The ages, metallicities, and star formation histories of early-type galaxies in the SDSS

We use the spectra of similar to 22,000 early-type galaxies, selected from the Sloan Digital Sky Survey, to infer the ages, metallicities, and star formation histories of these galaxies. We find clear evidence of downsizing,'' i.e., that galaxies with larger velocity dispersion have older stellar populations. In particular, we find that most early-type galaxies with velocity dispersion exceeding 200 km s(-1) formed more than 90% of their current stellar mass at redshift z > 2.5. Therefore, star formation was suppressed around this redshift. We also show that chemical enrichment was rapid, lasting 1-2 Gyr, and find evidence that [Fe/H] is subsolar. We study the robustness of these results by comparing three different approaches: (1) using Lick absorption line indices, (2) fitting a single-burst stellar population model to the whole spectrum (lines and continuum), and (3) reconstructing the star formation and metallicity histories in multiple age bins, providing a method to measure mass-weighted ages and metallicities. We find good agreement between the luminosity-weighted ages and metallicities computed with these three methods.