Colors, magnitudes, and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities

We present an analysis of the color - magnitude - velocity dispersion relation for a sample of 39,320 early-type galaxies within the Sloan Digital Sky Survey. We demonstrate that the color-magnitude relation is entirely a consequence of the fact that both the luminosities and colors of these galaxies are correlated with stellar velocity dispersions. Previous studies of the color-magnitude relation over a range of redshifts suggest that the luminosity of an early-type galaxy is an indicator of its metallicity, whereas residuals in color from the relation are indicators of the luminosity-weighted age of its stars. We show that this, when combined with our finding that velocity dispersion plays a crucial role, has a number of interesting implications. First, galaxies with large velocity dispersions tend to be older (i.e., they scatter redward of the color-magnitude relation). Similarly, galaxies with large dynamical mass estimates also tend to be older. In addition, at fixed luminosity galaxies that are smaller or have larger velocity dispersions or are more massive tend to be older. Second, models in which galaxies with the largest velocity dispersions are also the most metal-poor are difficult to reconcile with our data. However, at fixed velocity dispersion galaxies have a range of ages and metallicities: the older galaxies have smaller metallicities and vice versa. Finally, a plot of velocity dispersion versus luminosity can be used as an age indicator: lines of constant age run parallel to the correlation between velocity dispersion and luminosity.