The Color-Magnitude Relation in CL 1358+62 at Z = 0.33: Evidence for Significant Evolution in the S0 Population

We use a large, multicolor mosaic of HST WFPC2 images to measure the colors and morphologies of 194 spectroscopically confirmed members of the rich galaxy cluster CL 1358+62 at z = 0.33. We study the color-magnitude (CM) relation as a function of radius in the cluster to a limit of 4.6 arcmin from the center, equivalent to 1.6 h^-1₅₀ Mpc. The intrinsic scatter in the rest-frame B-V CM relation of the elliptical galaxies is very small, ~0.022 mag. The CM relation of the ellipticals does not depend significantly on the distance from the cluster center. In contrast, the CM relation for the S0 galaxies does depend on radius: the S0's in the core follow a CM relation similar to that of the ellipticals, but at large radii (R > 0.7 h^-1₅₀ Mpc) the S0's are systematically bluer and the scatter in the CM relation approximately doubles, to ~0.043 mag. The blueing of the S0's at large radii is significant at the 95% confidence level. These results imply that the S0 galaxies in the outer parts of the cluster have formed stars more recently than the S0's in the inner parts. A likely explanation is that clusters at z = 0.33 continue to accrete galaxies and groups from the field and that infall extinguishes star formation. The apparent homogeneity of the elliptical galaxy population implies that star formation in recently accreted ellipticals was terminated well before accretion occurred. We have constructed models to explore the constraints that these observations place on the star formation history of cluster galaxies. The best-constrained parameter is the scatter in the luminosity-weighted age Δτ_L/<τ_L>, which is less than 18% for the ellipticals and the S0's in the cluster core, and less than 35% for the S0's in the outer parts of the cluster. The constraints on the most recent period of star formation are model dependent, but we show that star formation in ellipticals likely ceased at z = 0.6 or higher. If we assume that the galaxies have a constant star formation rate up to a randomly distributed truncation time, we find that the S0's in the outer parts of the cluster have experienced star formation until the epoch of observation at z = 0.33. We conclude that the population of S0's in clusters is likely to evolve as star-forming galaxies are converted into passively evolving galaxies. Assuming a constant accretion rate after z = 0.33, we estimate that ~15% of the present-day early-type galaxy population in rich clusters was accreted between z = 0.33 and z = 0. The ellipticals (and the brightest S0's) are probably a more stable population, at least since z = 0.6.