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 wi th ground-based images of Coma (z = 0.02), have been used to make a di rect comparison of the structural properties of the early-type galaxie s in these clusters. This well-matched pair of images has similar rest -frame filter passband, nearly identical resolution in physical units (similar to 0.7 kpc), and samples a similar physical size in the clust er (similar to 1 Mpc). Furthermore, galaxies of a given luminosity hav e comparable signal-to-noise (integrated S/N = 100 at M(B) = -21) in e ach image. We apply a two-dimensional fitting algorithm convolving emp irical point-spread functions with analytical models for bulge (r(1/4) law) and disk (exponential) components. Some combination of these mod els provide a good representation of most of the galaxies. The luminos ity-effective radius relation for bright ellipticals (M(B) < -19.5) sh ows very small dispersion and has the same slope in both clusters emph asizing the cluster-to-cluster homogeneity of the early-type galaxy po pulation. However, the relations are offset from one another by 0.64 /- 0.26 +/- 0.08 mag (random and systematic uncertainties, respectivel y), 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 interp reted as the effect of the aging of the constituent stars in these gal axies, i.e., passive evolution between redshift 0.41 to 0.02. This eff ect is consistent with the change in luminosity predicted for a single burst stellar population formed at z similar to 1.2 or earlier. A pot entially important result detected at the 2 sigma level (which is subj ect 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 cent ral surface-brightness relation for local field galaxies.