The relative abundance of compact and normal massive early-type galaxies and its evolution from redshift z~2 to the present

Abstract: We report on the evolution of the number density and size of early-type galaxies from z~2 to z~0. We select a sample of 563 massive (M>10^{10} Msun), passively evolving (SSFR<10^{-2} Gyr^{-1}) and morphologically spheroidal galaxies at 0<z\<2.5, using the panchromatic photometry and spectroscopic redshifts available in the GOODS fields. We combine ACS and WFC3 HST images to study the morphology of our galaxies in their optical rest-frame in the whole 0<z\<2.5 range. We find that throughout the explored redshift range the passive galaxies selected with our criteria have weak morphological K-correction, with size being slightly smaller in the optical than in the UV rest-frame (by ~20 and ~10 at z\>1.2 and z<1.2, respectively). We measure a significant evolution of the mass-size relation of early-type galaxies, with the fractional increment that is almost independent on the stellar mass. Early-type galaxies (ETGs) formed at z>1 appear to be preferentially small, and the evolution of the mass-size relation at z<1 is driven by both the continuous size growth of the compact galaxies and the appearance of new ETGs with large sizes. We also find that the number density of all passive early-type galaxies increases rapidly, by a factor of 5, from z~2 to z~1, and then more mildly by another factor of 1.5 from z~1 to z~0. We interpret these results as the evidence that the bulk of the ETGs are formed at 1<z<3 through a mechanism that leaves very compact remnants. At z<1 the compact ETGs grow gradually in size, becoming normal size galaxies, and at the same time new ETGs with normal-large sizes are formed.