With the advent of large wide-field cameras and deep, CCD-based “all-sky” surveys, the study of the structure of the Milky Way (MW) through its stellar populations has become feasible. We are developing tools to apply CMD fitting techniques for this purpose. Here we present two applications. First we constrain the properties of the old and young stellar populations of the Canis Major overdensity. Secondly, we demonstrate how CMD fitting can be used to detect faint overdensities in the halo of the MW and determine their properties, using data from the Sloan Digital Sky Survey (SDSS).

Previous attempts to explain the observed mass–metallicity relation of galaxies invoked selective outflows that need to be tuned to the particular cases.

Here we demonstrate that the simple concept of a star-formation history (SFH) dependent galaxy-wide integrated galactic initial stellar mass function (the IGIMF) can easily and naturally explain the mass–metallicity relation.

The idea relies on the simple fact that stars form in clusters. So to construct the IGIMF one needs to add all the stellar IMFs in the clusters formed in one epoch. The SFH enters because a galaxy with a high SFR can form more-massive clusters than a galaxy with a low SFR. Dwarf galaxies therefore have, per unit stellar mass formed, far fewer if any supernovae of type II than massive galaxies.

This work rests on the recent paper by Köppen, Weidner & Kroupa (2007).

We present a new library of stellar evolution models for a large range of masses and chemical compositions, based on an up-to-date theoretical framework. We briefly discuss the physical inputs and the assumptions adopted in computing the models. The last developments of this database are also presented.

Do LSB galaxies have globular clusters? If they do, how do the main features of these systems compare to the ones observed in other galaxies? What are the implications of these findings on the formation of the halo and the star formation history of LSB galaxies? These are the questions we are looking to address with this study.

We present preliminary results of an ACS/HST survey for globular clusters on massive LSB galaxies.

We present preliminary long-slit spectroscopy of a sample of nearby cD galaxies recently observed with the William Herschel and GEMINI North and South Telescopes. So far, we have accumulated data for 27 cDs. More is being observed with the GEMINI telescopes, to create a statistically significant sample. Unresolved stellar populations will be used to investigate the history of when stars formed in these galaxies. Central and spatially resolved line strengths measured from these high signal-to-noise data will be interpreted via population modelling and used to derive luminosity weighted average ages and abundances. Comparison with previous results from other Hubble types of galaxies will highlight relative differences in their evolution.

We analyse a sample of 1,652 elliptical galaxies morphologically selected from the SDSS DR4 with respect to their stellar population properties such as luminosity weighted ages, metallicities and α/Fe element abundance ratios. We confirm and statistically strengthen earlier results that all three stellar population parameters correlate with velocity dispersion, hence galaxy mass. Most surprisingly and differently from previous work, however, these scaling relations turn out to be independent of environmental density for the bulk of the population. This implies that the formation epochs and timescales of the stellar populations in massive galaxies are entirely independent of the environmental density and only driven by galaxy mass. At the low-mass end some fraction of the galaxies show signs of recent star formation on top of the general downsizing trend. It is only the fractional amount of such rejuvenated galaxies that reveals strong dependence with environment. The influence of the environment apparently undergoes a phase transition at about z∼ 1. An early galaxy formation phase without environmental dependence is followed by a rejuvenation phase at redshifts below z∼ 1 affecting mostly low-mass galaxies, during which the environment plays a crucial role.

1. The data. We study detailed star formation histories (SFH) of 14 dwarf galaxies in the central parts (≤ 300 Kpc) of the two nearby galaxy groups: M 81 (D=3.6 Mpc) and Cen A (D = 3.8 Mpc). The images of 7 galaxies were selected from the sample of about 50 nearby dwarf galaxies observed with the Advanced Camera for Surveys (ACS) at the Hubble Space Telescope within our prog. 9771 & 10235, (PI I.Karachentsev). The rest of the images (ACS and WFPC2) were taken from the Hubble Archive (prog. 9884, 5898 and 6964).

The Indo-US coudé feed stellar spectral library (CFLIB) published recently by Valdes et al. (2004) contains spectra of 1273 stars in the spectral region 3460 to 9464 Å at a resolution of 1 Å. About 500 stars in this database have gaps ranging from a few Å to several tens of Å in this wavelength range. We use a variation of Principal Component Analysis (PCA) technique to fill gaps of up to 5Å in a subset of spectra from the CFLIB. We hope to exploit the full potential of the scheme and attempt to fill larger gaps in stellar spectra in a subsequent study.

We report preliminary results of the characterization of bulge and inner disk stellar populations for 8 nearby spiral galaxies using Gemini/GMOS. The long-slit spectra extend out to 1–2 disk scale lengths with S/N/Å>50. Two different model fitting techniques, absorption-line indices and full spectral synthesis, are found to weigh age, metallicity, and abundance ratios differently, but with careful attention to the data/model matching (resolution & flux calibration), we are able constrain real signatures of age and metallicity gradients in star-forming galaxies.

