Received; accepted

The Astrophysical Journal, 1997

We present new measurements of central line-strength indices (namely Mg 2 , Fe , and Hβ) and gradients for a sample of 6 bright spheroidal galaxies (Sph's) in the Virgo cluster. Comparison with similar measurements for elliptical galaxies (E's), galactic globular clusters (GGC's), and stellar population models yield the following results: (1) In contrast with bright E's, bright Sph's are consistent with solar abundance [Mg/Fe] ratios; (2) Bright Sph's exhibit metallicities ranging from values typical for metal-rich GGC's to those for E's; (3) Although absolute mean ages are quite model dependent, we find evidence that the stellar populations of some (if not all) Sph's look significantly younger than GGC's; and (4) Mg 2 gradients of bright Sph's are significantly shallower than those of E galaxies. We conclude that the dichotomy found in the structural properties of Sph and E galaxies is also observed in their stellar populations. A tentative interpretation in terms of differences in star formation histories is suggested.

The Astrophysical Journal, 2002

As a result of an extensive observational campaign targeting the Virgo cluster, we obtained integrated (drift-scan mode) optical spectra and multiwavelength (UV,U,B,V,H) photometry for 124 and 330 galaxies, respectively, spanning the whole Hubble sequence, and with m p ≤ 16 (M p ≤ −15). These data were combined to obtain galaxy Spectral Energy Distributions (SEDs) extending from 2000 to 22000Å. By fitting these SEDs with synthetic ones derived using Bruzual & Charlot population synthesis models we try to constrain observationally the Star Formation History (SFH) of galaxies in the rich cluster of galaxies nearest to us. Assuming a Salpeter IMF and an analytical form for the SFH, the fit free parameters are: the age (T) of the star formation event, its characteristic timescale (τ) and the initial metallicity (Z). In this work we test the (simplistic) case where all galaxies have a common age T =13 Gyr, exploring a SFH with "delayed" exponential form (which we call "a la Sandage"), thus allowing for an increasing SFR with time. This SFH is consistent with the full range of observed SEDs, provided that the characteristic timescale τ is let free to vary between 0.1 (quasi instantaneous burst) and 25 Gyr (increasing SFR) and Z between 1/50 and 2.5 Z⊙. Elliptical galaxies (including dEs) are best fitted with short time scales (τ ∼ 3 Gyr) and metallicity varying between 1/5 and 1 Z⊙. The model metallicity is found to increase as a function of H band luminosity. Spiral galaxies require that both τ and metallicity correlate with H band luminosity: low mass Im+BCD have sub-solar Z and τ ≥ 10 Gyr, whereas giant spirals have solar metallicities and τ ∼ 3 Gyr, consistent with elliptical galaxies. Moreover we find that the SFH of spiral galaxies in the Virgo cluster depends upon the presence at their interior of fresh gas capable of sustaining the star formation. In fact the residuals of the τ vs. L H relation depend significantly on the HI content. HI deficient galaxies have shorter (up to a factor of 4) τ (truncated SFH) than spirals with normal HI content.

The Astrophysical Journal, 2007

We report on the discovery of a faint (M V ∼ −10.6 ± 0.2) dwarf spheroidal galaxy on deep F606W and F814W Hubble Space Telescope images of a Virgo intracluster field. The galaxy is easily resolved in our images, as our color magnitude diagram (CMD) extends 1 magnitude beyond the tip of the red giant branch (RGB). Thus, it is the deepest CMD for a small dwarf galaxy inside a cluster environment. Using the colors of the RGB stars, we derive a metal abundance for the dwarf of [M/H]= −2.3 ± 0.3, and show that the metallicity dispersion is less than 0.6 dex at 95% confidence. We also use the galaxy's lack -2of AGB stars and the absence of objects brighter than M bol ∼ −4.1 ± 0.2 to show that the system is old (t 10 Gyr). Finally, we derive the object's structural parameters, and show that the galaxy displays no obvious evidence of tidal threshing. Since the tip of the red giant branch distance ((m−M) 0 = 31.23±0.17 or D = 17.6 ± 1.4 Mpc) puts the galaxy near the core of the Virgo cluster, one might expect the object to have undergone some tidal processing. Yet the chemical and morphological similarity between the dwarf and the dSph galaxies of the Local and M81 Group demonstrates that the object is indeed pristine, and not the shredded remains of a much larger galaxy. We discuss the possible origins of this galaxy, and suggest that it is just now falling into Virgo for the first time.

