2010A&A...515A...3H

Studying outliers from the bimodal distribution of galaxies in the color-mass space, such as morphological early-type galaxies residing in the blue cloud (blue E/S0s), can help for better understanding the physical mechanisms that lead galaxy migrations in this space. In this paper we try to bring new clues to studying the evolution of the properties of a significant sample of blue E/S0 galaxies in the COSMOS field. We define blue E/S0 galaxies as objects having a clear early-type morphology on the HST/ACS images (according to our automated classification scheme galSVM) but with a blue rest-frame color (defined by using the SED best-fit template on the COSMOS primary photometric catalogs). Combining these two measurements with spectroscopic redshifts from the zCOSMOS 10k release, we isolated 210 I_AB<22 blue early-type galaxies with M_*/M_☉>10¹⁰ in three redshift bins (0.2<z<0.55, 0.55<z<0.8, 0.8<z<1.4) and studied the evolution of their properties (number density, SFR, morphology, size). The threshold mass (M_t), defined at z=0 in previous studies as the mass below which the population of blue early-type galaxies starts to be abundant relative to passive E/S0s, evolves from log(M_*/M_☉)∼10.1±0.35 at z∼0.3 to log(M_*/M_☉)∼10.9±0.35 at z∼1. Interestingly, it follows the evolution of the crossover mass between the early and late type populations (bimodality mass) indicating that the abundance of blue E/S0 is another measure of the downsizing effect in the build-up of the red sequence. There seems to be a turn-over mass in the nature of blue E/S0 galaxies. Above log(M_*/M_☉)∼10.8 blue E/S0 resemble to merger remnants probably migrating to the red sequence on a time scale of ∼3Gyr. Below this mass, they seem to be closer to normal late-type galaxies, as if they were the result of minor mergers that triggered the central star formation and built a central bulge component or were (re)building a disk from the surrounding gas in a much longer time scale, suggesting that they are moving back or staying in the blue cloud. This turn-over mass does not seem to evolve significantly from z∼1 in contrast to the threshold mass and therefore does not seem to be linked with the relative abundance of blue E/S0s.