The assembly of early-type galaxies: combining clustering and luminosity function measurements

Abstract

We present a study of the evolution of early-type galaxies (ETGs) that combines luminosity funciton and clustering measurements. The latter allows us to infer the typical dark-matter halo mass of the hosts of the galaxies. Using LambdaCDM predictions, it is then possible to follow these haloes to a later time. This technique shows that ETGs at a given redshift evolve into brighter galaxies in the rest-frame passively evolved optical at lower redshifts. Notice that this indicates that a stellar-mass selection at different redshifts does not necessarily provide samples of galaxies in a progenitor-descendant relationship. The comparison between high redshift ETGs and their likely descendants at z=0 points to a higher number density for the progenitors by a factor 3-11, implying the need for mergers to decrease their number density by today. Because the progenitor-to-descendant ratios of luminosity density are consistent with the unit value, our results show no need for strong star-formation episodes in ETGs since z=1, which indicates that the needed mergers are dry, i.e. gas free.