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 massselected samples of galaxies at redshifts 0 < z < 1. The CDF-S sample at 0.6 < z < 1.0 contains 207 galaxies complete down toM = 4×10 M⊙ (for a “diet” Salpeter initial mass function), corresponding to a luminosity limit for red galaxies of MB = −20.1. The SDSS sample at 0.020 < z < 0.045 contains 2003 galaxies down to the same mass limit, which corresponds to MB = −19.3 for red galaxies. Morphologies are determined with an automated method, using the Sérsic 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 ∼ 0.03 and at z ∼ 0.8. Similarly, 65%−70% of the galaxies have n > 2.5. The fraction of E+S0 galaxies is 43%± 3% at z ∼ 0.03 and 48%± 7% at z ∼ 0.8; i.e., it has not changed significantly since z ∼ 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 ∼ 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 ∼ 1 must happen without large changes in the early-type galaxy fraction in a given environment. Subject headings: galaxies: clusters: general—galaxies: elliptical and lenticular, cD—galaxies: evolution—galaxies: formation—galaxies: fundamental parameters—galaxies: general—galaxies: photometry