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Within the Local Universe galaxies can be studied in great detail star by star. The Colour-Magnitude Diagram synthesis analysis method is well established as the most accurate way to determine the detailed star formation history of galaxies going back to the earliest times. This approach has benefited enormously from the exceptional data sets that wide field CCD imagers on the ground and the Hubble Space Telescope can provide. Spectroscopic studies using large ground based telescopes have allowed the determination of abundances and kinematics for significant samples of stars in nearby dwarf galaxies. These studies have shown directly how properties can vary spatially and temporally, and how these give important constraints to theories of galaxy formation and evolution.
The NIR Ca II triplet has proven to be an important tool for quantitative spectroscopy. Here we present results of synthetic spectral analysis for the Ca II triplet for low-metallicity red giant stars, combined with observational data. Our results start to deviate strongly from the widely-used and linear empirical calibrations below [Fe/H] = −2. We provide a new calibration for Ca II triplet studies which is valid down until [Fe/H] = −4 and apply this new calibration to current data sets. We suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought and discuss preliminary results and possibilities for follow-up observations of these extremely low-metallicity candidates.
The chemical compositions of the stars in Milky Way (MW) satellite galaxies reveals the history of gas flows and star formation (SF) intensity. This talk presented a Keck/DEIMOS spectroscopic survey of the Fe, Mg, Si, Ca, and Ti abundances of nearly 3000 red giants in eight MW dwarf satellites. The metallicity and alpha-to-iron ratio distributions obey the following trends: (1) The more luminous galaxies are more metal-rich, indicating that they retained gas more efficiently than the less luminous galaxies. (2) The shapes of the metallicity distributions of the more luminous galaxies require gas infall during their SF lifetimes. (3) At [Fe/H]< −1.5, [α/Fe] falls monotonically with increasing [Fe/H] in all MW satellites. One interpretation of these trends is that the SF timescale in any MW satellite is long enough that Type Ia supernovae exploded for nearly the entire SF lifetime.
The important role of Cepheid and RR Lyrae variable stars and what they teach us about dwarf galaxies is discussed. Despite ever improving star formation histories of Local Group dwarf galaxies uncertainties remain, in particular in the identification and characterisation of the oldest stellar populations. The old stellar populations sometimes can be hard to interpret, or even to detect, due to their inherent faintness and scarcity, and often a strong overlying young population makes the crowding due to much brighter stars difficult to overcome. Recent and some preliminary results for RR Lyrae searches (in M 32 and Leo A) carried out with HST and Gemini-North/GMOS are presented. In these cases variables represent the only way to constrain the nature and, sometimes, the presence of a stellar population ≥10 Gyrs old. The recent discovery of Cepheids in I Zw 18 with HST is discussed. This allowed the first accurate distance determination, enabling a more secure detection of the Tip of the Red Giant Branch, and confirmed the existence of a population ≥2 Gyrs old.
We examined the light variations of AGB variables of the two Local Group members NGC 147 and NGC 185. A photometric monitoring in the i-band was carried out with the Nordic Optical Telescope at La Palma over ≈ 2.5 yrs resulting in roughly 35 data points for the period analysis. We found 513 long period variables (LPVs) in NGC 185 and 213 LPVs in NGC 147. Additional single-epoch Ks-band photometry was obtained for 387 (NGC 185, 323 with periods) and 182 (NGC 147, 147 with periods) LPVs, respectively. For a large fraction of these stars we also have an indication of the atmospheric chemistry (carbon- or oxygen-rich) from narrow band photometry (see Nowotny et al. 2003). The resulting Ks − logP diagrams for both systems show a well populated sequence of fundamental mode pulsators (sequence C, cf. Ita et al. 2004). In NGC 185 we also see a number of LPVs pulsating in the first overtone mode (sequence C’). Interestingly, such stars are missing in NGC 147. We speculate that this lack of shorter period variables in NGC 147 could originate from a difference in the star formation history (SFH) of the two galaxies, with NGC 147 containing a smaller population of intermediate-age stars.
