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VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies: II. Implications for Understanding Galaxy Evolution  [PDF]
Eline Tolstoy,K. A. Venn,M. Shetrone,F. Primas,V. Hill,A. Kaufer,T. Szeifert
Physics , 2002, DOI: 10.1086/345967
Abstract: We have used UVES on VLT-UT2 to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax, Carina and Leo I. We measure the abundance variations of numerous elements in these low mass stars with a range of ages (1-15Gyr old). This means that we can effectively measure the chemical evolution of these galaxies WITH TIME. Our results show a significant spread in metallicity with age, but an overall trend consistent with what might be expected from a closed (or perhaps leaky) box chemical evolution scenario over the last 10-15Gyr. We notice that each of these galaxies show broadly similar abundance patterns for all elements measured. This suggests a fairly uniform progression of chemical evolution with time, despite quite a large range of star formation histories. It seems likely that these galaxies had similar initial conditions, and evolve in a similar manner with star formation occurring at a uniformly low rate, even if at different times. With our accurate measurements we find evidence for small variations in abundances which are correlated to variations in star formation histories. The alpha-elements suggest that dSph chemical evolution has not been affected by very high mass stars (>15-20 Msun). The abundance patterns we measure for stars in dwarf spheroidal galaxies are significantly different from those typically observed in the disk, bulge and inner-halo of our Galaxy. This suggests that it is NOT possible to construct a significant fraction of our Galaxy from STARS formed in these dwarf spheroidal galaxies which subsequently merged into our own. Any merger scenario involving dSph has to occur in the very early Universe whilst they are still gas rich, so the majority of mass transfer is gas, and few stars.
Chemical evolution of Local Group dwarf galaxies in a cosmological context -- I. A new modelling approach and its application to the Sculptor dwarf spheroidal galaxy  [PDF]
Donatella Romano,Else Starkenburg
Physics , 2013, DOI: 10.1093/mnras/stt1033
Abstract: We present a new approach for chemical evolution modelling, specifically designed to investigate the chemical properties of dwarf galaxies in a full cosmological framework. In particular, we focus on the Sculptor dwarf spheroidal galaxy as a test bed for our model. We select four candidate Sculptor-like galaxies from the satellite galaxy catalogue generated by implementation of a version of the Munich semi-analytic model for galaxy formation on the level 2 Aquarius dark matter simulations. We follow explicitly the evolution of several chemical elements, both in the cold gas out of which the stars form and in the hot medium residing in the halo. We take into account in detail the lifetimes of stars of different initial masses, the distribution of the delay times for type Ia supernova explosions and the dependency of the stellar yields from the initial metallicity of the stars. We allow large fractions of metals to be deposited into the hot phase, either directly as stars die or through reheated gas flows powered by supernova explosions. In order to reproduce both the observed metallicity distribution function and the observed abundance ratios of long-lived stars of Sculptor, large fractions of the reheated metals must never re-enter regions of active star formation. Our analysis sets further constraints on the semi-analytical models and, at large, on possible metal enrichment scenarios for the Sculptor dwarf spheroidal galaxy.
Estimating masses of dwarf spheroidal galaxies  [PDF]
Klaudia Kowalczyk,Ewa L. Lokas
Physics , 2014,
Abstract: Precise measurements of mass in dark matter dominated dwarf spheroidal galaxies are of great importance for testing the theories of structure formation. We use $N$-body simulations of the tidal evolution of a dwarf galaxy orbiting the Milky Way to generate mock kinematical data sets and use them to test the reliability of a simple mass estimator proposed by Wolf et al. The evolution of the initially disky dwarf galaxy embedded in a dark matter halo was traced for 10 Gyr on a rather tight orbit. After about half of the time a dwarf spheroidal galaxy is formed that retains some remnant rotation and a non-spherical shape. Observing the triaxial galaxy along each of its principal axes we measure its half-light radius and the line-of-sight velocity dispersion and use them to estimate the mass. We find that the mass is significantly overestimated when the dwarf is seen along the longest axis of the stellar component and underestimated when observed along the shortest axis. We provide a formula that quantifies the systematic error in the estimated mass with respect to the true one as a function of the galaxy shape and line of sight.
The Globular Clusters of Dwarf Spheroidal Galaxies  [PDF]
Sidney van den Bergh
Physics , 1998, DOI: 10.1086/311624
Abstract: The possibility that the Sagittarius dwarf spheroidal might have formed as a Searle-Zinn fragment in the outer halo of the Galaxy is discussed. Arguments in favor of this hypothesis are: (1) The luminosity distribution of globular clusters in both Sagittarius and in the outer halo (Rgc > 80 kpc), appear to be bimodal with peaks near MV ~ -5 and MV ~ -10, and (2) the globular clusters in both Sgr, and in the outer halo, have a significantly larger age spread than do the globulars in the inner halo of the Galaxy. However, a counter argument is that only one of the four globulars associated with Sgr has the large half-light diameter that is diagnostic of outer halo clusters. The absence of globular clusters from all Local Group dwarf spheroidal galaxies fainter than MV = -12 shows that their specific globular cluster frequency must be lower than it is in the Fornax dwarf spheroidal. This result suggests that Fornax may have had an unusual evolutionary history.
Dwarf Spheroidal Galaxies in the Virgo Cluster  [PDF]
S. Phillipps,Q. A. Parker,J. M. Schwartzenberg,J. B. Jones
Physics , 1997, DOI: 10.1086/311144
Abstract: We present a study of the smallest and faintest galaxies found in a very deep photographic R band survey of regions of the Virgo Cluster, totalling over 3 square degrees, made with the UK Schmidt Telescope. The objects we detect have the same physical sizes and surface brightnesses as Local Group dwarf spheroidal galaxies. The luminosity function of these extremely low luminosity galaxies (down to M_R =~ -11 or about 5 X 10^{-5} L*) is very steep, with a power law slope alpha =~ -2, as would be expected in many theories of galaxy formation via hierarchical clustering, supporting previous observational evidence at somewhat higher luminosities in other clusters.