The Astrophysical Journal, 2008

We present a study of the mass-metallicity (M -Z) relation and H II region physical conditions in a sample of 20 star-forming galaxies at 1.0 < z < 1.5 drawn from the DEEP2 Galaxy Redshift Survey. We find a correlation between stellar mass and gas-phase oxygen abundance in the sample and compare it with the one observed among UV-selected z ∼ 2 star-forming galaxies and local objects from the Sloan Digital Sky Survey (SDSS). This comparison, based on the same empirical abundance indicator, demonstrates that the zero point of the M -Z relationship evolves with redshift, in the sense that galaxies at fixed stellar mass become more metal-rich at lower redshift. Measurements of [O III]/Hβ and [N II]/Hα emission-line ratios show that, on average, objects in the DEEP2 1.0 < z < 1.5 sample are significantly offset from the excitation sequence observed in nearby H II regions and SDSS emission-line galaxies. In order to fully understand the causes of this offset, which is also observed in z ∼ 2 star-forming galaxies, we examine in detail the small fraction of SDSS galaxies that have similar diagnostic ratios to those of the DEEP2 sample. Some of these galaxies indicate evidence for AGN and/or shock activity, which may give rise to their unusual line ratios and contribute to Balmer emission lines at the level of ∼ 20%. Others indicate no evidence for AGN or shock excitation yet are characterized by higher electron densities and temperatures, and therefore interstellar gas pressures, than typical SDSS star-forming galaxies of similar stellar mass. These anomalous objects also have higher concentrations and star formation rate surface densities, which are directly connected to higher interstellar pressure. Higher star formation rate surface densities, interstellar pressures, and H II region ionization parameters may also be common at high redshift. These effects must be taken into account when using strong-line indicators to understand the evolution of heavy elements in galaxies. When such effects are included, the inferred evolution of the M -Z relation out to z ∼ 2 is more significant than previous estimates.

The Astrophysical Journal, 2006

We present the color distributions of globular cluster (GC) systems for 100 Virgo cluster earlytype galaxies observed in the ACS Virgo Cluster Survey, the deepest and most homogeneous survey of this kind to date. While the color distributions of individual GC systems can show significant variations from one another, their general properties are consistent with continuous trends across galaxy luminosity, color, and stellar mass. On average, galaxies at all luminosities in our study (−22 < M B < −15) appear to have bimodal or asymmetric GC color distributions. Almost all galaxies possess a component of metal-poor GCs, with the average fraction of metal-rich GCs ranging from 15 to 60%. The colors of both subpopulations correlate with host galaxy luminosity and color, with the red GCs having a steeper slope. The steeper correlation seen in the mean color of the entire GC system is driven by the increasing fraction of metal-rich GCs for more luminous galaxies.

The Astrophysical …, 2009

Monthly Notices of the Royal Astronomical Society, 1999

We present first results of a large photometric survey devoted to study the star formation history of the Sagittarius dwarf spheroidal galaxy (Sgr dSph).

Monthly Notices of the Royal Astronomical Society, 2010

We present a stellar population analysis of the absorption line strength maps for 48 earlytype galaxies from the SAURON sample. Using the line strength index maps of Hβ, Fe5015, and Mg b, measured in the Lick/IDS system and spatially binned to a constant signal-tonoise, together with predictions from up-to-date stellar population models, we estimate the simple stellar population-equivalent (SSP-equivalent) age, metallicity and abundance ratio [α/Fe] over a two-dimensional field extending up to approximately one effective radius. A discussion of calibrations and differences between model predictions is given. Maps of SSPequivalent age, metallicity and abundance ratio [α/Fe] are presented for each galaxy. We find a large range of SSP-equivalent ages in our sample, of which ∼40 per cent of the galaxies show signs of a contribution from a young stellar population. The most extreme cases of post-starburst galaxies, with SSP-equivalent ages of 3 Gyr observed over the full fieldof-view, and sometimes even showing signs of residual star-formation, are restricted to low mass systems (σ e 100 km s −1 or ∼2 ×10 10 M ⊙ ). Spatially restricted cases of young stellar populations in circumnuclear regions can almost exclusively be linked to the presence of star-formation in a thin, dusty disk/ring, also seen in the near-UV or mid-IR on top of an older underlying stellar population.

The Astrophysical Journal, 2009

We present a newly observed relation between galaxy mass and radial metallicity gradients of earlytype galaxies. Our sample of 51 early-type galaxies encompasses a comprehensive mass range from dwarf to brightest cluster galaxies. The metallicity gradients are measured out to one effective radius by comparing nearly all of the Lick absorption-line indices to recent models of single stellar populations. The relation shows very different behavior at low and high masses, with a sharp transition being seen at a mass of ∼ 3.5 × 10 10 M ⊙ (velocity dispersion of ∼ 140 km s −1 , M B ∼ −19). Low-mass galaxies form a tight relation with mass, such that metallicity gradients become shallower with decreasing mass and positive at the very low-mass end. Above the mass transition point several massive galaxies have steeper gradients, but a clear downturn is visible marked by a broad scatter. The results are interpreted in comparison with competing model predictions. We find that an early star-forming collapse could have acted as the main mechanism for the formation of low-mass galaxies, with star formation efficiency increasing with galactic mass. The high-mass downturn could be a consequence of merging and the observed larger scatter a natural result of different merger properties. These results suggest that galaxies above the mass threshold of ∼ 3.5 × 10 10 M ⊙ might have formed initially by mergers of gas-rich disk galaxies and then subsequently evolved via dry merger events. The varying efficiency of the dissipative merger-induced starburst and feedback processes have shaped the radial metallicity gradients in these high-mass systems.

The Astronomical Journal, 2012

Galaxies selected on the basis of their emission-line strength show low metallicities, regardless of their redshifts. We conclude this from a sample of faint galaxies at redshifts between 0.6 < z < 2.4, selected by their prominent emission lines in low-resolution grism spectra in the optical with the Advanced Camera for Surveys on the Hubble Space Telescope and in the near-infrared using Wide-Field Camera 3. Using a sample of 11 emission-line galaxies at 0.6 < z < 2.4 with luminosities of −22