New Constraints on the Luminosity Evolution of Spheroidal Galaxies in Distant Clusters

We investigate various probes of luminosity evolution in the rich cluster environment, concentrating in particular on the spheroidal (E/S0) galaxies, using a newly constructed catalog of morphologically classified Hubble Space Telescope (HST) images of galaxies observed in the cores of 13 clusters with redshifts 0.17 ≤ z ≤ 0.56. An important distinction of this study compared to earlier work is the availability for a substantial subset of our HST fields of new near-infrared ground-based photometry, which we have used to select and study the various populations. We find no significant change in the characteristic luminosity, M*K, of the spheroidal populations at redshifts of 0.31 and 0.56. As a more sensitive probe of luminosity evolution, we analyze the surface photometry of our HST-classified ellipticals by deriving effective metric radii, Re, and mean effective surface brightnesses, ⟨μ⟩e. At the standard condition corresponding to Re = 1 kpc, we find convincing evidence of evolutionary brightening in both rest frame B and K light, consistent with model predictions based on the passive evolution of stellar populations.