We present a new spectral library for old and intermediate-aged single-age, single-metallicity stellar populations for a wide metallicity range. The major ingredient of these models is a new empirical stellar library composed of 985 stars, whose main characteristic is its unprecedented stellar atmospheric parameters coverage. The model SEDs cover the spectral range 3540-7410A at 2.3A (FWHM). We present some advantages and applications of these models.

We present VLT spectroscopy of 13 RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from Sagittarius dSph galaxy. We observed these stars to measure their radial velocities and metal abundances and to complete a sample of stars from the overdensity detected by QUEST in this area of the sky. The full sample contains 42 stars, 29 observed with VLT and 13 observed with GEMINI, we report here the results from the VLT observations so far. We conclude that the new stars and stars reported before by Vivas, Zinn and Gallart (2005) are consistent with belonging to Sgr. We discuss the importance of the final third of the sample, the GEMINI targets, which will provide important constrains on models of the Sgr streams that assume different dark matter halo shapes.

We have undertaken a large, multi-colour, photometric, survey of distant (D > 500 kpc) Local Group dwarf galaxies with the Subaru SuprimeCam wide field camera. These data reach to below the horizontal branch level of these systems, to a depth equivalent to earlier HST WFPC2 studies but over an area ∼ 100 times larger, making them ideal for studies of the global stellar content and structural characteristics of these systems. All eight of our targets show radial gradients and spatially distinct, multiple structural components. This implies that dwarf galaxies in general possess multiple stellar components, akin to larger galaxies. Dynamical models of these systems which do not account independently and consistently for each individual density component may be limited in their applicability.

Recent spectral analysis of the SDSS DR3 has provided a wealth of information determined from the fossil record concerning star formation rates, stellar mass buildup, dust content and stellar mass functions. Essential to this work is an understanding of how model choice influences results gained from the analysis of galaxy spectra. We summarize the results of analyzing two plates of SDSS spectra with five different SSP models which appear in full in Panter et al. (2006) and discuss some residuals seen in a far larger sample of galaxies.

We present an overview and the first results of a large ACS survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N ≳ 10 for main sequence stars with masses ≳ 0.2M⊙ in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Our sample consists primarily of globular clusters within about 20 kpc of the Sun with fairly low reddening. We also added a few clusters of special interest toward the Galactic bulge as well as six clusters believed to be members of the Sagittarius dwarf spheroidal galaxy. This sample of 66 globular clusters was observed with HST using 134 orbits. We plan to explore a wide variety of scientific issues with these data including the formation timescale of the Milky Way halo, the age of the Universe, the dynamical evolution of stars in globular clusters (e.g. mass segregation), and the mass of the Milky Way and its gravitational potential field.

We present our on-going work on the determination of elemental abundances of giants in the Galactic Bulge by means of infrared spectroscopy. We show a preliminarily reduced spectrum and a synthetic spectrum fit of the Bulge giant Arp 4203 recorded with the near-infrared, high-resolution Crires spectrograph mounted on the VLT during its science verification run in August 2006. Abundances derived from this spectrum are discussed.

During Cycle 14 a total of 113 HST orbits were secured to observe five isolated dwarf galaxies, namely Tucana, LGS3, LeoA, IC1613, and Cetus. The aim of the project is a full characterization of the stellar content of these galaxies, in term of their SFH, radial distributions, halo populations and variable stars. Deep (V≈29) F475W, F814W data allowed us to fully sample all the evolutionary phases from the tip of the Red Giant Branch (RGB) to well below the old Main Sequence Turnoff (MSTO). Here we describe the observational design, and the reduction and calibration strategy adopted. A comparison of the results obtained using two different packages, ALLFRAME and Dolphot, is presented.

Recent stellar spectral libraries have sought higher resolution and the accurate determination of specific optical spectral indeces as stellar population indicators. But the value of accurate flux comparisons over wide wavelength regions should still be emphasized, particularly as more and better spectro-photometric data for composite populations becomes available.

The stellar population models dramatically progressed with the arrival of large and complete libraries, ELODIE, CFLIB (=Indo-US) and MILES at a relatively high resolution. We show that the quality of the fits is not anymore limited by the size of the stellar libraries in a large range of ages (0.1 to 10 Gyr) and metallicities (−2 to +0.4 dex). The main limitations of the empirical stellar libraries are (i) the coverage of the parameter space (lack of hot stars of low metallicity), (ii) the precision and homogeneity of the atmospheric parameters and (iii) the non-resolution of individual element abundances (in particular [α/Fe]). Detailed abundances measurements in the large libraries, and usage of theoretical libraries are probably the next steps, and we show that a combination between an empirical (ELODIE) and a theoretical library (Coelho et al. 2005) immediately improves the modeling of (α-enhanced) globular clusters.

