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We studied the stellar populations of Fornax dwarf spheroidal galaxy using HST/WFPC2 imaging of 7 galaxy fields. Our observations reach the oldest main-sequence turn-off, revealing distinct stars formation episodes and allowing us to address the evolution of this prototype dwarf spheroidal galaxy known to have experienced an extended history of star formation. From our HST data, spatial gradients in the stellar content of Fornax emerge with greater clarity. The outermost fields show only stars with ages between 7–12Gyr, while the intermediate region hosts a stellar population between 4–10Gyr, and stars younger of 2 Gyr are found in the innermost fields. A clearly visible gap in the subgiant branch points to bimodality in the main star formation episode. Our observations also indicate that the inner clump detected by Coleman et al. (2004) is characterized by the presence of young stars with age about 1.8 Gyr.
We present the internal kinematics of UCD3, the brightest known ultra-compact dwarf galaxy (UCD) in the Fornax cluster and the first UCD with spatially resolved spectroscopy. Based on seeing-limited observations with VLT/FLAMES, we measure a velocity field showing the signature of weak rotation and a radial velocity dispersion profile that is flatter than expected from an isotropic Jeans model where mass follows light. We find that both, a strongly anisotropic velocity distribution, and a dark matter component as predicted by simulations of galaxy threshing, can explain the observed flattening.
We analyze the stellar population parameters of seven ultra-compact dwarfs (UCDs) in the Virgo cluster. We find that the stellar population properties of UCDs are similar to those nuclei of early-type dwarf galaxies (dEs) that lie in the dense region of the cluster. This evidence suggests that the Virgo UCDs may have formed through the stripping of dE nuclei.
We have measured 3-D kinematics of stars in Kapteyn’s Selected Area (SA) 76 that were selected to be Anticenter Stream (ACS) members on the basis of their kinematics and CMD positions. Mean stream kinematics from the 31 identified ACS members produce an orbit inclined by ~30° to the well-defined spatial distribution of the stream. We have explored possible explanations for this, and suggest that our data in SA 76 are measuring the motion of a kinematically cold sub-stream among the ACS debris, which was likely a fragment of the same infalling structure that created the larger ACS system.
We present a study of the bar fraction in the Coma cluster galaxies based on a sample of ~190 galaxies selected from the SDSS-DR6 and observed with the Hubble Space Telescope (HST) Advanced Camera for Survey (ACS). We explore the presence of bars, detected by visual classification, throughout an unprecedented luminosity range of 9 mag (− 23 < Mr < −14). We find that bars are hosted by galaxies in a tight range of both luminosities (−22 < Mr < −17) and masses (109 < M∗/M⊙ < 1011). We find also that the bar fraction does not vary significantly with the distance to the cluster center, implying that cluster environment plays a second-order role in bar formation/evolution. The shape of the bar fraction distribution with respect to both luminosity and mass is well matched by the luminosity distribution of disk galaxies in Coma, indicating that bars are good tracers of cold stellar disks.
With the new generation of wide-field surveys, our understanding of the Andromeda satellite system has dramatically improved in recent years. Since 2004, 12 new dwarf galaxies have been discovered around Andromeda, doubling the number of previously known companions. What are the properties of these newly discovered dwarfs and what do they tell us about galaxy formation? Are these systems bound? Do they show evidence of multiple star formation epochs? To better understand the stellar populations of the faintest dwarfs around M 31, we have used the Large Binocular Camera (LBT) and Suprime-Cam (Subaru) to obtain photometric observations of And X, And XVII, And XVIII, And XIX, And XX, And XXI and And XXII. Reaching below the horizontal branch, these observations have allowed for accurate distance determinations to be made, together with metallicity estimates based on the red giant branch stars. Our analysis shows our 7 dwarfs to be metal poor and with large spreads in [Fe/H], it strongly suggests multiple generations of stars are present in And X, And XVII, And XVIII, and And XIX, And XXI.
Ultra-compact dwarf galaxies (UCDs) are predominatly found in the cores of nearby galaxy clusters. Besides the Fornax and Virgo cluster, UCDs have also been confirmed in the twice as distant Hydra I and Centaurus clusters. Having (nearly) complete samples of UCDs in some of these clusters allows the study of the bulk properties with respect to the environment they are living in. Moreover, the relation of UCDs to other stellar systems in galaxy clusters, like globular clusters and dwarf ellipticals, can be investigated in detail with the present data sets. The general finding is that UCDs seem to be a heterogenous class of objects. Their spatial distribution within the clusters is in between those of globular clusters and dwarf ellipticals. In the colour-magnitude diagram, blue/metal-poor UCDs coincide with the sequence of nuclear star clusters, whereas red/metal-rich UCDs reach to higher masses and might have originated from the amalgamation of massive star cluster complexes in merger or starburst galaxies.
