Early-type galaxies in the SDSS. 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. Color also correlates with magnitude 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. Chemical evolution and star formation histories of early-type galaxies are investigated using co-added spectra of similar objects. At fixed sigma, the population at z ~ 0.2 had weaker Mg_2 and stronger H_beta absorption compared to the population at z ~ 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 Gyrs ago, consistent with a Fundamental Plane analysis of this sample.

38 pages, 16 figures. Accepted by AJ (scheduled for April 2003). This paper is part IV of a revised version of astro-ph/0110344. The full version of Table 3 and the 182 composite spectra can be downloaded from http://astrophysics.phys.cmu.edu/~bernardi/SDSS/Etypes/SPECFITS A README file (bernardi4.README) explains the content of the directory