We describe the methodology required for estimation of photometric estimates of metallicity based on the SDSS gri passbands, which can be used to probe the properties of main-sequence stars beyond ~10 kpc, complementing studies of nearby stars from more metallicity-sensitive color indices that involve the u passband. As a first application of this approach, we determine photometric metal abundance estimates for individual main-sequence stars in the Virgo Overdensity, which covers almost 1000 deg2 on the sky, based on a calibration of the metallicity sensitivity of stellar isochrones in the gri filter passbands using field stars with well-determined spectroscopic metal abundances. Despite the low precision of the method for individual stars, internal errors of σ[Fe/H]~0.1 dex can be achieved for bulk stellar populations. The global metal abundance of the Virgo Overdensity determined in this way is 〈[Fe/H]〉 = −2.0±0.1 (internal) ±0.5 (systematic), from photometric measurements of 0.7 million stars with heliocentric distances from ~10 kpc to ~20 kpc. A preliminary metallicity map, based on results for 2.9 million stars in the northern SDSS DR-7 footprint, exhibits a shift to lower metallicities as one proceeds from the inner- to the outer-halo population, consistent with recent interpretation of the kinematics of local samples of stars with spectroscopically available metallicity estimates and full space motions.

We have completed a spectroscopic study of the “12.4 hr clump”, the second largest substructure in the Quasar Equatorial Survey Team (QUEST) catalog. First discovered as an over-density of RR Lyrae stars (Vivas et al. 2001, ApJL 554 33), the region containing the “12.4 hr clump” has generated much interest (Newberg et al. (2002), Martinez-Delgado et al. (2007), Juric et al. (2008), amongst many others). Our first spectroscopic study of this clump revealed the presence of a sharp peak in the radial velocity histogram for the candidate stars (Duffau et al. 2006). The combination of this result and metal abundance estimates for the sample was then interpreted as a signature of the presence of a stellar stream within the clump. This sub-structure was named the “Virgo Stellar Stream” (VSS), given its location in the direction of the Virgo Constellation, at approximately 20 kpc from the Sun. Several other groups have studied this region and have suggested that the over-density containing the VSS could extend to larger areas of the sky (outside QUEST's observing range). We present the complete spectroscopic follow up of the clump candidates present in QUEST and the composite of the studies we performed along the same l.o.s., including data at brighter magnitudes (Vivas et al. 2008). Our study confirmed the nature of the VSS, revealed its likely extent within the QUEST survey and defined a number of its relevant properties.

We present the deepest colour-magnitude diagram (CMD) of M32 to date, obtained from deep (F435W, F555W) photometry of HST ACS/HRC images. Due to the high resolution of our images, the small photometric errors, and the completeness level of our data we obtain the most detailed resolved photometric study of M32 to date. The CMD of M32 displays a wide colour distribution of red giant branch stars, mainly due to a metallicity spread, a strong red clump and bright asymptotic giant branch stars. The detection of a “blue plume” in M32 indicates the presence of a very young stellar population. There is not a noticeable presence of blue horizontal branch stars, suggesting that an old population with [Fe/H] < −1.5 does not significantly contribute to the light or mass of M32 in our observed fields.

Determining the kinematics of the dwarf Spheroidal galaxies (dSph) satellites of the Milky Way (MW) is crucial to estimate the mass of our galaxy, to understand its formation process and that of its satellites, to explain the origin of stellar streams in the MW's halo that seem to be related to these satellites, and to understand the role of tidal interactions in the evolution and star formation history of low mass galaxies and of the halo of our Galaxy. In what follows we briefly explain a ground-based astrometric project that will have an impact on these issues, and present some preliminary results.

We obtained long-slit spectra of high S/N of the galaxy M32 with the GMOS Spectrograph at the Gemini-North telescope. We analysed the integrated spectra by means of spectral fitting in order to extract the mixture of stellar populations that best represents its composite nature. As our main result, we propose that an ancient and an intermediate-age population co-exist in M32, and that the balance between these two populations change between the nucleus and outside one effective radius (1reff) in the sense that the contribution from the intermediate population is larger at the nuclear region. We retrieve a smaller signal of a young population at all radii whose origin is unclear, and may be a contamination from horizontal branch stars, blue stragglers or a true young population previously unidentified (Monachesi et al., this volume). We compare our metallicity distribution function for a region 1 to 2 arcmin from the centre to the one obtained with photometric data by Grillmair et al. Both distributions are broad, but our spectroscopically derived distribution has a significant component with [Z/Z⊙] ≤ −1, which is not found by Grillmair et al.

