We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 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
which 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.Comment: 12 pages, 9 figures. Accepted by A
