Evolution sincez= 1 of the Morphology‐Density Relation for Galaxies

We measure the morphology-density relation of galaxies at z=1 across the fullthree orders of magnitude in projected galaxy density available in low-redshiftstudies. Our study adopts techniques that are comparable with those applied atlower redshifts, allowing a direct investigation of how the morphologicalsegregation of galaxies has evolved over the last 8 Gyr. Although themorphology-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=0and z=0.5. In the highest density regions the early-type fraction has increasedsteadily with time from f(E+S0)=0.7+/-0.1 at z=1 to f(E+S0)=0.9+/-0.1 at thepresent epoch. However, in intermediate density regions corresponding to groupsand the accretion regions of rich clusters, significant evolution appears tobegin only after z=0.5. Finally, at the lowest densities, no evolution isobserved for the early-type fraction of field galaxies which remains constantat f(E+S0)=0.4+/-0.1 at all epochs. We examine a simple picture consistent withthese observations where the early-type population at z=1 is comprised largelyof elliptical galaxies. Subsequent evolution in both intermediate and denseregions is attributed to the transformation of spirals into lenticulars.Further progress in verifying our hypothesis may be achieved throughdistinguishing ellipticals and lenticulars at these redshifts through resolveddynamical studies of representative systems.Comment: 9 pages, 3 figures, submitted to Ap