THE RELATIONSHIP BETWEEN BARYONS AND DARK MATTER IN EXTENDED GALAXY HALOS

Using a sample of nearly 20,000 massive early-type galaxies selected from the Sloan Digital Sky Survey, we study the color-magnitude relation for the most luminous (L >~2.2L*) field galaxies in the redshift range 0.1 < z < 0.4 in several colors. The intrinsic dispersion in galaxy colors is quite small in all colors studied, but the 40 millimag (mmag) scatter in the bluest colors is a factor of 2 larger than the 20 mmag measured in the reddest bands. While each of three simple models constructed for the star formation history in these systems can satisfy the constraints placed by our measurements, none of them produce color distributions matching those observed. Subdividing by environment, we find the dispersion for galaxies in clusters to be about 11% smaller than that of more isolated systems. Finally, having resolved the red sequence, we study the color dependence of the composite spectra. Bluer galaxies on the red sequence are found to have more young stars than red galaxies; the extent of this spectral difference is marginally better described by passive evolution of an old stellar population than by a model consisting of a recent trace injection of young stars.