-luminosity correlation, in the sense that more luminous dE galaxies are redder in the mean. These observations are consistent with a model in which the intracluster gas exerted a confinement pressure (greatest near the cluster core), impeding the outflow of supernovae-driven metal-rich gas from the young dE galaxies. The spatial distribution of faint dEs is well fit by a standard King model with a core radius R_c = 18.7 arcmin ( =~ 0.44 Mpc), significantly larger than found for the brightest dEs and giant cluster galaxies, and consistent with tidal disruption of faint dEs in the dense cluster core. The composite luminosity function for Coma galaxies was modeled as the sum of a log-normal distribution for the giant galaxies and a Schechter function for the dE galaxies. Decomposing the galaxy luminosity function in this manner, I found that the early-type dwarf-to-giant ratio (EDGR) for the Coma cluster core is identical with that of the Virgo cluster. I proposed that the presence of substructure is an important factor in determining the cluster's EDGR, since during the merger of two or more richness-class 1 galaxy clusters, the total number of dwarf and giant galaxies will be conserved. Thus, this low EDGR

  6. Herschel's View of LITTLE THINGS Metal-Poor Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Cigan, Phil; Young, Lisa; Cormier, Diane; Lebouteiller, Vianney; Hunter, Deidre Ann; Madden, Suzanne; Little Things

    2015-01-01

    Dwarf galaxies present interesting challenges for the studies of various galaxy properties, due in part to their faintness and their typically low metal content. Low metallicity can lead to quite different physical conditions in the ISM of these systems, which can affect star formation and other processes. To determine the structure of star-forming molecular clouds at low metallicity and moderate star formation rates, far infrared (FIR) fine-structure lines were mapped with Herschel in selected regions of five dwarf irregular galaxies with metal abundances ranging from 13% down to 5% of solar. Abundances of [C II] 158, [O I] 63, [N II] 122, and [O III] 88 microns - the major FIR cooling lines - help to probe the conditions in the gas, and allow us to put these dwarfs in context with spirals and other galaxy types. We report our integrated fluxes and line ratios, and discuss the results: [C II] is the dominant FIR coolant in these systems, and it mostly originates in PDRs instead of the more diffuse phase. Funding for this project was provided by NASA JPL RSA grant 1433776.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. Galaxy And Mass Assembly (GAMA): the unimodal nature of the dwarf galaxy population

    NASA Astrophysics Data System (ADS)

    Mahajan, Smriti; Drinkwater, Michael J.; Driver, S.; Kelvin, Lee S.; Hopkins, A. M.; Baldry, I.; Phillipps, S.; Bland-Hawthorn, J.; Brough, S.; Loveday, J.; Penny, Samantha J.; Robotham, A. S. G.

    2015-01-01

    In this paper we aim to (i) test the number of statistically distinct classes required to classify the local galaxy population and (ii) identify the differences in the physical and star formation properties of visually distinct galaxies. To accomplish this, we analyse the structural parameters - effective radius (Reff), effective surface brightness within Reff (<μ>e), central surface brightness (μ0) and Sérsic index (n) - obtained by fitting the light profile of 432 galaxies (0.002 < z ≤ 0.02; Viking Z band), and their spectral energy distribution using multiband photometry in 18 broad-bands to obtain the stellar mass (M*), the star formation rate (SFR), the specific SFR (sSFR) and the dust mass (Mdust), respectively. We show that visually distinct, star-forming dwarf galaxies (irregulars, blue spheroids and low-surface-brightness galaxies) form a unimodal population in a parameter space mapped by <μ>e, μ0, n, Reff, SFR, sSFR, M*, Mdust and (g - i). The SFR and sSFR distribution of passively evolving (dwarf) ellipticals on the other hand, statistically distinguish them from other galaxies with similar luminosity, while the giant galaxies clearly segregate into star-forming spirals and passive lenticulars. We therefore suggest that the morphology classification scheme(s) used in literature for dwarf galaxies only reflect the observational differences based on luminosity and surface brightness among the apparent distinct classes, rather than any physical differences between them.

  12. 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

  13. 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.

  14. 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.

  15. The Structure of Dark Matter Halos in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Burkert, A.

    1995-07-01

    Recent observations indicate that dark matter halos have flat central density profiles. Cosmological simulations with nonbaryonic dark matter, however, predict self-similar halos with central density cusps. This contradiction has lead to the conclusion that dark matter must be baryonic. Here it is shown that the dark matter halos of dwarf spiral galaxies represent a one-parameter family with self-similar density profiles. The observed global halo parameters are coupled with each other through simple scaling relations which can be explained by the standard cold dark matter model if one assumes that all the halos formed from density fluctuations with the same primordial amplitude. We find that the finite central halo densities correlate with the other global parameters. This result rules out scenarios where the flat halo cores formed subsequently through violent dynamical processes in the baryonic component. These cores instead provide important information on the origin and nature of dark matter in dwarf galaxies.

  16. Early-type dwarf galaxies with multicomponent stellar structure: Are they remnants of disc galaxies strongly transformed by their environment?

    NASA Astrophysics Data System (ADS)

    Aguerri, J. Alfonso L.

    2016-03-01

    Context. The surface brightness distribution of ~30-40% of the early-type dwarf galaxies with - 18 ≤ MB ≤ -15 in the Virgo and the Coma clusters is fitted by models that include two structural components (Sérsic + exponential) as for bright disc galaxies. Aims: The goal of the present study is to determine whether early-type dwarf galaxies with a two-component stellar structure in the Virgo and the Coma clusters are low-luminosity copies of bright disc galaxies or are the remnants of bright galaxies strongly transformed by cluster environmental effects. Methods: I analysed the location of bright disc galaxies and early-type dwarfs in the rb,e/h- n plane. The location in this plane of the two-component dwarf galaxies was compared with the remnants of tidally disrupted disc galaxies reported by numerical simulations. Results: Bright unbarred disc galaxies show a strong correlation in the rb,e/h-n plane. Galaxies with larger Sérsic shape parameters show a higher rb,e/h ratio. In contrast, two-component early-type dwarf galaxies do not follow the same correlation. A fraction (~55%) of them are located outside the locus defined in this plane by having 95% of bright disc galaxies. This distribution indicates that they are not a low-mass replica of bright disc galaxies. The different location in the rb,e/h- n plane of two-component early-type dwarfs and bright galaxies can be qualitatively explain whether the former are remnants of disc galaxies strongly transformed by tidal processes. Conclusions: The progenitors of ~20-25% of early-type dwarf galaxies with - 18 ≤ MB ≤ -15 in the Virgo and Coma clusters could be bright disc galaxies transformed by effects of the environment. These tidally transformed galaxies can be selected according to their location in the rb,e/h-n plane.

  17. Star Formation Rate in Holmberg IX Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Andjelic, M. M.

    2011-12-01

    In this paper we use previously determined Hα fluxes for dwarf galaxy Holmberg IX (Arbutina et al. 2009) to calculate star formation rate (SFR) in this galaxy. We discuss possible contaminations of Hα flux and, for the first time, we take into account optical emission from supernova remnants (SNRs) as a possible source of contamination of Hα flux. Derived SFR for Holmberg IX is 3.4×10-4M_{⊙} yr-1. Our value is lower then in previous studies, due to luminous shock-heated source M&H 9-10, possible hypernova remnant, which we excluded from the total Hα flux in our calculation of SFR.

  18. A tidally distorted dwarf galaxy near NGC 4449.

    PubMed

    Rich, R M; Collins, M L M; Black, C M; Longstaff, F A; Koch, A; Benson, A; Reitzel, D B

    2012-02-01

    NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8 megaparsecs (ref. 3). It is wreathed in an extraordinary neutral hydrogen (H I) complex, which includes rings, shells and a counter-rotating core, spanning ∼90 kiloparsecs (kpc; refs 1, 4). NGC 4449 is relatively isolated, although an interaction with its nearest known companion--the galaxy DDO 125, some 40 kpc to the south--has been proposed as being responsible for the complexity of its H I structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of -13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxy's centre. PMID:22318602

  19. Spectroscopy of Giants of the Sagittarius Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Bonifacio, P.; Pasquini, L.; Molaro, P.; Marconi, G.

    The Sagittarius dwarf galaxy, at a distance of only 25 kpc, is undergoing strong tidal disruption and is probably in the process of merging with the Galaxy. This is likely to be just one of the many merger events which have contributed to shape the present day Galaxy. Photometric studies of the Saggitarius dwarf galaxy have suggested a spread in metallicity in the range -0.71 <= [Fe/H] <= -1.58, which may result from different bursts of star formation. From the deep CCD photometry of Marconi et al (1998) we have selected a sample of giants representative of the metallicity spread and acquired intermediate resolution (Δ λ ~3.5 AA) spectra using the ESO NTT telescope and the Multi Object capability of EMMI. These spectra have been used to measure radial velocities, to confirm the membership to the Sgr galaxy, and to provide a metallicity estimate for the sample stars by using spectral synthesis techniques. The present work will allow the selection of primary targets for detailed abundance analysis to be performed at high-resolution with the VLT.

  20. Absolute Proper Motions of Nearby Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Olszewski, Edward

    1997-07-01

    We propose to measure precise absolute proper motions for four dwarf spheroidal satellites of the Milky Way using spectroscopically-confirmed background QSOs to define a zero- velocity reference frame. Two epochs separated by 2 yrs will yield systemic tangential velocities of UMi, Car, Scl, {and For} to +/- 78 kms {+/- 130 kms}. These are worst-case velocity precisions and they are likely to be 2-4* smaller. Our long-term goal is to reduce them by an additional factor of several by obtaining data over the lifetime of WFPC2. With 2-3 QSOs per galaxy, we will still be confident of our motions with only 2 epochs. We will test whether the halo contains a small number of massive streams containing several dwarf galaxies, or whether the individual halo dwarfs are traveling along independent orbits. HST is essential to achieving the high precisions needed to conclusively compare the projected orbital motions of the individual galaxies; even with our conservative uncertainties, we are competitive with the best ground-based efforts with only a 2 year baseline. We will also use our results to improve our estimate of the mass of the Galaxy interior to 100 kpc. We believe that our project will show that astrometry has been a much ignored resource and power of HST. If HST performs as well as we suspect it can, it will be possible to measure the internal motions of stars in the dwarf spheroidals and the proper motions of all of the Local Group members over a timespan of 5 - 10 years.

  1. Dwarf Galaxies in the Local Group and in the Local Volume (Invited Talk)

    NASA Astrophysics Data System (ADS)

    Grebel, E. K.

    After summarizing the characteristics of different types of dwarf galaxies I briefly review our current state of knowledge of dwarf galaxy evolution in the Local Group, for which we now have a fairly detailed although by no means comprehensive picture. All Local Group dwarfs studied to date contain an old population, though its fraction varies considerably. The majority of the dwarf companions of the Milky Way shows evidence for a common epoch of ancient star formation. Spatial variations in star formation are frequently observed in many dwarf galaxies in the Local Group and beyond. These spatial variations range from a seemingly stochastic distribution of star-forming regions in gas-rich, high-mass dwarfs to radial gradients in low-mass dwarfs. The global mode of star formation may be either continuous with amplitude variations or episodic. High-mass dwarf galaxies tend to form stars over a Hubble time, whereas low-mass dwarfs eventually cease to form stars, possibly aided by environmental effects. Much less is known about the content and properties of dwarf galaxies in the Local Volume, which we are trying to remedy through a large observational effort. Dwarf galaxies in the Local Volume follow a similar trend with absolute magnitude, mean metallicity, and central surface brightness as the Local Group dwarfs do, and appear to be subject to morphological segregation.

  2. Ultradiffuse galaxies: the high-spin tail of the abundant dwarf galaxy population

    NASA Astrophysics Data System (ADS)

    Amorisco, N. C.; Loeb, A.

    2016-06-01

    Recent observations have revealed the existence of an abundant population of faint, low surface brightness (SB) galaxies, which appear to be numerous and ubiquitous in nearby galaxy clusters, including the Virgo, Coma and Fornax clusters. With median stellar masses of dwarf galaxies, these ultradiffuse galaxies (UDGs) have unexpectedly large sizes, corresponding to a mean SB of 24 ≲ <μe>r mag-1 arcsec2 ≲ 27 within the effective radius. We show that the UDG population represents the tail of galaxies formed in dwarf-sized haloes with higher-than-average angular momentum. By adopting the standard model of disc formation - in which the size of galaxies is set by the spin of the halo - we recover both the abundance of UDGs as a function of the host cluster mass and the distribution of sizes within the UDG population. According to this model, UDGs are not failed L* galaxies, but genuine dwarfs, and their low SB is not uniquely connected to the harsh cluster environment. We therefore expect a correspondingly abundant population of UDGs in the field, with possibly different morphologies and colours.

  3. Herschel Spectroscopic Observations of Little Things Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Cigan, Phil; Young, Lisa; Cormier, Diane; Lebouteiller, Vianney; Madden, Suzanne; Hunter, Deidre; Brinks, Elias; Elmegreen, Bruce; Schruba, Andreas; Heesen, Volker; the Little Things Team

    2016-01-01

    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.

  4. 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.

  5. 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.

  6. 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.

  7. 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

  8. 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.

    2016-04-01

    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' (2 kpc) from its centre. We perform a statistical analysis of the over-densities and find three distinct features, as well as an extended halo-like structure, to be significant at the 99.7% confidence level or higher. Unlike the extremely elongated and extended substructures surrounding the Hercules dwarf spheroidal galaxy, the over-densities 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' ± 7.1' (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.

  9. EXPLORING THE INTERSTELLAR MEDIA OF OPTICALLY COMPACT DWARF GALAXIES

    SciTech Connect

    Most, Hans P.; Cannon, John M.; Engstrom, Eric; Fliss, Palmer; Salzer, John J.; Rosenberg, Jessica L. E-mail: jcannon@macalester.edu E-mail: jrosenb4@gmu.edu

    2013-06-15

    We present new Very Large Array H I spectral line, archival Sloan Digital Sky Survey, and archival Spitzer Space Telescope imaging of eight star-forming blue compact dwarf galaxies that were selected to be optically compact (optical radii <1 kpc). These systems have faint blue absolute magnitudes (M{sub B} {approx}> -17), ongoing star formation (based on emission-line selection by the H{alpha} or [O III] lines), and are nearby (mean velocity = 3315 km s{sup -1} {approx_equal} 45 Mpc). One galaxy in the sample, ADBS 113845+2008, is found to have an H I halo that extends 58 r-band scale lengths from its stellar body. In contrast, the rest of the sample galaxies have H I radii to optical-scale-length ratios ranging from 9.3 to 26. The size of the H I disk in the 'giant disk' dwarf galaxy ADBS 113845+2008 appears to be unusual as compared with similarly compact stellar populations.

  10. 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.

  11. 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.

  12. Discovery of new dwarf galaxies around NGC4631 with Subaru/Hyper Suprime-Cam

    NASA Astrophysics Data System (ADS)

    Tanaka, Mikito; Komiyama, Yutaka; Chiba, Masashi

    2015-08-01

    We have been carrying out archaeological surveys of nearby galaxies using the Hyper Suprime-Cam (HSC) on the prime focus of the 8.2m Subaru telescope in order to understand an universal formation scenario of galactic halos, based on wide-field observations of the Local Group galaxies and the Local Volume galaxies. HSC consists of 104 effective 2048 x 4096 CCDs with a scale of 0.17 arcsec per pixel and covers a circular field of view with 1.5 degree in diameter. Especially, it is important to understand the variety of morphology of galactic halos through a detailed comparison of structures already found in the Local Group galaxies with structures recently detected in the Local Volume galaxies. In this conference, we report the discovery of new classical dwarf galaxies in the outskirts of NGC4631, which is a nearby edge-on Local Volume spiral galaxy interacting with the spiral NGC4656, using Subaru/HSC. We have confirmed dwarf galaxies detected by Karachentsev+14 and have newly found 8 uncatalogued dwarf galaxies based on visual inspection. We have measured physical parameters of these dwarf galaxies, such as a total magnitude, a half-light radius and a surface brightness profile described by a sersic parameter, based on our i-band HSC image. Furthermore, we show spatial distribution of blue young stars of each dwarf galaxy and comparisons with UV sources from GALEX. The relation between total absolute magnitude and half-light radius of dwarf galaxies of the NGC4631 group suggests that these dwarf galaxies with brighter total luminosity probably tend to be more extending. Finally, we conclude that provided that the luminosity to half-light radius relation of dwarf galaxies in the NGC4631 group is the same as that observed in the Local Group, the dwarf galaxy system of the NGC4631 group may have formed through the same manner as that of the Local Group.

  13. 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

    2013-01-01

    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. PMID:23282362

  14. DARK MATTER HEATING AND EARLY CORE FORMATION IN DWARF GALAXIES

    SciTech Connect

    Madau, Piero; Shen, Sijing; Governato, Fabio

    2014-07-01

    We present more results from a fully cosmological ΛCDM simulation of a group of isolated dwarf galaxies that has been shown to reproduce the observed stellar mass and cold gas content, resolved star formation histories, and metallicities of dwarfs in the Local Volume. Here we investigate the energetics and timetable of the cusp-core transformation. As suggested by previous work, supernova-driven gas outflows remove dark matter (DM) cusps and create kiloparsec-size cores in all systems having a stellar mass M {sub *} > 10{sup 6} M {sub ☉}. The {sup D}M core mass removal efficiency{sup —}dark mass ejected per unit stellar mass—ranges today from a few to a dozen, and increases with decreasing host mass. Because dwarfs form the bulk of their stars prior to redshift 1 and the amount of work required for DM heating and core formation scales approximately as M{sub vir}{sup 5/3}, the unbinding of the DM cusp starts early and the formation of cored profiles is not as energetically onerous as previously claimed. DM particles in the cusp typically migrate to 2-3 core radii after absorbing a few percent of the energy released by supernovae. The present-day slopes of the inner DM mass profiles, Γ ≡ dlog M/dlog R ≅ 2.5-3, of the simulated ''Bashful'' and ''Doc'' dwarfs are similar to those measured in the luminous Fornax and Sculptor dwarf spheroidals. None of the simulated galaxies has a circular velocity profile exceeding 20 km s{sup –1} in the inner 1 kpc, implying that supernova feedback is key to solve the ''too-big-to-fail'' problem for Milky Way subhalos.

  15. VizieR Online Data Catalog: Spectroscopy of Herschel Dwarf Galaxy Survey (Cormier+, 2015)

    NASA Astrophysics Data System (ADS)

    Cormier, D.; Madden, S. C.; Lebouteiller, V.; Abel, N.; Hony, S.; Galliano, F.; Remy-Ruyer, A.; Bigiel, F.; Baes, M.; Boselli, A.; Chevance, M.; Cooray, A.; de Looze, I.; Doublier, V.; Galametz, M.; Hugues, T.; Karczewski, O. L.; Lee, M.-Y.; Lu, N.; Spinoglio, L.

    2015-02-01

    Far-infrared line fluxes from the Herschel PACS instrument are provided for the 48 galaxies of the Dwarf Galaxy Survey. An atlas of images also shows spectral maps and line profiles for all sources. (9 data files).

  16. 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.

  17. Spectrophotometric Investigations of Blue Compact Dwarf Galaxies: Markarian 35

    NASA Astrophysics Data System (ADS)

    Cairós, Luz M.; Caon, Nicola; García-Lorenzo, Begoña; Monreal-Ibero, Ana; Amorín, Ricardo; Weilbacher, Peter; Papaderos, Polychronis

    2007-11-01

    We present results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy Mrk 35 (Haro 3), based on deep optical (BVRI) and near-IR (JHK) imaging, Hα narrowband observations, and long-slit spectroscopy. The optical emission of the galaxy is dominated by a central young starburst, with a barlike shape, while an underlying component of stars, with elliptical isophotes and red colors, extends more than 4 kpc from the galaxy center. High-resolution Hα and color maps allow us to identify the star-forming regions, to spatially discriminate them from the older stars, and to recognize several dust patches. We derive colors and Hα parameters for all the identified star-forming knots. Observables derived for each knot are corrected for the contribution of the underlying older stellar population, the contribution by emission lines, and from interstellar extinction, and compared with evolutionary synthesis models. We find that the contributions of these three factors are by no means negligible and that they significantly vary across the galaxy. Therefore, careful quantification and subtraction of emission lines, galaxy host contribution, and interstellar reddening at every galaxy position are essential to derive the properties of the young stars in blue compact dwarfs. We find that we can reproduce the colors of all the knots with an instantaneous burst of star formation and the Salpeter initial mass function with an upper mass limit of 100 Msolar. In all cases the knots are just a few Myr old. The underlying population of stars has colors consistent with being several Gyr old.

  18. DWARF GALAXIES WITH OPTICAL SIGNATURES OF ACTIVE MASSIVE BLACK HOLES

    SciTech Connect

    Reines, Amy E.; Greene, Jenny E.; Geha, Marla

    2013-10-01

    We present a sample of 151 dwarf galaxies (10{sup 8.5} ∼< M{sub *} ∼< 10{sup 9.5} M{sub ☉}) that exhibit optical spectroscopic signatures of accreting massive black holes (BHs), increasing the number of known active galaxies in this stellar-mass range by more than an order of magnitude. Utilizing data from the Sloan Digital Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have systematically searched for active BHs in ∼25,000 emission-line galaxies with stellar masses comparable to the Magellanic Clouds and redshifts z < 0.055. Using the narrow-line [O III]/Hβ versus [N II]/Hα diagnostic diagram, we find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad Hα emission. For these broad-line active galactic nucleus (AGN) candidates, we estimate BH masses using standard virial techniques and find a range of 10{sup 5} ∼< M{sub BH} ∼< 10{sup 6} M{sub ☉} and a median of M{sub BH} ∼ 2 × 10{sup 5} M{sub ☉}. We also detect broad Hα in 15 galaxies that have narrow-line ratios consistent with star-forming galaxies. Follow-up observations are required to determine if these are true type 1 AGN or if the broad Hα is from stellar processes. The median absolute magnitude of the host galaxies in our active sample is M{sub g} = –18.1 mag, which is ∼1-2 mag fainter than previous samples of AGN hosts with low-mass BHs. This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first supermassive BH seeds.

  19. The history of star formation in nearby dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel Ray

    2010-11-01

    We present detailed analysis of color-magnitude diagrams (CMDs) of resolved stellar populations in nearby dwarf galaxies based on observations taken with the Hubble Space Telescope (HST). From the positions of individual stars on a CMD, we are able to derive the star formation histories (SFHs), i.e., the star formation rate (SFR) as a function of time and metallicity, of the observed stellar populations. Specifically, we apply this technique to a number of nearby dwarf galaxies to better understand the mechanisms driving their evolution. The ACS Nearby Galaxy Survey Treasury program (ANGST) provides multi-color photometry of resolved stars in ˜ 60 nearby dwarf galaxies from images taken with HST. This sample contains 12 dSph, 5 dwarf spiral, 28 dIrr, 12 dSph/dIrr (transition), and 3 tidal dwarf galaxies. The sample spans a range of ˜ 10 in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best fit lifetime SFHs we find three significant results: (1) the average dwarf galaxy formed ˜ 60% of its stars by z ˜ 2 and 70% of its stars by z ˜ 1, regardless of morphological type, (2) the only statistically significant difference between the SFHs of different morphological types is within the most recent 1 Gyr (excluding tidal dwarf galaxies), and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single epoch SF or constant SFH. We then present the recent ( ≲ 1 Gyr) SFHs of nine M81 Group Dwarf Galaxies. Comparing the SFHs, birthrate parameters, fraction of stars formed per time interval, and spatial distribution of stellar components as a function of luminosity, we find only minor differences in SF characteristics among the M81 Group dIs despite a wide range of physical properties. We extend our comparison to select dIs in the Local Group (LG), with similar quality photometry, and again find only minor differences in SF parameters. The lack of a clear trend in SF parameters over

  20. X-RAYS FROM BLUE COMPACT DWARF GALAXIES

    SciTech Connect

    Kaaret, Philip; Schmitt, Joseph; Gorski, Mark

    2011-11-01

    We measured the X-ray fluxes from an optically selected sample of blue compact dwarf galaxies (BCDs) with metallicities <0.07 and solar distances less than 15 Mpc. Four X-ray point sources were observed in three galaxies, with five galaxies having no detectable X-ray emission. Comparing X-ray luminosity and star formation rate (SFR), we find that the total X-ray luminosity of the sample is more than 10 times greater than expected if X-ray luminosity scales with SFR according to the relation found for normal-metallicity star-forming galaxies. However, due to the low number of sources detected, one can exclude the hypothesis that the relation of the X-ray binaries to SFR in low-metallicity BCDs is identical to that in normal galaxies only at the 96.6% confidence level. It has recently been proposed that X-ray binaries were an important source of heating and reionization of the intergalactic medium at the epoch of reionization. If BCDs are analogs to unevolved galaxies in the early universe, then enhanced X-ray binary production in BCDs would suggest an enhanced impact of X-ray binaries on the early thermal history of the universe.

  1. Dwarf galaxies in the Virgo cluster. I - The systematic photometric properties of early-type dwarfs

    NASA Astrophysics Data System (ADS)

    Binggeli, B.; Cameron, L. M.