The UV upturn phenomenon found in giant elliptical galaxies through space observations has been a mystery. Recent GALEX observations have revealed new facts. The most notable is the rarity of UV upturn galaxies. Unlike previous beliefs, UV upturn is found only in less than 10 percent of giant ellitpical galaxies. Another notable finding is that the UV flux has been increasing for the last couple of billion years. This is consistent with the theoretical predictions that suggest hot horizontal branch stars are the main UV sources. Remaining theoretical and observational issues on this topic can be found in another recent review of mine (Yi 2008).

How did galaxies evolve from primordial fluctuations to the well-ordered but diverse population of disk and elliptical galaxies that we observe today? Stellar populations synthesis models have become a crucial tool in addressing this question by helping us to interpret the spectral energy distributions of present-day galaxies and their high redshift progenitors in terms of fundamental characteristics such as stellar mass and age. I will review our current knowledge on the evolution of stellar populations in early- and late type galaxies at z < 1 and the tantalizing – but incomplete – view of the stellar populations in galaxies at 1 < z < 3, during the global peak of star formation. Despite great progress, many fundamental questions remain: what processes trigger episodes of galaxy-scale star formation and what quenches them? To what degree does the star formation history of galaxies depend on the merger history, (halo) mass, or local environment? I will discuss some of the challenges posed in interpreting current data and what improved results might be expected from new observational facilities in the near- and more distant future.

We employ IRTF SpeX NIR (0.8–2.4μm) spectra to investigate the stellar population (SP), active galactic nuclei (AGN), featureless continuum (FC) and hot dust properties in 9 Sy 1 and 15 Sy 2 galaxies. Both the starlight code and the hot dust as an additional base element were used for the first time in this spectral range. Our synthesis shows significant differences between Sy 1 and Sy 2 galaxies: the hot dust component is required to fit the K-band spectra of ~90% of the Sy 1 galaxies, and only of ~25% of the Sy 2; about 50% of the Sy 2 galaxies require an FC component contribution ≳20%; this fraction increases to about 60% in the Sy 1. In about 50% of the Sy2, the combined FC and young components contribute with more than 20%, while this occurs in 90% of the Sy1, suggesting recent star formation in the central region. The central few hundred parsecs of our galaxy sample contain a substantial fraction of intermediate-age SPs with a mean metallicity near solar. Our SP synthesis confirms that the 1.1μm CN band can be used as a tracer of intermediate-age stellar populations.

Multi-wavelength photometry of early-type galaxies (ETGs) in the COSMOS survey is used to demonstrate that the low-level star formation activity in the ETG population at late epochs (z < 1) is likely to be driven by repeated minor mergers. While relaxed ETGs are almost entirely contained within the UV red sequence, their morphologically disturbed counterparts are largely found in the blue cloud, regardless of luminosity. Since empirically determined major-merger rates in the redshift range z < 1 are a few factors too low to account for the number fraction of disturbed ETGs, this suggests that minor mergers are the principal mechanism that drives star formation activity in ETGs at low and intermediate redshift.

We present a detailed study of the stellar populations (SPs) and kinematics of the bulge and inner disk regions of nearby spiral galaxies (Sa-Sd) based on deep long-slit Gemini/GMOS data. We find that the SPs of spiral galaxies are not well matched by single episodes of star formation; representative SPs must involve average SP values integrated over the star formation history (SFH) of the galaxy, such as those derived from the “full population synthesis” method used here. Our spiral bulges follow the same correlations of increasing light-weighted age and metallicity with central velocity dispersion as those of elliptical galaxies and early-type bulges found in other studies, but when SFHs more complex and realistic than a single burst are invoked, the trend with age is shallower and its scatter much reduced. In a mass-weighted context, all bulges are predominantly composed of old and metal-rich SPs. Bulge formation appears to be dominated by early processes that are common to all spheroids, whether they currently reside in disks or not. While monolithic collapse cannot be ruled out in some cases, merging must be invoked to explain the SP gradients in most bulges. Further bulge growth via secular processes, or “rejuvenated” star formation, generally contributes minimally to the stellar mass budget. We also demonstrate how the combination of our full population synthesis modeling of high-quality optical spectra of integrated SPs along with optical-NIR broad-band imaging can single out potential model weaknesses and help determine the reliability of the inferred SFHs.

DEEP, AEGIS, and CATS are examples of spectroscopic, multiwavelength, and adaptive optics surveys, respectively, that are pushing the frontiers of stellar population studies of distant galaxies. Together, these surveys affirm the unprecedented richness of high quality information that can already be gathered using today's generation of ground and space telescopes. We highlight several results in extracting the stellar and dynamical masses, chemical abundances, ages, and frequency of galactic winds for galaxies at redshifts up to z ~ 1.4.

