GALAXY DOWNSIZING EVIDENCED BY HYBRID EVOLUTIONARY TRACKS

Abstract

An unified picture of stellar and halo mass build-up as a function of mass is presented. Inferred stellar-dark halo mass relations of galaxies, Ms-Mh, out to z=4 together with average LCDM halo mass aggregation histories (MAHs) are used for inferring average Ms growth histories, the Galaxian Hybrid Evolutionary Tracks (GHETs). The more massive the galaxy, the earlier transited in average from an active regime of Ms growth to a passive one: log(Mtran/Msun)=10.30+0.55z ("population downsizing"), where Mtran is the typical transition stellar mass. This result agrees with independent observational determinations based on the evolution of the galaxy stellar mass function decomposition into blue and red galaxies. The specific star formation rate, SSFR, predicted from the derivative of the GHET is consistent with direct measures of the SSFR for galaxies at different z's. The average GHETs of galaxies smaller than Mtran at z=0 (Ms~10^10.3 Msun) did not reach the quiescent regime, and for them, the lower the mass, the faster the later Ms growth rate ("downsizing in SSFR"). The GHETs allow to predict the transition rate in number density of active to passive population; the predicted values agree with direct estimates of growth rate in number density for the (massive) red population up to z~1. We show that LCDM-based models of disk galaxy evolution are able to reproduce the low-mass side of the Ms-Mh relation at z~0, but at higher z's disagree strongly with the GHETs: models do not reproduce the downsizing in SSFR and the high SSFR of low mass galaxies. (Abridged)