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 relationfor a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.We demonstrate that the color-magnitude relation is entirely a consequence ofthe fact that both the luminosities and colors of these galaxies are correlatedwith stellar velocity dispersions. Previous studies of the color-magnituderelation over a range of redshifts suggest that the luminosity of an early-typegalaxy is an indicator of its metallicity, whereas residuals in color from therelation are indicators of the luminosity-weighted age of its stars. We showthat this, when combined with our finding that velocity dispersion plays acrucial role, has a number of interesting implications. First, galaxies withlarge velocity dispersions tend to be older (i.e., they scatter redward of thecolor-magnitude relation). Similarly, galaxies with large dynamical massestimates also tend to be older. In addition, at fixed luminosity, galaxieswhich are smaller, or have larger velocity dispersions, or are more massive,tend to be older. Second, models in which galaxies with the largest velocitydispersions are also the most metal poor are difficult to reconcile with ourdata. However, at fixed velocity dispersion, galaxies have a range of ages andmetallicities: the older galaxies have smaller metallicities, and vice-versa.Finally, a plot of velocity dispersion versus luminosity can be used as an ageindicator: lines of constant age run parallel to the correlation betweenvelocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A