The presence of blue straggler stars (BSs) as secure members of Galactic open clusters (OCs) poses a major challenge to the conventional picture of evolutionary population synthesis (EPS) based on the stellar evolution theory of single stars, since the major formation ways of BSs are all correlated with stellar interactions. With a working sample including 100 Galactic OCs with age ranging from 0.1 to 10 Gyr, the contributions of BSs to the integrated light of their host clusters are calculated on an individual cluster base. We also show in this work that the intrinsic evolutionary stages of OCs can be enormously misunderstood by the conventional simple stellar population (SSP) models, if the real/observed integrated spectral energy distributions (ISEDs) of OCs are the only thing that we can depend on and are fitted with ISEDs of conventional SSPs, and consequently the huge uncertainties in age and/or metallicity can be conservatively estimated at 50%. Thus, we strongly confirm that when the conventional EPS technique is used to study the properties of stellar populations in complicated systems such as galaxies, the contributions of BSs should be definitely taken into account in order to avoid big fitting mistake.

We are using Subaru/Suprime-Cam to survey the luminous resolved stellar populations in the outskirts of galaxies lying beyond the Local Group. We report here on first results from our analysis of the low mass edge-on galaxy, NGC 4244, lying at 4.4 Mpc. Red giant branch stars are clearly resolved in the outer disk and extraplanar regions and our preliminary stellar density maps suggest the presence of an extended and asymmetric stellar component reaching significant distances above the disk plane.

Long-slit spectra have been obtained with the Keck telescope for a sample of 11 early-type galaxies covering a range in luminosity. Rotation velocity and velocity dispersions, together with 20 Lick line-strength gradients have been measured to two effective radius. We transform line-strength gradients into age, metallicity and α/Fe. Galaxies show very shallow age gradients, strong metallicity gradients, and both positive and negative α/Fe gradients, which cannot be explained with simple outside-in scenarios of galaxy formation. We explore the correlation of the gradients with other global parameters of the galaxies, finding different behaviour for the two families of galaxies, luminous (boxy, core inner profile) and less luminous (disky, cuspy inner profile) galaxies.

We report about the results of the Stellar Evolution Challenge, initiated at the Leiden Lorentz Center meeting “Fine-tuning stellar population models” (June, 2006).

We present infrared observations with the integral field spectrometer SINFONI on the ESO-VLT (Very Large Telescope) Yepun of obscured stellar clusters in HII regions. The classification in spectral subclasses of the detected early type stars together with a determination of the interstellar extinction and the stellar apparent magnitudes enable us to obtain spectrophotometric distances, which are independent and complementary to kinematic distances and free from the ambiguity inherent to the determination of the latter. This study is part of a larger project, investigating the spatial distribution of Galactic Hii regions. Our primary aim is obtaining a more complete and less biased picture of the Milky Way's spiral structure.

In the last years, observations made by large international consortia made available hundreds of thousands of high-resolution spectra of galaxies in the local and distant Universe. The interpretation of these spectra in terms of stellar ages and metallicities is the key to reconstructing the star formation and chemical enrichment histories of the Universe. However, current spectral models rely on libraries of observed stars in the solar neighborhood, hampering our hability in interpreting populations that had undergone different star formation histories. Although it is available in the literature Lick/IDS model indices with variable chemical mixtures, we were still lacking high resolution spectral energy distribution models. Those models are required if we want to exploit the total amount of information contained in observed galaxy spectra. We present here our work aiming at the development of such models, constructed with a consistent set of synthetic stellar library and new stellar evolution models.

It is well established that mass loss from AGB stars due to dust driven winds cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars using detailed frequency-dependent radiation hydrodynamics including dust formation. We present a study of the thresholds for the mass loss rate as a function of stellar parameters based on a subset of a larger grid of such models and compare these results to previous theoretical work. Furthermore, we demonstrate the impact of the pulsation mechanism and dust formation for the creation of a stellar wind and how it affects these thresholds and briefly discuss the consequences for stellar evolution.

I review efforts to determine the ages and chemical compositions of early-type galaxies from their integrated spectra. After reviewing the history of integrated light analyses, we consider how reliably the light-weighted mean ages and chemical compositions of galaxies can be extracted from current state-of-the-art population synthesis modelling and observational data. Particular attention is given to assessing the best strategies for pursuing integrated spectrum analysis, both from observational and modelling standpoints. Finally, we consider efforts to move beyond light-weighted mean ages and metallicities, specifically, to constrain star formation histories with multiple episodes of star formation.

We present the first results of our study of stellar populations in the Large and Small Magellanic Clouds based on multi-band WFPC2 observations of “random” fields taken as part of the “pure parallel” programme carried out with the HST as a service to the community.

