Early-type galaxies in the Sloan Digital Sky Survey. IV. Colors and chemical evolution

Abstract

The colors and chemical abundances of early-type galaxies at redshifts z < 0.3 are studied using a sample of nearly 9000 galaxies, selected from the Sloan Digital Sky Survey using morphological and spectral criteria. In this sample, redder galaxies have larger velocity dispersions: g*-r* proportional to sigma(0.26+/-0.02). Color also correlates with magnitude, g* - r* proportional to (-0.025+/-0.003) Mr(*), and size, but these correlations are entirely due to the L-sigma and R-o-sigma relations: the primary correlation is color-sigma. The red light in early-type galaxies is, on average, slightly more centrally concentrated than the blue. Because of these color gradients, the strength of the color-magnitude relation depends on whether or not the colors are defined using a fixed metric aperture; the color-sigma relation is less sensitive to this choice. Chemical evolution and star formation histories of early-type galaxies are investigated using co-added spectra of similar objects. The resulting library of co-added spectra contains spectra that represent a wide range of early-type galaxies. Chemical abundances correlate primarily with velocity dispersion: H beta proportional to sigma(-0.24+/-0.03), Mg-2 proportional to sigma(0.20+/-0.02), Mg b proportional to sigma(0.32+/-0.03), and [Fe] proportional to sigma(0.11+/-0.03). At fixed sigma, the population at z similar to 0.2 had weaker Mg-2 and stronger H beta absorption compared to the population at z similar to 0. It was also bluer. Comparison of these colors and line strengths and their evolution with single-burst stellar population models suggests a formation time of 9 Gyr ago, consistent with a fundamental plane analysis of this sample. Although the fundamental plane shows that galaxies in dense regions are slightly different from galaxies in less dense regions, the co-added spectra and color-magnitude relations show no statistically significant dependence on environment.