  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.
