ON THE SIZE AND COMOVING MASS DENSITY EVOLUTION OF EARLY-TYPE GALAXIES

We present a simple, empirically motivated model that simultaneously predicts the evolution of the mean size and the comoving mass density of massive (> 10(11) M-circle dot) early-type galaxies from z = 2 to the present. First, we demonstrate that some size evolution of the population can be expected simply due to the continuous emergence of early-type galaxies. The Sloan Digital Sky Survey (SDSS) data reveal that in the present-day universe more compact early-type galaxies with a given dynamical mass have older stellar populations. This implies that with increasing look-back time, the more extended galaxies will be more and more absent from the population. In contrast, at a given stellar velocity dispersion, SDSS data show that there is no relation between size and age, which implies that the velocity dispersion can be used to estimate the epoch at which galaxies stopped forming stars, turning into early-type galaxies. Based on this, we define an empirically motivated, redshift-dependent velocity dispersion threshold above which galaxies do not form stars at a significant rate, which we associate with the transformation into early-type galaxies. Applying this formation criterion to a large sample of nearby early-type galaxies, we predict the redshift evolution in the size distribution and the comoving mass density. The resulting evolution in the mean size is roughly half of the observed evolution. Then we include a prescription for the merger histories of galaxies between the formation redshift and the present, based on cosmological simulations of the assembly of dark matter halos. Such mergers after the transformation into an early-type galaxy are presumably dissipationless (dry), where the increase in size is expected to be approximately proportional to the increase in mass. This model successfully reproduces the observed evolution since z similar to 2 in the mean size and in the comoving mass density of early-type galaxies with mass M > 10(11) M-circle dot. We conclude that the recently measured, substantial size evolution of early-type galaxies can be explained by the combined effect of the continuous emergence of galaxies as early types and their subsequent growth through dry merging.