We present results from new deep HST/ACS photometry of I Zw 18, the most metal-poor blue compact dwarf galaxy in the nearby universe. It has been previously argued that this is a very young system that started forming stars only ≲500 Gyr ago, but other work has hinted that older (≳1 Gyr) red giant branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal evidence for an RGB. Underlying old (≳1 Gyr) populations are therefore present in even the most metal-poor systems, implying that star formation started at z ≳ 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away (D = 19.0 ± 1.8 Mpc) than previously believed.

The outer regions of M31 contain significant substructure. Here we investigate the origins of the substructure in three fields close to the major axis of M31 using colour-magnitude diagrams produced from HST-ACS.

Photometric studies on globular cluster systems in early-type galaxies are widely used to resolve stellar populations in these galaxies. The knowledge of the age and metalicity structure within the globular cluster systems allows to set constraints on the major star formation episodes in the galaxy. Here we present our method of detecting age sub-populations in globular cluster systems based on optical/near-infrared observations and Monte-Carlo simulations. It is based on the determination of the cumulative age distribution and its comparison to simulated globular cluster systems. On various galaxies we will demonstrate the ability of our method to detect age sub-populations and show how we use the derived age structure to find correlations between various galaxy parameters and the formation/evolution of a given galaxy.

We present a new numerical model of star formation history and chemical evolution using the color-magnitude diagrams of resolved stars in galaxies. Our model differs from previous models in that it generates the simulated color-magnitude diagrams with the metal enrichment law calculated from the galactic chemical evolution model. We present a case of applying our model to a resolved galaxy in the Local Group: the Sextans dwarf spheroidal galaxy. It is found the the primary origin of the radial gradient of stellar population in this galaxy is star formation followed by metallicity.

Main results from a study of extremely metal-poor stars are briefly summarised.

The Canis Major overdensity (CMa) was initially proposed to be the remnant of a tidally disrupting dSph galaxy. Since its nature is still subject of debate, the goal of the present work was to conduct a large-scale RR Lyrae survey in CMa, in order to see if there is an overdensity of these stars. The survey spans a total area of ∼ 34 sq. deg. with observations in V and R filters, made with the 1.0m Jürgen Stock Schmidt telescope at the National Astronomical Observatory of Venezuela. Current results in a subregion, including spectroscopic observations, show that the small number of RR Lyrae stars found can be accounted for by the halo and thick disk components of our Galaxy.

Here we present new empirical fitting functions for the Lick/IDS line-strength indices as measured in MILES (Medium-resolution INT Library of Empirical Spectra; Sánchez-Blázquez et al., Cenarro et al. 2007). Following previous work in the field (Gorgas et al. 1993; Worthey et al. 1994; Worthey & Ottaviani 1997), these functions describe the empirical behavior of the line-strength indices with the atmospheric stellar parameters. In order to derive the fitting functions we have devised a new procedure which, being fully automatic, provides a better description of the line-strength index variations in the stellar parameter space.

We present a brief summary of our current results on the stellar distribution and population gradients of the resolved stars in the surroundings of ∼50 nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based telescopes (Subaru, VLT, BTA, Palomar, CFHT, & INT). We examine the radial (in-plane) and vertical (extraplanar) distributions of resolved stars as a function of stellar age and metallicity by tracking changes in the color-magnitude diagram of face-on and edge-on galaxies. Our data show, that the scale length and height of a stellar population increases with age, with the oldest detected stellar populations identified at a large galactocentric radius or extraplanar height, out to typically a few kpc. In the most massive of the studied galaxies there is evidence for a break in number density and color gradients of evolved stars, which plausibly correspond to the thick disk and halo components of the galaxies. The ratio of intermediate-age to old stars in the outermost fields correlate with the gas fraction, while relative sizes of the thick-to-thin disks anticorrelate with galactic circular velocity.

This is a review of the progress of ESO/WFI Imaging and VLT/FLAMES spectroscopy of large numbers of individual stars in nearby dwarf spheroidal galaxies primarily by the Dwarf Abundances and Radial-velocities Team (DART). These observations have allowed us to show that neither the kinematics nor the abundance nor the spatial distributions are easy to explain in a straight forward manner for these smallest galaxies. The main result is that dwarf galaxies show complex and highly specific evolutionary and metal enrichment processes, especially at ancient times. This conclusively proves that these small galaxies are not the building blocks of the larger galaxies in the Local Group.

We present our UBVR photometry observations of giant HII regions in NGC 628 in comparison with a detailed grid of evolutionary models of stellar clusters to interpret the observed colours of young stellar populations in terms of IMFs and SFRs parameters and dust content. Observed colours are complicate functions of searched IMF, age, regime of star formation as well as chemical abudances and interstellar reddening. A content of database of observed properties is crucial to get significant results. To exclude age-reddening and age-metallicity degenerations we carried out UBVR photometry of 127 giant HII regions are star formation complexes (SFCs) with independently measured chemical abundancies Z and interstellar reddening in NGC 628 by Belly & Roy (1992).

We have combined B, V, I time-series photometry from three different telescopes (SUBARU, TNG and HST) to study the variable star population of the remote Galactic globular cluster NGC 2419 and derive hints on a possible extragalactic origin for this cluster. We have discovered 40 new variables (34 RR Lyrae stars, 3 SX Phoenicis stars, Scuti 1 eclipsing binary system and 2 red irregular variables) doubling the number of known variable stars in NGC 2419.

We present a study of 413 early-type dwarf (dE) galaxies in the Virgo Cluster with SDSS multicolour imaging. Several dE subclasses show systematic differences in shape, clustering properties, and stellar content. We find different colour-magnitude relations for nucleated and non-nucleated dEs, as well as for dE samples populating regions of different densities.

