Evolution since Z=1 of the Morphology–density Relation for Galaxies

We measure the morphology–density relation of galaxies at z=1 across the full three orders of magnitude in projected galaxy density available in low–redshift studies. Our study adopts techniques that are comparable with those applied at lower redshifts, allowing a direct investigation of how the morphological segregation of galaxies has evolved over the last 8 Gyr. Although the morphology–density relation, as described by the fraction of early–type (E+S0) galaxies, was in place at z=1, its form differs from that observed at both z=0 and z=0.5. In the highest density regions the early–type fraction has increased steadily with time from fE+S0=0.7±0.1 at z=1 to fE+S0=0.9±0.1 at the present epoch. However, in intermediate density regions corresponding to groups and the accretion regions of rich clusters, significant evolution appears to begin only after z=0.5. Finally, at the lowest densities, no evolution is observed for the early type fraction of field galaxies which remains constant at fE+S0=0.4±0.1 at all epochs. We examine a simple picture consistent with these observations where the early– type population at z=1 is comprised largely of elliptical galaxies. Subsequent evolution in both intermediate and dense regions is attributed to the transformation of spirals into lenticulars. Further progress in verifying our hypothesis may be achieved through distinguishing ellipticals and lenticulars at these redshifts through resolved dynamical studies of representative systems. Subject headings: galaxies: clusters: general — galaxies: formation — galaxies: evolution — galaxies: structure