The small (LV ≤ 107LV,⊙) baryonic components of the Milky Way’s dwarf spheroidal (dSph) satellites contribute small “backgrounds” with regards to estimates of dSph dark matter density profiles and any related predictions of dark matter decay/annihilation signals. As dSphs gain attention from a broad range of physicists interested in dark matter, it becomes important to examine the robustness – with respect to details like modeling assumptions and observational uncertainties – of conclusions regarding the dark matter content of dSphs and the scaling relations derived therefrom. Indeed dSphs seem to contain copious amounts of dark matter. However, the standard Jeans analysis constrains the dark mass enclosed only within the luminous scale radius, and does not reveal the internal dark matter distribution in a model-indepdendent way. Furthermore, new observational results help to identify complexities in internal dSph kinematics and cast doubt on previously apparent similarities among dSph dark matter halos. Further progress in understsanding dark matter on small scales will require consideration of dSphs more as individual galaxies and less as laboratories.
A widely held view is that a “mini halo” with much less than 108M⊙ in dark matter is unlikely to retain any baryons because even a single SN event is expected to sweep out all of the gas. But we show that a clumpy medium is much less susceptible to SN sweeping (particularly if it is off-centred) because the coupling efficiency of the explosive energy is much lower than for a diffuse interstellar medium. With the aid of the sophisticated 3D hydro code Fyris, we show that baryons are retained and stars are formed in dark matter haloes down to 3 × 106M⊙. The gas survives the SN explosion and is enriched with specific abundance yields of the discrete events. The smallest galaxies may not contribute a large fraction of baryons and dark matter to the formation of galaxies. But they are likely to carry important chemical signatures that were laid down in the earliest epochs of star formation, as we show. We discuss the results in light of the newly discovered, very metal poor, damped Lyα systems and ultra-faint dwarf galaxies.
The study of dwarf galaxies in groups is a powerful tool for investigating galaxy evolution, chemical enrichment and environmental effects on these objects. Here we present results obtained for dwarf galaxies in the Centaurus A complex, a dense nearby (∼4 Mpc) group that contains two giant galaxies and about 60 dwarf companions of different morphologies and stellar contents. We use archival optical (HST/ACS) and near-infrared (VLT/ISAAC) data to derive physical properties and evolutionary histories from the resolved stellar populations of these dwarf galaxies. In particular, for early-type dwarfs we are able to construct metallicity distribution functions, find population gradients and quantify the intermediate-age star formation episodes. For late-type dwarfs, we compute recent (∼1 Gyr) star formation histories and study their stellar distribution. We then compare these results with properties of the dwarfs in our Milky Way and in other groups. Our work will ultimately lead to a better understanding of the evolution of dwarf galaxies.
In recent years the census of known satellites in our own Local Group and in nearby galaxy groups has increased substantially due to sensitive wide-area surveys. In the Local Group these surveys have more than doubled its galaxy content and extended the galaxy luminosity function to very faint total magnitudes. Deep ground-based imaging and spectroscopic observations as well as high-resolution imaging with the Hubble Space Telescope have revolutionized our understanding of the chemical evolution and star formation histories of the satellites. We often find long-lasting star formation episodes with low star formation efficiencies. There is evidence for localized, stochastic enrichment, and recent searches are now beginning to uncover even extremely metal-deficient stars. In many satellites evidence for two or more distinct stellar subpopulations is found. Differing fractions of old populations have been detected in all satellites studied in sufficient detail so far. Kinematic measurements support a picture in which satellites are dark-matter dominated, although recent results indicate that the proposed common mass scale may not hold for very low-mass satellites. When considering satellite ensembles, we find global morphology–distance and gas-content–distance relations in all groups studied thus far, but individual star formation histories seem to also strongly depend on a given satellite’s intrinsic properties.
Carina is a small and faint classical dwarf spheroidal galaxy in the halo of the Milky Way with a highly episodic star formation history (e.g., Hurley-Keller et al. 1998). Using VLT/FLAMES in high resolution mode, we significantly increase the sample of stars with abundance determinations in Carina, adding to earlier work with UVES. Here we present our preliminary abundances for Fe, Mg, Ca and Ba in 23 red giant branch stars in the centre of Carina.
Using VIMOS (VLT) we carried out a spectroscopic survey around seven late-type spiral galaxies and we discovered 55 satellites. After obtaining the radial velocities and spatial distribution of the satellites, we analyze their properties.
