NEW CONSTRAINTS ON THE LUMINOSITY EVOLUTION OF SPHEROIDAL GALAXIES INDISTANT CLUSTERS

We investigate various probes of luminosity evolution in the rich clus ter environment, concentrating in particular on the spheroidal (E/SO) galaxies, using a newly constructed catalog of morphologically classif ied Hubble Space Telescope (HST) images of galaxies observed in the co res of 13 clusters with redshifts 0.17 less than or equal to z less th an or equal to 0.56. An important distinction of this study compared t o earlier work is the availability for a substantial subset of our HST fields of new near-infrared ground-based photometry, which we have us ed to select and study the various populations. We find no significant change in the characteristic luminosity, M-K, of the spheroidal popu lations at redshifts of 0.31 and 0.56. As a more sensitive probe of lu minosity evolution, we analyze the surface photometry of our HST-class ified ellipticals by deriving effective metric radii, R-e, and mean ef fective surface brightnesses, <mu >(e). At the standard condition corr esponding to R-e =1 kpc, we find convincing evidence of evolutionary b rightening in both rest frame B and K light, consistent with model pre dictions based on the passive evolution of stellar populations.