Evolution of Cluster and Field Elliptical Galaxies at 0.2 < [ITAL]z[/ITAL] < 0.6 in the CNOC Cluster Survey

Two-dimensional surface photometry has been done for 166 {\em early-type}galaxies (bulge/total luminosity $B/T>0.6$) in 3 fields of the Canadian Networkfor Observational Cosmology (CNOC) cluster survey. These galaxies are eitherspectroscopically confirmed members of clusters at $z=0.23$ (45 galaxies),$0.43$ (22), and $0.55$ (16) or field galaxies in the same redshift range. Anadditional 51 early-type galaxies in the rich cluster Abell 2256 at $z=0.06$were analysed with the same technique. The resulting structural and surfacebrightness measurements show that, in the plane of absolute magnitude$M_{AB}(B)$ versus $\log R_e$ (half-light radius), the locus of clusterellipticals shifts monotonically with redshift so that at redshifts of (0.23,0.43, 0.55), galaxies of a given size are more luminous by ($-0.25\pm0.10,-0.55\pm 0.12,-0.74\pm 0.21$) magnitudes with respect to the same relationmeasured at $z=0.06$ (adopting $q_\circ=0.5$). There is no evidence thatearly-type galaxies in the field evolve differently from those in clusters. Ifdynamical processes do not substantially modify the size-luminosity relationfor early-type galaxies over the observed redshift range, then these galaxieshave undergone significant luminosity evolution over the past half of the ageof the universe. The amount of brightening is consistent with passive evolutionmodels of old, single-burst stellar populations.