GALAXY STELLAR MASS ASSEMBLY BETWEEN 0.2 < z < 2 FROM THE S-COSMOS SURVEY

We follow the galaxy stellar mass assembly by morphological and spectral type in the COSMOS 2 deg(2) field. We derive the stellar mass functions and stellar mass densities from z = 2 to z = 0.2 using 196,000 galaxies selected at F-3.6 (mu m) > 1 mu Jy with accurate photometric redshifts (sigma((Zphot-Zspec)/(1+Zspec)) = 0.008 at i(+) < 22.5). Using a spectral classification, we find that z similar to 1 is an epoch of transition in the stellar mass assembly of quiescent galaxies. Their stellar mass density increases by 1.1 dex between z = 1.5-2 and z = 0.8-1 (Delta t similar to 2.5 Gyr), but only by 0.3 dex between z = 0.8-1 and z similar to 0.1 (Delta t similar to 6 Gyr). Then, we add the morphological information and find that 80%-90% of the massive quiescent galaxies (log M similar to 11) have an elliptical morphology at z < 0.8. Therefore, a dominant mechanism links the shutdown of star formation and the acquisition of an elliptical morphology in massive galaxies. Still, a significant fraction of quiescent galaxies present a Spi/Irr morphology at low mass (40%-60% at log M similar to 9.5), but this fraction is smaller than predicted by semi-analytical models using a halo quenching recipe. We also analyze the evolution of star-forming galaxies and split them into intermediate activity and high activity galaxies. We find that the most massive high activity galaxies end their high star formation rate phase first. Finally, the space density of massive star-forming galaxies becomes lower than the space density of massive elliptical galaxies at z < 1. As a consequence, the rate of wet mergers involved in the formation of the most massive ellipticals must decline very rapidly at z < 1, which could explain the observed slow down in the assembly of these quiescent and massive sources.