Using a huge library of 1-Å resolution spectra over a wide range of log Teff, log g and [Fe/H] and for both solar and α-enhanced abundance ratios [α/Fe], we derive the response functions (s) which are commonly used to correct indices with solar α-enhanced ratios to indices with α-enhanced>0. The s vary not only with the type of star but also with metallicity. With the aid of this and the fitting functions (s), we derive the indices for single stellar populations. The new indices for single stellar populations are used to derive the age, metallicity, and degree of enhancement first for sample of Galactic globular clusters for which these key parameters have been independently derived from the colour-magnitude diagram and/or spectroscopic studies, and second to study the classical two index diagram.

The combination of huge databases of galaxy spectra and advances in evolutionary synthesis models in the past few years has renewed interest in an old question: How to estimate the star formation history of a galaxy out of its integrated spectrum? Fresh approaches to this classical problem are making it possible to extract the best of both worlds, producing exquisite pixel-by-pixel fits to galaxy spectra with state-of-the-art stellar population models while at the same time exploring the fabulous statistics of mega-surveys to derive the star-formation and chemical enrichment histories of different types of galaxies with an unprecedented level of detail. This review covers some of these recent advances, focusing on results for late-type, star-forming galaxies, and outlines some of the issues which will keep us busy in the coming years.

We present deep B,V,I photometry of three wide fields within the core of the Sagittarius dwarf spheroidal galaxy (Sgr dSph). We provide accurate templates for some well identified sequences of the CMD of the galaxy (Blue HB, Red HB, etc.) as tools to recognize portion of the tidal Stream detected by various surveys, to compare the stellar populations in the main body of Sgr and in its Stream, and to derive accurate distances to Stream stars.

We present a detailed study of stellar line indices along the bar region for a sample of six early-type galaxies. We find positive gradients within the bar region in the metal indices in four of the six galaxies, and opposite trends in the other two. These latter two galaxies are classified as SAB and they present exponential bar light profiles. For all the galaxies, we find a positive gradient in the Balmer indices. There is a clear correlation between the position of morphological features inside the bar region with changes in the slope and value of the indices, which indicate, using stellar population analysis, changes in the stellar population. Therefore, it seems that the bar regions show a gradient in both age and metallicity, changing radially to younger and more metal rich populations for all the galaxies except for those two with exponential profiles. This is the first time such an analysis of the stellar populations in bars has been performed. The radial distribution of the indices is related to the star formation history of the bar, understanding these trends will help us to understand how bars are formed and how they evolve.

We are calculating stellar spectra for types A through K using Kurucz codes, Castelli models, and Kurucz laboratory lines plus guessed identifications for other lines in the spectra. Weighted coadditions of these spectra are being constructed to match spectra observed in integrated light of old stellar systems such as elliptical galaxies and globular clusters. Grids of theoretical spectra, both stellar and composite, that include an enhancement of light elements and span a wide metallicity range will be calculated over 2200Å–9000Å, and will be archived on MAST at the Hubble Space Telescope website. Here we summarize our results and describe how we automate the fit to our grid of an observed high-resolution stellar or globular-cluster spectrum, to determine the stellar parameters or to break the age-metallicity degeneracy.

We try to find some colors that have the potential to study stellar age and metallicity from UBV RIJHK and ugriz colors using the simple population synthesis model of Bruzual & Charlot 2003 (BC03). The principal component analysis (PCA) (See Kong & Cheng 2001) and relative sensitive parameter techniques (see Worthey 1994) are used in this work. As a result, some colors such as (B – K), (R – K), (I – H), (B – V), (U – R), (R – I), are found to have the ability to break the stellar age-metallicity degeneracy partially and can possibly be used for studying stellar populations of galaxies.

Integrated spectra of 17 blue Large Magellanic Cloud (LMC) clusters were obtained in the (3600-6800 Å) range using the CASLEO (Argentina) 2.15 m telescope. The typical resolution and dispersion were 12 Å and 3.5 Å/pixel, respectively. Cluster ages were derived by means of two methods: the template matching, in which the observed spectra are compared and matched to template spectra with well-known determined properties, and the equivalent width (EW) method, in which diagnostic diagrams involving the sum of EWs of selected spectral lines were employed together with their calibrations with age and metallicity given by Santos & Piatti (2004), hereafter SP. The spectra were normalized to Fλ = 1 at ∼ 5870 Å. The EWs of H Balmer, KCaII, G band and MgI were measured within the spectral windows defined by Bica & Alloin (1986). We then obtained the sum of EWs of the 3 metallic lines (Sm) and of the 3 Balmer lines Hβ, Hγ and Hδ (Sh). As a first approach to get cluster ages, we used the diagnostic diagrams defined by SP. The clusters were then age-ranked according to the SP's calibrations. We used Sm to get a first age estimate using: log t(Gyr) = a0 + a1Sm + a2Sm2, where a0 = −2.18 ± 0.38, a1 = 0.188 ± 0.080 and a2 = −0.0030 ± 0.0032. We then used Sh to get a second age estimate guided by the previous Sm estimate, since from Sh two solutions are possible: log t(Gyr) = { − b ± [b2 − 4a(c − Sh)]1/2}/2a, where a = −6.35 ± 0.18, b = −8.56 ± 0.35 and c = 23.32 ± 0.20. The average of these two estimates is listed in column 7 of Table 1.