    1991-12-01

    The azimuthally averaged surface brightness profiles of 200 faint early-type Virgo cluster galaxies have been analyzed. Faint dwarfs are very well described by an exponential or a King model. The magnitudes of the nuclei vary greatly at a given galaxian magnitude, but the maximum nuclear luminosity is a strong function of M(T). In the 0.1-1 kpc radius range, the logarithmically plotted profiles of all early-type galaxies come in two well-defined classes identified with classical types versus dwarf types. The former are all classified E or S0, while the latter comprise all galaxies classified dE or dS0, all morphologically 'intermediate' types, and even two classified 'E'. The mean SB profiles of dS0 galaxies are indistinguishable from bright dE profiles. In 2D, the dS0s appear highly flattened and/or show asymmetric and irregular features which may indicate their disk nature.

  2. Dwarf galaxies in the Perseus Cluster: further evidence for a disc origin for dwarf ellipticals

    NASA Astrophysics Data System (ADS)

    Penny, Samantha J.; Forbes, Duncan A.; Pimbblet, Kevin A.; Floyd, David J. E.

    2014-10-01

    We present the results of a Keck-ESI (Echellette Spectrograph and Imager) spectroscopic study of six dwarf elliptical (dE) galaxies in the Perseus Cluster core, and confirm two dwarfs as cluster members for the first time. All six dEs follow the size-magnitude relation for dE/dSph galaxies. Central velocity dispersions are measured for three Perseus dwarfs in our sample, and all lie on the σ-luminosity relation for early-type, pressure-supported systems. We furthermore examine SA 0426-002, a unique dE in our sample with a bar-like morphology surrounded by low surface brightness wings/lobes (μB = 27 mag arcsec-2). Given its morphology, velocity dispersion (σ0 = 33.9 ± 6.1 km s-1), velocity relative to the brightest cluster galaxy NGC 1275 (2711 km s-1), size (Re = 2.1 ± 0.10 kpc), and Sérsic index (n = 1.2 ± 0.02), we hypothesize the dwarf has morphologically transformed from a low-mass disc to dE via harassment. The low surface brightness lobes can be explained as a ring feature, with the bar formation triggered by tidal interactions via speed encounters with Perseus Cluster members. Alongside spiral structure found in dEs in Fornax and Virgo, SA 0426-002 provides crucial evidence that a fraction of bright dEs have a disc infall origin, and are not part of the primordial cluster population.

  3. 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.

  4. 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

  5. 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

  6. 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>.

  7. Andromeda IV, a solitary gas-rich dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Karachentsev, I. D.; Chengalur, J. N.; Tully, R. B.; Makarova, L. N.; Sharina, M. E.; Begum, A.; Rizzi, L.

    2016-03-01

    Observations are presented of the isolated dwarf irregular galaxy And IV made with the Hubble Space Telescope Advanced Camera for Surveys and the Giant Metrewave Radio Telescope in the 21 cm H I line. We determine the galaxy distance of 7.17{±}0.31 Mpc using the Tip of Red Giant Branch method. The galaxy has a total blue absolute magnitude of -12.81 mag, linear Holmberg diameter of 1.88 kpc, and an H I-disk extending to 8.4 times the optical Holmberg radius. The H I mass-to-blue luminosity ratio for And IV amounts 12.9M⊙/L⊙. From the GMRT data we derive the rotation curve for the H I and fit it with different mass models. We find that the data are significantly better fit with an iso-thermal dark matter halo, than by an NFW halo. We also find that MOND rotation curve provides a very poor fit to the data. The fact that the iso-thermal dark matter halo provides the best fit to the data supports models in which star formation feedback results in the formation of a dark matter core in dwarf galaxies. The total mass-to-blue luminosity ratio of 162M⊙/L⊙ makes And IV among the darkest dIrr galaxies known. However, its baryonic-to-dark mass ratio (M_gas+M*)/M_T = 0.11 is close to the average cosmic baryon fraction of 0.15.

  8. Mapping the HI Neighborhood Around Starburst Dwarf Galaxies NGC 1569, NGC 4214 and NGC 4163

    NASA Astrophysics Data System (ADS)

    Johnson, Megan C.; LITTLE THINGS Team

    2013-01-01

    Dwarf galaxies are believed to be the building blocks of larger galaxies. However, there are some studies that indicate the dwarf galaxies observed in the nearby universe may have formed later, after the most massive galaxies coalesced. Dwarf galaxy formation and evolution is important for our understanding of cosmology. If dwarf galaxies mimic their more massive counterparts, then starburst dwarfs may be present day merger remnants and provide information on the building block hypothesis. The origins of starburst dwarf irregular galaxies of the Magellanic type (dIm) are not well known. The role of interactions and mergers as mechanisms to create these systems has been hypothesized, but not well studied. We present deep HI maps around three starburst dwarf galaxies NGC 1569, NGC 4214 and NGC 4163. The purpose of these maps is to determine if there are tenuous HI structures around these objects that would indicate a recent interaction or merger. We detect HI filamentary structures that may be connected with NGC 1569 thereby indicating a recent interaction with nearby dwarf irregular UGCA 92. However, our map of NGC 4163 and NGC 4214 does not show any tenuous HI at our 5σ sensitivity limit of 1 x 10^18 for a 25 km/s line width.

  9. Dark matter searches with Cherenkov telescopes: nearby dwarf galaxies or local galaxy clusters?

    NASA Astrophysics Data System (ADS)

    Sánchez-Conde, Miguel A.; Cannoni, Mirco; Zandanel, Fabio; Gómez, Mario E.; Prada, Francisco

    2011-12-01

    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

  10. 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

  11. The Stellar Content of 10 Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Schulte-Ladbeck, Regina E.; Hopp, Ulrich

    1998-12-01

    We examine the stellar content of 10 dwarf irregular galaxies, the broadband CCD photometry of which was published in 1995 by Hopp & Schulte-Ladbeck. We also present Hα images for several of these galaxies. The galaxies in the sample are located outside of the Local Group. Yet they are still close enough to be resolved into single stars from the ground but only the brightest stars (or star clusters) are detected and there is severe crowding. The sample galaxies were selected to be isolated from massive neighbors; about half of them are (mostly peripheral) members of groups, the other half is located in the field. We discuss the vicinity of the sample galaxies to other dwarf galaxies. In order to interpret single-star photometry and draw conclusions about the stellar content or other distance-dependent quantities, it is crucial that accurate distances to the galaxies be known. The distances to the sample galaxies are not well known since all but one have not had a primary distance indicator measured. We make an attempt to constrain the distances by identifying the envelope of the brightest supergiants in B, B-R and R, B-R color-magnitude diagrams, but the results are not very accurate (we estimate the minimal error on the distance modulus is 1.36 mag). Nevertheless, the fact that the sample galaxies are resolved with direct ground-based imaging indicates that they are sufficiently nearby to represent good candidates for observations with instruments that provide high spatial resolution, e.g., adaptive optics systems on large ground-based telescopes, or the Hubble Space Telescope. We discuss the morphologies, color-magnitude diagrams, and frequencies of the resolved stars together with the morphology of the ionized gas, as well as the surface brightness profiles and colors of the underlying light distributions of unresolved stars. We point out the occurrence in half of the galaxies studied of H II regions and young stellar associations located well outside of the

  12. 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

  13. 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.

  14. 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.

  15. Calibrating UV Star Formation Rates for Dwarf Galaxies from STARBIRDS

    NASA Astrophysics Data System (ADS)

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

    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. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA), and the Canadian Astronomy Data Centre (CADC/NRC/CSA).

  16. Black holes at the centers of nearby dwarf galaxies

    SciTech Connect

    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⩽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{sub BH}⩽10{sup 6} M{sub ⊙}. 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] λ5007 luminosities of the Seyfert nuclei in our sample have a median value of L{sub 5007}=2×10{sup 5} L{sub ⊙} and extend down to ∼10{sup 4} L{sub ⊙}. Using published data for broad-line IMBH candidates, we have derived an [O iii] bolometric correction of log(L{sub bol}/L{sub 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 10{sup 3} M{sub ⊙}–10{sup 4} M{sub ⊙} 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

  17. 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.

  18. Chemical enrichment in Ultra-Faint Dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Romano, Donatella

    2016-08-01

    Our view of the Milky Way's satellite population has radically changed after the discovery, ten years ago, of the first Ultra-Faint Dwarf galaxies (UFDs). These extremely faint, dark-matter dominated, scarcely evolved stellar systems are found in ever-increasing number in our cosmic neighbourhood and constitute a gold-mine for studies of early star formation conditions and early chemical enrichment pathways. Here we show what can be learned from the measurements of chemical abundances in UFD stars read through the lens of chemical evolution studies, point out the limitations of the classic approach, and discuss the way to go to improve the models.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. X-ray sources in dwarf galaxies in the Virgo cluster and the nearby field

    NASA Astrophysics Data System (ADS)

    Papadopoulou, Marina; Phillipps, S.; Young, A. J.

    2016-08-01

    The extent to which dwarf galaxies represent essentially scaled down versions of giant galaxies is an important question with regards the formation and evolution of the galaxy population as a whole. Here, we address the specific question of whether dwarf galaxies behave like smaller versions of giants in terms of their X-ray properties. We discuss two samples of around 100 objects each, dwarfs in the Virgo cluster and dwarfs in a large Northern hemisphere area. We find nine dwarfs in each sample with Chandra detections. For the Virgo sample, these are in dwarf elliptical (or dwarf lenticular) galaxies and we assume that these are (mostly) low-mass X-ray binaries (LMXB) [some may be nuclear sources]. We find a detection rate entirely consistent with scaling down from massive ellipticals, viz. about one bright (i.e. LX > 1038 erg s-1) LMXB per 5 × 109 M⊙ of stars. For the field sample, we find one (known) Seyfert nucleus, in a galaxy which appears to be the lowest mass dwarf with a confirmed X-ray emitting nucleus. The other detections are in star-forming dwarf irregular or blue compact dwarf galaxies and are presumably high-mass X-ray binaries (HMXB). This time, we find a very similar detection rate to that in large late-type galaxies if we scale down by star formation rate, roughly one HMXB for a rate of 0.3 M⊙ per year. Nevertheless, there does seem to be one clear difference, in that the dwarf late-type galaxies with X-ray sources appear strongly biased to very low metallicity systems.

  4. An inefficient dwarf: chemical abundances and the evolution of the Ursa Minor dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Ural, Uğur; Cescutti, Gabriele; Koch, Andreas; Kleyna, Jan; Feltzing, Sofia; Wilkinson, Mark I.

    2015-05-01

    We present detailed chemical element abundance ratios of 17 elements with eight ≤ Z ≤ 60 in three metal-poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine with extant data from the literature to assess the predictions of a novel suite of galaxy chemical evolution models. The spectroscopic data were obtained with the Keck/High-Resolution Echelle Spectrograph instrument and revealed low metallicities of [Fe/H] = -2.12, -2.13 and -2.67 dex. While the most metal-poor star in our sample shows an overabundance of [Mn/Fe] and other Fe-peak elements, our overall findings are in agreement with previous studies of this galaxy: elevated values of the [α/Fe] ratios that are similar to, or only slightly lower than, the halo values but with SN Ia enrichment at very low metallicity, as well as an enhancement of the ratio of first to second peak neutron capture elements [Y/Ba] with decreasing metallicity. The chemical evolution models which were tailored to reproduce the metallicity distribution function of the dwarf spheroidal, indicate that Ursa Minor had an extended star formation which lasted nearly 5 Gyr with low efficiency and are able to explain the [Y/Ba] enhancement at low metallicity for the first time. In particular, we show that the present-day lack of gas is probably due to continuous loss of gas from the system, which we model as winds.

  5. The Origin of Prolate Rotation in Dwarf Spheroidal Galaxies Formed by Mergers of Disky Dwarfs

    NASA Astrophysics Data System (ADS)

    Ebrová, Ivana; Łokas, Ewa L.

    2015-11-01

    Motivated by the discovery of prolate rotation of stars in Andromeda II (And II), a dwarf spheroidal companion of M31, we study its origin via mergers of disky dwarf galaxies. We simulate merger events between two identical dwarfs changing the initial inclination of their disks with respect to the orbit and the amount of orbital angular momentum. On radial orbits, the amount of prolate rotation in the merger remnants correlates strongly with the inclination of the disks and is well understood as due to the conservation of the angular momentum component of the disks along the merger axis. For non-radial orbits, prolate rotation may still be produced if the orbital angular momentum is initially not much larger than the intrinsic angular momentum of the disks. The orbital structure of the remnants with significant rotation is dominated by box orbits in the center and long-axis tubes in the outer parts. The frequency analysis of stellar orbits in the plane perpendicular to the major axis reveals the presence of two families roughly corresponding to inner and outer long-axis tubes. The fraction of inner tubes is largest in the remnant forming from disks that are initially oriented most vertically, and is responsible for the boxy shape of the galaxy. We conclude that prolate rotation results from mergers with a variety of initial conditions and no fine tuning is necessary to reproduce this feature. We compare the properties of our merger remnants to those of dwarfs resulting from the tidal stirring scenario and the data for And II.

  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 violent interstellar medium of the dwarf galaxy IC 2574.

    NASA Astrophysics Data System (ADS)

    Walter, F.; Brinks, E.

    The authors present VLA H I-synthesis observations of the Violent Interstellar Medium of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) at high spatial and velocity resolution. The H I-observations show a stunning amount of detail in the form of H I shells and holes in the neutral interstellar medium of IC 2574, ranging in size from 100 to 1500 pc. The most likely explanation, as has been proposed by previous studies, is combined effects of stellar winds and supernova-explosions of the most massive stars, blowing holes and shells into the interstellar medium. This picture is confirmed by a striking correlation between Hα emission and H I-shells: the smaller holes tend to be filled with Hα emission whereas for the larger H I holes the Hα seems to be restricted to the edges.

  9. 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.

  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. Metals Removed by Outflows from Milky Way Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    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. 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.

  12. A violent interaction between the dwarf galaxy UGC 7636 and the giant elliptical galaxy NGC 4472

    NASA Technical Reports Server (NTRS)

    Mcnamara, Brian R.; Sancisi, Renzo; Henning, Patricia A.; Junor, William

    1994-01-01

    We present new U, B, R, and H I imagery of the Virgo Cluster giant elliptical galaxy NGC 4472 and its interacting dwarf companion galaxy UGC 7636. Using a composite image reconstruction technique, we show that a trail of debris approx. 5 arcmin in length and approx. 1 arcmin in width (30x6 kpc for a Virgo cluster distance of 20 Mpc) is projected northward from the dwarf galaxy. A cloud of H I is projected along the northwest edge of the debris between the dwarf and gE. The dwarf's nuclear morphology is irregular and bow-shaped on what appears to be its leading edge. Apart from a number of isolated blue regions, most of of the trailing debris is similar in color to the dwarf's nucleus. Only a modest enhancement of star formation appears to have been induced by the interaction. Although separated by 15 kpc, the H I and stellar morphologies are remarkably similar. The stars and H I appear to have been tidally distorted in situ, prior to the cloud's removal by ram pressure. If the H I has maintained its shape by magnetic support, a magnetic field strength an order of magnitude larger than the galaxy's is required. Ram pressure deceleration due to the cloud's motion through NGC 4472's x-ray-emitting interstellar medium shold be sufficient for the cloud to become gravitationally bound to NGC 4472. The H I cloud is not self-gravitating and may fragment and be destroyed in the interaction. UGC 7636 will probably be disrupted by NGC 4472's strong tidal forces; the stellar debris will disperse into the Virgo cluster or become bound to NGC 4472's halo on eccentric orbits. The debris captured in the collision will have a negligible impact on NGC 4472's stellar and gaseous content. On the other hand, if similar interactions are common in giant elliptical galaxies, they could alter or deplete surrounding dwarf galaxy populations, fuel bursts of nuclear activity, and perhaps provide a source of magnetic energy to their interstellar media.

  13. A violent interaction between the dwarf galaxy UGC 7636 and the giant elliptical galaxy NGC 4472

    NASA Astrophysics Data System (ADS)

    McNamara, Brian R.; Sancisi, Renzo; Henning, Patricia A.; Junor, William

    1994-09-01

    We present new U, B, R, and H I imagery of the Virgo Cluster giant elliptical galaxy NGC 4472 and its interacting dwarf companion galaxy UGC 7636. Using a composite image reconstruction technique, we show that a trail of debris approx. 5 arcmin in length and approx. 1 arcmin in width (30x6 kpc for a Virgo cluster distance of 20 Mpc) is projected northward from the dwarf galaxy. A cloud of H I is projected along the northwest edge of the debris between the dwarf and gE. The dwarf's nuclear morphology is irregular and bow-shaped on what appears to be its leading edge. Apart from a number of isolated blue regions, most of of the trailing debris is similar in color to the dwarf's nucleus. Only a modest enhancement of star formation appears to have been induced by the interaction. Although separated by 15 kpc, the H I and stellar morphologies are remarkably similar. The stars and H I appear to have been tidally distorted in situ, prior to the cloud's removal by ram pressure. If the H I has maintained its shape by magnetic support, a magnetic field strength an order of magnitude larger than the galaxy's is required. Ram pressure deceleration due to the cloud's motion through NGC 4472's x-ray-emitting interstellar medium should be sufficient for the cloud to become gravitationally bound to NGC 4472. The H I cloud is not self-gravitating and may fragment and be destroyed in the interaction. UGC 7636 will probably be disrupted by NGC 4472's strong tidal forces; the stellar debris will disperse into the Virgo cluster or become bound to NGC 4472's halo on eccentric orbits. The debris captured in the collision will have a negligible impact on NGC 4472's stellar and gaseous content. On the other hand, if similar interactions are common in giant elliptical galaxies, they could alter or deplete surrounding dwarf galaxy populations, fuel bursts of nuclear activity, and perhaps provide a source of magnetic energy to their interstellar media.

  14. The Origin of Dwarf Early-Type Galaxies

    NASA Astrophysics Data System (ADS)

    Toloba, Elisa; Boselli, A.; Gorgas, J.

    2013-01-01

    The physical mechanisms involved in the formation and evolution of dwarf early-type galaxies (dEs) are not well understood yet. Whether these objects, that outnumber any other class of object in clusters, are the low luminosity extension of massive early-type galaxies, i.e. formed through similar processes, or are a different group of objects possibly formed through the transformation of low luminosity spiral galaxies, is still an open debate. Studying the kinematic properties of dEs is a powerful way to distinguish between these two scenarios. In my PhD, awarded with a Fulbright postdoctoral Fellowship and with the 2011 prize to the best Spanish PhD dissertation in Astronomy, we used this technique to make a spectrophotometric analysis of 18 dEs in the Virgo cluster. I found some differences for these dEs within the cluster. The dEs in the outer parts of Virgo have rotation curves with shapes and amplitudes similar to late-type galaxies of the same luminosity. They 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. Ram pressure stripping, which removes the gas of galaxies leaving the stars untouched, 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 disky structures of these galaxies. I am conducting new analysis with 20 new dEs to throw some light in this direction. I also analysed the Faber-Jackson and the Fundamental Plane relations, and I found that 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. This indicates that dEs have a non-negligible dark matter

  15. Evolution of dwarf galaxies simulated in the cosmological LCDM scenario

    NASA Astrophysics Data System (ADS)

    Gonzalez, Alejandro; Colin, Pedro; Avila-Reese, Vladimir; Rodriguez-Puebla, Aldo; Valenzuela, Octavio

    2014-03-01

    We present results from numerical simulations of low-mass galaxies with the aim to explore the way their stellar masses are assembled. We analyze how the mass assembly histories of the parent halo determine the growth of their host galaxy and its implications on the current paradigm of formation and evolution of low-mass structures in the LCDM scenario. We have found that low-mass galaxies simulated in this scenario assemble their stellar masses following roughly the dark matter halo assembly, which seems to be in tension with the downsizing trend suggested by current observational inferences. We show that there is no more room to increase the strength of feedback from astrophysical processes in order to deviate strongly the stellar mass assembly from the dark halo one, as has been recently invoked to solve some of the potential issues faced by CDM-based simulations of dwarf galaxies. Alejandro González acknowledges finacial support from UNAM, Fundacion UNAM, and the APS to attend this meeting.

  16. THE STELLAR AND GASEOUS CONTENTS OF THE ORION DWARF GALAXY

    SciTech Connect

    Cannon, John M.; Haynes, Korey; Most, Hans; Haugland, Kaitlin; Scudder, Jillian; Weindling, Jacob; Salzer, John J.; Sugden, Arthur

    2010-06-15

    We present new Kitt Peak National Observatory 0.9 m optical and Very Large Array H I spectral line observations of the Orion dwarf galaxy. This nearby (D {approx_equal} 5.4 Mpc), intermediate-mass (M{sub dyn{approx_equal}} 1.1 x 10{sup 10} M{sub sun}) dwarf displays a wealth of structure in its neutral interstellar medium, including three prominent 'hole/depression' features in the inner H I disk. We explore the rich gas kinematics, where solid-body rotation dominates and the rotation curve is flat out to the observed edge of the H I disk ({approx}6.8 kpc). The Orion dwarf contains a substantial fraction of dark matter throughout its disk: comparing the 4.7 x 10{sup 8} M{sub sun} of detected neutral gas with estimates of the stellar mass from optical and near-infrared imaging (3.7 x 10{sup 8} M{sub sun}) implies a mass-to-light ratio {approx_equal}13. New H{alpha} observations show only modest-strength current star formation (SF; {approx}0.04 M{sub sun} yr{sup -1}); this SF rate is consistent with our 1.4 GHz radio continuum non-detection.

  17. 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.

  18. A spectroscopic binary in the Hercules dwarf spheroidal galaxy

    SciTech Connect

    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 a{sub p} sin i = 38 R {sub ☉}. In fact, a very close orbit could inhibit the production of heavier elements through s-process nucleosynthesis, leading to the very low abundances of neutron-capture elements that are found in this star. We discuss the further implications for the chemical enrichment history of the Hercules dSph, but find no compelling binary scenario that could reasonably explain the full, peculiar abundance pattern of the Hercules dSph galaxy.

  19. Metal Diffusion in Smoothed Particle Hydrodynamics Simulations of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Williamson, David; Martel, Hugo; Kawata, Daisuke

    2016-05-01

    We perform a series of smoothed particle hydrodynamics simulations of isolated dwarf galaxies to compare different metal mixing models. In particular, we examine the role of diffusion in the production of enriched outflows and in determining the metallicity distributions of gas and stars. We investigate different diffusion strengths by changing the pre-factor of the diffusion coefficient, by varying how the diffusion coefficient is calculated from the local velocity distribution, and by varying whether the speed of sound is included as a velocity term. Stronger diffusion produces a tighter [O/Fe]–[Fe/H] distribution in the gas and cuts off the gas metallicity distribution function at lower metallicities. Diffusion suppresses the formation of low-metallicity stars, even with weak diffusion, and also strips metals from enriched outflows. This produces a remarkably tight correlation between “metal mass-loading” (mean metal outflow rate divided by mean metal production rate) and the strength of diffusion, even when the diffusion coefficient is calculated in different ways. The effectiveness of outflows at removing metals from dwarf galaxies and the metal distribution of the gas is thus dependent on the strength of diffusion. By contrast, we show that the metallicities of stars are not strongly dependent on the strength of diffusion, provided that some diffusion is present.

  20. Metal Diffusion in Smoothed Particle Hydrodynamics Simulations of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Williamson, David; Martel, Hugo; Kawata, Daisuke

    2016-05-01

    We perform a series of smoothed particle hydrodynamics simulations of isolated dwarf galaxies to compare different metal mixing models. In particular, we examine the role of diffusion in the production of enriched outflows and in determining the metallicity distributions of gas and stars. We investigate different diffusion strengths by changing the pre-factor of the diffusion coefficient, by varying how the diffusion coefficient is calculated from the local velocity distribution, and by varying whether the speed of sound is included as a velocity term. Stronger diffusion produces a tighter [O/Fe]-[Fe/H] distribution in the gas and cuts off the gas metallicity distribution function at lower metallicities. Diffusion suppresses the formation of low-metallicity stars, even with weak diffusion, and also strips metals from enriched outflows. This produces a remarkably tight correlation between “metal mass-loading” (mean metal outflow rate divided by mean metal production rate) and the strength of diffusion, even when the diffusion coefficient is calculated in different ways. The effectiveness of outflows at removing metals from dwarf galaxies and the metal distribution of the gas is thus dependent on the strength of diffusion. By contrast, we show that the metallicities of stars are not strongly dependent on the strength of diffusion, provided that some diffusion is present.

  1. 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.

  2. Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

    NASA Technical Reports Server (NTRS)

    Lin, Douglas N. C.; Murray, Stephen D.

    1991-01-01

    Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

  3. Conversion Problems: How (Not) to Determine Molecular Masses in Dwarf Galaxies (Oral Contribution)

    NASA Astrophysics Data System (ADS)

    Hüttemeister, S.

    The determination of molecular gas masses in star forming dwarf irregular galaxies is crucial to assess the star formation process in these objects. But the derivation of the molecular gas content of dwarf galaxies has been a long-standing problem. CO, as the only practical tracer of cold molecular gas, has been (and to some extent still is) notoriously diffucult to detect. Yet, star formation clearly takes place in many dwarf irregulars. This conference contribution contrasts a number of methods commonly used to derive the molecular gas contents of dwarf galaxies based on CO observations: Procedures based on the virial theorem and those relying on radiative transfer arguments. It is shown that both classes of methods have serious drawbacks. Still, examples show that there seem to be real differences in the `correct' conversion factor both between and within star forming dwarf irregular galaxies.

