The Effects of Environment on Morphological Evolution at 0<z<1.2 in the COSMOS Survey

Abstract

We explore the evolution of the morphology-density relation using the COSMOS Advanced Camera for Surveys (ACS) 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 that galaxies are transformed from late- (spiral and irregular) to early-type (E+S0) galaxies more rapidly in dense than in 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 Mpc^-2, no evolution is found at z>0.4. In contrast, the star formation-density relation shows strong evolution at all densities and redshifts, suggesting that different physical mechanisms are responsible for the morphological and star formation transformation. We show that photometric redshifts can measure local galaxy environment, but that the present results are limited by photometric redshift error to densities above Σ=3 galaxies Mpc^-2.

Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555 the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the MegaPrime/MegaCam, a joint project of the Canada-France-Hawaii Telescope (CFHT) and CEA/DAPNIA, at the CFHT, which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche, and the University of Hawaii; and Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which is operated by AURA under cooperative agreement with the National Science Foundation.