EVOLUTION OF CLUSTER AND FIELD ELLIPTICALS AT 0.2 < z < 0.6 IN THE CNOC CLUSTER SURVEY

Two-dimensional surface photometry has been done for 166 early-type galaxies (bulge/total luminosity B/T > 0.6) in 3 fields of the Canadian Network for Observational Cosmology (CNOC) cluster survey. These galaxies are either spectroscopically 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. An additional 51 early-type galaxies in the rich cluster Abell 2256 at z = 0.06 were analysed with the same technique. The resulting structural and surface brightness measurements show that, in the plane of absolute magnitude MAB(B) versus logRe (half-light radius), the locus of cluster ellipticals 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±0.10,−0.55±0.12,−0.74±0.21) magnitudes with respect to the same relation measured at z = 0.06 (adopting q◦ = 0.5). There is no evidence that early-type galaxies in the field evolve differently from those in clusters. If dynamical processes do not substantially modify the size-luminosity relation for early-type galaxies over the observed redshift range, then these galaxies have undergone significant luminosity evolution over the past half of the age of the universe. The amount of brightening is consistent with passive evolution models of old, single-burst stellar populations. Subject headings: galaxies:evolution—galaxies:fundamental parameters Visiting Astronomer, Canada-France-Hawaii Telescope, which is operated by the National Research Council of Canada, the Centre Nationale de la Recherche Scientifique of France, and the University of Hawaii. 2 also Instituto Nacional de Astrof́ısica Optica y Electrónica (INAOE), Tonantzintla, México Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.