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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~2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z~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 measurements are taken from the literature for 50 morphologically selected 0.8<z<1.2 field and cluster early-type galaxies with typical masses 2e11 Msol. Sizes are determined with ACS imaging. We compare the distant sample with a large sample of nearby (0.04<z<0.08) early-type galaxies extracted from the SDSS 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 structural difference between the nearby and distant samples, regardless of sample selection effects. The implied evolution in size 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~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.