  4. X-RAY DISCOVERY OF A DWARF-GALAXY-GALAXY COLLISION

    SciTech Connect

    Garmire, Gordon P.

    2013-06-10

    We report the discovery of a probable dwarf galaxy colliding with NGC 1232. This collision is visible only in the X-ray spectral band, and it is creating a region of shocked gas with a temperature of 5.8 MK covering an impact area 7.25 kpc in diameter. The X-ray luminosity is 3.7 Multiplication-Sign 10{sup 38} erg s{sup -1}. The long lifetime of this gas against radiative and adiabatic cooling should permit the use of the luminous afterglow from such collisions to be used as a way of estimating their importance in galaxy evolution.

  5. 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.

  6. 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

  7. Alignment of Red-Sequence Cluster Dwarf Galaxies: From the Frontier Fields to the Local Universe

    NASA Astrophysics Data System (ADS)

    Barkhouse, Wayne Alan; Archer, Haylee; Burgad, Jaford; Foote, Gregory; Rude, Cody; Lopez-Cruz, Omar

    2015-08-01

    Galaxy clusters are the largest virialized structures in the universe. Due to their high density and mass, they are an excellent laboratory for studying the environmental effects on galaxy evolution. Numerical simulations have predicted that tidal torques acting on dwarf galaxies as they fall into the cluster environment will cause the major axis of the galaxies to align with their radial position vector (a line that extends from the cluster center to the galaxy's center). We have undertaken a study to measure the redshift evolution of the alignment of red-sequence cluster dwarf galaxies based on a sample of 57 low-redshift Abell clusters imaged at KPNO using the 0.9-meter telescope, and 64 clusters from the WINGS dataset. To supplement our low-redshift sample, we have included galaxies selected from the Hubble Space Telescope Frontier fields. Leveraging the HST data allows us to look for evolutionary changes in the alignment of red-sequence cluster dwarf galaxies over a redshift range of 0 < z < 0.35. The alignment of the major axis of the dwarf galaxies is measured by fitting a Sersic function to each red-sequence galaxy using GALFIT. The quality of each model is checked visually after subtracting the model from the galaxy. The cluster sample is then combined by scaling each cluster by r200. We present our preliminary results based on the alignment of the red-sequence dwarf galaxies with: 1) the major axis of the brightest cluster galaxy, 2) the major axis of the cluster defined by the position of cluster members, and 3) a radius vector pointing from the cluster center to individual dwarf galaxies. Our combined cluster sample is sub-divided into different radial regions and redshift bins.

  8. 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.

  9. 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

    2016-08-01

    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 halos, 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 observed 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.

  10. CHEMICAL ABUNDANCE PATTERNS AND THE EARLY ENVIRONMENT OF DWARF GALAXIES

    SciTech Connect

    Corlies, Lauren; Johnston, Kathryn V.; Bryan, Greg; Tumlinson, Jason

    2013-08-20

    Recent observations suggest that abundance pattern differences exist between low metallicity stars in the Milky Way stellar halo and those in the dwarf satellite galaxies. This paper takes a first look at what role the early environment for pre-galactic star formation might have played in shaping these stellar populations. In particular, we consider whether differences in cross-pollution between the progenitors of the stellar halo and the satellites could help to explain the differences in abundance patterns. Using an N-body simulation, we find that the progenitor halos of the main halo are primarily clustered together at z = 10 while the progenitors of the satellite galaxies remain on the outskirts of this cluster. Next, analytically modeled supernova-driven winds show that main halo progenitors cross-pollute each other more effectively while satellite galaxy progenitors remain more isolated. Thus, inhomogeneous cross-pollution as a result of different high-z spatial locations of each system's progenitors can help to explain observed differences in abundance patterns today. Conversely, these differences are a signature of the inhomogeneity of metal enrichment at early times.

  11. A Neighboring Dwarf Irregular Galaxy Hidden by the Milky Way

    NASA Astrophysics Data System (ADS)

    Massey, Philip; Henning, P. A.; Kraan-Korteweg, R. C.

    2003-11-01

    We have obtained VLA and optical follow-up observations of the low-velocity H I source HIZSS 3 discovered by Henning et al. and Rivers in a survey for nearby galaxies hidden by the disk of the Milky Way. Its radio characteristics are consistent with this being a nearby (~1.8 Mpc) low-mass dwarf irregular galaxy (dIm). Our optical imaging failed to reveal a resolved stellar population but did detect an extended Hα emission region. The location of the Hα source is coincident with a partially resolved H I cloud in the 21 cm map. Spectroscopy confirms that the Hα source has a similar radial velocity to that of the H I emission at this location, and thus we have identified an optical counterpart. The Hα emission (100 pc in diameter and with a luminosity of 1.4×1038 ergs s-1) is characteristic of a single H II region containing a modest population of OB stars. The galaxy's radial velocity and distance from the solar apex suggests that it is not a Local Group member, although a more accurate distance is needed to be certain. The properties of HIZSS 3 are comparable to those of GR 8, a nearby dIm with a modest amount of current star formation. Further observations are needed to characterize its stellar population, determine the chemical abundances, and obtain a more reliable distance estimate.

  12. Diffuse Ionized Gas inside the Dwarf Irregular Galaxy NGC 6822

    NASA Astrophysics Data System (ADS)

    Hidalgo-Gámez, A. M.; Peimbert, A.

    2007-05-01

    We have studied the differences between the diffuse ionized gas (DIG) and the H II regions along a slit position in the local dwarf irregular galaxy NGC 6822. The slit position passes through the two most prominent H II regions: Hubble V and Hubble X. Important differences have been found in the excitation, ionization, and [N II] λ6584/Hα and [S II] λ6717/Hα line ratios between the DIG and the H II locations. Moreover, the values of all the line ratios are not similar to those in the DIG locations of spiral galaxies but are very similar to the values in other irregular galaxies, such as IC 10. We also determined the rate of recombination using the He I λ5875 line. Finally, we obtained a picture of the ionization sources of the DIG. We consider that the leakage of photons from the H II regions might explain most of the line ratios, except [N II]/Hα, which might be explained by turbulence. Based on observations collected at the European Southern Observatory, Chile, proposal 69.C-0203(A).

  13. 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.

  14. 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

  15. Age and metallicity gradients in early-type galaxies: a dwarf-to-giant sequence

    NASA Astrophysics Data System (ADS)

    Koleva, Mina; Prugniel, Philippe; de Rijcke, Sven; Zeilinger, Werner W.

    2011-11-01

    We studied the stellar populations of 40 early-type galaxies using medium-resolution long-slit spectroscopy along their major axes (and along the minor axis for two of them). The sample, including elliptical and lenticular galaxies as well as dwarf galaxies, is combined with other previously published data in order to discuss the systematics of the radial gradients of age and metallicity over a large mass range, from 107 M⊙ to 1012 M⊙ (-9.2 > MB > -22.4 mag). The well-known mass-metallicity relation is continuous throughout the whole mass range, in the sense that more massive galaxies are more metal-rich. The age-mass relation is consistent with the idea of downsizing: smaller galaxies have more extended star formation histories than more massive ones. The transition-type dwarfs (intermediate between dwarf irregular and dwarf elliptical galaxies) deviate from this relation having younger mean age, and the low-mass dwarf spheroidals have older ages, marking a discontinuity in the relation, possibly due to selection effects. In all mass regimes, the mean metallicity gradients are approximately -0.2 and the mean age gradients +0.1 dex per decade of radius. The individual gradients are widely spread: -0.1 < ∇Age < 0.4 and -0.54 < ∇[Fe/H] < +0.2. We do not find evidence for a correlation between the metallicity gradient and luminosity, velocity dispersion, central age or age gradient. Likewise, we do not find a correlation between the age gradient and any other parameter in bright early-type galaxies. In faint early-types with MB≳-17 mag, on the other hand, we find a strong correlation between the age gradient and luminosity: the age gradient becomes more positive for fainter galaxies. Together with the observed downsizing phenomenon this indicates that, as time passes, star formation persists in dwarf galaxies and becomes more centrally concentrated. However, this prolonged central star formation is not reflected in the metallicity profiles of the dwarfs in

  16. 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.

  17. Photometry of resolved galaxies. I - The Pegasus dwarf irregular

    NASA Technical Reports Server (NTRS)

    Hoessel, J. G.; Mould, J. R.

    1982-01-01

    Color-magnitude diagrams for resolved stars in the Pegasus dwarf galaxy in the green, red, and infrared passbands of the extended Gunn photometric system are presented. The evolved nature of the upper main sequence and the lack of luminous red supergiants indicate that recent star formation in Pegasus has been very subdued. Three star clusters are identified. Their red colors indicate they are of intermediate age or older. Two features of the color-magnitude diagrams are of interest: a group of red stars with I at about 21.3, which may be the counterparts of the carbon stars found in the Magellanic Clouds, and a clump in the (I, R-I)-diagram, which is probably the tip of the old giant branch. A provisional distance estimate for Pegasus of 1.7 Mpc is derived, placing it at the outer margins of the Local Group.

  18. 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.

  19. 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.

  20. The Main-Sequence Stars of the Sagittarius Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Fahlman, G. G.; Mandushev, G.; Richer, H. B.; Thompson, I. B.; Sivaramakrishnan, A.

    1996-03-01

    The Sagittarius dwarf galaxy (SDG) is visible in the background field of the globular cluster M55. We present a deep VI color-magnitude diagram (CMD) of M55, which shows a prominent sequence of stars some 3.5 mag below the cluster main sequence. Through a comparison with a similar CMD for the globular cluster M4, we show that the M55 background field is not the Galactic bulge or spheroid. The SDG main sequence is almost as blue as that of M55 and thus, if it is metal rich, it must be younger than M55, a typical old Galactic globular cluster. The results from isochrone fitting indicate that the age of the SDG is 10--14 Gyr, similar to the ages inferred for the two associated globular clusters Ter 7 and Arp 2.

  1. The cometary blue compact dwarf galaxies Mkn 59 and Mkn 71. A clue to dwarf galaxy evolution?

    NASA Astrophysics Data System (ADS)

    Noeske, K. G.; Guseva, N. G.; Fricke, K. J.; Izotov, Y. I.; Papaderos, P.; Thuan, T. X.

    2000-09-01

    ``Cometary'' Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by an off-center starburst close to the end of their elongated stellar bodies. This rare phenomenon may carry some clues on how collective star formation ignites and propagates in gas-rich low-mass stellar systems. This off-center burst may be a fortuitous enhancement of the otherwise moderate star-forming activity of a dwarf irregular (dI), or may be caused by a set of special properties of such systems or their environment. We attempt here a first investigation of this issue by analysing two prototypical examples of cometary dwarf galaxies, the nearby iI,C BCDs Markarian 59 and Markarian 71, both containing an extraordinarily luminous H Ii region in the outskirts of a dI-like host. Using deep ground-based spectrophotometric data and HST images, we study the physical state of the starburst regions and the structural properties of the underlying irregular galaxies. We find that the average metallicities show small scatter in the vicinity of the star-forming regions and along the major axis of Mkn 59 which suggests that mixing of heavy elements must have been efficient on scales of several kpc. The azimuthally averaged radial intensity distributions of the underlying host galaxies in either iI,C BCD can be approximated by an exponential law with a central surface brightness and scale length that is intermediate between typical iE/nE BCDs and dwarf irregulars. Spectral population synthesis models in combination with colour magnitude diagrams and colour profiles yield a most probable formation age of ~ 2 Gyr for the low surface brightness (LSB) host galaxies in both iI,C BCDs, with upper age limits of ~ 4 Gyr for Mkn 59 and ~ 3 Gyr for Mkn 71, i.e. significantly lower than the typical age of several Gyr derived for the LSB component of iE/nE BCDs. These findings raise the question whether iI,C systems form a distinct physical class within BCDs with respect to the age and structural properties of

  2. The extended structure of the dwarf irregular galaxy Sagittarius

    NASA Astrophysics Data System (ADS)

    Beccari, G.; Bellazzini, M.; Fraternali, F.; Battaglia, G.; Perina, S.; Sollima, A.; Oosterloo, T. A.; Testa, V.; Galleti, S.

    2014-10-01

    We present a detailed study of the stellar and H i structure of the dwarf irregular galaxy Sagittarius. We use new deep and wide field photometry to trace the surface brightness profile of the galaxy out to ≃5.0' (corresponding to ≃1600 pc) and down to μV ≃ 30.0 mag/arcsec2, thus showing that the stellar body of the galaxy is much more extended than previously believed, and it is similarly (or more) extended than the overall H i distribution. The whole major-axis profile is consistent with a pure exponential, with a scale radius of ≃340 pc. The surface density maps reveal that the distribution of old and intermediate-age stars is smooth and remarkably flattened out to its edges, while the associated H i has a much rounder shape, is off-centred and presents multiple density maxima and a significant hole. No clear sign of systemic rotation is detectable in the complex H i velocity field. No metallicity gradient is detected in the old and intermediate age population of the galaxy, and we confirm that this population has a much more extended distribution than young stars (age ≲ 1 Gyr). Based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, under the Program 089.D-0052(A).Table of stellar photometry (IN6) 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/570/A78

  3. The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Cohen, Judith G.; Huang, Wenjin

    2009-08-01

    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 α-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. Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.

  4. 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.

  5. Star formation in blue compact dwarf (BCD) galaxies

    NASA Astrophysics Data System (ADS)

    Sethuram, Ramya

    I present the optical photometry, spectroscopy using HCT of Blue compact dwarf (BCD) galaxies. The BCDs are observed through UBVRI filters. The colour-colour diagram (CCD) of U-B vs V-I and U-B vs B-V are created from integrated magnitudes. We have for the first time employed the mixed population technique using CCDs to find out ages of the underlying population intermixed with the starburst regions. A combination of few Gyr population, few hundred million years population and an young burst of age 5-15 Myr is detected for most of these galaxies. The structure of BCDs is a smooth background of low surface brightness (LSB) old stellar population and intermixed with the clumps of H II regions. The spectra of BCDs are typical H II region kind of spectra. The electron densities are < 100 cm^{-3} and temperatures are in the range 6000-15000 K. The oxygen abundance is calculated using the standard bright line methods. The oxygen abundance (log[O/H] + 12) of the sample of BCDs varies from as low as 7.9 to as high as 8.5. So these galaxies are (1/5 - 1/2.5) times metal poor when compared to solar. The BCDs are observed through H_α filters installed in the HFOSC system. The H_α fluxes and luminosities are used to calculate star formation rates (SFR) that are in the range 0.01 to 0.1 M_⊙/yr for individual H II regions and ranges between 0.1 to 1.0 M_⊙/yr for the whole galaxy. The archival data from 2MASS is used to obtain the integrated colour-colour diagrams of (J-H) vs (V-K). NIR colours are less affected by reddening and are better signatures of old stellar population. The results are compared with optical CCD, the interesting dichotomy are presented.

  6. A spatial characterization of the Sagittarius dwarf galaxy tidal tails

    SciTech Connect

    Newby, Matthew; Cole, Nathan; Newberg, Heidi Jo; Willett, Benjamin; Desell, Travis; Magdon-Ismail, Malik; Szymanski, Boleslaw; Varela, Carlos; Yanny, Brian E-mail: heidi@rpi.edu

    2013-06-01

    We measure the spatial density of F turnoff stars in the Sagittarius dwarf tidal stream, from Sloan Digital Sky Survey data, using statistical photometric parallax. We find a set of continuous, consistent parameters that describe the leading Sgr stream's position, direction, and width for 15 stripes in the north Galactic cap, and three stripes in the south Galactic cap. We produce a catalog of stars that has the density characteristics of the dominant leading Sgr tidal stream that can be compared with simulations. We find that the width of the leading (north) tidal tail is consistent with recent triaxial and axisymmetric halo model simulations. The density along the stream is roughly consistent with common disruption models in the north, but possibly not in the south. We explore the possibility that one or more of the dominant Sgr streams has been misidentified, and that one or more of the ''bifurcated'' pieces is the real Sgr tidal tail, but we do not reach definite conclusions. If two dwarf progenitors are assumed, fits to the planes of the dominant and ''bifurcated'' tidal tails favor an association of the Sgr dwarf spheroidal galaxy with the dominant southern stream and the ''bifurcated'' stream in the north. In the north Galactic cap, the best fit Hernquist density profile for the smooth component of the stellar halo is oblate, with a flattening parameter q = 0.53, and a scale length of r {sub 0} = 6.73. The southern data for both the tidal debris and the smooth component of the stellar halo do not match the model fits to the north, although the stellar halo is still overwhelmingly oblate. Finally, we verify that we can reproduce the parameter fits on the asynchronous MilkyWay@home volunteer computing platform.

  7. THE DEARTH OF NEUTRAL HYDROGEN IN GALACTIC DWARF SPHEROIDAL GALAXIES

    SciTech Connect

    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 M{sub H} {sub I}{sup 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 L{sub V} and dynamical mass M {sub dyn}, we find M{sub H} {sub I}{sup lim}/L{sub V}∼10{sup −3} M{sub ⊙}/L{sub ⊙} and M{sub H} {sub I}{sup lim}/M{sub dyn}∼5×10{sup −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 M{sub H} {sub I}{sup 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.

  8. A Spatial Characterization of the Sagittarius Dwarf Galaxy Tidal Tails

    NASA Astrophysics Data System (ADS)

    Newby, Matthew; Cole, Nathan; Newberg, Heidi Jo; Desell, Travis; Magdon-Ismail, Malik; Szymanski, Boleslaw; Varela, Carlos; Willett, Benjamin; Yanny, Brian

    2013-06-01

    We measure the spatial density of F turnoff stars in the Sagittarius dwarf tidal stream, from Sloan Digital Sky Survey data, using statistical photometric parallax. We find a set of continuous, consistent parameters that describe the leading Sgr stream's position, direction, and width for 15 stripes in the north Galactic cap, and three stripes in the south Galactic cap. We produce a catalog of stars that has the density characteristics of the dominant leading Sgr tidal stream that can be compared with simulations. We find that the width of the leading (north) tidal tail is consistent with recent triaxial and axisymmetric halo model simulations. The density along the stream is roughly consistent with common disruption models in the north, but possibly not in the south. We explore the possibility that one or more of the dominant Sgr streams has been misidentified, and that one or more of the "bifurcated" pieces is the real Sgr tidal tail, but we do not reach definite conclusions. If two dwarf progenitors are assumed, fits to the planes of the dominant and "bifurcated" tidal tails favor an association of the Sgr dwarf spheroidal galaxy with the dominant southern stream and the "bifurcated" stream in the north. In the north Galactic cap, the best fit Hernquist density profile for the smooth component of the stellar halo is oblate, with a flattening parameter q = 0.53, and a scale length of r 0 = 6.73. The southern data for both the tidal debris and the smooth component of the stellar halo do not match the model fits to the north, although the stellar halo is still overwhelmingly oblate. Finally, we verify that we can reproduce the parameter fits on the asynchronous MilkyWay@home volunteer computing platform.

  9. A Case Study for a Tidal Interaction between Dwarf Galaxies in UGC 6741

    NASA Astrophysics Data System (ADS)

    Paudel, Sanjaya; Duc, P. A.; Ree, C. H.

    2015-03-01

    We present a case study of the tidal interaction between low-mass star-forming galaxies initially found in the Sloan Digital Sky Survey (SDSS) images and further analyzed with SDSS spectroscopy and UV GALEX photometry. With a luminosity of Mr = -17.7 mag and exhibiting a prominent tidal filament, UGC 6741 appears as a scaled down version of massive gas-rich interacting systems and mergers. The stellar disk of the smaller companion, UGC 6741_B, which is three times less massive, has likely already been destroyed. Both galaxies, which are connected by a 15 kpc long stellar bridge, have similar oxygen abundances of 12 +log(O/H)˜8.3. Several knots of star-forming regions are identified along the bridge, some with masses exceeding ˜107 M ⊙ . The most compact of them, which are unresolved, may evolve into globular clusters or ultra compact dwarf galaxies. This would be the first time progenitors of such objects are detected in mergers involving dwarf galaxies. UGC 6741 currently has the color and star formation properties of blue compact dwarf galaxies (BCDs). However, analysis of its surface photometry suggests that the galaxy lies within the scaling relations defined by early-type dwarf galaxies (dEs). Thus, UGC 6741 appears as a promising system for studying the possible transformation of BCDs into dEs, possibly through a merger episode. The frequency of such dwarf-dwarf mergers should now be explored.

  10. THE EXTENSIVE AGE GRADIENT OF THE CARINA DWARF GALAXY

    SciTech Connect

    Battaglia, G.; Irwin, M.; Tolstoy, E.; De Boer, T.; Mateo, M.

    2012-12-20

    The evolution of small systems such as dwarf spheroidal galaxies (dSphs) is likely to have been a balance between external environmental effects and internal processes within their own relatively shallow potential wells. Assessing how strong such environmental interactions may have been is therefore an important element in understanding the baryonic evolution of dSphs and their derived dark matter distribution. Here we present results from a wide-area CTIO/MOSAIC II photometric survey of the Carina dSph, reaching down to about two magnitudes below the oldest main-sequence turnoff (MSTO). This data set enables us to trace the structure of Carina in detail out to very large distances from its center, and as a function of stellar age. We observe the presence of an extended structure made up primarily of ancient MSTO stars, at distances between 25' and 60' from Carina's center, confirming results in the literature that Carina extends well beyond its nominal tidal radius. The large number statistics of our survey reveals features such as isophote twists and tails that were undetected in other previous, shallower surveys. This is the first time that such unambiguous signs of tidal disruption have been found in a Milky Way 'classical' dwarf other than Sagittarius. We also demonstrate the presence of a negative age gradient in Carina directly from its MSTOs, and trace it out to very large distances from the galaxy center. The signs of interaction with the Milky Way make it unclear whether the age gradient was already in place before Carina underwent tidal disruption.

  11. Ejection of Supernova-enriched Gas from Dwarf Disk Galaxies

    NASA Astrophysics Data System (ADS)

    Fragile, P. Chris; Murray, Stephen D.; Lin, Douglas N. C.

    2004-12-01

    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 108 and 109 Msolar with supernova rates of 30, 300, and 3000 Myr-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, however, we find the loss of enriched material to be much less efficient 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 predictions indicates that when undergoing self-regulated star formation, galaxies in the mass range considered will efficiently retain the products of Type II supernovae.

  12. 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).

  13. 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

  14. Nucleosynthesis and the Inhomogeneous Chemical Evolution of the Carina Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

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

    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 [α/Fe] ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in α-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 interesting

  15. 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

  16. Exploring Optically Compact Dwarf Galaxies for Kinematic Structures and Extended HI Halos

    NASA Astrophysics Data System (ADS)

    Most, Hans; Cannon, J. M.; Salzer, J. J.; Rosenberg, J. L.

    2012-01-01

    We present Very Large Array H I spectral line and optical imaging of eight optically compact (optical radii <1 kpc), star-forming dwarf galaxies. These galaxies were chosen because of their optically compact stellar distributions, faint blue magnitudes, ongoing star formation, and relative proximity. The sample includes ADBS 113845+2008, which was found to have an HI halo that extends nearly 40 optical scale lengths from the stellar body (Cannon et al. 2009). Using this larger sample, we are working to discern if the "giant gas disk" dwarf galaxy is common or rare. We are also exploring the kinematics and dark matter contents of each of the sample galaxies.

  17. Exploring the Extended Structure of the Sculptor Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Westfall, K. B.; Ostheimer, J. C.; Frinchaboy, P. M.; Patterson, R. J.; Majewski, S. R.; Kunkel, W. E.

    2000-12-01

    We have undertaken a large area (>3 deg2) survey of the Sculptor dSph using the 1-m Swope telescope. The region surveyed includes roughly 1 deg2 centered on the Sculptor core, with the remaining survey area extending to the east and stretching to almost twice the tidal radius (rt=76.5m) to the northeast and southeast. We have imaged in the Washington M,T2 and DDO51 filters, a combination that allows us to discriminate dwarf and giant stars based on the gravity sensitivity of DDO51. The extended structure of Sculptor can be mapped via those stars selected both as giant stars and as having a combination of M and M-T2 consistent with the red giant branch of Sculptor. We also make use of the areal distribution of blue horizontal branch stars, which delineate the extended structure of Sculptor relatively well in this field at high Galactic latitude. Using the HYDRA spectrograph on the Blanco 4-m, we have obtained more than a dozen radial velocities for candidate Sculptor stars that we have identified well outside (1) the core radius, and (2) the radii explored by previous surveys. A preliminary conclusion from our work so far is that Sculptor does not show as extensive a population of extratidal stars as we have identified in similar work we have conducted around the Carina (Majewski et al. 2000, AJ, 119, 760) and Ursa Minor (Palma et al. 2000, BAAS) dwarf galaxies. Indeed, if a lack of significant extended material around Sculptor is borne out by further study over more area and other position angles, then an interesting correlation begins to emerge: Among four galaxies we have surveyed in this way (Car, UMi, Leo II, and Scl), the relative fraction of the dSph's found outside the nominal tidal radius appears to correlate with the published values of M/L. This may suggest that the derived masses for the dwarf spheroidals may be systematically overestimated to a degree set by the amount of dynamical non-equilibrium in the system. This work was supported by NSF, NASA, the

  18. Hot gas outflow in the blue compact dwarf galaxy VII Zw 403

    NASA Technical Reports Server (NTRS)

    Papaderos, P.; Fricke, K. J.; Thuan, T. X.; Loose, H.-H.

    1994-01-01

    We have observed the Blue Compact Dwarf Galaxy VII Zw 403 with the Position Sensitive Proportional Counter (PSPC) camera onboard ROSAT. We found a total X-ray luminosity of 1.94 x 10(exp 38) erg/s distributed in a central core to which are connected three elongated structures. We interpret this X-ray morphology as the result of a hot gas outflow from the core of the dwarf galaxy powered by the present starburst.

  19. Populations of Dwarfs in Clusters of Galaxies: Environmental Connections (Oral Contribution)

    NASA Astrophysics Data System (ADS)

    Gallagher, J. S.; Conselice, C. J.; Wyse, R. F. G.

    Despite their apparent fragile appearance, dwarf spheroidals are the most common galaxy type in clusters. In this paper we consider some of the issues associated with two major models for the origin of these dwarfs: primeval galaxies which formed with the cluster and the modification of accreted systems. We argue that the present observational evidence, derived from the Virgo and Perseus clusters, points to infall as the origin of many of these objects.

  20. How similar is the stellar structure of low-mass late-type galaxies to that of early-type dwarfs?

    NASA Astrophysics Data System (ADS)

    Janz, J.; Laurikainen, E.; Laine, J.; Salo, H.; Lisker, T.

    2016-09-01

    We analyse structural decompositions of 500 late-type galaxies (Hubble T-type ≥6) from the Spitzer Survey of Stellar Structure in Galaxies (S4G; Salo et al.), spanning stellar mass range of about 107 to a few times 1010 M⊙. Their decomposition parameters are compared with those of the early-type dwarfs in the Virgo cluster from Janz et al. They have morphological similarities, including the fact that the fraction of simple one-component galaxies in both samples increases towards lower galaxy masses. We find that in the late-type two-component galaxies both the inner and outer structures are by a factor of 2 larger than in the early-type dwarfs, for the same stellar mass of the component. While dividing the late-type galaxies to low- and high-density environmental bins, it is noticeable that both the inner and outer components of late types in the high local density galaxies are smaller, and lie closer in size to those of the early-type dwarfs. This suggests that, although structural differences between the late- and early-type dwarfs are observed, environmental processes can plausibly transform their sizes sufficiently, thus linking them evolutionarily.

  1. 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.

  2. Dwarf spheroidal galaxies as degenerate gas of free fermions

    NASA Astrophysics Data System (ADS)

    Domcke, Valerie; Urbano, Alfredo

    2015-01-01

    In this paper we analyze a simple scenario in which Dark Matter (DM) consists of free fermions with mass mf. 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 mf. 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 mfsimeq 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.

  3. On the assembly of dwarf galaxies in clusters and their efficient formation of globular clusters

    NASA Astrophysics Data System (ADS)

    Mistani, Pouria A.; Sales, Laura V.; Pillepich, Annalisa; Sanchez-Janssen, Rubén; Vogelsberger, Mark; Nelson, Dylan; Rodriguez-Gomez, Vicente; Torrey, Paul; Hernquist, Lars

    2016-01-01

    Galaxy clusters contain a large population of low-mass dwarf elliptical galaxies whose exact origin is unclear: their colours, structural properties and kinematics differ substantially from those of dwarf irregulars in the field. We use the Illustris cosmological simulation to study differences in the assembly histories of dwarf galaxies (3 × 108 < M*/M⊙ < 1010) according to their environment. We find that cluster dwarfs achieve their maximum total and stellar mass on average ˜8 and ˜4.5 Gyr ago (or redshifts z = 1.0 and 0.4, respectively), around the time of infall into the clusters. In contrast, field dwarfs not subjected to environmental stripping reach their maximum mass at z = 0. These different assembly trajectories naturally produce a colour bimodality, with blue isolated dwarfs and redder cluster dwarfs exhibiting negligible star formation today. The cessation of star formation happens over median times 3.5-5 Gyr depending on stellar mass, and shows a large scatter (˜1-8 Gyr), with the lower values associated with starburst events that occur at infall through the virial radius or pericentric passages. We argue that such starbursts together with the early assembly of cluster dwarfs can provide a natural explanation for the higher specific frequency of globular clusters (GCs) in cluster dwarfs, as found observationally. We present a simple model for the formation and stripping of GCs that supports this interpretation. The origin of dwarf ellipticals in clusters is, therefore, consistent with an environmentally driven evolution of field dwarf irregulars. However, the z = 0 field analogues of cluster dwarf progenitors have today stellar masses a factor of ˜3 larger - a difference arising from the early truncation of star formation in cluster dwarfs.

  4. H I IN LOCAL GROUP DWARF GALAXIES AND STRIPPING BY THE GALACTIC HALO

    SciTech Connect

    Grcevich, Jana; Putman, Mary E E-mail: mputman@astro.columbia.edu

    2009-05-01

    We examine the H I content and environment of all of the Local Group dwarf galaxies (M {sub tot} < 10{sup 10} M {sub sun}), including the numerous newly discovered satellites of the Milky Way and M31. All of the new dwarfs, with the exception of Leo T, have no detected H I. The majority of dwarf galaxies within {approx}270 kpc of the Milky Way or Andromeda are undetected in H I (<10{sup 4} M {sub sun} for Milky Way dwarfs), while those further than {approx}270 kpc are predominantly detected with masses {approx}10{sup 5} to 10{sup 8} M {sub sun}. Analytical ram-pressure arguments combined with velocities obtained via proper motion studies allow for an estimate of the halo density of the Milky Way at several distances. This halo density is constrained to be greater than 2x 10{sup -4}-3 x 10{sup -4} cm{sup -3} out to distances of at least 70 kpc. This is broadly consistent with theoretical models of the diffuse gas in a Milky Way-like halo and is consistent with this component hosting a large fraction of a galaxy's baryons. Accounting for completeness in the dwarf galaxy count, gasless dwarf galaxies could have provided at most 2.1 x 10{sup 8} M {sub sun} of H I gas to the Milky Way, which suggests that most of our Galaxy's star formation fuel does not come from accreted small satellites in the current era.

  5. Study of the nature of dark matter in halos of dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Karmakar, Pradip; Chattopadhyay, Tanuka; Chattopadhyay, Asis Kumar

    2015-08-01

    The kinematics of dwarf galaxies are strongly influenced by dark matter down to small galactocentric radii. So they are good candidates to investigate the nature of Dark Matter. In the present work we have carried out mass modeling of a number of recently observed dwarf galaxies Swaters et al. in Astron. Astrophys. 493:871, 2009. We have used a Navarro-Frenk-White (NFW) halo, Freeman disc along with a gaseous disc for modeling the observed rotation curves of those dwarf galaxies. For comparison we also used a Burkert halo, Freeman disc and gaseous disc. For both the scenario we have performed Kolmogorov-Smirnov (KS) test between the observed and predicted rotational velocity profiles. The tests are rejected for NFW halo almost in 50 per cent cases but they are accepted almost for all cases for Burkert halo, preferring a Burkert halo model generally for dwarf galaxies. The above results reveal a constant density core of dark matter (DM) in the halos of dwarf galaxies compared to a cuspy nature of NFW halo and a possible challenge to -CDM scenario for the nature of dark matter in most of the dwarf galaxies.

  6. The H I chronicles of LITTLE THINGS blue compact dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Ashley, Trisha Lynn

    Star formation occurs when the gas (mostly atomic hydrogen; H I) in a galaxy becomes disturbed, forming regions of high density gas, which then collapses to form stars. In dwarf galaxies it is still uncertain which processes contribute to star formation and how much they contribute to star formation. Blue compact dwarf (BCD) galaxies are low mass, low shear, gas rich galaxies that have high star formation rates when compared to other dwarf galaxies. What triggers the dense burst of star formation in BCDs but not other dwarfs is not well understood. It is often suggested that BCDs may have their starburst triggered by gravitational interactions with other galaxies, dwarf-dwarf galaxy mergers, or consumption of intergalactic gas. However, there are BCDs that appear isolated with respect to other galaxies, making an external disturbance unlikely. Here, I study six apparently isolated BCDs from the LITTLE THINGS sample in an attempt to understand what has triggered their burst of star formation. LITTLE THINGS is an H I survey of 41 dwarf galaxies. Each galaxy has high angular and velocity resolution H I data from the Very Large Array (VLA) telescope and ancillary stellar data. I use these data to study the detailed morphology and kinematics of each galaxy, looking for signatures of starburst triggers. In addition to the VLA data, I have collected Green Bank Telescope data for the six BCDs. These high sensitivity, low resolution data are used to search the surrounding area of each galaxy for extended emission and possible nearby companion galaxies. The VLA data show evidence that each BCD has likely experienced some form of external disturbance despite their apparent isolation. These external disturbances potentially seen in the sample include: ongoing/advanced dwarf-dwarf mergers, an interaction with an unknown external object, and external gas consumption. The GBT data result in no nearby, separate H I companions at the sensitivity of the data. These data therefore

  7. 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

  8. Variable Stars in the Sextans Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Mateo, Mario; Fischer, Philippe; Krzeminski, Wojtek

    1995-11-01

    We describe a survey for variable stars in the Sextans dwarf spheroidal (dSph) galaxy based on the analysis of 113 B and 48 V CCD images of four fields covering a total area of 18' x 18'. We have identified 44 variables: 36 RR Lyr star, 6 anomalous Cepheids, one long-period red variable, all probable members of Sextans, and one foreground contact binary. We have used the pulsating stars to derive a true distance modulus of 19.67 +/- 0.15 for Sextans (or D = 86 +/-6 kpc), where the error is primarily due to uncertainties in the luminosity-metallicity relation for RR Lyr stars. Based on our new data we conclude that [Fe/H]_Sex_ = -1.6 +/- 0.2, somewhat higher than the value from Suntzeff et al. (ApJ, 418,208(1993)] obtained from the analysis of fiber spectroscopy of the near-IR Calcium triplet. We present a new deep color- magnitude diagram for Sextans which reveals the presence of a metal-poor population containing stars as young as 2-4 Gyr, consistent with the presence of anomalous Cepheids in the galaxy. This young population may represent as much as 25% of the total stellar content of Sextans. We find a surprisingly strong correlation between the frequency of anomalous Cepheids in dSph galaxies and galaxian luminosity and speculate on the possible origin of this strange effect. The RR Lyr stars in Sextans do not exhibit the Oosterhoff dichotomy observed in globular clusters and in the Galactic halo field.

  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. CO at Low-metallicity: Molecular Clouds in the dwarf galaxy WLM

    NASA Astrophysics Data System (ADS)

    Hunter, Deidre Ann; Rubio, Monica; Cigan, Phil; Cortes, Juan R.; Elmegreen, Bruce; Brinks, Elias; Simpson, Caroline E.; Young, Lisa

    2015-01-01

    Metallicity is not a passive result of galaxy evolution, but a crucial driver. Dwarf galaxies are low in heavy elements, which has important consequences for the ability to form cold, dense clouds that form stars. Molecular cores shrink and atomic envelopes grow in star-forming clouds as the metallicity drops. We are testing this picture of changing structure with metallicity with Herschel [CII]158 micron images of the photo-dissociation regions and ALMA maps of CO in star-forming regions in 4 dwarf irregular galaxies. These galaxies cover a range in metallicity from 13% solar to 5% solar. Here we report on the structure of the molecular clouds in WLM, a dwarf galaxy at 13% solar abundance where we for the first time detected CO emission at such a low heavy element abundance.The Herschel part of this work was supported by grant RSA #1433776 from JPL.

  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. Dark influences II. Gas and star formation in minor mergers of dwarf galaxies with dark satellites

    NASA Astrophysics Data System (ADS)

    Starkenburg, T. K.; Helmi, A.; Sales, L. V.

    2016-03-01

    Context. It has been proposed that mergers induce starbursts and lead to important morphological changes in galaxies. Most studies so far have focused on large galaxies, but dwarfs might also experience such events, since the halo mass function is scale-free in the concordance cosmological model. Notably, because of their low mass, most of their interactions will be with dark satellites. Aims: In this paper we follow the evolution of gas-rich disky dwarf galaxies as they experience a minor merger with a dark satellite. We aim to characterize the effects of such an interaction on the dwarf's star formation, morphology, and kinematical properties. Methods: We performed a suite of carefully set-up hydrodynamical simulations of dwarf galaxies that include dark matter, gas, and stars merging with a satellite consisting solely of dark matter. For the host system we vary the gas fraction, disk size and thickness, halo mass, and concentration, while we explore different masses, concentrations, and orbits for the satellite. Results: We find that the interactions cause strong starbursts of both short and long duration in the dwarfs. Their star formation rates increase by factors of a few to 10 or more. They are strongest for systems with extended gas disks and high gas fractions merging with a high-concentration satellite on a planar, radial orbit. In contrast to analogous simulations of Milky Way-mass galaxies, many of the systems experience strong morphological changes and become spheroidal even in the presence of significant amounts of gas. Conclusions: The simulated systems compare remarkably well with the observational properties of a large selection of irregular dwarf galaxies and blue compact dwarfs. This implies that mergers with dark satellites might well be happening but not be fully evident, and may thus play a role in the diversity of the dwarf galaxy population.

  14. 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.

  15. Young tidal dwarf galaxies cannot be used to probe dark matter in galaxies

    NASA Astrophysics Data System (ADS)

    Flores, H.; Hammer, F.; Fouquet, S.; Puech, M.; Kroupa, P.; Yang, Y.; Pawlowski, M.

    2016-03-01

    The location of dark-matter free, tidal dwarf galaxies (TDGs) in the baryonic Tully-Fisher (bTF) diagram has been used to test cosmological scenarios, leading to various and controversial results. Using new high-resolution 3D spectroscopic data, we re-investigate the morpho-kinematics of these galaxies to verify whether or not they can be used for such a purpose. We find that the three observed TDGs are kinematically not virialized and show complex morphologies and kinematics, leading to considerable uncertainties about their intrinsic rotation velocities and their locations on the bTF. Only one TDG can be identify as a (perturbed) rotation disc that it is indeed a sub-component of NGC5291N and that lies at <1σ from the local bTF relation. It results that the presently studied TDGs are young, dynamically forming objects, which are not enough virialized to robustly challenge cosmological scenarios.

  16. 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.

  17. 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.

  18. Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Kormendy, John; Freeman, K. C.

    2015-04-01

    Dark matter (DM) halos of Sc-Im galaxies satisfy structural scaling laws analogous to the fundamental plane relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii rc , higher central densities ρ^, and smaller central velocity dispersions σ. If dwarf spheroidal (dSph) and dwarf Magellanic irregular (dIm) galaxies lie on the extrapolations of these correlations, then we can estimate their baryon loss relative to that of Sc-Im galaxies. We find that, if there had been no enhanced baryon loss relative to Sc-Im galaxies, typical dSph and dIm galaxies would be brighter by ΔMB ~= -4.0 mag and ΔMB ~= -3.5 mag, respectively. Instead, the galaxies lost or retained as gas (in dIm galaxies) baryons that could have formed stars. Also, dSph and dIm galaxies have DM halos that are more massive than we thought, with σ ~ 30 km s-1 or circular-orbit rotation velocities V circ ~ 42 km s-1. Comparison of DM and visible matter parameter correlations confirms that, at MV >~ -18, dSph and dIm galaxies form a sequence of decreasing baryon-to-DM mass ratios in smaller dwarfs. We show explicitly that galaxy baryon content goes to (almost) zero at V circ <~ 42 +/- 4 km s-1, in agreement with V circ as found from our estimate of baryon depletion. Our results suggest that there may be a large population of DM halos that are dark and undiscovered. This helps to solve the problem that the initial fluctuation spectrum of cold dark matter predicts more dwarf galaxies than we observe.

  19. Spectrophotometric Investigation of a Sample of Tidal Dwarf Galaxies (Oral Contribution)

    NASA Astrophysics Data System (ADS)

    Weilbacher, P. M.; Duc, P.-A.

    We define a Tidal Dwarf Galaxy (TDG) as a self-gravitating entity of dwarf-galaxy mass built from tidal material expelled during interactions. We then summarize our findings on broad-band imaging and spectroscopy of a sample of TDG candidates in a sequence of interacting systems. Evidence for decoupled kinematics in the ionized gas have been found in several objects. This could indicate that they are bound galaxies and therefore genuine TDGs. As a detailed example we analyze the system AM 1159-530, where surprisingly high velocity gradients have been measured.

  20. 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.

  1. M dwarfs, microlensing, and the mass budget of the Galaxy

    NASA Technical Reports Server (NTRS)

    Bahcall, John N.; Flynn, Chris; Gould, Andrew; Kirhakos, Sofia

    1994-01-01

    We show that faint red stars do not contribute significantly to the mass budget of the Galaxy or to microlensing statistics. Our results are obtained by analyzing two long exposures of a high-latitude field taken with the Wide Field Camera (WFC) on the newly repaired Hubble Space Telescope (HST). Stars are easily distinguished from galaxies essentially to the limiting magnitudes of the images. We find five stars with 2.0 less than V - I less than 3.0 and I less than 25.3 and no stars with V - I greater than 3.0. Therefore, main-sequence stars with M(sub I) greater than 10 that are above the hydrogen-burning limit in the dark halo or the spheroid contribute less than 6% of the unseen matter. Faint red disk stars, M-dwarfs, contribute at most 15% to the mass of the disk. We parameterize the faint end of the cumulative distribution of stars, Phi, as a function of luminosity L(sub V), d Phi/d ln L(sub V) proportional to L(sub V exp -gamma). For spheroid stars, gamma less than 0.32 over the range 6 less than M(sub V) less than 17, with 98% confidence. The disk luminosity function falls, gamma less than 0, for 15 approximately less than M(sub V) approximately less than 19. Faint red stars in the disk or thick disk, and stars with M(sub V) less than 16 in the spheroid contribute tau less than 10(exp -8) to the optical depth to microlensing toward the Large Magellanic Cloud.

  2. Disk dwarf galaxy as the progenitor of the Andromeda giant stream

    NASA Astrophysics Data System (ADS)

    Kirihara, Takanobu; Miki, Yohei; Mori, Masao; Kawaguchi, Toshihiro

    2016-08-01

    We present a study of the morphology of a progenitor galaxy that has been disrupted and formed a giant southern stellar stream in the halo of Andromeda galaxy(M31). N-body simulations of a minor merger of M31 with a dwarf galaxy suggest that the progenitor's rotation plays an important role in the formation of an asymmetric surface brightness distribution of the stream.

  3. 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.

  4. Stellar Populations and Chemical Evolution of Late-Type Dwarf Galaxies (Invited Talk)

    NASA Astrophysics Data System (ADS)

    Tosi, M.

    Some aspects of the chemical evolution of late-type dwarf galaxies are reviewed, together with their implications on three issues of cosmological relevance: similarity to primeval galaxies, derivation of the primordial helium abundance, contribution to the excess of faint blue galaxies. A more detailed approach to model their evolution is suggested. The importance of deriving the star formation history in these systems by studying their resolved stellar populations is emphasized.

  5. A Chemical Evolution Model for the Fornax Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Yuan, Zhen; Qian, Yong-Zhong; Jing, Yi Peng

    2016-02-01

    Fornax is the brightest Milky Way (MW) dwarf spheroidal galaxy and its star formation history (SFH) has been derived from observations. We estimate the time evolution of its gas mass and net inflow and outflow rates from the SFH usinga simple star formation law that relates the star formation rate to the gas mass. We present a chemical evolution model on a 2D mass grid with supernovae (SNe) as sources of metal enrichment. We find that a key parameter controlling the enrichment is the mass Mx of the gas to mix with the ejecta from each SN. The choice of Mx depends on the evolution of SN remnants and on the global gas dynamics. It differs between the two types of SNe involved and between the periods before and after Fornax became an MW satellite at time t = tsat. Our results indicate that due to the global gas outflow at t > tsat, part of the ejecta from each SN may directly escape from Fornax. Sample results from our model are presented and compared with data.

  6. 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).

  7. Searching for Tidal Tails in Galactic Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Martínez-Delgado, David; Aparicio, A.; Gomez-Flechoso, Maria A.

    The formation of the Galactic halo is currently best explained by the combination of two scenarios which previously were regarded as competing models. Based on the kinematics of metal-poor halo field stars, Eggen, Lynden-Bell & Sandage (ELS, 1962) proposed that the halo formed during a rapid, smooth collapse from a homogeneous primordial medium. Searle & Zinn (SZ, 1978) argued a halo formation via the gradual merging of many sub-galactic fragments. The SZ scenario has been strengthened by the observational evidence accumulated during the past decade. The discovery of the Sagittarius dwarf galaxy (Ibata, Gilmore & Irwin 1994), in a process of dissolving into the Galactic halo, argued in favour that accretion events can take place in the Milky Way. The possibility that accretion events may leave observable fossil records in the halo is also supported by theoretical models of tidally disrupted dSph satellites (Johnston, Spergel & Hernquist 1995; Oh, Lin & Aarseth 1995; Piatek & Pryor 1995). We present our preliminary results of a long-term project to investigate the process of accretion and tidal disruption of dSph satellites in the Galactic halo and, in particular, to search for new tidal tails in a sample of nearby dSph satellites of the Milky Way. The presence of a possible tidal debris in Ursa Minor and Sculptor dSphs and the results of our survey for a tidal extension along the NW semimajor axis of Saggitarius is discussed.

  8. On the r-process Enrichment of Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Bramante, Joseph; Linden, Tim

    2016-07-01

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

  9. 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.

  10. DWARFS GOBBLING DWARFS: A STELLAR TIDAL STREAM AROUND NGC 4449 AND HIERARCHICAL GALAXY FORMATION ON SMALL SCALES

    SciTech Connect

    Martinez-Delgado, David; Rix, Hans-Walter; Maccio, Andrea V.; Romanowsky, Aaron J.; Arnold, Jacob A.; Brodie, Jean P.; Annibali, Francesca; Fliri, Juergen; Zibetti, Stefano; Van der Marel, Roeland P.; Aloisi, Alessandra; Chonis, Taylor S.; Carballo-Bello, Julio A.; Gallego-Laborda, J.; Merrifield, Michael R.

    2012-04-01

    A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the BlackBird 0.5 m telescope, and high-resolution wide-field images from the 8 m Subaru Telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The stellar mass ratio between the two galaxies is {approx}1:50, while the indirectly measured dynamical mass ratio, when including dark matter, may be {approx}1:10-1:5. This system may thus represent a 'stealth' merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

  11. 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. PMID:26354481

  12. Searching for Dwarf Spheroidal Galaxies with DES and the Fermi-LAT

    NASA Astrophysics Data System (ADS)

    Drlica-Wagner, Alex; DES Collaboration, Fermi-LAT Collaboration

    2016-01-01

    The population of Milky Way satellite galaxies includes the least luminous, least chemically evolved, and most dark matter dominated galaxies in the known universe. Due to their proximity, high dark matter content, and lack of astrophysical backgrounds, dwarf spheroidal galaxies are promising targets for the indirect detection of dark matter via gamma rays. Prior to 2015, roughly two dozen dwarf spheroidal galaxies were known to surround the Milky Way. From combined observations of these objects, the dark matter annihilation cross section has been constrained to be less than the generic thermal relic cross section for dark matter particles with mass < 100 GeV. Since the beginning of 2015, new optical imaging surveys have discovered over twenty new dwarf galaxy candidates, potentially doubling the population of Milky Way satellite galaxies in a single year. I will discuss recent optical searches for dwarf galaxies, focusing specifically on results from the Dark Energy Survey (DES) and the implications for gamma-ray searches for dark matter annihilation with the Fermi Large Area Telescope.

  13. 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.

  14. 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.

  15. Mass assembly history and infall time of the Fornax dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Wang, Mei-Yu; Strigari, Louis E.; Lovell, Mark R.; Frenk, Carlos S.; Zentner, Andrew R.

    2016-04-01

    We use cosmological simulations to identify dark matter subhalo host candidates of the Fornax dwarf spheroidal galaxy using the stellar kinematic properties of Fornax. We consider cold dark matter (CDM), warm dark matter (WDM), and decaying dark matter (DDM) simulations for our models of structure formation. The subhalo candidates in CDM typically have smaller mass and higher concentrations at z = 0 than the corresponding candidates in WDM and DDM. We examine the formation histories of the ˜100 Fornax candidate subhaloes identified in CDM simulations and, using approximate luminosity-mass relationships for subhaloes, we find two of these subhaloes that are consistent with both the Fornax luminosity and kinematics. These two subhaloes have a peak mass over 10 times larger than their z = 0 mass. We suggest that in CDM the dark matter halo hosting Fornax must have been severely stripped of mass and that it had an infall time into the Milky Way of ˜9 Gyr ago. In WDM, we find that candidate subhaloes consistent with the properties of Fornax have a similar infall time and a similar degree of mass-loss, while in DDM we find a later infall time of ˜3-4 Gyr ago and significantly less mass-loss. We discuss these results in the context of the Fornax star formation history, and show that these predicted subhalo infall times can be linked to different star formation quenching mechanisms. This emphasizes the links between the properties of the dark matter and the mechanisms that drive galaxy evolution.

  16. 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.

  17. Anomalous Evolution of the Dwarf Galaxy HIPASS J1321-31

    NASA Astrophysics Data System (ADS)

    Pritzl, Barton J.; Knezek, Patricia M.; Gallagher, John S., III; Grossi, Marco; Disney, Mike J.; Minchin, Robert F.; Freeman, Kenneth C.; Tolstoy, Eline; Saha, A.