The structural parameters of bulges, disks and bars of a sample of nearly 1000 nearby galaxies are being determined through sophisticated image decomposition in the g, r and i bands. The sample is carefully drawn from the Sloan Digital Sky Survey Data Release 2 (SDSS DR2), contains 963 galaxies, of which 407 host AGN, and we show that it is representative of the galaxy and AGN populations in the local universe. A large number of other physical properties have also been determined for these galaxies within the SDSS collaboration. With these data, we reinforce several recent studies and find a number of results leading to new insights into how the different galaxy components relate to each other and the extent to which the hosts galaxies of AGN differ from quiescent galaxies.

The present study tries to identify the mechanism responsible for the hostility to star formation (SF) found in galaxies of dense environments. Among the several candidate mechanisms, the gas-rich merger of galaxies stands as a plausible option since the realisation that the AGN-feedback, which follows the merger, can deplete the gas and truncate the SF of the resulting spheroid. Our strategy is to look for traces of SF truncation in the elliptical galaxies of Hickson compact groups (HCGs), because these are the ideal environments for mergers to occur. The stellar populations of 22 elliptical galaxies in HCGs and 12 in the field are compared using three different SSP models and two different emission correction procedures. Along the study, an extensive use of the [Mg/Fe] abundance ratio is made, because it contains useful information on the timescale of the SF history of the galaxies. Systematic differences of [Mg/Fe] and [Z/H] between both galaxy subsamples (field and HCGs) suggest that SF truncation events have affected the intermediate-mass galaxies of the HCGs. Our interpretation is that when a field galaxy enters the dense HCG environment it merges with another galaxy and ends up as a passive elliptical with traces of SF truncation.

Our knowledge about unresolved stellar systems comes from comparing integrated-light properties to SSP models. Therefore it is crucial to calibrate the latter as well as possible by integrated-light colors of clusters that have reliable ages and metallicities (deep CMDs and/or spectroscopy of individual giants). This is especially true for the NIR and MIR, which contain important population synthesis diagnostics and are often used to derive masses of stellar systems at hight redshifts. Here we present integrated colors of MC clusters using new VRI photometry and 2MASS data. In the imminent future we will include MIR data from Spitzer/IRAC. We compare our new colors with popular SSP models to illustrate their strengths and weaknesses.

On the basis of large photometric and spectroscopic datasets, we investigated the general properties of the different sub-populations of ω Centauri in different regions of the color-magnitude diagram. In particular, we analysed the morphology, the structure and the chemical properties of the stellar populations of the system in variuos evolutionary stages. Such a large observational effort allowed us to constraint the different hypotesis on the origin and chemical evolution of this peculiar stellar system.

We spectroscopically characterize the Galactic Bulge to infer its star formation timescale, compared to the other Galactic components, through the chemical signature on its individual stars. O, Na, Mg, Al were obtained for 50 K giants in four fields towards the Galactic bulge from UVES spectra (R=45,000), while Fe was measured in more than 400 stars with a slightly low resolution (R=20,000) and the GIRAFFE spectrograph at VLT. Oxygen and Magnesium show a well defined trend with [Fe/H], with abundances larger than those measured in both thin and thick disk stars, supporting a scenario in which the bulge formed before and more rapidly than the disk. On the other hand the iron distribution peaks at solar metallicity and it is slightly narrower than that measured in previous works. Part of the present results have been published by Zoccali et al. (2006) and Lecureur et al. (2006), and part will be discussed in forthcoming papers.

We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the one chip per galaxy we studied so far, a total of 230 and 80 candidates variables were found, respectively. For Tucana, we identified 134 of them as RR Lyrae stars (RRL) pulsating in the fundamental mode (RRab), 51 in the first-overtone mode (RRc), and 37 in both modes simultaneoulsy (RRd), as well as four candidate anomalous Cepheids (AC). In the case of LGS3, we found 45 RRab and 5 RRc, plus three candidates RRd and five candidate AC. The metallicities obtained from the mean period of the RRab are [Fe/H]Tuc = − 1.7 and [Fe/H]LGS3 = − 1.8.

A new method to extract information from color-magnitude diagrams (CMDs) is presented. We employ a Richardson-Lucy (RL) algorithm to the analysis of the observational uncertainties. The resulting reconstruction is a cleaned data set with which to perform analyses of the star formation history. For this task, synthetic stellar populations are built and compared with the data CMD. As an application, the star formation rate in the solar vicinity is recovered.

Recent observations have revealed a class of unusually HI-rich early-type galaxies. By combining observations of their morphology, stellar populations and neutral hydrogen we aim to understand how these galaxies fit into the hierarchical formation paradigm. Here we present the result of our radio and optical observations of a test case galaxy, the E/S0 IC 4200.

We adopted uvby Strömgren photometry to investigate the metallicity distribution of ω Cen Red Giant (RG) stars. We provided a new empirical calibration of the Strömgren m1≡(v − b) − (b − y) metallicity index based on cluster stars. The new calibration has been applied to a sample of ω Cen RGs. The shape of the estimated metallicity distribution is clearly asymmetric, with a sharp cut-off at low metallicities ([Fe/H] < −2.0) and a metal-rich tail up to [Fe/H] ∼ 0.0. Two main metallicity peaks have been identified, around [Fe/H]≈ −1.9 and −1.3 dex, and a metal-rich shoulder at ≈ −0.2 dex.