Supernova Enrichment of Dwarf Spheroidal Galaxies  [PDF]
P. Chris Fragile,Stephen D. Murray,Peter Anninos,Douglas N. C. Lin
Physics , 2003, DOI: 10.1086/375183
Abstract: (Abridged) Many dwarf galaxies exhibit sub-Solar metallicities, with some star-to-star variation, despite often containing multiple generations of stars. The total metal content in these systems is much less than expected from the heavy element production of massive stars in each episode of star formation. Such a deficiency implies that a substantial fraction of the enriched material has been lost from these small galaxies. Mass ejection from dwarf galaxies may have important consequences for the evolution of the intergalactic medium and for the evolution of massive galaxies, which themselves may have formed via the merger of smaller systems. We report here the results of three-dimensional simulations of the evolution of supernova-enriched gas within dwarf spheroidal galaxies (dSph's), with the aim of determining the retention efficiency of supernova ejecta. We consider two galaxy models, selected to represent opposite ends of the dSph sequence. For each model galaxy we investigate a number of scenarios, ranging from a single supernova in smooth gas distributions to more complex multiple supernovae in highly disturbed gas distributions. The results of these investigations suggest that, for low star-formation efficiencies, it is difficult to completely expel the enriched material from the galaxy. Most of the enriched gas is, however, lost from the core of the galaxy following multiple supernovae, especially if the interstellar medium is already highly disturbed by processes such as photo-ionization and stellar winds. If subsequent star formation occurs predominantly within the core where most of the residual gas is concentrated, then these results could explain the poor self-enrichment efficiency observed in dwarf galaxies.
Resonant stripping as the origin of dwarf spheroidal galaxies  [PDF]
Elena D'Onghia,Gurtina Besla,Thomas J. Cox,Lars Hernquist
Physics , 2009, DOI: 10.1038/nature08215
Abstract: Dwarf spheroidal galaxies are the most dark matter dominated systems in the nearby Universe and their origin is one of the outstanding puzzles of how galaxies form. Dwarf spheroidals are poor in gas and stars, making them unusually faint, and those known as ultra-faint dwarfs have by far the lowest measured stellar content of any galaxy. Previous theories require that dwarf spheroidals orbit near giant galaxies like the Milky Way, but some dwarfs have been observed in the outskirts of the Local Group. Here we report simulations of encounters between dwarf disk galaxies and somewhat larger objects. We find that the encounters excite a process, which we term ``resonant stripping'', that can transform them into dwarf spheroidals. This effect is distinct from other mechanisms proposed to form dwarf spheroidals, including mergers, galaxy-galaxy harassment, or tidal and ram pressure stripping, because it is driven by gravitational resonances. It may account for the observed properties of dwarf spheroidals in the Local Group, including their morphologies and kinematics. Resonant stripping predicts that dwarf spheroidals should form through encounters, leaving detectable long stellar streams and tails.
The Andromeda Dwarf Spheroidal Galaxies  [PDF]
Taft E. Armandroff,Gary S. Da Costa
Physics , 1998,
Abstract: Our current knowledge of M31's dwarf spheroidal companions is reviewed. Two topics of recent interest constitute the bulk of this review. First, color-magnitude diagrams reaching below the horizontal branch have been constructed for two M31 dwarf spheroidals based on images from HST/WFPC2. The horizontal branches are predominantly red in both galaxies, redder than expected for their metallicity based on Galactic globular clusters. Thus, the second parameter effect is seen in the M31 halo. Second, recent surveys have revealed three new dwarf spheroidal companions to M31. Thus, dwarf spheroidal galaxies are not as rare around M31 as previously thought and as a result, some properties of the M31 companion system have changed.
Improved Searches for HI in Three Dwarf Spheroidal Galaxies  [PDF]
L. M. Young
Physics , 1999, DOI: 10.1086/300820
Abstract: Previous searches for HI in our Galaxy's dwarf spheroidal companions have not been complete enough to settle the question of whether or not these galaxies have HI, especially in their outer parts. We present VLA observations of three dwarf spheroidals: Fornax, Leo II, and Draco, all of which have known stellar velocities. The new data show no HI emission or absorption. Column density limits in emission are 4--7 x 10^18 atoms/cm^2 in the centers of the galaxies. The importance of the new observations is that they cover larger areas than previous searches and they are less plagued by confusion with foreground (Galactic) HI. The apparent absence of neutral gas in the Fornax dwarf spheroidal is especially puzzling because recent photometry shows evidence of stars only 10^8 years old. We discuss whether the VLA observations could have missed significant amounts of HI.
RR Lyrae Stars in Dwarf Spheroidal Galaxies  [PDF]
Gisella Clementini
Physics , 2010,
Abstract: With ages comparable to the age of the Universe, the variable stars of RR Lyrae type have eyewitnessed the formation of their host galaxies, and thus can provide information on the processes that led to the assembling of large galaxies such as the Milky Way and the Andromeda galaxy. The present knowledge of the RR Lyrae population in Local Group dwarf spheroidal galaxies is reviewed,calling attention to the "ultra-faint" spheroidal systems recently discovered around the Milky Way by the Sloan Digital Sky Survey. The properties of the RR Lyrae stars and the Oosterhoff dichotomy observed for Galactic globular clusters and field RR Lyrae stars are discussed, since they put constrain on the possibility that the Milky Way and Andromeda halos were built up from protogalactic fragments resembling the dwarf spheroidals we observe today.
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