    2003-10-01

    We present Hubble Space Telescope/WFPC2 observations of the dwarf galaxy HIPASS J1321-31. This unusual galaxy lies in the direction of the Centaurus A group of galaxies and has a color-magnitude diagram with a distinctive red plume of luminous stars. This feature could arise from (1) a red giant branch if the galaxy were much nearer than previously recognized, (2) a peculiar asymptotic giant branch, or (3) an ~1 Gyr old population of intermediate-mass red supergiants, which we find to be the most likely explanation. However, the lack of equally luminous blue stars requires that the star formation has dropped substantially since these stars were formed. Evidently HIPASS J1321-31 experienced an episode of enhanced star formation rather recently in its star formation history followed by a period of relative quiescence that has led to the evolution of the main-sequence stars into the red supergiant branch. The stellar populations in HIPASS J1321-31 reflect a star formation history that is uncommon in star-forming dwarf galaxies. This is the first time such a star formation history has been noted, although the literature contains a small number of other dwarf galaxies with similar color-magnitude diagrams. Therefore, HIPASS J1321-31 and these other galaxies represent a different path of dwarf galaxy evolution that has not been well explored and an important probe into how dwarf galaxies evolve. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.

  18. 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).

  19. The same with less: the cosmic web of warm versus cold dark matter dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Reed, Darren S.; Schneider, Aurel; Smith, Robert E.; Potter, Doug; Stadel, Joachim; Moore, Ben

    2015-08-01

    We explore fundamental properties of the distribution of low-mass dark matter haloes within the cosmic web using warm dark matter (WDM) and cold dark matter (CDM) cosmological simulations. Using self-abundance-matched mock galaxy catalogues, we show that the distribution of dwarf galaxies in a WDM universe, wherein low-mass halo formation is heavily suppressed, is nearly indistinguishable to that of a CDM universe whose low-mass haloes are not seen because galaxy formation is suppressed below some threshold halo mass. However, if the scatter between dwarf galaxy luminosity and halo properties is large enough, low-mass CDM haloes would sometimes host relatively bright galaxies thereby populating CDM voids with the occasional isolated galaxy and reducing the numbers of completely empty voids. Otherwise, without high mass to light scatter, all mock galaxy clustering statistics that we consider - the auto-correlation function, the numbers and radial profiles of satellites, the numbers of isolated galaxies, and the probability distribution function of small voids - are nearly identical in CDM and WDM. WDM voids are neither larger nor emptier than CDM voids, when constructed from abundance-matched halo catalogues. It is thus a challenge to determine whether the CDM problem of the overabundance of small haloes with respect to the number density of observed dwarf galaxies has a cosmological solution or an astrophysical solution. However, some clues about the dark matter particle and the scatter between the properties of dwarf galaxies and their dark matter halo hosts might be found in the cosmic web of galaxies in future surveys of the local volume.

  20. How the First Stars Shaped the Faintest Gas-dominated Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    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.

  1. 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

  2. Gravitational lensing by globular clusters and dwarf galaxies-- the explanation of quasar-galaxy associations

    NASA Astrophysics Data System (ADS)

    Yushchenko, A.; Kim, C.; Sergeev, A.

    2003-04-01

    Quasar-galaxy associations can be explained as gravitational lensing by globular clusters, located in the halos of the foreground galaxies and dwarf galaxies in small groups of galaxies. We propose an observational test for checking this hypothesis. We used the SUPERCOSMOS sky survey to find the overdensities of star-like sources with zero proper motions in the vicinities of the~foreground galaxies from the CfA3 catalog. The results obtained for 19413 galaxies are presented. We show the results of calculations of number densities of star-like sources with zero proper motions in the vicinity of 19413 galaxies. Two different effects can explain the observational data: lensing by globular clusters and lensing by dwarf galaxies. We carried out the CCD 3-color photometry with the 2.0-m telescope of the~Terskol Observatory and the 1.8-m telescope of the Bohyunsan Observatory (South Korea) to select extremely lensed objects around several galaxies for future spectroscopic observations. From ads Wed Jan 12 06:25:17 2005 Return-Path: Received: (from ads@localhost) by head.cfa.harvard.edu (d/w) id j0CBPHjt007159; Wed, 12 Jan 2005 06:25:17 -0500 (EST) Received: from cfa.harvard.edu (cfa.harvard.edu [131.142.10.1]) by head.cfa.harvard.edu (d/w) with ESMTP id j0CBOuKD007095 for ; Wed, 12 Jan 2005 06:24:56 -0500 (EST) Received: from uqbar.mao.kiev.ua (mao.gluk.org [194.183.183.193]) by cfa.harvard.edu (8.12.9-20030924/8.12.9/cfunix Mast-Sol 1.0) with ESMTP id j0CBOgRv026875 for ; Wed, 12 Jan 2005 06:24:51 -0500 (EST) Received: from maoling.mao.kiev.ua (root@maoling.mao.kiev.ua [194.44.216.101]) by uqbar.mao.kiev.ua (8.11.6/8.11.6) with ESMTP id j0CBOdv08381 for ; Wed, 12 Jan 2005 13:24:39 +0200 Received: from maoling.mao.kiev.ua (gallaz@localhost [127.0.0.1]) by maoling.mao.kiev.ua (8.12.3/8.12.3/Debian-7.1) with ESMTP id j0CBObPb014682 for ; Wed, 12 Jan 2005 13:24:37 +0200 Received: (from gallaz

  3. 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

  4. H II Regions and Abundances in the ``Dark Galaxy'' DDO 154 and the Chemical Evolution of Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Kennicutt, Robert C., Jr.; Skillman, Evan D.

    2001-03-01

    We present Hα imaging and optical spectrophotometry of H II regions in the low surface brightness dwarf irregular galaxy DDO 154. The galaxy possesses a very small population of faint discrete H II regions and larger diffuse H II regions and ionized shells. We confirm the very low star formation rate and extremely long gas consumption times reported previously by van Zee, Haynes, & Salzer. The current star formation rate is ~2-4 times lower than its average past rate, confirming the previous characterization of DDO 154 as a ``quiescent'' dwarf irregular galaxy. Spectrophotometry of two of the brightest H II regions yields a relatively low oxygen abundance of 0.055+/-0.008 (O/H)solar, in agreement with the previous determination by van Zee et al., and in accordance with the previously determined metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 0.037+/-0.003, which is marginally higher than the typical value of 0.025 found in low-metallicity blue compact galaxies. Although DDO 154 has been labeled ``the dark galaxy'' and is a prototype for low surface brightness galaxies with large H I content, its chemical abundances are consistent with an average, low-mass, dwarf irregular galaxy. Assuming that the neutral gas is chemically homogeneous, we derive an effective oxygen yield of roughly 50% of the solar value, a value that is close to the theoretically favored values for the true oxygen yield. Thus, it is possible that DDO 154 is evolving nearly as a closed system. On the other hand, if the abundances in the extended H I disk are lower than in the H II regions, the derived value of the effective yield has been artificially inflated, and DDO 154 may have experienced significant loss of metal-enriched gas. Observations reported in this paper were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution.

  5. Properties of Outflows from Dwarf Galaxies: Insights into the Evolution of the Star Formation Rate

    NASA Astrophysics Data System (ADS)

    Martin, C. L.

    1997-12-01

    Stellar winds and supernovae from massive stars have a strong impact on the interstellar medium. In dwarf galaxies, for example, the supernova explosions following a burst of star formation are predicted to drive any remaining interstellar gas out of the galaxy (Larson 1974; Dekel & Silk 1986). Uncertainties about the role of this feedback process -- and related but less violent activity -- in regulating the star formation rate in a galaxy pose a critical problem for theories of galaxy formation and evolution. I will present measurements of disk mass-loss rates in 15 nearby dwarf galaxies, examine the efficiency of mass ejection relative to the star formation rate, and discuss the effect of the halo potential on the fate of the outflowing gas.

  6. Chemistry of Stars in the Sculptor Dwarf Galaxy from VLT-FLAMES

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.; Hill, V.

    The chemical composition of 91 stars in the Sculptor dwarf spheroidal galaxy is presented as determined from spectra taken with the FLAMES multiobject spectrograph in the Medusa mode. The analysis methods are outlined. The [α/Fe] ratios are shown for Mg, Ca, and Ti, and compared with those of Galactic stars. Heavy element abundance ratios (Y, Ba, and Eu) are also presented. Since the Sculptor dwarf galaxy has had a significantly different star formation history and chemical evolution than the Galaxy, then comparison of Sculptor's metal-poor (old) stars to similar metallicity stars in the Galaxy can be used to discuss galaxy formation scenarios, as well as test some of our fundamental assumptions in stellar nucleosynthesis.

  7. 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.

  8. 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

    2011-02-01

    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 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. PMID:21217688

  9. 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.

  10. 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.

  11. 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. PMID:20075915

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

    SciTech Connect

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

    2011-11-20

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

  13. PAndAS' CUBS: DISCOVERY OF TWO NEW DWARF GALAXIES IN THE SURROUNDINGS OF THE ANDROMEDA AND TRIANGULUM GALAXIES

    SciTech Connect

    Martin, Nicolas F.; McConnachie, Alan W.; Irwin, Mike; Chapman, Scott; Widrow, Lawrence M.; Ferguson, Annette M. N.; Ibata, Rodrigo A.; Dubinski, John; Babul, Arif; Navarro, Julio; Fardal, Mark; Lewis, Geraint F.; Rich, R. Michael

    2009-11-01

    We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey, a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam Wide-Field Camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H] = -1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (M{sub V} = -9.9 +- 0.6), albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II, and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (M{sub V} = -6.5 +- 0.8) and lies a lot closer in projection to M33 than it does to M31 (42 versus 224 kpc), suggesting that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20.

  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. 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. PMID:27350244

  16. 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.

  17. The effect of central starbursts on the interstellar medium of dwarf galaxies

    NASA Technical Reports Server (NTRS)

    De Young, David S.; Heckman, Timothy M.

    1994-01-01

    Major starburst events can last tens of millions of years, and in the process they can deposit significant amounts of energy into the surrounding interstellar medium. This energy from supernova and stellar winds imparts enough momentum to the interstellar medium (ISM) that portions of the ISM can become unbound and leave the parent galaxy, taking the metal-enriched stellar debris along. In dwarf galaxies, starbursts can produce enough total energy to unbind most or all of the ambient ISM. Whether this actually occurs is a strong function of the ellipticity of the ISM distribution, with flat disks and spheres being the limiting cases. We calculate whether 'blow out' along the symmetry axis of 'blow away' of the entire ISM occurs during a central starburst in dwarf galaxies as a function of galactic mass, starburst energy, ISM density, and ISM ellipticity. The calculations cover a range of 10(exp 7) to 10(exp 9) solar mass for dwarf galaxies and include 'normal' galaxies of 10(exp 11) solar mass as well. No massive dark matter halos are assumed to be present. We find that for physically reasonable values of total ISM mass and starburst energy a blow out along the symmetry axis occurs in the majority of cases, though a significant fraction of small dwarf galaxies can lose most of their ISM. As no dark matter halos or clumpy ISM distributions are included, it is apparent that the ISM in most dwarf galaxies may be generally resistant to significant disruption by a central starburst event. The effects of this range of behavi or on the metallicities that would be observed in these galaxies is discussed.

  18. 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

  19. Stellar Populations in the Dwarf Spheroidal Galaxy Leo I

    NASA Astrophysics Data System (ADS)

    Caputo, Filippina; Cassisi, Santi; Castellani, Marco; Marconi, Gianni; Santolamazza, Patrizia

    1999-05-01

    We present a detailed study of the color-magnitude diagram (CMD) of the dwarf spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our photometric analysis, confirming previous results on the brighter portion of the CMD, also allow us to obtain an accurate sampling of the stellar populations at the faint magnitudes corresponding to the main sequence. By adopting a homogeneous and consistent theoretical scenario for both hydrogen and central helium-burning evolutionary phases, the various features observed in the CMD are interpreted and reliable estimates for both the distance modulus and the age(s) of the main stellar components of Leo I are derived. In more detail, from the upper luminosity of the red giant branch and the lower luminosity of the subgiant branch we simultaneously constrain the galaxy distance and the age of the oldest stellar population in Leo I. In this way we obtain a distance modulus (m-M)_V=22.00+/-0.15 mag and an age of 10-15 Gyr or 9-13 Gyr, adopting a metallicity of Z=0.0001 or 0.0004, respectively. The reliability of this distance modulus has been tested by comparing the observed distribution of the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted boundaries of the instability strip as given by convective pulsating models. A detailed investigation of the age(s) of the Leo I stellar populations is then performed by comparing the CMD with a suitable set of theoretical isochrones and central helium-burning models. By taking into account all the various features, including the lack of RR Lyrae variables, we conclude that the star formation process in Leo I started ~10 Gyr (with Z=0.0001) or ~13 Gyr (with Z=0.0004) ago, and stopped about 1 Gyr ago. Some evidence is reported supporting the mild metal deficiency (Z=0.0004), whereas no clear indication has been found supporting a star formation history characterized by episodic bursts. The adoption of updated physics, including the inward diffusion of

  20. H I properties of dwarf irregular galaxies in the Virgo Cluster

    NASA Technical Reports Server (NTRS)

    Hoffman, G. L.; Helou, G.; Salpeter, E. E.; Sandage, A.

    1985-01-01

    A neutral hydrogen survey was carried out at Arecibo on 91 dwarf irregular galaxies, with morphological types Sdm through Im, in and around the direction of the Virgo Cluster. Only nine of these were found to be background galaxies, and 19 remain undetected, i.e., most of the candidate galaxies are indeed members of the Virgo Cluster or Supercluster. The distribution of positions and systemic velocities (compared with large spirals) shows no evidence for mass segregation. The H I depletion for dwarfs in the cluster core is only moderate, no more than for spirals. The magnitude-velocity width correlation is continous from spirls to dwarfs. Satistics on H I masses agree only partially with a simple stochastic star formation model.

  1. Stable State Simulations of Andromeda Dwarf Spheroidal Satellite Galaxies Using MOND

    NASA Astrophysics Data System (ADS)

    Walentosky, Matthew; Blankartz, Benjamin; Alexander, Stephen; Messinger, Justin; Staron, Alex

    2016-01-01

    We present the results of numerical simulations of the stable state condition of several dwarf spheroidal galaxies orbiting the Andromeda galaxy. Using Modified Newtonian Dynamics, we calculate the motion of ten thousand stars in a spherically symmetric Hernquist potential to obtain both the line of sight bulk velocity dispersion and the dispersion profile, i.e. the velocity dispersion as a function of distance from the galactic center. Our results for the bulk dispersion show excellent agreement with observed values and previously published theoretical results and provide reliable estimates of the mass to luminosity ratio. We predict relatively flat radial dispersion profiles for several of the Andromeda dwarf spheroidal galaxies that are similar to those measured for the Milky Way dwarf spheroidals .

  2. Faint Companions in the Close Environment of Starforming Dwarf Galaxies: possible overlooked starburst triggers? (Oral Contribution)

    NASA Astrophysics Data System (ADS)

    Noeske, K. G.; Iglesias-Páramo, J.; Vílchez, J. M.; Papaderos, P.; Fricke, K. J.

    Using the NASA Extragalactic Database, we have searched the close environment of 98 star-forming dwarf galaxies (SFDGs) from field- and low density environments for companion galaxies. Most of the found companions are dwarf galaxies, previously disregarded in environmental studies of SFDGs. Using a subsample at low redshifts, cz < 2000 km/s, i.e. less biased against dwarf companions, we find that 30% of the SFDGs have close companions within a projected linear separation s_p < 100 kpc and a redshift difference of (Delta cz) < 500 km/s. This fraction must be considered a lower limit, given the incompleteness of the available data sets and the non-negligible frequency of HI clouds in the vicinity of SFDGs, so that the majority of SFDGs should not be considered isolated. The redshift differences between companion candidates and sample SFDGs are typically smaller than ~250 km/s and concentrated towards lower values. This is similarly observed for dwarf satellites of spiral galaxies and suggests a physical association between the companion candidates and the sample SFDGs. SFDGs with a close companion do not show significant differences in their H(beta) equivalent widths and B-V colours as compared to isolated ones. However, the available data do not allow to rule out close dwarf companions as an influencing factor for star formation activity.

  3. 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. PMID:21635025

  4. 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.

  5. 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

  6. Photometric Calibration of DECam Images of the Sextans Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Howard, Brittany; Vivas, Kathy

    2016-01-01

    As part of an ongoing study on the variable star population of the Sextans Dwarf Spheroidal Galaxy, we present here details on the photometric calibration of the data, which were obtained with the Dark Energy Camera (DECam) at the Blanco 4mTelescope at the Cerro Tololo Interamerican Observatory. Since DECam is a relatively new instrument, we tested different calibration strategies including calibrating each chip individually and all together. Our results indicate that the color terms and zero points are constant across the camera, at least in the g, r and i bands. We present preliminary results on the location of variable stars in the Sextans dwarf galaxy.

  7. Spatially resolved Lyman-alpha emission from a virtual dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Verhamme, Anne

    2015-02-01

    In the context of the first light of MUSE, Integral Field Unit (IFU) spectrograph of second generation installed recently at VLT, we compute mock IFU Lyman-alpha (lyα) observations of a virtual dwarf galaxy, to help understanding and interpreting forthcoming observations. This study is an extension of the work carried out in Verhamme et al. (2012), where we studied the spatially integrated lyα properties of a dwarf galaxy. With the same data, we now investigate the spatial variations of lyα spectra.

  8. 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.

  9. Dwarf galaxies in the Coma cluster - II. Spectroscopic and photometric fundamental planes

    NASA Astrophysics Data System (ADS)

    Kourkchi, E.; Khosroshahi, H. G.; Carter, D.; Mobasher, B.

    2012-03-01

    We present a study of the Fundamental Plane (FP) for a sample of 71 dwarf galaxies in the core of the Coma cluster in the magnitude range -21 < MI < -15. Taking advantage of the high-resolution DEIMOS spectrograph on Keck II for measuring the internal velocity dispersion of galaxies and high-resolution imaging of the Hubble Space Telescope (HST)/ACS, which allows an accurate surface brightness modelling, we extend the FP of galaxies to luminosities of ˜1 mag fainter than all the previous studies of the FP in the Coma cluster. We find that the scatter about the FP depends on the faint-end luminosity cut-off, such that the scatter increases for fainter galaxies. The residual from the FP correlates with the galaxy colour, with bluer galaxies showing larger residuals from the FP. We find M/L ∝ M-0.15±0.22 in the F814W band, indicating that in faint dwarf ellipticals, the M/L ratio is insensitive to the mass. We find that less massive dwarf ellipticals are bluer than their brighter counterparts, possibly indicating ongoing star formation activity. Although tidal encounters and harassment can play a part in removing stars and dark matter from the galaxy, we believe that the dominant effect will be the stellar wind associated with the star formation, which will remove material from the galaxy, resulting in larger M/L ratios. We attribute the deviation of a number of faint blue dwarfs from the FP of brighter ellipticals to this effect. We also study other scaling relations involving galaxy photometric properties including the Photometric Plane. We show that compared to the FP, the scatter about the Photometric Plane is smaller at the faint end.

  10. Globular Clusters, Dwarf Galaxies, and the Assembly of the M87 Halo

    NASA Astrophysics Data System (ADS)

    Peng, Eric W.; Zhang, Hong-Xin; Liu, Chengze; Liu, Yiqing

    2016-08-01

    At the center of the nearest galaxy cluster, the Virgo cluster, lies the massive cD galaxy, M87 (NGC 4486). Using data from the Next Generation Virgo Cluster Survey, we investigate the relationship between M87, its globular clusters (GCs), and satellite dwarf galaxies. We find that the kinematics of GCs and ultra-compact dwarfs (UCDs) are different, indicating that UCDs are not simply massive GCs. We also identify a morphological sequence of envelope fraction around UCDs correlated with cluster-centric distance that suggest UCDs are the result of tidal stripping. Lastly, we find that the [α/Fe] abundance ratios of low-mass early-type galaxies in Virgo exhibit a strong negative gradient within ~ 400 kpc of M87, where the galaxies closest to M87 have the highest values. These satellite galaxies are likely the surviving counterparts of accreted dwarfs that contribute stars to the metal-poor, α-rich stellar halos of massive galaxies. Together, these results describe a dense environment that has had a strong and continuing impact on the evolution of its low-mass neighbors.

  11. 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.

  12. FCC046: A CANDIDATE GASEOUS POLAR RING DWARF ELLIPTICAL GALAXY IN THE FORNAX CLUSTER

    SciTech Connect

    De Rijcke, S.; Buyle, P.; Koleva, M.

    2013-06-20

    FCC046 is a Fornax Cluster dwarf elliptical galaxy. Optical observations have shown that this galaxy, besides an old and metal-poor stellar population, also contains a very young centrally concentrated population and is actively forming stars, albeit at a very low level. Here, we report on 21 cm observations of FCC046 with the Australia Telescope Compact Array which we conducted in the course of a small survey of Fornax Cluster early-type dwarf galaxies. We have discovered a {approx}10{sup 7} M{sub Sun} H I cloud surrounding FCC046. We show that the presence of this significant gas reservoir offers a concise explanation for this galaxy's optical morphological and kinematical properties. Surprisingly, the H I gas, as evidenced by its morphology and its rotational motion around the galaxy's optical major axis, is kinematically decoupled from the galaxy's stellar body. This is the first time such a ring of gaseous material in minor-axis rotation is discovered around a dwarf galaxy.

  13. Star Formation in Dwarf Galaxies as a Function of Cluster-Centric Radii

    NASA Astrophysics Data System (ADS)

    Rude, Cody; Barkhouse, Wayne

    2015-01-01

    Galaxy clusters form the largest structures in the universe. The cluster galaxy population differs both by morphology and star formation histories relative to the field population. Several physical mechanisms have been proposed to account for these differences, including ram pressure stripping due to the intracluster medium, and harassment from close encounters with other galaxies. Dwarf galaxies could prove to be particularly important as their low mass makes them more susceptible to external influences. This study looks for evidence of enhanced/quenching of star formation in dwarf galaxies using photometric u- and r-band data of several Abell clusters taken with the CFHT. From the combined sample, scaled by r200, composite luminosity functions (LFs) and histograms of galaxy color at various cluster-centric radii are constructed. An increase in the faint-end slope of the u-band LF relative to the r-band is a possible indicator of enhanced star formation. Comparisons of the inner and outer regions of the cluster sample may yield insights into the physical mechanisms that affect star formation of infalling cluster dwarf galaxies.

  14. 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.

  15. 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. PMID

  16. 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.

  17. Comparing the Observable Properties of Dwarf Galaxies on and off the Andromeda Plane

    NASA Astrophysics Data System (ADS)

    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.

    2015-01-01

    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.

  18. Constraints on Environmental and Secular Effects on the Chemodynamical Evolution of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Leaman, Ryan

    This thesis presents observations and analysis relating to the understanding of processes that govern the formation and evolution of low mass galactic systems. In particular we have focused on separating out the contribution to the chemical and dynamical evolution of dwarf galaxies due to solely secular (internal) processes compared to external effects from the local environment a galaxy resides in. Our observational data focus on an extremely isolated dwarf galaxy, WLM, which we demonstrate has had a uniquely quiescent tidal history, thereby making it an excellent test case for such a study. With spectroscopic and photometric observations of the resolved stars and neutral gas in WLM we have been able to characterize the chemical, structural and kinematic properties of this gas rich dwarf galaxy. As WLM has not been subject to strong tidal or ram-pressure stripping of its stellar and gaseous populations, we have been able to compare the dynamical evolution and chemical history of WLM to theoretical models which are environment independent. A differential comparison of WLM to more environmentally processed dwarf galaxies in the Local Group has revealed that WLM's structural and dynamical state is far from the idealized picture of dIrrs as thin gas-rich rotating systems. The stellar component of WLM shows equal parts rotation and dispersion, and both the gaseous and stellar structural properties show an intrinsically thick axisymmetric configuration. The time evolution of the random (dispersion) component of the stellar orbital energy shows an increase with stellar age, which we show is consistent with secular processes alone - such as disk heating from giant molecular clouds and dark matter substructure. While the degree to which the thick structural and dynamically hot configuration for WLM is surprising, its chemical properties show remarkably consistent values with other galaxies of the same halo mass. Comparing the spatial chemical trends in WLM with other dwarf

  19. 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.

  20. 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. PMID:24572352

  1. Spectroscopic Studies of Starburst Galaxies; the Dynamical Structure of Blue Compact Dwarf Galaxy Haro 6

    NASA Astrophysics Data System (ADS)

    Chun, Mun-Suk; Moon, Honh-Kyu; Sung, Eon-Chang

    1995-06-01

    We carried out photometric and spectroscopic observations of the blue compact dwarf galaxy Haro 6 in the Virgo Cluster of Galaxies. The long-slit spectroscopy was employed at three position angles, ¥Õ = 0¡Æ, ¥Õ = 30¡Æ, and ¥Õ = 120¡Æwith CCD camera mounted on the Cassegrain Spectrograph. Based on the mean intrinsic axial ratio q0=0.3, we derived inclination i of the system as 44¡Æusing our composite V-band CCD image. Careful analysis on the velocity field of the system chows an asymptotically flat rotation curve with the maximum rotational velocity V(r)max reaches about 12 km/sec. The calculation of the dynamical mass of Haro 6 with a simple mass model is briefly discussed with emphasis on the mass to luminosity ratio. From the IRAS Point Source Catalogue, we derived dust-to-gas ratio which indicates relatively low dust content, thus tempting us to conjecture the youth of the system.

