Luminosity Evolution in Cluster Galaxies from z = 0.41 to z = 0.02* ** ***

HST archive images of the cluster CL 0939+4713 (z = 0.41), together with ground-based images of Coma (z = 0.02), have been used to make a direct comparison of the structural properties of the early-type galaxies in these clusters. This well-matched pair of images has similar rest-frame filter passband, nearly identical resolution in physical units (~0.7 kpc), and samples a similar physical size in the cluster (~1 Mpc). Furthermore, galaxies of a given luminosity have comparable signal-to-noise (integrated S/N = 100 at MB = -21) in each image. We apply a two-dimensional fitting algorithm convolving empirical point-spread functions with analytical models for bulge (r1/4 law) and disk (exponential) components. Some combination of these models provide a good representation of most of the galaxies. The luminosity-effective radius relation for bright ellipticals (MB < -19.5) shows very small dispersion and has the same slope in both clusters emphasizing the cluster-to-cluster homogeneity of the early-type galaxy population. However, the relations are offset from one another by 0.64 ± 0.26 ± 0.08 mag (random and systematic uncertainties, respectively), so that a galaxy of a given size in CL 0939+4713 is more luminous by this amount than its counterpart in Coma. This result may be interpreted as the effect of the aging of the constituent stars in these galaxies, i.e., passive evolution between redshift 0.41 to 0.02. This effect is consistent with the change in luminosity predicted for a single burst stellar population formed at z ~ 1.2 or earlier. A potentially important result detected at the 2 σ level (which is subject to contamination by field galaxies) is that the disk components of galaxies in the higher redshift cluster are brighter by 0.50 ± 0.26 mag than those in Coma, which follow reasonably well the Freeman central surface-brightness relation for local field galaxies.