Analyzing a volume-limited sample of > 14,000 early-type galaxies from the SDSS, Clemens et al. 2009a find that their ages, metallicities, and -element enhancement increase with their mass (using velocity dispersion, , as indicator). For galaxies with > 180 km s-1, the mean age decreases with decreasing galactocentric radius, while the metallicity increases. Clemens et al. suggest that the massive early-type galaxies were assembled at z 3.5, merging with low-mass halos that began to form at z ~ 10. These subhalos contributed older, metal-poor stars that are still distributed over large radii. Gas-rich mergers, very frequent at early times, contributed fuel for intense star formation in the central regions of the galaxies, while mergers at later times were increasingly gas-poor or dry. Clemens et al. find these radial age and metallicity gradients in early-type galaxies regardless of environment, although massive ellipticals in clusters are on average ~ 2 Gyr older than those in the field, supporting the trends expected for downsizing.
4.1. Environment and rejuvenation
From an analysis of Spitzer Space Telescope (SST) data of 50 early-type galaxies in the Coma cluster, Clemens et al. (2009b) find that while the majority is passive, some ~ 30% of the galaxies are either younger than 10 Gyr or were rejuvenated in the last few Gyr.
Combining near-UV photometry from the Galaxy Evolution Explorer (GALEX) satellite with SDSS data of a volume-limited sample of 839 luminous early-type galaxies, Schawinski et al. (2007) conclude that ~ 30% of these objects show evidence of recent (< 1 Gyr) star formation (~ 29% ellipticals, ~ 39% lenticulars). Moreover, they show that that low-density environments contain ~ 25% more UV-bright early-type galaxies.
Thomas et al. (2010) analyze low-redshift > 3000 early-type galaxies from the SDSS and infer that intermediate-mass and low-mass galaxies show evidence for a secondary peak of more recent star formation around ~ 2.5 Gyr ago. They find that the fraction of these rejuvenated galaxies becomes larger with decreasing galaxy mass and with decreasing environmental density, reaching up to 45% at low masses and low densities. Thomas et al. conclude that the impact of environment increases with decreasing galaxy mass via mergers and interactions and has done so since z ~ 0.2.
4.2. E+A galaxies
An interesting class of rejuvenated early-type galaxies are the so-called "E+A" galaxies, ellipticals that show the typical K-star spectra with Mg, Ca, and Fe absorption lines as well as strong Balmer lines akin to A-stars (Dressler & Gunn (1983)), indicating that in addition to the usual passive evolution, they experienced star formation within the last Gyr. The absence of [O II] and H emission lines shows that there is no ongoing star formation. These post-starburst galaxies are observed both in clusters and in the field.
SDSS studies support suggestions that the E+A phenomenon is created by interactions and/or mergers. About 30% of the E+A galaxies show disturbed morphologies or tidal tails (Goto 2005). The analysis of 660 E+A galaxies revealed that these objects have a 54% higher probability of having close companion galaxies than normal galaxies (~ 8% vs. ~ 5%; Yamauchi et al. 2008).