RECENT STRUCTURAL EVOLUTION OF EARLY-TYPE GALAXIES: SIZE GROWTH FROM z=1 TO z=0

Strong size and internal density evolution of early-type galaxies between z similar to 2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z similar to 1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion (sigma) measurements are taken from the literature for 50 morphologically selected 0.8 < z < 1.2 field and cluster early-type galaxies with typical masses M-dyn = 2 x 10(11) M-circle dot. Sizes (R-eff) are determined with Advanced Camera for Surveys imaging. We compare the distant sample with a large sample of nearby (0.04 < z < 0.08) early-type galaxies extracted from the Sloan Digital Sky Survey for which we determine sizes, masses, and densities in a consistent manner, using simulations to quantify systematic differences between the size measurements of nearby and distant galaxies. We find a highly significant difference between the sigma-R-eff distributions of the nearby and distant samples, regardless of sample selection effects. The implied evolution in R-eff at fixed mass between z = 1 and the present is a factor of 1.97 +/- 0.15. This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since z similar to 2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations.