The ages and metallicities of galaxies in the local universe

We derive stellar metallicities, light-weighted ages and stellar masses for a magnitude-limited sample of 175 128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute the median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we derive are set by the simultaneous fit of five spectral absorption features, which are well reproduced by our population synthesis models. By design, these constraints depend only weakly on the alpha/Fe element abundance ratio. Our sample includes galaxies of all types spanning the full range in star formation activity, from dormant early-type to actively star-forming galaxies. By analysing a subsample of 44 254 high-quality spectra, we show that, in the mean, galaxies follow a sequence of increasing stellar metallicity, age and stellar mass at increasing 4000-angstrom break strength. For galaxies of intermediate mass, stronger Balmer absorption at fixed 4000-angstrom break strength is associated with higher metallicity and younger age. We investigate how stellar metallicity and age depend on total galaxy stellar mass. Low-mass galaxies are typically young and metal-poor, massive galaxies old and metal-rich, with a rapid transition between these regimes over the stellar mass range 3 x 109 less than or similar to M-* less than or similar to 3 x 10(10) M-circle dot. Both high- and low-concentration galaxies follow these relations, buts there is a large dispersion in stellar metallicity at fixed stellar mass, especially for low-concentration galaxies of intermediate mass. Despite the large scatter, the relation between stellar metallicity and stellar mass is similar to the correlation between gas-phase oxygen abundance and stellar mass for star-forming galaxies. This is confirmed by the good correlation between stellar metallicity and gas-phase oxygen abundance for galaxies with both measures. The substantial range in stellar metallicity at fixed gas-phase oxygen abundance suggests that gas ejection and/or accretion are important factors in galactic chemical evolution.