EVOLUTION OF GALAXIES IN CLUSTERS

We use new models of stellar population synthesis to estimate the fraction of stars formed during the last major bursts of star formation in E/SO galaxies in low-redshift clusters (z less than or similar to 0.4) from the spectral signatures of intermediate-age stars. We find that the mass fraction of stars formed in late bursts in early-type galaxies in clusters must have decreased smoothly with time, from about 8% at z approximate to 1 to less than 1% at z approximate to 0. This result, which we interpret as a constraint on stellar mass added in mergers, is nearly independent of the assumed ages and morphological types of the progenitor galaxies prior to the last major bursts of star formation. We then compute the implied color and 4000 Angstrom break evolution for progenitors of E/SO galaxies in clusters at redshifts z less than or similar to 1. We investigate a conservative model in which all present-day E/SO galaxies are assumed to be initially elliptical galaxies and then to undergo bursts of star formation at the rate estimated from the signatures of intermediate-age stars at low redshifts. This model predicts that the progenitors of present-day E/SO galaxies could be among the galaxies with the bluest colors and smallest 4000 A breaks observed in high-redshift clusters. However, the predicted fraction of blue galaxies at high redshifts is significantly lower than is observed. The most natural explanation for this shortfall is an increase with redshift of the fraction of blue spiral galaxies in clusters, as suggested independently by recent high-resolution observations of intermediate-redshift clusters.