How do galaxies accrete their mass? Quiescent and star-forming massive galaxies at high redshift

Abstract

In recent years, several surveys have shown that massive galaxies have undergone a major evolution during the epoch corresponding to the redshift range 1.5–3, assembling a significant fraction of their stellar mass at that epoch. To understand the origin of this rapid rise, a closer scrutiny on the nature and physical properties of massive galaxies at high redshift is needed. I will present our recent results based on the analysis of the 24 μm MIPS data of the GOODS-S field, that allow to trace star formation (or the lack of it) in high redshift galaxies without biases due to dust extinction. I will show the results of our analysis focusing in particular on the fraction of quiescent galaxies as a function of redshift and the evolution of the specific star formation rate as a function of redshift and stellar mass. The scenario emerging from these data will be compared with recent predictions of theoretical models to discuss the validity of their physical ingredients. The main results of this work are: a) the fraction of quiescent galaxies decreases with redshift, and a non-negligible fraction (∼15%) is already in place at z ∼ 3; b) massive star-forming galaxies are vigorously forming stars (∼ 300M⊙yr−8) at z ∼ 2, and during this epoch they assemble a substantial part of their final stellar mass; c) the specific star formation rate shows a bimodal distribution up to z ∼ 2; d) theoretical predictions, although qualitatevely in agreement, are unable to quantitatively reproduce these observations.