GALAXY STELLAR MASS ASSEMBLY BETWEEN 0.2 &lt;<i>z</i>&lt; 2 FROM THE S-COSMOS SURVEY

Abstract

We follow the galaxy stellar mass assembly by morphological and spectral type\nin the COSMOS 2-deg^2 field. We derive the stellar mass functions and stellar\nmass densities from z=2 to z=0.2 using 196,000 galaxies selected at F(3.6\nmicron) &gt; 1 microJy with accurate photometric redshifts\n(sigma_((zp-zs)/(1+zs))=0.008 at i&lt;22.5). Using a spectral classification, we\nfind that z~1 is an epoch of transition in the stellar mass assembly of\nquiescent galaxies. Their stellar mass density increases by 1.1 dex between\nz=1.5-2 and z=0.8-1 (Delta t ~2.5 Gyr), but only by 0.3 dex between z=0.8-1 and\nz~0.1 (Delta t ~ 6 Gyr). Then, we add the morphological information and find\nthat 80-90% of the massive quiescent galaxies (log(M)~11) have an elliptical\nmorphology at z&lt;0.8. Therefore, a dominant mechanism links the shutdown of star\nformation and the acquisition of an elliptical morphology in massive galaxies.\nStill, a significant fraction of quiescent galaxies present a Spi/Irr\nmorphology at low mass (40-60% at log(M)~9.5), but this fraction is smaller\nthan predicted by semi-analytical models using a ``halo quenching'' recipe. We\nalso analyze the evolution of star-forming galaxies and split them into\n``intermediate activity'' and ``high activity'' galaxies. We find that the most\nmassive ``high activity'' galaxies end their high star formation rate phase\nfirst. Finally, the space density of massive star-forming galaxies becomes\nlower than the space density of massive elliptical galaxies at z&lt;1. As a\nconsequence, the rate of ``wet mergers'' involved in the formation of the most\nmassive ellipticals must decline very rapidly at z&lt;1, which could explain the\nobserved slow down in the assembly of these quiescent and massive sources.\n