The Evolution of the Field and Cluster Morphology-Density Relation for Mass-Selected Samples of Galaxies

The Sloan Digital Sky Survey (SDSS) and photometric/spectroscopic surveys in the GOODS-South field (the Chandra Deep Field-South, CDF-S) are used to construct volume-limited, stellar-mass-selected samples of galaxies at redshifts 0 < z < 1. The CDF-S sample at 0.6 < z < 1.6 contains 207 galaxies complete down to M = 4 x 10 super(10) M [unk] (for a "diet" Salpeter initial mass function), corresponding to a luminosity limit for red galaxies of M sub(B) = -20.1. The SDSS sample at 0.020 < z < 0.045 contains 2003 galaxies down to the same mass limit, which corresponds to M sub(B) = -19.3 for red galaxies. Morphologies are determined with an automated method, using the Sersic parameter n and a measure of the residual from the model fits, called "bumpiness," to distinguish different morphologies. These classifications are verified with visual classifications. In agreement with previous studies, 65%-70% of the galaxies are located on the red sequence, both at z similar to 0.03 and at z similar to 0.8. Similarly, 65%-70% of the galaxies have n > 2.5. The fraction of E + S0 galaxies is 43% plus or minus 3% at z similar to 0.03 and 48% plus or minus 7% at z similar to 0.8; i.e., it has not changed significantly since z similar to 0.8. When combined with recent results for cluster galaxies in the same redshift range, we find that the morphology-density relation for galaxies more massive than 0.5M* has remained constant since at least z similar to 0.8. This implies that galaxies evolve in mass, morphology, and density such that the morphology-density relation does not change. In particular, the decline of star formation activity and the accompanying increase in the stellar mass density of red galaxies since z similar to 1 must happen without large changes in the early-type galaxy fraction in a given environment.