  2. The Holistic Evolution of Dwarf Galaxies: Internal and External Processes in NGC 6822

    NASA Astrophysics Data System (ADS)

    Cannon, John

    2010-09-01

    Low-mass galaxy evolution is driven by both internal {e.g., star formation and feedback} and external {e.g., tidal interaction} processes; however, few nearby systems show evidence of both mechanisms operating in tandem. The Local Group dwarf galaxy NGC 6822 presents a unique opportunity to study important evolutionary processes because of proximity and timing: it harbors one of the largest known holes in its neutral ISM {a signpost of violent stellar feedback}, and it appears to be undergoing a tidal interaction with a very low mass companion dwarf galaxy. To capitalize on this opportunity, we propose to undertake an ACS and WFC3 imaging study of the stellar populations throughout NGC 6822. We will study the role of both internal and external processes by sampling the stellar populations associated with the main body {using archival WFPC2 data}, the giant HI hole, the companion dwarf galaxy, and the tidal material at the ends of the disk. From these data we will extract precise color magnitude diagrams; we will measure the full 13 Gyr star formation history of each field, with high time resolution {20-250 Myr} over the past 1 Gyr, and quantify the relative contributions of internal and external drivers of dwarf galaxy evolution. Specifically, we will study: 1} the nature of the companion object {differentiating between infalling gas and a genuine low-mass galaxy}; 2} the energetics of giant HI hole creation; 3} the role of interactions in the system's evolution; 4} the nature of feedback in governing the star formation process; and 5} the patterns of star formation over time. We have optimized our field placements to extract the most information possible about the competing forces that drive galaxy evolution; in only six orbits, we will gain fundamental insights into the contributions of both internal and external processes that bear on the evolution of low-mass galaxies.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. HST Imaging of the Local Volume Dwarf Galaxies Pisces A and B: Prototypes for Local Group Dwarfs

    NASA Astrophysics Data System (ADS)

    Tollerud, Erik J.; Geha, Marla C.; Grcevich, Jana; Putman, Mary E.; Weisz, Daniel R.; Dolphin, Andrew E.

    2016-08-01

    We present observations of the Pisces A and B galaxies with the Advanced Camera for Surveys on the Hubble Space Telescope. Photometry from these images clearly resolves a red giant branch (RGB) for both objects, demonstrating that they are nearby dwarf galaxies. We describe a Bayesian inferential approach to determining the distance to these galaxies using the magnitude of the tip of the RGB, and then apply this approach to these galaxies. This reveals the distance to these galaxies as {5.64}-0.15+0.13 {{Mpc}} and {8.89}-0.85+0.75 {{Mpc}} for Pisces A and B, respectively, placing both within the Local Volume but not the Local Group (LG). We estimate the star formation histories of these galaxies, which suggests that they have recently undergone an increase in their star formation rates. Together these yield luminosities for Pisces A and B of {M}V=-{11.57}-0.05+0.06 and ‑12.9 ± 0.2, respectively, and estimated stellar masses of {log}({M}* /{M}ȯ )={7.0}-1.7+0.4 and {7.5}-1.8+0.3. We further show that these galaxies are likely at the boundary between nearby voids and higher-density filamentary structure. This suggests that they are entering a higher-density region from voids, where they would have experienced delayed evolution, consistent with their recent increased star formation rates. If this is indeed the case, they are useful for study as proxies of the galaxies that later evolved into typical LG satellite galaxies.

  8. A Universal Mass Profile for Dwarf Spheroidal Galaxies?

    NASA Astrophysics Data System (ADS)

    Walker, Matthew G.; Mateo, Mario; Olszewski, Edward W.; Peñarrubia, Jorge; Wyn Evans, N.; Gilmore, Gerard

    2009-10-01

    We apply the Jeans equation to estimate masses for eight of the brightest dwarf spheroidal (dSph) galaxies. For Fornax, the dSph with the largest kinematic data set, we obtain a model-independent constraint on the maximum circular velocity, V max = 20+4 -3 km s-1. Although we obtain only lower limits of V max >~ 10 km s-1 for the remaining dSphs, we find that in all cases the enclosed mass at the projected half-light radius is well constrained and robust to a wide range of halo models and velocity anisotropies. We derive a simple analytic formula that estimates M(r half) accurately with respect to results from the full Jeans analysis. Applying this formula to the entire population of Local Group dSphs with published kinematic data, we demonstrate a correlation such that M(r half) vprop r 1.4±0.4 half, or in terms of the mean density interior to the half-light radius, langρrang vprop r -1.6±0.4 half. This relation is driven by the fact that the dSph data exhibit a correlation between global velocity dispersion and half-light radius. We argue that tidal forces are unlikely to have introduced this relation, but tides may have increased the scatter and/or altered the slope. While the data are well described by mass profiles ranging over a factor of lsim2 in normalization (V max ~ 10-20 km s-1), we consider the hypothesis that all dSphs are embedded within a "universal" dark matter halo. We show that in addition to the power law M vprop r 1.4, viable candidates include a cuspy "Navarro-Frenk-White" halo with V max ~ 15 km s-1 and scale radius r 0 ~ 800 pc, as well as a cored halo with V max ~ 13 km s-1 and r 0 ~ 150 pc. Finally, assuming that their measured velocity dispersions accurately reflect their masses, the smallest dSphs now allow us to resolve dSph densities at radii as small as a few tens of pc. At these small scales, we find mean densities as large as langρrang <~ 5 M sun pc-3 (lsim200 GeV cm-3). This paper presents data gathered with the Magellan

  9. Machine Learning Identification of Dwarf Galaxy Satellites around Milky Way Analogs

    NASA Astrophysics Data System (ADS)

    Sandford, Emily; Geha, M. C.; Wechsler, R. H.; Tollerud, E. J.; Marshall, P. J.; Cunha, C. E.

    2014-01-01

    The Milky Way galaxy (MW) hosts approximately two dozen known dwarf galaxy satellites, ranging from the bright Magellanic Clouds, to the fainter dwarf spheroidal galaxies, including Leo I and Fornax, to ultra-faint dwarfs. Galaxy formation simulations predict that, down to the luminosity of Fornax, roughly three times more satellites should exist than we observe. However, the MW is a small and possibly biased sample. No MW analog satellite populations have yet been studied to these luminosities, because identifying the satellites around a MW analog requires spectroscopic observations of all objects within the virial radius to distinguish the satellites from the significantly more numerous background galaxies. Here, we apply machine learning techniques, specifically the use of an artificial neural network (ANN), to the problem of identifying satellites around MW analogs based on photometric observables, which are more easily obtained than spectroscopy. The ANN is trained on a set of labeled satellites and background galaxies around ~100 MW-like hosts, then applied to a validation data set to evaluate its performance. This work is carried out in parallel with a long-term spectroscopic observing program of several MW analogs, which will increase the training set size and improve the ANN’s performance.

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

    NASA Astrophysics Data System (ADS)

    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 2.95 σ 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 2.2 σ (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.

  12. Compact Neutral Hydrogen Clouds: Searching for Undiscovered Dwarf Galaxies and Gas Associated with an Algol-type Variable Star

    NASA Astrophysics Data System (ADS)

    Grcevich, Jana; Berger, Sabrina; Putman, Mary E.; Eli Goldston Peek, Joshua

    2016-01-01

    Several interesting compact neutral hydrogen clouds were found in the GALFA-HI (Galactic Arecibo L-Band Feed Array HI) survey which may represent undiscovered dwarf galaxy candidates. The continuation of this search is motivated by successful discoveries of Local Volume dwarfs in the GALFA-HI DR1. We identify additional potential dwarf galaxies from the GALFA-HI DR1 Compact Cloud Catalog which are indentified as having unexpected velocities given their other characteristics via the bayesian analysis software BayesDB. We also present preliminary results of a by-eye search for dwarf galaxies in the GALFA-HI DR2, which provides additional sky coverage. Interestingly, one particularly compact cloud discovered during our dwarf galaxy search is spatially coincident with an Algol-type variable star. Although the association is tentative, Algol-type variables are thought to have undergone significant gas loss and it is possible this gas may be observable in HI.

  13. The dynamical and chemical evolution of dwarf spheroidal galaxies with GEAR

    NASA Astrophysics Data System (ADS)

    Revaz, Y.; Jablonka, P.

    2012-02-01

    We present the fully parallel chemo-dynamical Tree/SPH code GEAR, which allows us to perform high resolution simulations with detailed chemical diagnostics. Starting from the public version of Gadget-2, we included the complex treatment of the baryon physics: gas cooling, star formation law, chemical evolution, and supernova feedback. We qualified the performances of GEAR in the case of dwarf spheroidal galaxies (dSphs) galaxies. Our code GEAR conserves the total energy budget of the systems to better than 5% over 14 Gyr and provides an excellent convergence of the results with numerical resolution. We showed that models of dSphs in a static Euclidean space, where the expansion of the universe is neglected are valid. In addition, we tackled some existing open questions in the field, such as the stellar mass fraction of dSphs and its link to the predicted dark matter halo mass function, the effect of supernova feedback, the spatial distribution of the stellar populations, and the origin of the diversity in star formation histories and chemical abundance patterns. Strong supernova-driven winds seem incompatible with the observed metallicities and luminosities. Despite newly formed stars being preferentially found in the galaxy central parts, turbulent motions in the gas can quickly erase any metallicity gradient. The diversity in properties of dSph are related to a range of total masses, as well as a range of dispersion in the central densities, which is also seen in the halos emerging from a ΛCDM cosmogony. Appendices A and B are available in electronic form at http://www.aanda.org

  14. 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

  15. Delayed Star Formation in Isolated Dwarf galaxies: Hubble Space Telescope Star Formation History of the Aquarius Dwarf Irregular

    NASA Astrophysics Data System (ADS)

    Cole, Andrew A.; Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; McConnachie, Alan W.; Brooks, Alyson M.; Leaman, Ryan

    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 H I /M sstarf, 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations were obtained under program GO

  16. 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

  17. The imprint of reionization on the star formation histories of dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Benítez-Llambay, A.; Navarro, J. F.; Abadi, M. G.; Gottlöber, S.; Yepes, G.; Hoffman, Y.; Steinmetz, M.

    2015-07-01

    We use a compilation of star formation histories (SFHs) and cosmological simulations to explore the impact of cosmic reionization on nearby isolated dwarf galaxies. Nearby dwarfs show a wide diversity of SFHs; from ancient systems that completed their star formation (SF) ˜10 Gyr ago to young dwarfs that formed the majority of their stars in the past ˜5 Gyr to `two-component' systems characterized by the overlap of old and young stars. As an ensemble, SF in nearby dwarfs dips to lower-than-average rates at intermediate times (4 < t/Gyr < 8), a feature caused in the simulation by cosmic reionization. Reionization heats the gas and drives it out of low-mass haloes, affecting especially systems with virial temperatures of ˜2 × 104 K at zreion. SF begins before zreion in systems above this threshold; its associated feedback compounds the effects of reionization, emptying the haloes of gas and leaving behind old stellar systems. In haloes below the threshold at zreion, reionization leads to a delay in the onset of SF that lasts until the halo grows massive enough to allow gas to cool and form stars, leading to a system with a prominent young stellar component. `Two-component' systems may be traced to late accretion events that allow young stars to form in systems slightly above the threshold at zreion. The dearth of intermediate-age stars in nearby dwarfs might be the clearest signature of the imprint of cosmic reionization on the SFHs of dwarf galaxies.

  18. Stellar kinematics and dark matter in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Battaglia, Giuseppina

    2016-08-01

    In this review I will discuss the current status on determinations of the dark matter content and distribution in Milky Way dwarf spheroidals, for which the available data-sets allow the application of sophisticated mass modeling techniques.

  19. 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.

  20. Constraints on MACHO Dark Matter from Compact Stellar Systems in Ultra-faint Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Brandt, Timothy D.

    2016-06-01

    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 10 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.

  1. 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. PMID:25230660

  2. Beacons in the Dark: Using Novae and Supernovae to Detect Dwarf Galaxies in the Local Universe

    NASA Astrophysics Data System (ADS)

    Conroy, Charlie; Bullock, James S.

    2015-05-01

    We propose that luminous transients, including novae and supernovae (SNe), can be used to detect the faintest galaxies in the universe. Beyond a few megaparsecs, dwarf galaxies with stellar masses ≲ {{10}6} {{M}⊙ } will likely be too faint and/or too low in surface brightness to be directly detected in upcoming large area ground-based photometric surveys. However, single-epoch Large Synoptic Survey Telescope photometry will be able to detect novae to distances of ˜30 Mpc and SNe to gigaparsec-scale distances. Depending on the form of the stellar mass-halo mass relation and the underlying star formation histories of low-mass dwarfs, the expected nova rates will be a few to ˜100 yr-1 and the expected SN rates (including both type Ia and core-collapse) will be ˜ {{10}2}-{{10}4} within the observable (4π sr) volume. The transient rate associated with intrahalo stars will be comparably large, but these transients will be located close to bright galaxies, in contrast to the dwarfs, which should trace the underlying large-scale structure of the cosmic web. Aggressive follow-up of hostless transients has the potential to uncover the predicted enormous population of low-mass field dwarf galaxies.

  3. 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

  4. 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.

  5. 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.

  6. A Radio Continuum Study of Dwarf Galaxies: 6 cm imaging of LITTLE THINGS

    NASA Astrophysics Data System (ADS)

    Kitchener, Ben; Brinks, Elias; Heesen, Volker; Hunter, Deidre Ann; Zhang, Hongxin; Rau, Urvashi; Rupen, Michael P.; Little Things Collaboration

    2015-01-01

    To bypass uncertainties introduced by extinction caused by dust at optical wavelengths, we examine to what extent the radio continuum can probe star formation (SF) in dwarf galaxies. We provide VLA 6-cm C-array (4 to 8 GHz) radio continuum images with integrated flux densities for 40 dwarf galaxies taken from LITTLE THINGS. We find 27 harbor significant emission coincident with SF tracers; 17 are new detections. We infer the average thermal fraction to be 39 +- 25%. The LITTLE THINGS galaxies follow the Condon radio continuum - star formation rate (SFR) relation down to an SFR of 0.1 Msol/yr. At lower rates they follow a power-law characterized by a slope of 1.2 +- 0.1 with a scatter of 0.2 dex . We interpret this as an underproduction of the non-thermal radio continuum component. When considering the non-thermal radio continuum to star formation rate slope on its own, we find the slope to be 1.2. The magnetic field strength we find is typically 9.4 +- 3.8 muG in and around star forming regions which is similar to that in spiral galaxies. In a few dwarfs, the magnetic field strength can reach as high as 30 muG in localized 100 pc star forming regions. The underproduction of non-thermal radio continuum is likely due to the escape of Cosmic Ray electrons from the galaxy. The LITTLE THINGS galaxies are consistent with the radio continuum - far infrared luminosity relation. We observe a power-law slope of 1.06 +- 0.08 with a scatter of 0.24 dex which suggests that the 'conspiracy' of the radio continuum - far infrared relation continues to hold even for dwarf galaxies.

  7. Finding Gas-rich Dwarf Galaxies Betrayed by their Ultraviolet Emission

    NASA Astrophysics Data System (ADS)

    Donovan Meyer, Jennifer; Peek, J. E. G.; Putman, Mary; Grcevich, Jana

    2015-08-01

    We present ultraviolet (UV) follow-up of a sample of potential dwarf galaxy candidates selected for their neutral hydrogen (HI) properties, taking advantage of the low UV background seen by the GALEX satellite and its large and publicly available imaging footprint. The HI clouds, which are drawn from published Galactic Arecibo L-band Feed Array and Arecibo Legacy Fast Arecibo L-band Feed Array HI survey compact cloud catalogs, are selected to be galaxy candidates based on their spatial compactness and non-association with known high-velocity cloud complexes or Galactic HI emission. Based on a comparison of their UV characteristics to those of known dwarf galaxies, half (48%) of the compact HI clouds have at least one potential stellar counterpart with UV properties similar to those of nearby dwarf galaxies. If they are galaxies, then the star formation rates, HI masses, and star formation efficiencies of these systems follow the trends seen for much larger galaxies. The presence of UV emission is an efficient method to identify the best targets for spectroscopic follow-up, which is necessary to prove that the stars are associated with compact HI. Furthermore, searches of this nature help to refine the salient HI properties of likely dwarfs (even beyond the Local Group). In particular, HI compact clouds considered to be velocity outliers relative to their neighbor HI clouds have the most significant detection rate of single, appropriate UV counterparts. Correcting for the sky coverage of the two all-Arecibo sky surveys yielding the compact HI clouds, these results may imply the presence of potentially hundreds of new tiny galaxies across the entire sky.

  8. 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.

  9. Families of ellipsoidal stellar systems adn the formation of dwarf elliptical galaxies

    SciTech Connect

    Kormendy, J.

    1985-08-01

    Core radii and central surface brightnesses of bulges and elliptical galaxies are measured using CCD photometry obtained with the Canada-France-Hawaii Telescope (scale = 0''.22 pixel/sup -1/; seeing = 0''.45--1''.0 FWHM). The correlations between core parameters are derived and compared for ellipticals, bulges, dwarf spheroidal galaxies, dwarf irregular galaxies, and globular clusters. The results are as follows. 1. Ihe data confirm the existence of well-defined correlations between the core parameters of elliptical galaxies. More luminous ellipticals have larger core radii r/sub c/ and lower central surface brightnesses ..mu../sub 0v/. Galaxies with larger core radii have larger central velocity dispersions. The small, bright core of M32 is normal for a galaxy of M/sub B/ = -15.2. Radio ellipticals and brightest cluster galaxies satisfy the correlations. 2. The bulges of disk galaxies are basically similar to elliptical galaxies. Their cores have slightly smaller r/sub c/ and brighter ..mu../sub 0v/ than ellipticals of the same luminosity, because their nonisothermal profiles rise more rapidly toward the center and because they often contain extra nuclei superposed on their cores. 3. There is a large discontinuity between the parameter correlations for bright ellipticals, including M32, and those for dwarf spheroidals. Seven dE's in the Local Group and three in the Virgo Cluster have core parameters which are correlated, but not as in ordinary ellipticals. More luminous dE's have larger r/sub c/ and brighter ..mu../sub 0v/.

  10. The same with less: The cosmic web of warm versus cold dark matter dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Reed, Darren; Schneider, Aurel; Smith, Robert E.; Stadel, Joachim; Moore, Ben

    2015-01-01

    We explore fundamental properties of the distribution of low mass dark matter halos within the cosmic web using warm dark matter (WDM) and cold dark matter (CDM) cosmological simulations. Using self abundance-matched mock galaxy catalogs, we show that the distribution of dwarf galaxies in a WDM universe wherein low mass halo formation is heavily suppressed, is nearly indistinguishable to that of a CDM universe whose low mass halos are not seen because galaxy formation is suppressed below some threshold halo mass. However, if the scatter between dwarf galaxy luminosity and halo properties is large enough, low mass CDM halos would sometimes host relatively bright galaxies thereby populating CDM voids with the occasional isolated galaxyand reducing the numbers of completely empty voids. Otherwise, without high mass to light scatter, all mock galaxy clustering statistics that we consider-the auto-correlation function, the numbers and radial profiles of satellites, the numbers of isolated galaxies, and the PDF of small voids-are nearly identical in CDM and WDM. WDM voids are neither larger nor emptier than CDM voids, when constructed from abundance- matched halo catalogs. It is thus a challenge to determine whether the CDM problem of the over-abundance of small halos with respect to the number density of observed dwarf galaxies has a cosmological solution or an astrophysical solution. However, some clues about the dark matter particle and the scatter between the properties of dwarfgalaxies and their dark matter halo hosts might be found in the cosmic web of galaxies in future surveys of the local volume.

  11. Dwarf Galaxies in the Leo I Group: the Group Luminosity Function beyond the Local Group (Oral Contribution)

    NASA Astrophysics Data System (ADS)

    Flint, K.; Bolte, M.; Mendes de Oliveira, C.

    We present first results of a survey of the Leo I group at 10 Mpc for M_R < -10 dwarf galaxies. This is part of a larger program to measure the faint end of the galaxy luminosity function in nearby poor groups. Our method is optimized to find Local-Group-like dwarfs down to dwarf spheroidal surface brightnesses, but we also find very large LSB dwarfs in Leo I with no Local Group counterpart. A preliminary measurement of the luminosity function yields a slope consistent with that measured in the Local Group.

  12. The Most Metal-poor Damped Lyα Systems: An Insight into Dwarf Galaxies at High-redshift

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

    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] sime -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 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. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (VLT program IDs: 60.A-9022(A), 65.O-0063(B), 65.O-0296(A), 67.A-0022(A), 67.A-0078(A), 68.A-0600(A), 68.B-0115(A), 70.A-0425(C), 078.A-0185(A), 080.A-0014(A), 082.A-0544(A), 083.A-0042(A), 083.A-0454(A), 085.A-0109(A), 086.A-0204(A)), and at the

  13. The baryon cycle of dwarf galaxies: dark, bursty, gas-rich polluters

    SciTech Connect

    Shen, Sijing; Madau, Piero; Conroy, Charlie; Governato, Fabio; Mayer, Lucio

    2014-09-10

    We present results from a fully cosmological, very high-resolution, ΛCDM simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M {sub vir} = 4.4 × 10{sup 8}-3.6 × 10{sup 10} M {sub ☉}. The simulation includes a blastwave scheme for supernova feedback, a star-formation recipe based on a high gas density threshold, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, and a uniform UV background. The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well. All three halos with M {sub vir} < 10{sup 9} M {sub ☉} are devoid of stars, as they never reach the density threshold for star formation of 100 atoms cm{sup –3}. The other four, M {sub vir} > 10{sup 9} M {sub ☉} dwarfs have blue colors, low star-formation efficiencies, high cold gas-to-stellar mass ratios, and low stellar metallicities. Their bursty star-formation histories are characterized by peak specific star-formation rates in excess of 50-100 Gyr{sup –1}, far outside the realm of normal, more massive galaxies. The median stellar age of the simulated galaxies decreases with decreasing halo mass, with the two M {sub vir} ≅ 2-3 × 10{sup 9} M {sub ☉} dwarfs being predominantly young, and the two more massive systems hosting intermediate and older populations. The cosmologically young dwarfs are lit up by tidal interactions, have compact morphologies, and have metallicities and cold gas fractions similar to the relatively quiescent, extremely metal-deficient dwarf population. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a megaparsec-size region of the intergalactic medium, but are not sufficient to completely quench star-formation activity and are absent in the faintest dwarfs.

  14. Molecular gas and star formation in the tidal dwarf galaxy VCC 2062

    NASA Astrophysics Data System (ADS)

    Lisenfeld, U.; Braine, J.; Duc, P. A.; Boquien, M.; Brinks, E.; Bournaud, F.; Lelli, F.; Charmandaris, V.

    2016-05-01

    The physical mechanisms driving star formation (SF) in galaxies are still not fully understood. Tidal dwarf galaxies (TDGs), made of gas ejected during galaxy interactions, seem to be devoid of dark matter and have a near-solar metallicity. The latter makes it possible to study molecular gas and its link to SF using standard tracers (CO, dust) in a peculiar environment. We present a detailed study of a nearby TDG in the Virgo Cluster, VCC 2062, using new high-resolution CO(1-0) data from the Plateau de Bure, deep optical imaging from the Next Generation Virgo Cluster Survey (NGVS), and complementary multiwavelength data. Until now, there was some doubt whether VCC 2062 was a true TDG, but the new deep optical images from the NGVS reveal a stellar bridge between VCC 2062 and its parent galaxy, NGC 4694, which is clear proof of its tidal origin. Several high-resolution tracers (Hα, UV, 8 μm, and 24 μm) of the star formation rate (SFR) are compared to the molecular gas distribution as traced by the CO(1-0). Coupled with the SFR tracers, the NGVS data are used with the CIGALE code to model the stellar populations throughout VCC 2062, yielding a declining SFR in the recent past, consistent with the low Hα/UV ratio, and a high burst strength. HI emission covers VCC 2062, whereas the CO is concentrated near the HI maxima. The CO peaks correspond to two very distinct regions: one with moderate SF to the NE and one with only slightly weaker CO emission but with nearly no SF. Even where SF is clearly present, the SFR is below the value expected from the surface density of the molecular and the total gas as compared to spiral galaxies and other TDGs. After discussing different possible explanations, we conclude that the low surface brightness is a crucial parameter to understand the low SFR. The reduced data cubes are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A92

  15. 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.

  16. Simulations of the Sagittarius Dwarf Galaxy Collision with the Milky Way

    NASA Astrophysics Data System (ADS)

    Myers, Jeannette; Snyder, Bart; Rusthoven, Mary; The, Lih-Sin; Hartmann, Dieter

    2007-11-01

    The Sagittarius Dwarf Galaxy is one of the satellite galaxies interacting with the Milky Way. Discovered to be located just below the galactic center; this galaxy is currently being tidally disrupted as it approaches the Milky Way disk. We performed a series of N-body/SPH simulations of the interaction between Sagittarius and the Milky Way over a 1 Gyr time period leading up to today's position. These simulations include an updated Milky Way model, gas and star formation. We highlight the creation of the tidal tails and compare them to the known tidal structures we observe today.

