THE EVOLUTION OF EARLY-TYPE GALAXIES IN DISTANT CLUSTERS

We present results from an optical-infrared photometric study of early -type (E+S0) galaxies in 19 galaxy clusters out to z = 0.9. The galaxy sample is selected on the basis of morphologies determined from Hubbl e Space Telescope (HST) WFPC2 images and is photometrically defined in the K band in order to minimize redshift-dependent selection biases. Using new ground-based photometry in five optical and infrared bands f or each cluster, we examine the evolution of the color-magnitude relat ion for early-type cluster galaxies, considering its slope, intercept, and color scatter around the mean relation. New multiwavelength photo metry of galaxies in the Coma Cluster is used to provide a baseline sa mple at z approximate to 0 with which to compare the distant clusters. The optical-IR colors of the early-type cluster galaxies become bluer with increasing redshift in a manner consistent with the passive evol ution of an old stellar population formed at an early cosmic epoch. Th e degree of color evolution is similar for clusters at similar redshif t and does not depend strongly on the optical richness or X-ray lumino sity of the cluster, which suggests that the history of early-type gal axies is relatively insensitive to environment, at least above a certa in density threshold. The slope of the color-magnitude relationship sh ows no significant change out to z = 0.9, which provides evidence that it arises from a correlation between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in the optical-IR colors of the g alaxies is small and nearly constant with redshift, which indicates th at the majority of giant, early-type galaxies in clusters share a comm on star formation history, with little perturbation due to uncorrelate d episodes of later star formation. Taken together, our results are co nsistent with models in which most early-type galaxies in rich cluster s are old, formed the majority of their stars at high redshift in a we ll-synchronized fashion, and evolved quiescently thereafter. We consid er several possible effects that may be introduced by the choice of mo rphologically recognizable elliptical and SO galaxies in dense environ ments as a subject for study. In particular, the inclusion of SO galax ies, which might be undergoing morphological transformation in cluster s as part of the Butcher-Oemler effect, may influence the results of o ur investigation.