The Star Formation History (SFH) of the Phoenix dwarf galaxy has been obtained using IAC-star and IAC-pop. The observations are based on WFPC2@HST. The results show a Star Formation Rate (SFR) decreasing with time and that star formation has happened in Phoenix up to 0.5 Gyr ago. The metallicity decrease with the stellar age: z>0.0007 for stars younger than 5 Gyr and z<0.0004 for stars older than 10 Gyr. Using the SFH as a function of galactocentric radius, the length scale αN of the star density distribution profile has been obtained as a function of the time. The results show that αN decreases smoothly with time, suggesting a sort of shrinking scenario for the evolution of Phoenix.

We summarise the current status of our project to identify and classify ~ 6-10 galaxies thanks to strong gravitational lensing. Building on the detailed work by Richard et al. (2006), we present results from new follow-up observations undertaken with the ACS/HST and the Spitzer space telescope and compare our results with findings from the Hubble Ultra-Deep Field (UDF).

We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628. We investigate the role of the individual Hii regions together with the large-scale gravitational mechanisms which govern star formation and overall evolution in spiral galaxies. Our kinematical analysis (reinforced by literature maps in HI and CO at lower angular resolution) enables us to verify the presence of an inner rapidly rotating inner disk-like component which we attribute to long term secular evolution of the large-scale spiral arms and oval structure. We find that gas is falling in from the outer parts towards the bluer central regions. This could be an early phase in the formation of a pseudo-bulge. We find signatures of radial motions caused by an m = 2 perturbation, which are likely to be responsible for the inflow of material forming the circumnuclear ring and the rapidly rotating inner structure.

Spectra in the range 4000-7000 Å were obtained for a sample of bulge stars using the GMOS-Gemini low resolution spectrograph. The sample stars were selected from a VLT-FLAMES project for the observation of 1000 bulge stars, for which abundance ratios have been derived. Our aim is to study old stellar populations in external galaxies.

We present a grid of 440 spectro-photometric models for simulating spiral and irregular galaxies. They have been consistently calculated with evolutionary synthesis models which use as input the information proceeding from chemical evolution models. The model predictions are spectral energy distributions, brightness and color profiles and radial distributions of spectral absorption stellar indices which are in agreement with observations

We present HST/ACS observations of five regions in the outskirts of M31. Four are centered on prominent stellar overdensities roughly aligned with the major axis. Well-populated colour magnitude diagrams (CMDs) show that these fields differ in morphology from the fifth (Minor Axis) field. Comparisons with isochrones present tantalising evidence that they contain a small contingent of very young (≤ 1Gyr) stars, at odds with what may be expected of typical halo populations, suggesting that this low-latitude substructure could feasibly have formed in the disk.

We show initial results from our ongoing HST/ACS GHOSTS survey of the resolved stellar envelopes of 14 nearby, massive disk galaxies. In hierarchical galaxy formation the stellar halos and thick disks of galaxies are formed by accretion of minor satellites and therefore contain valuable information about the (early) assembly process of galaxies. We detect for the first time the very small halo of NGC 4244, a low mass edge-on galaxy. We find that massive galaxies have very extended halos, with equivalent surface brightnesses of 28-29 V-mag arcsec−2 at 20-30 kpc from the disk. The old RGB stars of the thick disk in the NGC 891 and NGC 4244 edge-on galaxies truncate at the same radius as the young thin disk stars, providing insights into the formation of both disk truncations and thick disks. We furthermore present the stellar populations of a very low surface brightness stream around M83, the first such a stream resolved into stars beyond those of the Milky Way and M31.

Choosing either an empirical or a theoretical stellar library in stellar population models is a subject of debate. Theoretical models do not perfectly match high-resolution spectra of real stars, but we can use them to overcome the natural limitations of empirical libraries on covering abundance patterns that differ from solar neighbourhood stars. Here, we will mix stellar population synthesis models based on both theoretical and empirical libraries, in order to explore a new method to constrain the enhanced α-elements pattern on galaxies.

Chemodynamical disc galaxy simulations calibrated with stellar populations synthesis models allow to build mock multi-wavelength images from numerical simulations. The morphological evolution of the mass and colour distributions can thus be simultaneously followed. We present the evolution of the stellar disc and bar structural parameters and summarize a number of important results we got in the recent past.

We report the results of our project devoted to study the chemical enrichment history of the field population in the Magellanic Clouds using Ca II triplet spectroscopy.

We present a preliminary reconstruction of the star formation history of the Sagittarius dwarf irregular galaxy (SagDIG), a dIrr galaxy at the border of the Local Group. SagDIG is a lively star-forming dwarf galaxy with very low metallicity and an abundant gas reservoir, for which deep HST ACS observations have been obtained (Momany et al. 2005). We have built synthetic color-magnitude diagrams in the intrinsic ACS bands F606W and F814W and compared them with our HST/ACS observations to derive the global star formation history of the galaxy. We find a broad episode of star formation between 3 and 8 Gyr ago, with a low-intensity tail of star formation at older ages, and a SF enhancement in the last Gyr. An upper limit to the old populations is set by modeling the old HB. Our modeling of the color-magnitude diagram also provides some constraints on the chemical enrichment history of SagDIG.