Making use of public spectra from Cimatti et al. (2008), we measure for the first time the velocity dispersion of spheroid-like massive (M* ~ 1011M⊙) galaxies at z ~ 1.6. By comparing with galaxies of similar stellar mass at lower redshifts, we find evidence for a mild evolution in velocity dispersion, decreasing from ~240 kms−1 at z ~ 1.6 down to ~180 km s−1 at z ~ 0. Such mild evolution contrasts with the strong change in size (a factor of ~4) found for these type of objects in the same cosmic time, and it is consistent with a progressive larger role, at lower redshift, of the dark matter halo in setting the velocity dispersion of these galaxies. We discuss the implications of our results within the context of different scenarios proposed for the evolution of these massive objects.

This is a pilot study part of a larger program devoted to understand the influence of bars in the evolution of galaxy disks. Some of the goals of this observational program are to compare the stellar metallicity gradients of galaxies with and without bars, the role of bars in the redistribution of mass and angular momentum and to study the evolution of the metallicity gradients with time. Some promising results are presented as well as some words of caution.

An essential use of stellar population synthesis methods is to interpret the observations of galaxies, to infer their star-formation and assembly histories. I describe here some of the uses of these techniques over the past few years, primarily in the context of local galaxy redshift surveys. For this purpose, such surveys have both advantages and disadvantages relative to higher signal-to-noise but smaller studies, which I discuss. After discussing general issues, I describe how the substantial model uncertainties impact the analysis of galaxy spectra. Then, I discuss the special case of elliptical galaxies, arguing that investigators have made interesting discoveries even in the absence of perfect (or even good) models. I then describe the desiderata for massive higher redshift surveys and the trade-offs that must be made between cosmological and galaxy-focused science, in qualitative terms.

The age and chemical composition of the stars in present-day galaxies carry important clues about their star formation processes. The latest generation of population synthesis models have allowed to derive age and stellar metallicity estimates for large samples of low-redshift galaxies. After reviewing the main results about the distribution in ages and metallicities as a function of galaxy mass, I will concentrate on recent analysis that aims at disentangling the dependences of stellar populations properties on environment and on galaxy stellar mass. Finally, new models that predict the response of the full spectrum to variations in [α/Fe] will allow us to derive accurate estimates of elements abundance ratios and gain deeper insight into the timescales of star formation cessation.

We study the evolution of the red sequence and the blue cloud since z ~ 1 to present in GOODS-S field for MB ≤ −20 and M* ≥ 1010M⊙ selected galaxies. We segregate the galaxies in early-type galaxies (ET, E/S0/Sa), and late-type galaxies (LT, Sb-Irr) by their position in the concentration-asymmetry plane, while in passive and star-forming by their rest-frame NUV – R color and their 24μm properties.

We find that red sequence (passive early types) comoving number density rise with cosmic time in both samples, while blue cloud (star-forming late types) number density descent in luminosity-selected samples and increase in mass-selected sample. We also find a third population that comprises star-forming early-type galaxies.

Comparing the observed evolution with the disk-disk (wet) major merger rate in GOODS-S, we infer that only ~20% of M* ≥ 1010M⊙ red sequence galaxies appeared since z = 1 can be explained by this kind of mergers, suggesting that minor mergers and secular evolution are the main processes in the red sequence evolution of M* ≥ 1010M⊙ galaxies since z ~ 1.

A large sample of low surface brightness (LSB) disk galaxies is selected from SDSS with B-band central surface brightness μ0(B) from 22 to 24.5 mag arcsec−2. Some of their properties are studied, such as magnitudes, surface brightness, scalelengths, colors, metallicities, stellar populations, stellar masses and multiwavelength SEDs from UV to IR etc. These properties of LSB galaxies have been compared with those of the galaxies with higher surface brightnesses. Then we check the variations of these properties following surface brightness.

Many of the advances of the last decade in this subject have been the result of large spectroscopic surveys of galaxies on the one hand, and multi-wavelength surveys of galaxies on the other. This talk highlights applications that exploit a combination of spectroscopic and multi-wavelength diagnostics. Most applications to date have been based on spatially integrated measurements of galaxies, but the introduction of wide-field integral field spectrometers promises to extend ths approach to spatially-resolved analyses of galaxies.

Understanding the details of how the red sequence is built is a key question in galaxy evolution. What are the relative roles of gas-rich vs. dry mergers, major vs. minor mergers or galaxy mergers vs. gas accretion? In a recent paper (Wild et al. 2009), we compare hydrodynamic simulations with observations to show how gas-rich major mergers result in galaxies with strong post-starburst spectral features, a population of galaxies easily identified in the real Universe using optical spectra. Using spectra from the VVDS deep survey with <z> = 0.7, and a principal component analysis technique to provide indices with high enough SNR, we find that 40% of the mass flux onto the red-sequence could enter through a strong post-starburst phase, and thus through gas-rich major mergers. The deeper samples provided by next generation galaxy redshift surveys will allow us to observe the primary physical processes responsible for the shut-down in starformation and build-up of the red sequence.

I summarize the main characteristics of cosmological galaxy formation models and discuss some of the main results obtained in the last years.