We present detailed star formation and chemical enrichment histories obtained for six isolated Local Group galaxies in the context of the LCID (Local Cosmology from Isolated Dwarfs) project. One of the main goals of the LCID project was to investigate the influence of global processes (like cosmic reionization) or internal mechanisms (such as SNe feedback) on the evolution of dwarf galaxies. We show that all the galaxies in the sample, including those that ended star formation very early on, formed most of their stars after reionization was complete. This demonstrates that reionization alone was not able to stop star formation in these galaxies, as had been expected.
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.
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.
Chemical abundances and radial velocities for ∼210 stars in WLM, ∼130 new stars from DEIMOS spectroscopy, and 80 stars from Leaman et al. (2009) are presented. This sample shows that this isolated dwarf has chemically, dynamically, and spatially distinct stellar populations similar to the nearby dwarf spheroidals. The kinematics of the stars appear to be decoupled from the HI gas, which shows a higher rotational velocity profile. This thicker stellar configuration for a distant, isolated dwarf irregular may have implications for timescales of tidal transformation scenarios of infalling galaxies in the Local Group.
We report on discovery of first unambiguous evidence of the presence of ancient stars in galaxies beyond the Local Group. In two Sculptor Group galaxies, ESO 294-010 and ESO 410-005, we have used deep multi-epoch Hubble Space Telescope ACS observations to image stellar populations fainter than the horizontal branch magnitude. In both galaxies we uncovered a well populated blue horizontal branch, as well as numerous RR Lyrae variable stars. The presence of these low mass star testifies that the star formation in dwarf galaxies started at least 10 Gyr ago.
We have monitored, during four years, a 32′× 32′ area of the Local Group dwarf galaxy NGC 6822. Observations lead to the discovery of 30 Miras and 24 semi-regular and irregular variables. When relating the periods of Miras to their age, we find a continuous star formation during the last 3 Gyr. This result is basically consistent with the hypothesis that the intermediate-age population of NGC 6822 was born following the “polar ring” formation.
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.
Recent observations of UGC 4879 with the Advanced Camera for Surveys on the Hubble Space Telescope confirm that it is a nearby isolated dwarf irregular galaxy. We measure a distance of 1.36 ± 0.03 Mpc using the Tip of the Red Giant Branch method. This distance puts UGC 4879 beyond the radius of first turnaround of the Local Group and ~700 kpc from its nearest neighbor Leo A. This isolation makes this galaxy an ideal laboratory for studying pristine star formation uncomplicated by interactions with other galaxies. We present the star formation history of UGC 4879 derived from simulated color-magnitude diagrams.
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 a new catalog of dwarf galaxies in the Virgo cluster using SDSS DR7 data. Basically, morphologies of galaxies are classified based on the image and spectral feature of SDSS data. We secure 994 dwarf galaxies as members of the Virgo cluster, from which 492 galaxies are new objects not included in the previous catalog. By matching with GALEX GR5 data, we present ultraviolet properties of various dwarf galaxies in the Virgo cluster. We suggest that there are different star formation histories among different subclasses of early-type dwarf galaxies in the Virgo cluster related with their morphology and environment.
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.
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.
The Sculptor dwarf Spheroidal is a Milky Way satellite with a dominant old (>10 Gyr) population that displays at least two distinct stellar populations. We present new sensitive imaging data from CTIO/MOSAIC and make a detailed study of the stellar populations of Sculptor, including the main sequence turnoff (MSTO) properties. We present the Star Formation History (SFH) and Metallicity Distribution Function (MDF) of the Sculptor dSph and find that a well-defined radial gradient is present not only in metallicity but now also in age.
We present the radial velocity (RV) distribution of Carina dSph, obtained using low-, medium- and high-resolution spectroscopic data collected with FORS2 and GIRAFFE at the VLT. We found that regions where the mean RV is smaller the dispersion is also smaller, and the RV distribution is sharper (i.e. the kurtosis attains larger values). The mean RV, when moving to lower Galactic latitude, decreases by ∼ 5kms−1, attains a well defined minimum outside the galaxy center, and a constant value at larger distances. The RV gradient appears to be in the direction of the Carina proper motion. However, this parameter is affected by large uncertainties to reach a firm conclusion. These findings are probably reminiscent of a substructure with transition properties, already detected in dwarf ellipticals, and call for independent confirmations.