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC), using the IRAC and MIPS instruments on board the Spitzer Space Telescope (Spitzer). Meixner et al. (2006) provides an overview of the project and initial results and their Table 1 (repeated here) outlines the survey's salient characteristics. In this project, we are surveying the agents of a galaxys evolution (SAGE), i.e. the interstellar medium (ISM) and stars, and their interaction on the galaxy wide scale of the LMC. Spitzer IRAC and MIPS images provide key insights into the life cycle of matter in a galaxy because the infrared emission from dust grains is an effective tracer of the ISM, star formation, and stellar mass-loss. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities > 1.2×1021 H cm−2 permits detailed studies of dust processes in the ISM. SAGE's point source sensitivity enables a complete census of newly formed stars with masses >3 M⊙ that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass loss rates > 10−8 M⊙ yr−1 will quantify the rate at which evolved stars inject mass into the ISM of the LMC (Blum et al. 2006). The SAGE data are nonproprietary. The preliminary SAGE catalog of epoch 1 photometry, prepared by the SAGE Team and released to the public on January 3, 2006, contains over 4 million IRAC sources, band merged with 2MASS photometry and over 60,000 MIPS 24 micron sources. Preliminary estimates indicate that foreground Milky Way stars and background galaxies may comprise as much as 18% and 12%, respectively, of these catalogs. To learn more about the SAGE project: http://sage.stsci.edu/.

The Ks-band magnitude distribution of carbon-rich and oxygen-rich asymptotic giant branch stars within Local Group galaxies like the Magellanic Clouds, NGC 6822, M33 and SagDIG is easily obtained from ground-based observations. Appropriate stellar evolutionary models covering a range of metallicities and star formation rates are used to produce theoretical distributions that allow us to derive the history of star formation across these galaxies. I will show the result of these studies and discuss the application of this technique to more distant systems as well as deeper observations, like those that VISTA will provide, to improve our understanding of, in particular, the Magellanic Clouds.

Secular evolution theory predicts that spheroidal components are formed thanks to bars that can transport gas to the central regions of the galaxies. This activity would trigger the star formation needed to generate galactic bulges. According to this idea, bars at smaller scales may carry material to the galactic center and feed active galactic nuclei. For the first time, we have carried out a complete kinematical and stellar population analysis of a double-barred galaxy (NGC 357) to determine if this system of two bars is efficient enough to cause that kind of effects. We have measured suitable line-strength indices on high quality spectra to obtain the relative ages and metallicities for the different structural components and thus constrain the star formation history. First results seems to indicate that in the case of NGC 357 the nuclear bar hasn't played an important role in fueling the star formation in the center.

How clumpy are galactic halos? Recent observations around both the Milky Way and the Andromeda galaxy (M31) have revealed numerous faint stellar streams and dwarf galaxies, leading to the belief that more of these may yet remain undetected. In this contribution, we present the map produced from the Megacam/CFHT survey that our group has undertaken in the outer halo of M31 and that, for the first time, gives a deep panoramic view of a significant region of the outer halo of a spiral galaxy. This panoramic survey, which covers ∼ 60 sq. deg. of the southern quadrant of the M31 halo, extends the WFC/INT survey of the inner halo (Ferguson et al. 2002) from a projected distance of ∼ 50 to ∼ 150 kpc. It is deep enough to cover three magnitudes below the tip of the red giant branch of stellar populations at the distance of M31. The survey reveals:

• • Three faint dwarf galaxies with absolute magnitudes in the range -7.3 < M_V < -6.4 and the most remote M31 globular cluster at a projected distance of ∼ 120 kpc from M31 (see Martin et al. 2006 for more details).

• • That the giant stream of Ibata et al. (2001) covers a much wider area than previously expected from shallower surveys, has an apocenter at 125±25 kpc from M31 and is probably due to the accretion of a small disk galaxy.

• • A new stellar stream or shell approximately perpendicular to the minor axis of M31 at a projected distance of 120 kpc and with a metallicity of [Fe/H] ∼ -1.5 (assuming it is at the distance of M31).

• • A new stellar stream along the ma jor axis of M31 that extends to at least 100 kpc from M31 with [Fe/H] ∼ -1.3 (once again assuming it is at the distance of M31).

• • Regions void of any stellar structure brighter than 34-35 mag/arcsec2 at a distance of 100 to 130 kpc from M31.

The survey shows that the outer halo of the Andromeda galaxy is very structured, in qualitative agreement with recent cosmological simulations (e.g. Bullock & Johnston, 2005).