The Assembly of Massive Galaxies from Near-Infrared Observations of the Hubble Deep Field-South

Abstract

We use a deep K_AB<=25 galaxy sample in the Hubble Deep Field-South to trace the evolution of the cosmological stellar mass density from z~=0.5 to z~=3. We find clear evidence for a decrease of the average stellar mass density at high redshift, 2<=z<=3.5, that is 15⁺²⁵_-5% of the local value, 2 times higher than observed in the Hubble Deep Field-North. To take into account for the selection effects, we define a homogeneous subsample of galaxies with 10¹⁰ M_solar<=M_*<=10¹¹ M_solar: in this sample, the mass density at z>2 is 20⁺²⁰_-5% of the local value. In the mass-limited subsample at z>2, the fraction of passively fading galaxies is at most 25%, although they can contribute up to about 40% of the stellar mass density. On the other hand, star-forming galaxies at z>2 form stars with an average specific rate of at least *>~=4×10^-10 yr^-1, 3 times higher than the z<=1 value. This implies that UV-bright star-forming galaxies are substancial contributors to the rise of the stellar mass density with cosmic time. Although these results are globally consistent with Λ-CDM scenarios, the present rendition of semianalytic models fails to match the stellar mass density produced by more massive galaxies present at z>2.