  17. Simulating the Interactions of the Sagittarius Dwarf Galaxy and the Milky Way

    NASA Astrophysics Data System (ADS)

    Rusthoven, Mary; Myers, Jeannette; Snyder, Bart; The, Lih-Sin; Hartmann, Dieter

    2007-11-01

    The Sagittarius Dwarf Galaxy is one of the satellite galaxies interacting with the Milky Way. Discovered to be located just below the galactic center; this galaxy is currently being tidally disrupted as it approaches the Milky Way disk. We performed a series of four N-body simulations of the interaction between Sagittarius and the Milky Way over a 1 Gyr time period leading up to today's position. Two of the simulations were designed to test the initial conditions used for the Milky Way model. The other two simulations were designed to test what happens when the Milky Way is treated as a static system with only the Sagittarius system evolving with time.

  18. The Local Group Dwarf Irregular Galaxy NGC 6822: new insight on its star formation history .

    NASA Astrophysics Data System (ADS)

    Fusco, F.; Buonanno, R.; Bono, G.; Cassisi, S.; Monelli, M.; Pietrinferni, A.; Hidalgo, S. L.; Aparicio, A.

    We present a new photometric analysis of the Local Group Dwarf Irregular Galaxy NGC 6822 based on archival Hubble Space Telescope Advanced Camera for Surveys images. The data correspond to three fields covering the south-east region of the galaxy; for each field F475W and F814W HST bands are available. For each field an accurate color magnitude diagram (F814W, F475W-F814W) has been obtained. Preliminary hints on the galaxy star formation history are presented based on the comparison with isochrones from "A Bag of Stellar Tracks and Isochrones" (BaSTI) database.

  19. The sensitivity of harassment to orbit: mass loss from early-type dwarfs in galaxy clusters

    NASA Astrophysics Data System (ADS)

    Smith, R.; Sánchez-Janssen, R.; Beasley, M. A.; Candlish, G. N.; Gibson, B. K.; Puzia, T. H.; Janz, J.; Knebe, A.; Aguerri, J. A. L.; Lisker, T.; Hensler, G.; Fellhauer, M.; Ferrarese, L.; Yi, S. K.

    2015-12-01

    We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed.

  20. 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.

  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. INTERGALACTIC GAS IN GROUPS OF GALAXIES: IMPLICATIONS FOR DWARF SPHEROIDAL FORMATION AND THE MISSING BARYONS PROBLEM

    SciTech Connect

    Freeland, E.; Wilcots, E. E-mail: ewilcots@astro.wisc.edu

    2011-09-10

    Radio galaxies with bent jets are predominantly located in groups and clusters of galaxies. We use bent-double radio sources, under the assumption that their jets are bent by ram pressure, to probe intragroup medium (IGM) gas densities in galaxy groups. This method provides a direct measurement of the intergalactic gas density and allows us to probe intergalactic gas at large radii and in systems whose IGM is too cool to be detected by the current generation of X-ray telescopes. We find gas with densities of 10{sup -3} to 10{sup -4} cm{sup -3} at group radii from 15 to 700 kpc. A rough estimate of the total baryonic mass in intergalactic gas is consistent with the missing baryons being located in the IGM of galaxy groups. The neutral gas will be easily stripped from dwarf galaxies with total masses of 10{sup 6}-10{sup 7} M{sub sun} in the groups studied here. Indications are that intragroup gas densities in less-massive systems like the Local Group should be high enough to strip gas from dwarfs like Leo T and, in combination with tides, produce dwarf spheroidals.

  3. Molecular Gas and Star Formation in Tidal Dwarf Galaxies (Oral Contribution)

    NASA Astrophysics Data System (ADS)

    Lisenfeld, U.; Braine, J.; Duc, P.-A.; Charmandaris, V.; Vallejo, O.; Leon, S.; Brinks, E.

    Tidal Dwarf Galaxies (TDGs) are objects presently forming from gas which has been expelled from their parent galaxies during an interaction. We observed CO emission of a sample of 11 TDGs, of which 8 were detected. The CO is found at the peak of the HI observations and has and has the same line velocity and with, indicating that the molecular gas is forming in situ instead of being torn from the parent galaxies. The presence of Ha emission furthermore shows that stars are forming from this molecular gas. In order too investigate star formation in TDGs further, we compared their molecular gas content and star formation rate (SFR), traced by Ha, to those of spiral galaxies and classical dwarfs. The major difference between TDGs and classical dwarfs is the lower metallicity of the later. The star formation efficiency (SFR per molecular gas mass) of TDGs lies in the range typical of spiral galaxies indicating that star formation is proceeding in a normal fashion from molecular gas.

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

    NASA Astrophysics Data System (ADS)

    Vikram, Vinu; Cabré, Anna; Jain, Bhuvnesh; VanderPlas, J. T.

    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 & 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.

  5. H-alpha Imaging Survey of Low-Redshift Cluster Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Barkhouse, Wayne; Kalawila, Sandanuwan; Rude, Cody; Sultanova, Madina; Archer, Haylee Nichole; Foote, Gregory

    2016-01-01

    We describe our on-going H-alpha imaging survey to measure the star formation activity of dwarf galaxies selected from a sample of low-redshift (0.02 < z< 0.15) galaxy clusters using the KPNO 4-meter telescope+Mosaic camera. H-alpha observations are obtained using the narrow-band BATC filters centered on the redshifted H-alpha emission line. The continuum-subtracted H-alpha images allow us to constrain star formation rates via the correlation between star formation and H-alpha luminosity and equivalent width. The impact of the cluster environment can be quantified using radial-dependent measures of the star formation rate within individual clusters, and by comparing clusters within our sample on a cluster-to-cluster basis. Comparison of our H-alpha measurements to CFHT u-band imaging data of our cluster sample, permits us to explore the correlation between the UV continuum and H-alpha emission of the dwarf galaxy population. The goal of our survey is to further understand the mechanism that is responsible for the enhancement/quenching of star formation as dwarf galaxies fall into the galaxy cluster environment.

  6. 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.

  7. THE SMALL ISOLATED GAS-RICH IRREGULAR DWARF (SIGRID) GALAXY SAMPLE: DESCRIPTION AND FIRST RESULTS

    SciTech Connect

    Nicholls, David C.; Dopita, Michael A.; Jerjen, Helmut; Meurer, Gerhardt R.

    2011-09-15

    Using an optically unbiased selection process based on the HIPASS neutral hydrogen survey, we have selected a sample of 83 spatially isolated, gas-rich dwarf galaxies in the southern hemisphere with cz between 350 and 1650 km s{sup -1}, and with R-band luminosities and H I masses less than that of the Small Magellanic Cloud. The sample is an important population of dwarf galaxies in the local universe, all with ongoing star formation, and most of which have no existing spectroscopic data. We are measuring the chemical abundances of these galaxies, using the integral-field spectrograph on the Australian National University 2.3 m telescope, the Wide-Field Spectrograph. This paper describes our survey criteria and procedures, lists the survey sample, and reports on initial observations.

  8. 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.

  9. 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

  10. The Sagittarius Dwarf Galaxy Tidal Debris in the south Galactic Cap

    NASA Astrophysics Data System (ADS)

    Thompson, Jeffery; Newby, M.; Newberg, H. J.; Desell, T.

    2014-01-01

    We characterize the spatial properties of the Sagittarius dwarf galaxy tidal debris, both primary and secondary (bifurcated) tidal tails, in the south Galactic cap. The Sagittarius dwarf galaxy is currently being ripped apart by tidal forces from the Milky Way galaxy. The spatial density of turnoff stars from the Sloan Digital Sky Survey Data Release 8 are fit using statistical photometric parallax with half a petaFLOPS of computing power from the MilkyWay@home volunteer computing platform. The secondary tail appears to be significantly wider than the originally detected primary tail. These results are compared with the leading tidal tail stream density measured in the north Galactic cap. This research was funded by NSF grant AST 10-09670.

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

    NASA Astrophysics Data System (ADS)

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

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

  12. 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.

  13. Cosmological Evolution of Dwarf Galaxies: The Influence of Star Formation and the Multiphase Interstellar Medium

    NASA Astrophysics Data System (ADS)

    Spaans, Marco; Norman, Colin A.

    1997-07-01

    A model is developed to explain the cosmological evolution of dwarf galaxies. The population of small galaxies is found to evolve rapidly for z < 1, which provides a natural explanation for the evolution observed in the galaxy luminosity function. A tail is found in the redshift distribution of the faint blue excess that can extend to a redshift of 2. The star formation history is followed in detail for these objects. Constraints on the metallicity are identified for which stars are formed with much higher efficiency in a multiphase interstellar medium than in massive galaxies. Blue dwarf galaxies at the current epoch are identified with this starburst mode. The collapse of 1 and 2 σ perturbations of the initial density fluctuation spectrum is followed using the extended standard hierarchical clustering formalism. The collapse of these perturbations is normally associated with the formation of dwarf galaxies. These objects have shallow gravitational potential wells, and their evolution strongly depends upon the cooling time of the gas. The latter is determined by the ionization and chemical equilibrium of the gas in the presence of the intergalactic and local stellar radiation fields. The latter generally dominates and creates a feedback mechanism that regulates the evolutionary timescale. To improve upon previous models, essential new astrophysical ingredients are incorporated, such as a more detailed description of the physical processes regulating the multiphase structure of the interstellar medium in dwarf galaxies and the effects of evolution in the galaxy's metallicity on the formation of stars in molecular clouds. It is found that for a low star formation rate of 0.1 M⊙ yr-1, the cooling time of interstellar gas is longer than the local Hubble time until z ~ 1. At this epoch, a two-phase medium makes the dwarf interstellar medium less fragile against supernova explosions, and the volume filling factor of the hot phase (107 K) becomes of order unity. The

  14. The fate of heavy elements in dwarf galaxies - the role of mass and geometry

    NASA Astrophysics Data System (ADS)

    Recchi, S.; Hensler, G.

    2013-03-01

    Context. Energetic feedback from supernovae and stellar winds can drive galactic winds. Dwarf galaxies, due to their shallower potential wells, are assumed to be more vulnerable to this phenomenon. Metal loss through galactic winds is also commonly invoked to explain the low metal content of dwarf galaxies. Aims: Our main aim in this paper is to show that galactic mass cannot be the only parameter determining the fraction of metals lost by a galaxy. In particular, the distribution of gas must play an equally important role. Methods: We performed 2D chemo-dynamical simulations of galaxies characterized by different gas distributions, masses, and gas fractions. Results: The gas distribution can change the fraction of lost metals through galactic winds by up to one order of magnitude. In particular, disk-like galaxies tend to lose metals more easily than roundish ones. Consequently, the final metallicities attained by models with the same mass but with different gas distributions can also vary by up to one dex. Confirming previous studies, we also show that the fate of gas and freshly produced metals strongly depends on the mass of the galaxy. Smaller galaxies (with shallower potential wells) more easily develop large-scale outflows, so that the fraction of lost metals tends to be higher.

  15. 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%.

  16. A Comparative Study of Star-forming and Quiescent Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Simpson, Caroline E.; Gottesman, S. T.

    2000-12-01

    We present the results from a comparative study of the atomic hydrogen (H I) and optical properties of a sample of 16 dwarf galaxies, chosen to investigate the effects of star formation on the properties of low-mass systems. The violent star formation bursts believed to occur in these low-mass systems suggest a possible connection between the actively star-forming blue compact dwarfs (BCDs), and the quiescent low surface brightness dwarfs (LSBDs). It has been suggested that LSBDs, upon undergoing a burst of star formation, will evolve into BCDs and then back into LSBDs when the star formation slows or stops as the H I column density falls below the critical threshold necessary to support it. We have examined the location and kinematics of H I in eight BCDs and eight LSBDs of similar H I masses and a range of color indices to investigate this ``evolutionary'' sequence. The starburst episodes in these low-mass galaxies should lead to (1) a dispersal/depletion of the H I seen in the eight LSB dwarfs and (2) more centrally concentrated and agitated H I in the eight BCDs. The results of this project indicate that the quiescent LSBD galaxies have more diffuse H I distributions and often show a ringlike structure, while the active galaxies have more highly centrally concentrated H I reservoirs. The bluer, more recently active systems of both types also have higher internal H I velocity dispersions, indicating that energy has been pumped into the interstellar medium of these galaxies. These observations are consistent with an evolutionary scheme wherein the H I reservoirs in these galaxies take on different characteristics depending upon their star formation histories.

  17. 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.

  18. 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.

  19. Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. H II region observations and chemical abundances

    SciTech Connect

    Nicholls, David C.; Dopita, Michael A.; Sutherland, Ralph S.; Jerjen, Helmut; Kewley, Lisa J.; Basurah, Hassan

    2014-05-10

    In this paper we present the results of observations of 17 H II regions in thirteen galaxies from the SIGRID sample of isolated gas-rich irregular dwarf galaxies. The spectra of all but one of the galaxies exhibit the auroral [O III] 4363 Å line, from which we calculate the electron temperature, T{sub e} , and gas-phase oxygen abundance. Five of the objects are blue compact dwarf galaxies, of which four have not previously been analyzed spectroscopically. We include one unusual galaxy which exhibits no evidence of the [N II] λλ 6548,6584 Å lines, suggesting a particularly low metallicity (< Z {sub ☉}/30). We compare the electron temperature based abundances with those derived using eight of the new strong-line diagnostics presented by Dopita et al. Using a method derived from first principles for calculating total oxygen abundance, we show that the discrepancy between the T{sub e} -based and strong-line gas-phase abundances have now been reduced to within ∼0.07 dex. The chemical abundances are consistent with what is expected from the luminosity-metallicity relation. We derive estimates of the electron densities and find them to be between ∼5 and ∼100 cm{sup –3}. We find no evidence for a nitrogen plateau for objects in this sample with metallicities 0.5 > Z {sub ☉} > 0.15.

  20. Northern dwarf and low surface brightness galaxies. IV - The large-scale space distribution

    NASA Technical Reports Server (NTRS)

    Thuan, Trinh X.; Alimi, Jean-Michel; Gott, J. Richard, III; Schneider, Stephen E.

    1991-01-01

    Results are reported from a statistical analysis of published observational data on a sample of 860 northern dwarf and low-surface-brightness (D/LSB) galaxies with delta = 0 deg or greater and b between -40 and 40 deg, selected from the Uppsala General Catalogue of Galaxies (Nilson et al., 1973). The results are presented in extensive redshift/space maps, histograms, graphs and tables and characterized in detail. It is shown that the distribution of D/LSB galaxies closely resembles that of bright galaxies, apparently ruling out biased-star-formation models predicting a uniform distribution of D/LSBs. Although bright galaxies outside clusters are somewhat more clustered than the H I-rich D/LSBs, the latters' pairwise peculiar velocity (460 + or - 50 km/sec) is similar to that of the former.

  1. Detection of CO (J=1-0) in the dwarf elliptical galaxy NGC 185

    NASA Technical Reports Server (NTRS)

    Wiklind, Tommy; Rydbeck, Gustaf

    1987-01-01

    The detection of CO (J = 1-0) emission in the dwarf elliptical galaxy NGC 185 is reported. The presence of massive molecular clouds in this early-type galaxy supports the idea of recent or ongoing stellar formation indicated by the population of blue stars in the center. The CO was detected in two positions in the galaxy, the center, and a prominent dustcloud. The emission profile has two peaks, roughly centered around the systemic velocity. It is found that NGC 185 is overluminous in blue light for its CO luminosity compared with Sc galaxies. This might indicate a higher star-formation efficiency for NGC 185 than for the late-type galaxies.

  2. PAndAS' CUBS: Discovery of Two New Dwarf Galaxies in the Surroundings of the Andromeda and Triangulum Galaxies

    NASA Astrophysics Data System (ADS)

    Martin, Nicolas F.; McConnachie, Alan W.; Irwin, Mike; Widrow, Lawrence M.; Ferguson, Annette M. N.; Ibata, Rodrigo A.; Dubinski, John; Babul, Arif; Chapman, Scott; Fardal, Mark; Lewis, Geraint F.; Navarro, Julio; Rich, R. Michael

    2009-11-01

    We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey, a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam Wide-Field Camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H] = -1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (MV = -9.9 ± 0.6), albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II, and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (MV = -6.5 ± 0.8) and lies a lot closer in projection to M33 than it does to M31 (42 versus 224 kpc), suggesting that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of

  3. 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.

  4. The Celestial Buffet: multiple populations and globular cluster formation in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Maxwell, Aaron J.; Wadsley, James; Couchman, H. M. P.; Sills, Alison

    2014-04-01

    We present a framework that explains the commonly observed variation in light element abundances in globular clusters. If globular clusters form in the centres of dwarf galaxies, they will be pumped on to larger orbits as star formation progresses. The potential well will only retain the moderate velocity asymptotic giant branch (AGB) ejecta, the expected source of enrichment, but not supernova ejecta. There is no need to increase the initial cluster mass, a requirement of self-enrichment scenarios, as all the stars within the dwarf can contribute. As the clusters move through the dwarf centre they sweep up a mix of AGB ejecta and in-falling pristine gas to form a second generation of stars. The specific mix will vary in time and is thus able to explain the spread in second generation abundances observed in different clusters. The globular clusters will survive to the present day or be stripped as part of the hierarchical merging process of larger galaxies. We illustrate how this process may operate using a high-resolution simulation of a dwarf galaxy at high redshift.

  5. 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.

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

    SciTech Connect

    Hooper, Dan; Linden, Tim

    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.

  7. Scaling Laws for Dark Matter Halos in Late-type and Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Kormendy, John; Freeman, K. C.

    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 MV > -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 Vcirc of test particles in their DM halos. Baryons become unimportant at Vcirc = 42 ± 4 km s-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 MB ˜ -5 to -22. This implies a Faber-Jackson law with halo mass M ∝ (halo dispersion)4.

  8. White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy

    NASA Astrophysics Data System (ADS)

    Torres, S.; Camacho, J.; Isern, J.; García-Berro, E.

    2010-02-01

    Context. The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass substellar objects and stars with masses larger than 1 Msun have been ruled out as major components of a massive astrophysical compact halo object (MACHO) galactic halo, while stars of half a solar mass seem to be viable candidates. Main sequence stars have been already discarded, and there are tight restrictions on the role played by white dwarfs with hydrogen-dominated atmospheres. Aims: In this paper we evaluate the contribution to the dark matter content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. Methods: For this purpose we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres. We also take into account detailed descriptions of the thick disk and the halo of our Galaxy as well as of a reliable model of the LMC. Results: We find that the contribution of white dwarfs with hydrogen-deficient atmospheres moderately increases the theoretical estimate of the optical depth with respect to the value obtained when only hydrogen-rich white dwarfs are considered. We also find that the contribution of the thick disk population of white dwarfs is comparable to the halo contribution. However, the contributions of both the halo and the thick disk white-dwarf populations are still insufficient to explain the number of events observed by the MACHO team. Conclusions: Finally, we find that the contribution to the halo dark matter of the entire population under study is less than 10% at the 95% conficence level.

  9. Are the globular clusters with significant internal [Fe/H] spreads all former dwarf galaxy nuclei?

    NASA Astrophysics Data System (ADS)

    Da Costa, Gary Stewart

    2015-08-01

    In this presentation I will advance the idea that the 'globular clusters' with significant internal [Fe/H] dispersions are in fact the former nuclei or nuclear star clusters of dwarf galaxies that have been disrupted during the formation of the Galactic halo. I'll discuss the characteristics of these clusters, particularly their common properties, and indicate that at present the number of such clusters known is broadly consistent with the disruption hypothesis. Identification of significantly more clusters with large internal [Fe/H] abundance ranges may, however, cast doubt on the hypothesis, if the stars of the disrupted dwarfs remain in the Galactic halo.

  10. 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) HI 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 dIrr. Both tidal knots are located within a prominent HI tidal tail, appear to have sufficient mass (M_gas~10^8 M_sol) 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 HI 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 HI 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 to 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.

  11. 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.

  12. AN UPPER LIMIT ON THE MASS OF THE BLACK HOLE IN URSA MINOR DWARF GALAXY

    SciTech Connect

    Lora, V.; Sanchez-Salcedo, F. J.; Raga, A. C.; Esquivel, A. E-mail: jsanchez@astroscu.unam.mx E-mail: esquivel@nucleares.unam.mx

    2009-07-10

    The well-established correlations between the mass of massive black holes (BHs) in the nuclei of most studied galaxies and various global properties of their hosting galaxy lend support to the idea that dwarf galaxies and globular clusters could also host a BH in their centers. Direct kinematic detection of BHs in dwarf spheroidal (dSph) galaxies is seriously hindered by the small number of stars inside the gravitational influence region of the BH. The aim of this Letter is to establish an upper dynamical limit on the mass of the putative BH in the Ursa Minor (UMi) dSph galaxy. We present direct N-body simulations of the tidal disruption of the dynamical fossil observed in UMi, with and without a massive BH. We find that the observed substructure is incompatible with the presence of a massive BH of (2-3) x 10{sup 4} M {sub sun} within the core of UMi. These limits are consistent with the extrapolation of the M {sub BH}-{sigma} relation to the M {sub BH} < 10{sup 6} M {sub sun} regime. We also show that the BH may be off-center with respect to the center of symmetry of the whole galaxy.

  13. 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

  14. A XMM-Newton observation of a sample of four close dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Manni, L.; Nucita, A. A.; De Paolis, F.; Testa, V.; Ingrosso, G.

    2015-08-01

    We present the results of the analysis of deep archival XMM-Newton observations towards the dwarf spheroidal galaxies Draco, Leo I, Ursa Major II (UMa II) and Ursa Minor (UMi) in the Milky Way neighbourhood. The X-ray source population is characterized and cross-correlated with available databases to infer their nature. We also investigate if intermediate-mass black holes are hosted in the centre of these galaxies. For Draco, we detect 96 high-energy sources, two of them possibly being local stars, while no evidence for any X-ray emitting central compact object is found. Towards the Leo I and UMa II fields of view, we reveal 116 and 49 X-ray sources, respectively. None of them correlates with the putative central black holes and only one is likely associated with a UMa II local source. The study of the UMi dwarf galaxy found 54 high-energy sources and a possible association with a source at the dwarf spheroidal galaxy centre. We put an upper limit on the luminosity of the central compact object of 4.02 × 1033 erg s-1. Furthermore, via the correlation with a radio source near the galactic centre, the putative black hole should have a mass of (2.76^{+32.00}_{-2.54})× 10^6 M_{{{⊙}}} and be radiatively inefficient. This confirms a previous result obtained using Chandra data alone.

  15. 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.

  16. 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.

  17. The universal stellar mass-stellar metallicity relation for dwarf galaxies

    SciTech Connect

    Kirby, Evan N.; Bullock, James S.; Cohen, Judith G.; Guhathakurta, Puragra; Gallazzi, Anna

    2013-12-20

    We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the Milky Way and M31: Z{sub ∗}∝M{sub ∗}{sup 0.30±0.02}. The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass-stellar metallicity relation for galaxies as massive as M {sub *} = 10{sup 12} M {sub ☉}. Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping.

  18. INTERNAL DYNAMICS AND DYNAMICAL FRICTION EFFECTS IN THE DWARF SPHEROIDAL GALAXY IN FORNAX

    SciTech Connect

    Cowsik, Ramanath; Wagoner, Kasey; Sircar, Amit; Berti, Emanuele

    2009-07-10

    In the Fornax dwarf spheroidal galaxy the globular clusters are distributed widely, without any significant central concentration. Oh et al. pointed out that such a distribution is paradoxical: dynamical friction effects estimated using single-component King models would have forced the globular clusters to spiral down to the center of the galaxy well within a Hubble time. This paper is devoted to a discussion of this paradox. We describe a model in which the stars of the dwarf spheroidal galaxy are embedded in a cloud of dark matter, and each of these components is specified by its own phase-space distribution function. This model allows us to fit self-consistently the observed luminosity profile and the spatial variation of the velocity dispersion of the stars. This fitting yields two basic parameters, related to the central density and velocity dispersion, that characterize the phase-space distribution of dark matter. The dynamical friction effects calculated on the basis of this self-consistent model are small enough that the observed spatial distribution of the globular clusters poses no difficulty, and the apparent paradox is resolved. Thus, we have at hand a model for Fornax that reproduces the main observed features of this dwarf spheroidal galaxy.

  19. The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Cohen, Judith G.; Guhathakurta, Puragra; Cheng, Lucy; Bullock, James S.; Gallazzi, Anna

    2013-12-01

    We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the Milky Way and M31: Z_* \\propto M_*^{0.30+/- 0.02}. The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass-stellar metallicity relation for galaxies as massive as M * = 1012 M ⊙. Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping. 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.

