The Effects of Environment on Morphological Evolution between 0 < Z < 1.2 in the Cosmos Survey

We explore the evolution of the morphology density relation using the COSMOS-ACS survey and previous cluster studies. The Gini parameter measured in a Petrosian aperture is found to be an effective way of selecting early-type galaxies free from systematic effects with redshift. We find galaxies are transformed from late (spiral and irregular) to early (E+S0) type galaxies more rapidly in dense than sparse regions. At a given density, the early-type fraction grows constantly with cosmic time, but the growth rate increases with density as a power law of index 0.29±0.02. However, at densities below 100 galaxies per Mpc no evolution is found at z > 0.4. In contrast the star-formation-density relation shows strong evolution at all densities and redshifts, suggesting different physical mechanisms are responsible for the morphological and star formation transformation. We show photometric redshifts can measure local galaxy environment, but the present results are limited by photometric redshift error to densities above Σ = 3 galaxies per Mpc. Subject headings: galaxies: evolution — galaxies: formation — cosmology: observations — cosmology: large scale structure — galaxies: structure — galaxies: clusters