Here we present the results of an observational program aimed at providing a stellar library in the K band with an appropriate coverage of physical stellar parameters (effective temperature, gravity and metallicity) to be used for stellar population synthesis models. In particular, we study the behavior of the CO feature at 2.3 μm as a function of the stellar parameters and we will compute empirical fitting functions that can be easily implemented into stellar population models to provide accurate predictions for integrated CO strengths that will help to face outstanding problems in galaxy formation and evolution.

A new theoretical calibration of surface-brightness fluctuations (SBF) for single age, single metallicity stellar populations is presented for the optical and near-IR broad-band filters, as well as for the HST WFPC2 filters. The IAC-star code is used together with two Padua and the Teramo stellar evolution libraries. A set of single-burst stellar populations with a wide range of ages (3Gy-15Gy) and metallicities (Z=0.0001-0.03) have been computed. The present theoretical calibration shows that the analysis of near-IR SBF provides a very powerful tool in the study and characterization of unresolved stellar populations.

We present the preliminary detection of a stellar halo in the nearby, edge-on galaxy, NGC 4244. Using data from the Hubble Space Telescope's Advanced Camera for Surveys (HST/ACS), we have examined number counts of red giant branch (RGB) stars along the minor axis of the galaxy out to 10 kpc, with a limiting surface brightness of μR ∼ 31 mag/arcsec2. At distances more than 2.5 kpc above the disk, a second stellar component is clearly visible. This component is well fit by either an exponential or a power law. The best-fit exponential scale height is similar to the scale length of the galaxy disk, suggesting that we are indeed detecting a halo component. Furthermore, the color of the RGB stars above 2.5 kpc are significantly bluer than those in the disk, suggesting a more metal-poor population. Outside the local group, this finding represents one of the first detections of a resolved stellar halo in a spiral galaxy.

To this contribution we present a brief review of our recent abundance surveys (Reddy et al. 2003; Reddy et al. 2006) of the Milky Way galaxy. Survey focussed on controlled samples of stars selected based on their kinematic properties to belong either thin disk, thick disk or halo components of the Galaxy. Abundance and kinematic results were obtained for about 400 F-, G- and K- dwarfs. Abundances for 22 elements representing different production mechanisms (α-process, p-capture, Fe-peak, s- and r-process) and sites (AGB, SNIa, SNII) were obtained using high quality and high resolution echelle spectra.

Results showed thin and thick disk components are distinct stellar populations with different chemical history. The ratios of α-elements and a few other elements (like Al, V, and Co) are clearly enhanced for stars in the thick disk compared to thin disk stars at given metallicity. Abundance ratios for halo and thick disk stars are very similar. Dispersion in [X/Fe] ratios at given metallicity is comparable to measurement errors inferring lack of cosmic' scatter. Thick disk stars are older (10-13 Gyrs) compared to their counter parts (1-10 Gyrs) in the thin disk. Abundance results for thin and thick disk stars favor the models of hierarchical formation of the galaxy.

The spatial distributions of the mean luminosity-weighted stellar age, metallicity, and α/Fe ratio along both photometric axes of two nearby elliptical galaxies have been obtained using Lick index measurements on long slit spectra in order to reconstruct the star formation history in their kinematically distinct subsystems. Lick indexes were compared with those of single-aged stellar population (SSP) models. A population synthesis method was also applied in order to help disentangling the age-metallicity degeneracy. The stars characteristics are associated with their kinematics: they are older and α-enhanced in the not rotating bulge of NGC 1052 and counter rotating core of NGC 7796, while they show a strong spread of α/Fe and age along the rotating disk of NGC 1052 and an outwards radial decreasing of them outside the core of NGC 7796.

In the framework of an ongoing ACS@HST project (HST program #10586, PI: Aloisi) we have obtained deep multi-color imaging of the very metal-poor Blue Compact Dwarf galaxy IZw18. The data were acquired in time-series fashion to allow the identification of Classical Cepheids (CCs). The main aim of this project is to constrain both the distance and the Star Formation History of the galaxy. However, as a byproduct these data also provide new insights into the properties of CCs at very low metallicities. We have identified 24 candidate CCs in IZw18. New theoretical pulsation models of CCs specifically for the low metallicity of this primordial galaxy (Z=0.0004, Y=0.24) have been computed to interpret our results.

Using Yunnan evolutionary population synthesis (EPS) models, we present integrated colours, integrated spectral energy distributions (ISEDs) and absorption-line indices defined by the Lick Observatory image dissector scanner (Lick/IDS) system, for an extensive set of instantaneous-burst binary stellar populations (BSPs) with interactions. By comparing the results for populations with and without interactions we show that the inclusion of binary interactions makes the appearance of the population substantially bluer. This effect raises the derived age and metallicity of the population.

To be used in the studies of modern spectroscopic galaxy surveys at intermediate/high spectral resolution, we also present intermediate- (3 Å) and high-resolution (∼ 0.3 Å) ISEDs and Lick/IDS absorption-line indices for BSPs. To directly compare with observations the Lick/IDS absorption indices are also presented by measuring them directly from the ISEDs.

We investigate how morphological information of galaxies help us to improve photometric redshift estimation. Using a catalog of morphologically classified bright galaxies derived from the Third Data Release of the Sloan Digital Sky Survey, the statistical properties of different photometrical parameters as a function of morphology are examined. Parameters that best correlates with morphology are used as an additional information to better estimate the redhift. Improvements of several percent are obtained.