  20. THE LINK BETWEEN PLANETARY SYSTEMS, DUSTY WHITE DWARFS, AND METAL-POLLUTED WHITE DWARFS

    SciTech Connect

    Debes, John H.; Walsh, Kevin J.; Stark, Christopher

    2012-03-10

    It has long been suspected that metal-polluted white dwarfs (types DAZ, DBZ, and DZ) and white dwarfs with dusty disks possess planetary systems, but a specific physical mechanism by which planetesimals are perturbed close to a white dwarf has not yet been fully posited. In this paper, we demonstrate that mass loss from a central star during post-main-sequence evolution can sweep planetesimals into interior mean motion resonances with a single giant planet. These planetesimals are slowly removed through chaotic excursions of eccentricity that in time create radial orbits capable of tidally disrupting the planetesimal. Numerical N-body simulations of the solar system show that a sufficient number of planetesimals are perturbed to explain white dwarfs with both dust and metal pollution, provided other white dwarfs have more massive relic asteroid belts. Our scenario requires only one Jupiter-sized planet and a sufficient number of asteroids near its 2:1 interior mean motion resonance. Finally, we show that once a planetesimal is perturbed into a tidal crossing orbit, it will become disrupted after the first pass of the white dwarf, where a highly eccentric stream of debris forms the main reservoir for dust-producing collisions. These simulations, in concert with observations of white dwarfs, place interesting limits on the frequency of planetary systems around main-sequence stars, the frequency of planetesimal belts, and the probability that dust may obscure future terrestrial planet finding missions.

  1. Mining the Treasuries: Dwarf Galaxies at 0.5 < z < 1 as Lynchpins for Galaxy Formation and Feedback

    NASA Astrophysics Data System (ADS)

    Guo, Yicheng

    2014-10-01

    Distant dwarf galaxies {DGs}, with stellar masses 100-1000 times less than that of our Milky Way, are lynchpins for understanding galaxy formation and feedback. Like gems, they are small, hard-to-find, but precious by being the most sensitive probes of both the macro-physics of dark matter halos and the micro-physics of the feedback and regulation of star formation. We propose to undertake a comprehensive study of dwarf galaxies at 0.5galaxy formation models to trace the stellar mass-halo mass relation, test viable feedback mechanisms, and track star formation and assembly histories of DGs.

  2. ARE THE ULTRA-FAINT DWARF GALAXIES JUST CUSPS?

    SciTech Connect

    Zolotov, Adi; Hogg, David W.; Willman, Beth

    2011-01-20

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

  3. A Chemical Confirmation of the Faint Boötes II Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Koch, Andreas; Rich, R. Michael

    2014-10-01

    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.

  4. 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.

  5. Constraining ultracompact dwarf galaxy formation with galaxy clusters in the local universe

    NASA Astrophysics Data System (ADS)

    Pfeffer, J.; Hilker, M.; Baumgardt, H.; Griffen, B. F.

    2016-05-01

    We compare the predictions of a semi-analytic model for ultracompact dwarf galaxy (UCD) formation by tidal stripping to the observed properties of globular clusters (GCs) and UCDs in the Fornax and Virgo clusters. For Fornax we find the predicted number of stripped nuclei agrees very well with the excess number of GCs+UCDs above the GC luminosity function. GCs+UCDs with masses >107.3 M⊙ are consistent with being entirely formed by tidal stripping. Stripped nuclei can also account for Virgo UCDs with masses >107.3 M⊙ where numbers are complete by mass. For both Fornax and Virgo, the predicted velocity dispersions and radial distributions of stripped nuclei are consistent with that of UCDs within ˜50-100 kpc but disagree at larger distances where dispersions are too high and radial distributions too extended. Stripped nuclei are predicted to have radially biased anisotropies at all radii, agreeing with Virgo UCDs at clustercentric distances larger than 50 kpc. However, ongoing disruption is not included in our model which would cause orbits to become tangentially biased at small radii. We find the predicted metallicities and central black hole masses of stripped nuclei agree well with the metallicities and implied black hole masses of UCDs for masses >106.5 M⊙. The predicted black hole masses also agree well with that of M60-UCD1, the first UCD with a confirmed central black hole. These results suggest that observed GC+UCD populations are a combination of genuine GCs and stripped nuclei, with the contribution of stripped nuclei increasing towards the high-mass end.

  6. 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).

  7. 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.

  8. Early-type dwarf galaxies in clusters: A mixed bag with various origins?

    NASA Astrophysics Data System (ADS)

    Lisker, T.

    2009-12-01

    The formation of early-type dwarf (dE) galaxies, the most numerous objects in clusters, is believed to be closely connected to the physical processes that drive galaxy cluster evolution, like galaxy harassment and ram-pressure stripping. However, the actual significance of each mechanism for building the observed cluster dE population is yet unknown. Several distinct dE subclasses were identified, which show significant differences in their shape, stellar content, and distribution within the cluster. Does this diversity imply that dEs originate from various formation channels? Does ``cosmological'' formation play a role as well? I try to touch on these questions in this brief overview of dEs in galaxy clusters.

  9. ALFALFA DISCOVERY OF THE NEARBY GAS-RICH DWARF GALAXY LEO P. II. OPTICAL IMAGING OBSERVATIONS

    SciTech Connect

    Rhode, Katherine L.; Salzer, John J.; Haurberg, Nathalie C.; Van Sistine, Angela; Young, Michael D.; Haynes, Martha P.; Giovanelli, Riccardo; Adams, Elizabeth A. K.; Cannon, John M.; Skillman, Evan D.; McQuinn, Kristen B. W. E-mail: slaz@astro.indiana.edu E-mail: haynes@astro.cornell.edu E-mail: jcannon@macalester.edu E-mail: kmcquinn@astro.umn.edu

    2013-06-15

    We present results from ground-based optical imaging of a low-mass dwarf galaxy discovered by the ALFALFA 21 cm H I survey. Broadband (BVR) data obtained with the WIYN 3.5 m telescope at Kitt Peak National Observatory (KPNO) are used to construct color-magnitude diagrams of the galaxy's stellar population down to V{sub o} {approx} 25. We also use narrowband H{alpha} imaging from the KPNO 2.1 m telescope to identify a H II region in the galaxy. We use these data to constrain the distance to the galaxy to be between 1.5 and 2.0 Mpc. This places Leo P within the Local Volume but beyond the Local Group. Its properties are extreme: it is the lowest-mass system known that contains significant amounts of gas and is currently forming stars.

  10. Surprising Image Revises Understanding Of Dwarf Galaxies -- Building Blocks of the Universe

    NASA Astrophysics Data System (ADS)

    2003-01-01

    An intensive study of a neighboring dwarf galaxy has surprised astronomers by showing that most of its molecular gas -- the raw material for new stars -- is scattered among clumps in the galaxy's outskirts, not near its center as they expected. Composite view of galaxy Composite view of the galaxy IC 10. Optical view in blue; Ionized hydrogen (H-alpha) in red; and molecular gas (CO) in green. CREDIT: OVRO, Caltech, NOAO, KPNO "This tells us that the galaxies we call dwarf irregulars are even more irregular than we thought," said Fabian Walter, of the National Radio Astronomy Observatory (NRAO) in Socorro, NM. "Our new work also shows that these galaxies probably are useful 'laboratories' for studying how stars were formed when the Universe was young," Walter added. Walter worked with Christopher Taylor of the University of Massachusetts and Nick Scoville of Caltech. The scientists presented their results at the American Astronomical Society's meeting in Seattle, WA. Using the millimeter-wave interferometer at Caltech's Owens Valley Radio Observatory, the astronomers combined 15 smaller images into a single mosaic to produce an image showing the location of Carbon Monoxide (CO) gas throughout a galaxy called IC 10, some 2.5 million light-years away. IC 10 is one of the Local Group of galaxies of which our own Milky Way is part. The telescope system was tuned to a frequency near 115 GigaHertz, where the CO molecule naturally emits radio waves. "We found the clumps of CO gas far from the galaxy's center, and not near the regions of current star formation," Walter said. "This tells us that stars may, in fact, form way out there in the outskirts of the galaxy, where we didn't expect," he added. Most of the galaxy's gas is atomic Hydrogen, composed of single Hydrogen atoms. Most of the galaxy's molecular gas is composed of Hydrogen molecules with two atoms each. Atomic Hydrogen can be seen with radio telescopes because it naturally emits at a radio frequency of 1420 Mega

  11. Effects of star-formation stochasticity on the Lyα and Lyman continuum emission from dwarf galaxies during reionization

    NASA Astrophysics Data System (ADS)

    Forero-Romero, Jaime E.; Dijkstra, Mark

    2013-01-01

    Observations of distant galaxies play a key role in improving our understanding of the Epoch of Reionization (EoR). The observed Lyα emission line strength - quantified by its restframe equivalent width (EW) - provides a valuable diagnostic of stellar populations and dust in galaxies during and after the EoR. In this paper, we quantify the effects of star-formation stochasticity on the predicted Lyα EW in dwarf galaxies, using the publicly available code slug (used to `Stochastically Light Up Galaxies'). We compute the number of hydrogen ionizing photons, as well as flux in the Far-UV for a set of models with star-formation rates (SFR) in the range 10-3-1 M⊙ yr-1. From these fluxes, we compute the luminosity, Lα, and the EW of the Lyα line. We find that stochasticity alone induces a broad distribution in Lα and EW at a fixed SFR, and that the widths of these distributions decrease with increasing SFR. We parametrize the EW probability density function as an SFR-dependent double power law. We find that it is possible to have EW as low as ˜EW0/4 and as high as ˜3 × EW0, where EW0 denotes the expected EW in the absence of stochasticity. We argue that stochasticity may therefore be important when linking drop-out and narrow-band selected galaxies, when identifying Population III galaxies, and that it may help to explain the large EW (EW ≳ 100-200 Å) observed for a fraction of Lyα selected galaxies. Finally, we show that stochasticity can also affect the inferred escape fraction of ionizing photons from galaxies. In particular, we argue that stochasticity may simultaneously explain the observed anomalous ratios of the Lyman continuum flux density to the (non-ionizing) UV continuum density in the so-called Lyman-Bump galaxies at z = 3.1, as well as the absence of such objects among a sample of z = 1.3 drop-out galaxies.

  12. A Tidally Disrupting Dwarf Galaxy in the Halo of NGC 253

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    We report the discovery of Scl-MM-Dw2, a new dwarf galaxy at a projected separation of ∼50 kpc from NGC 253, as part of the Panoramic Imaging Survey of Centaurus and Sculptor project (PISCeS). We measure a tip of the red giant branch (RGB) distance of 3.12 ± 0.30 Mpc, suggesting that Scl-MM-Dw2 is likely a satellite of NGC 253. We qualitatively compare the distribution of RGB stars in the color–magnitude diagram with theoretical isochrones and find that it is consistent with an old, ∼12 Gyr, and metal-poor, -2.3 \\lt [Fe/H] \\lt -1.1, stellar population. We also detect a small number of asymptotic giant branch stars consistent with a metal-poor 2–3 Gyr population in the center of the dwarf. Our non-detection of HI in a deep Green Bank Telescope spectrum implies a gas fraction {M}{HI}/{L}V\\lt 0.02 {M}ȯ /{L}ȯ . The stellar and gaseous properties of Scl-MM-Dw2 suggest that it is a dwarf spheroidal galaxy. Scl-MM-Dw2 has a luminosity of {M}V=-12.1+/- 0.5 mag and a half-light radius of rh =2.94+/- 0.46 {{kpc}}, which makes it moderately larger than dwarf galaxies in the Local Group of the same luminosity. However, Scl-MM-Dw2 is very elongated (ε =0.66+/- 0.06), and it has an extremely low surface brightness ({μ }0,V=27.7+/- 0.6 mag arcsec‑2). Its elongation and diffuseness make it an outlier in the ellipticity–luminosity and surface brightness–luminosity scaling relations. These properties suggest that this dwarf is being tidally disrupted by NGC 253.

  13. 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.

  14. Forged in FIRE: cusps, cores and baryons in low-mass dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Oñorbe, Jose; Boylan-Kolchin, Michael; Bullock, James S.; Hopkins, Philip F.; Kereš, Dušan; Faucher-Giguère, Claude-André; Quataert, Eliot; Murray, Norman

    2015-12-01

    We present multiple ultrahigh resolution cosmological hydrodynamic simulations of M⋆ ≃ 104-6.3 M⊙ dwarf galaxies that form within two Mvir = 109.5-10 M⊙ dark matter halo initial conditions. Our simulations rely on the Feedback in Realistic Environments (FIRE) implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve structure on the ˜200 pc scales. The resultant galaxies sit on the M⋆ versus Mvir relation required to match the Local Group stellar mass function via abundance matching. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. We demonstrate that it is possible to create a large (˜1 kpc) constant-density dark matter core in a cosmological simulation of an M⋆ ≃ 106.3 M⊙ dwarf galaxy within a typical Mvir = 1010 M⊙ halo - precisely the scale of interest for resolving the `too big to fail' problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late (z ≲ 2), after the early epoch of cusp building mergers has ended. Our M⋆ ≃ 104 M⊙ dwarf retains a cuspy dark matter halo density profile that matches that of a dark-matter-only run of the same system. Though ancient, most of the stars in our ultrafaint form after reionization; the ultraviolet field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the protodwarf.

  15. A Tidally Disrupting Dwarf Galaxy in the Halo of NGC 253

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    We report the discovery of Scl-MM-Dw2, a new dwarf galaxy at a projected separation of ˜50 kpc from NGC 253, as part of the Panoramic Imaging Survey of Centaurus and Sculptor project (PISCeS). We measure a tip of the red giant branch (RGB) distance of 3.12 ± 0.30 Mpc, suggesting that Scl-MM-Dw2 is likely a satellite of NGC 253. We qualitatively compare the distribution of RGB stars in the color-magnitude diagram with theoretical isochrones and find that it is consistent with an old, ˜12 Gyr, and metal-poor, -2.3 < [Fe/H] < -1.1, stellar population. We also detect a small number of asymptotic giant branch stars consistent with a metal-poor 2-3 Gyr population in the center of the dwarf. Our non-detection of HI in a deep Green Bank Telescope spectrum implies a gas fraction {M}{HI}/{L}V< 0.02 {M}⊙ /{L}⊙ . The stellar and gaseous properties of Scl-MM-Dw2 suggest that it is a dwarf spheroidal galaxy. Scl-MM-Dw2 has a luminosity of {M}V=-12.1+/- 0.5 mag and a half-light radius of rh =2.94+/- 0.46 {{kpc}}, which makes it moderately larger than dwarf galaxies in the Local Group of the same luminosity. However, Scl-MM-Dw2 is very elongated (ɛ =0.66+/- 0.06), and it has an extremely low surface brightness ({μ }0,V=27.7+/- 0.6 mag arcsec-2). Its elongation and diffuseness make it an outlier in the ellipticity-luminosity and surface brightness-luminosity scaling relations. These properties suggest that this dwarf is being tidally disrupted by NGC 253.

  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. COMPLEXITY ON DWARF GALAXY SCALES: A BIMODAL DISTRIBUTION FUNCTION IN SCULPTOR

    SciTech Connect

    Breddels, Maarten A.; Helmi, Amina

    2014-08-10

    In our previous work, we presented Schwarzschild models of the Sculptor dwarf spheroidal galaxy demonstrating that this system could be embedded in dark matter halos that are either cusped or cored. Here, we show that the non-parametric distribution function recovered through Schwarzschild's method is bimodal in energy and angular momentum space for all of the best-fitting mass models explored. We demonstrate that this bimodality is directly related to the two components known to be present in Sculptor through stellar population analysis, although our method is purely dynamical in nature and does not use this prior information. It therefore constitutes independent confirmation of the existence of two physically distinct dynamical components in Sculptor and suggests a rather complex assembly history for this dwarf galaxy.

  18. 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.

  19. The impact of galaxy harassment on the globular cluster systems of early-type cluster dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Smith, R.; Sánchez-Janssen, R.; Fellhauer, M.; Puzia, T. H.; Aguerri, J. A. L.; Farias, J. P.

    2013-02-01

    The dynamics of globular cluster systems (GCSs) around galaxies are often used to assess the total enclosed mass, and even to constrain the dark matter distribution. The GCS of a galaxy is typically assumed to be in dynamical equilibrium within the potential of the host galaxy. However cluster galaxies are subjected to a rapidly evolving and, at times, violently destructive tidal field. We investigate the impact of the harassment on the dynamics of GCs surrounding early-type cluster dwarfs, using numerical simulations. We find that the dynamical behaviour of the GCS is strongly influenced by the fraction of bound dark matter fDM remaining in the galaxy. Only when fDM falls to ˜15 per cent do stars and GCs begin to be stripped. Still the observed GC velocity dispersion can be used to measure the true enclosed mass to within a factor of 2, even when fDM falls as low as ˜3 per cent. This is possible partly because unbound GCs quickly separate from the galaxy body. However even the distribution of bound GCs may spatially expand by a factor of 2-3. Once fDM falls into the <3 per cent regime, the galaxy is close to complete disruption, and GCS dynamics can no longer be used to reliably estimate the enclosed mass. In this regime, the remaining bound GCS may spatially expand by a factor of 4 to 8. It may be possible to test if a galaxy is in this regime by measuring the dynamics of the stellar disc. We demonstrate that if a stellar disc is rotationally supported, it is likely that a galaxy has sufficient dark matter that the dynamics of the GCS can be used to reliably estimate the enclosed mass.

  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

    NASA Astrophysics Data System (ADS)

    Elson, E. C.; de Blok, W. J. G.; Kraan-Korteweg, R. C.

    2012-01-01

    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 μm 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. 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.

  3. VLA-ANGST: A HIGH-RESOLUTION H I SURVEY OF NEARBY DWARF GALAXIES

    SciTech Connect

    Ott, Juergen; Stilp, Adrienne M.; Dalcanton, Julianne J.; Warren, Steven R.; Skillman, Evan D.; Walter, Fabian; De Blok, W. J. G.; Koribalski, Baerbel; West, Andrew A. E-mail: adrienne@astro.washington.edu E-mail: warren@astro.umn.edu E-mail: walter@mpia.de E-mail: Baerbel.Koribalski@csiro.au

    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{sup -1}) and spatial ({approx}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 {approx}< 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{sup -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

  4. 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

  5. INSIGHTS INTO PRE-ENRICHMENT OF STAR CLUSTERS AND SELF-ENRICHMENT OF DWARF GALAXIES FROM THEIR INTRINSIC METALLICITY DISPERSIONS

    SciTech Connect

    Leaman, Ryan

    2012-12-01

    Star clusters are known to have smaller intrinsic metallicity spreads than dwarf galaxies due to their shorter star formation timescales. Here we use individual spectroscopic [Fe/H] measurements of stars in 19 Local Group dwarf galaxies, 13 Galactic open clusters, and 49 globular clusters to show that star cluster and dwarf galaxy linear metallicity distributions are binomial in form, with all objects showing strong correlations between their mean linear metallicity Z-bar and intrinsic spread in metallicity {sigma}(Z){sup 2}. A plot of {sigma}(Z){sup 2} versus Z-bar shows that the correlated relationships are offset for the dwarf galaxies from the star clusters. The common binomial nature of these linear metallicity distributions can be explained with a simple inhomogeneous chemical evolution model, where the star cluster and dwarf galaxy behavior in the {sigma}(Z){sup 2}- Z-bar diagram is reproduced in terms of the number of enrichment events, covering fraction, and intrinsic size of the enriched regions. The inhomogeneity of the self-enrichment sets the slope for the observed dwarf galaxy {sigma}(Z){sup 2}- Z-bar correlation. The offset of the star cluster sequence from that of the dwarf galaxies is due to pre-enrichment, and the slope of the star cluster sequence represents the remnant signature of the self-enriched history of their host galaxies. The offset can be used to separate star clusters from dwarf galaxies without a priori knowledge of their luminosity or dynamical mass. The application of the inhomogeneous model to the {sigma}(Z){sup 2}- Z-bar relationship provides a numerical formalism to connect the self-enrichment and pre-enrichment between star clusters and dwarf galaxies using physically motivated chemical enrichment parameters. Therefore we suggest that the {sigma}(Z){sup 2}- Z-bar relationship can provide insight into what drives the efficiency of star formation and chemical evolution in galaxies, and is an important prediction for galaxy

  6. ANDROMEDA XXIX: A NEW DWARF SPHEROIDAL GALAXY 200 kpc FROM ANDROMEDA

    SciTech Connect

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

    2011-11-20

    We report the discovery of a new dwarf galaxy, Andromeda XXIX (And XXIX), using data from the recently released Sloan Digital Sky Survey Data Release 8, and confirmed by Gemini North telescope Multi-Object Spectrograph imaging data. And XXIX appears to be a dwarf spheroidal galaxy, separated on the sky by a little more than 15 Degree-Sign from M31, with a distance inferred from the tip of the red giant branch of 730 {+-} 75 kpc, corresponding to a three-dimensional separation from M31 of 207{sup +20}{sub -2} kpc (close to M31's virial radius). Its absolute magnitude, as determined by comparison to the red giant branch luminosity function of the Draco dwarf spheroidal, is M{sub V} = -8.3 {+-} 0.4. And XXIX's stellar populations appear very similar to Draco's; consequently, we estimate a metallicity for And XXIX of [Fe/H] {approx}-1.8. The half-light radius of And XXIX is 360 {+-} 60 pc and its ellipticity is 0.35 {+-} 0.06, typical of dwarf satellites of the Milky Way and M31 at this absolute magnitude range.

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

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

    Based on high-resolution observed spectra, the non-local thermodynamic equilibrium (NLTE) line formation, and precise stellar atmosphere parameters, we present the first complete sample of dwarf spheroidal galaxies (dSphs) with accurate chemical abundances in the very metal-poor (VMP) regime. The obtained stellar elemental ratios are compared with chemical enrichment models, and we show that NLTE is a major step forward for studies of the dSph and the Milky Way (MW) chemical evolution.

  8. 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.

  9. Sowing the seeds of massive black holes in small galaxies: Young clusters as the building blocks of ultracompact dwarf galaxies

    SciTech Connect

    Amaro-Seoane, Pau; Konstantinidis, Symeon; Freitag, Marc Dewi; Coleman Miller, M.; Rasio, Frederic A. E-mail: simos@ari.uni-heidelberg.de E-mail: miller@astro.umd.edu

    2014-02-20

    Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses ∼10{sup 4}-10{sup 6} M {sub ☉} in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies. Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs.

  10. A dwarf galaxy's transformation and a massive galaxy's edge: detailed modeling of the extended stream in NGC1097

    NASA Astrophysics Data System (ADS)

    Cristiano Amorisco, Nicola; Martinez-Delgado, David

    2015-08-01

    Low surface brightness tidal features around massive galaxies are the smoking gun of hierarchical galaxy formation. These debris are informative of: (i) the evolutionary struggles of the progenitor dwarf galaxies, transformed and partially destroyed by the tides; (ii) the formation history of the massive host, its halo populations and the structure of its dark matter halo. However, extracting reliable measurements of the progenitor’s initial mass, infall time, host halo mass and density profile has so far been difficult, as the parameter space is too wide to explore with N-body simulations.We use new deep imaging data of the extended, X shaped stream in NGC1097 [1,2] and a new dynamical technique to quantitatively reconstruct: (i) the density profile of the massive spiral host (inferred virial mass M200=1012.25±0.1 M⊙) ; and (ii) the dramatic evolution of the progenitor galaxy; by modeling its stream within a fully statistical framework. I will show that the current location of the remnant coincides with a nucleated dwarf Spheroidal, with a luminosity of ~3.3x106LV,⊙ [3], and a predicted total mass of M(<0.45±0.2 kpc)=107.8±0.6 M⊙. This is the result of a strong transformation: at its first interaction with the host, 4.4±0.4 Gyr and three pericentric passages ago, the progenitor was over two orders of magnitude more massive, with Mtot(3.2±0.7 kpc)=1010.4±0.2 M⊙. Its orbit has a pericenter of a few kpc, but reaches out to 150±12 kpc. In this range the stream’s morphology allows us to see the total density slope of the host bending and steepening towards large radii. For the first time in a single galaxy (rather than on stacked data), both central and outer slope are constrained by observations and can be compared to LCDM expectations [4]. Finally, I will discuss prospects of applying this technique to more known streams, to map the structure of a wider sample of galaxy haloes and unveil the evolutionary histories of more individual dwarf galaxies

  11. Optical and NIR Photometry of the Interacting Dwarf Galaxies IIZw70 / IIZw71 (Poster)

    NASA Astrophysics Data System (ADS)

    Papaderos, P.; Noeske, K. G.; Cairós, L. M.; Vílchez, J. M.; Fricke, K. J.

    We obtained deep optical and NIR images of the pair of blue compact dwarf (BCD) galaxies II Zw 70 and II Zw 71 in order to study the effects of interaction on the structural properties of their stellar low-surface-brightness (LSB) component. We find that within their Holmberg radius the interacting BCDs under study do not differ significantly in terms of the central surface brightness and exponential scale length of their LSB hosts from typical iE/nE systems. In the faint outskirts (26.2 < mu_B [mag arcsec^-2^] < 28.5) of both systems, however, the present data reveal conspicuous morphological distortions, most notably an extended feature protruding as far as ~9 kpc from the starburst region of II Zw 70 in the direction of II Zw 71. The relatively blue colors of this stellar extension, together with its apparent spatial coincidence with the massive HI streamer connecting the dwarf galaxies, are consistent with the hypothesis that it originates from recent star formation within the gaseous halo of II Zw 70, rather than from stellar matter torn out of the LSB host of the BCD during the interaction.The results presented here support the view that important signatures of the dynamical response and secular evolution of the stellar LSB component in interacting dwarf galaxies can be found in their very faint outskirts.

  12. 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.

  13. 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.

  14. The Star Formation Histories of Local Group Dwarf Galaxies. III. Characterizing Quenching in Low-mass Galaxies