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* ∝ σ^0.26±0.02. Color also correlates with magnitude, g* - r* ∝ (-0.025 ± 0.003)M, and size, but these correlations are entirely due to the L-σ and R_o-σ relations: the primary correlation is color-σ. 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-σ 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: Hβ ∝ σ^-0.24±0.03, Mg₂ ∝ σ^0.20±0.02, Mg b ∝ σ^0.32±0.03, and Fe ∝ σ^0.11±0.03. At fixed σ, the population at z ~ 0.2 had weaker Mg₂ and stronger Hβ 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 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.