THE COLOR-MAGNITUDE RELATION IN CL-1358- EVIDENCE FOR SIGNIFICANT EVOLUTION IN THE S0 POPULATION(62 AT Z=0.33 )

We use a large, multicolor mosaic of HST WFPC2 images to measure the c olors and morphologies of 194 spectroscopically confirmed members of t he rich galaxy cluster CL 1358 + 62 at z = 0.33. We study the color-ma gnitude (CM) relation as a function of radius in the cluster to a limi t of 416 from the center, equivalent to 1.6 h(50)(-1) Mpc. The intrins ic scatter in the rest-frame B-V CM relation of the elliptical galaxie s is very small, similar to 0.022 mag. The CM relation of the elliptic als does not depend significantly on the distance from the cluster cen ter. In contrast, the CM relation for the SO galaxies does depend on r adius: the SO's in the core follow a CM relation similar to that of th e ellipticals, but at large radii (R > 0.7 h(50)(-1) Mpc) the SO's are systematically bluer and the scatter in the CM relation approximately doubles, to similar to 0.043 mag. The blueing of the SO's at large ra dii is significant at the 95% confidence level. These results imply th at the SO galaxies in the outer parts of the cluster have formed stars more recently than the SO's in the inner parts. A likely explanation is that clusters at z = 0.33 continue to accrete galaxies and groups f rom the field and that infall extinguishes star formation. The apparen t homogeneity of the elliptical galaxy population implies that star fo rmation in recently accreted ellipticals was terminated well before ac cretion occurred. We have constructed models to explore the constraint s that these observations place on the star formation history of clust er galaxies. The best-constrained parameter is the scatter in the lumi nosity-weighted age Delta tau(L)/<tau(L)>, which is less than 18% for the ellipticals and the SO's in the cluster core, and less than 35% fo r the SO's in the outer parts of the cluster. The constraints on the m ost recent period of star formation are model dependent, but we show t hat 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 SO's in th e outer parts of the cluster have experienced star formation until the epoch of observation at z = 0.33. We conclude that the population of SO's in clusters is likely to evolve as star-forming galaxies are conv erted into passively evolving galaxies. Assuming a constant accretion rate after z = 0.33, we estimate that similar to 15% of the present-da y early-type galaxy population in rich clusters was accreted between z = 0.33 and z = 0. The ellipticals land the brightest SO's) are probab ly a more stable population, at least since z = 0.6.