We present new results of a long-term investigation of the stellar content of the Large and the Small Magellanic Cloud. The main aim of the project is to obtain a comprehensive knowledge of the star formation history of these two nearby galaxies. Here we discuss the latest findings obtained using new data and a variety of different observational approaches, based on wide-field ground-based photometry and low-resolution CaT spectra. We find evidence of two main star formation events across the LMC disk. We also found evidence that the young and intermediate age population (<5 Gyr) presents a steeper gradient with radius than the old one (>8 Gyr). In particular, the young (<2 Gyr) component seems to extend to 8 degrees from the center, distance at which it decreases abruptly. At larger distance, the old population dominates, extending to at least 13.5 kpc. We also report the discovery of a population of stars located across the Magellanic Bridge, compatible with the old population of the LMC.
We present recent results on the oxygen and nitrogen chemical abundances in the extremely compact, low-mass starburst galaxies at redshifts 0.1–0.3 usually referred to as “green pea” galaxies. We show that they are metal-poor galaxies (~1/5 solar) with lower oxygen abundances than star-forming galaxies of similar mass and N/O ratios unusually high for galaxies of the same metallicity. Recent, rapid, and massive inflows of cold gas, possibly coupled with enriched outflows from supernova winds, are used to explain the results. This is consistent with the known “pea” galaxy properties and suggest that these rare objects are experiencing a short and extreme phase in their evolution.
We investigate the possibility that Hercules, a recently discovered Milky Way satellite, is a stellar stream in the process of formation. This hypothesis is motivated by Hercules’ highly elongated shape as well as the measurement of a tentative radial velocity gradient along its body (Adén et al. 2009a). The application of simple analytical techniques (Jin & Lynden-Bell 2007) on radial velocity data of its member stars provides tight constraints on the tangential velocity of the system (−16−22+6 kms−1, relative to the Galactic Standard of Rest). Combining this with its large receding velocity (145 km s−1) and distance (138 kpc) yields an orbit that would have taken Hercules to within 6−2+9 kpc of the Galactic centre approximately 0.6 Gyr ago. This very small perigalacticon can naturally explain the violent tidal destruction of the dwarf galaxy in the Milky Way’s gravitational potential, inducing its transformation into a stellar stream.
We present a study of two dE/dSph members of the nearby M 81 group of galaxies, KDG 61 and KDG 64. Direct HST/ACS images and integrated-light spectra of 6 m telescope of Special Astrophysical Observatory (Russia) have been used for quantitative star formation history analysis. The spectroscopic and colour-magnitude diagrams analysis gives consistent results. These galaxies appear to be dominated by an old population (12–14 Gyr) of low metallicity ([Fe/H] ∼−1.5). Stars of ages about 1 to 4 Gyr have been detected in both galaxies. The later population shows marginal metal enrichment. Our radial velocity measurement suggests that the HII knot on the line-of-sight of KDG 61 is not gravitationally attached to the galaxy.
We studied the impact of the revisited values for the LSR circular velocity of the Milky Way (Reid et al. 2004) on the formation of the Magellanic Stream. The LSR circular velocity was varied within its observational uncertainties as a free parameter of the interaction between the Large (LMC) and the Small (SMC) Magellanic Clouds and the Galaxy. We have shown that the large–scale morphology and kinematics of the Magellanic Stream may be reproduced as tidal features, assuming the recent values of the proper motions of the Magellanic Clouds (Kallivayalil 2006). Automated exploration of the entire parameter space for the interaction was performed to identify all parameter combinations that allow for modeling the Magellanic Stream. Satisfactory models exist for the dynamical mass of the Milky Way within a wide range of 0.6 ⋅ 1012M⊙ to 3.0 ⋅ 1012M⊙ and over the entire 1–σ errors of the proper motions of the Clouds. However, the successful models share a common interaction scenario. The Magellanic Clouds are satellites of the Milky Way, and in all cases two close LMC–SMC encounters occurred within the last 4 Gyr at t < −2.5 Gyr and t ≈ −150 Myr, triggering the formation of the Stream and of the Magellanic Bridge, respectively. The latter encounter is encoded in the observed proper motions and inevitable in any model of the interaction. We conclude that the tidal origin of the Magellanic Stream implies the previously introduced LMC/SMC orbital history, unless the parameters of the interaction are revised substantially.
We present color-magnitude diagrams (CMD) reaching the oldest main sequence turn offs for three fields in the Fornax dwarf spheroidal galaxy, obtained with FORS1 at the VLT. One of them is situated at the center of the galaxy while the other two are located at a distance of 10′ form the center. We determine the full star formation history (SFH) of the central field, extending to the first star formation events.
Besides giant elliptical galaxies, a number of low-mass stellar systems inhabit the cores of galaxy clusters, such as dwarf elliptical galaxies (dEs/dSphs), ultra-compact dwarf galaxies (UCDs), and globular clusters. The detailed morphological examination of faint dwarf galaxies has, until recently, been limited to the Local Group (LG) and the two very nearby galaxy clusters Virgo and Fornax. Here, we compare the structural parameters of a large number of dEs/dSphs in the more distant clusters Hydra I and Centaurus to other dynamically hot stellar systems.