Early-Type Galaxies in the Sloan Digital Sky Survey. IV. Colors and Chemical Evolution

The colors and chemical abundances of early-type galaxies at redshifts z<0.3are studied using a sample of nearly 9000 galaxies, selected from the SloanDigital Sky Survey using morphological and spectral criteria. In this sample,redder galaxies have larger velocity dispersions. Color also correlates withmagnitude and size, but these correlations are entirely due to the L-sigma andR_o-sigma relations: the primary correlation is color-sigma. The red light inearly-type galaxies is, on average, slightly more centrally concentrated thanthe blue. Because of these color gradients, the strength of thecolor--magnitude relation depends on whether or not the colors are definedusing a fixed metric aperture. Chemical evolution and star formation historiesof early-type galaxies are investigated using co-added spectra of similarobjects. At fixed sigma, the population at z ~ 0.2 had weaker Mg_2 and strongerH_beta absorption compared to the population at z ~ 0. It was also bluer.Comparison of these colors and line-strengths, and their evolution, withsingle-burst stellar population models suggests a formation time of 9 Gyrs ago,consistent with a Fundamental Plane analysis of this sample.