The Assembly History of the Stellar Mass in Galaxies: from the Young to the Old Universe

We present a detailed analysis of the Galaxy Stellar Mass Function (GSMF) of galaxies up to z = 2.5 as obtained from the VIMOS VLT Deep Survey (VVDS). Our survey offers the possibility to investigate it using two different samples: (1) an optical (I-selected 17.5 < IAB < 24) main spectroscopic sample of about 6500 galaxies over 1750 arcmin 2 and (2) a near-IR (K-selected KAB < 22.34 & KAB < 22.84) sample of about 10200 galaxies, with photometric redshifts accurately calibrated on the VVDS spectroscopic sample, over 610 arcmin. We apply and compare two different methods to estimate the stellar mass Mstars from broad-band photometry based on different assumptions on the galaxy star-formation history. We find that the accuracy of the photometric stellar mass is overall satisfactory, and show that the addition of secondary bursts to a continuous star formation history produces systematically higher (up to 40%) stellar masses. We derive the cosmic evolution of the GSMF, the galaxy number density and the stellar mass density in different mass ranges. At low redshift (z ≃ 0.2) we find a substantial population of low-mass galaxies (< 10M⊙) composed by faint blue galaxies (MI −MK ≃ 0.3). In general the stellar mass function evolves slowly up to z ∼ 0.9 and more significantly above this redshift, in particular for low mass systems. Conversely, a massive population is present up to z = 2.5 and have extremely red colours (MI−MK ≃ 0.7−0.8). We find a decline with redshift of the overall number density of galaxies for all masses (59± 5% for Mstars > 10 M⊙ at z = 1), and a mild mass-dependent average evolution (‘mass-downsizing’). In particular our data are consistent with mild/negligible (< 30%) evolution up to z ∼ 0.7 for massive galaxies (> 6× 10M⊙). For less massive systems the no-evolution scenario is excluded. Specifically, a large fraction (≥ 50%) of massive galaxies have been already assembled and converted most of their gas into stars at z ∼ 1, ruling out the ‘dry mergers’ as the major mechanism of their assembly history below z ≃ 1. This fraction decreases to ∼ 33% at z ∼ 2. Low-mass systems have decreased continuously in number density (by a factor up to 4.1 ± 0.9) from the present age to z = 2, consistently with a prolonged mass assembly also at z < 1. The evolution of the stellar mass density is relatively slow with redshift, with a decrease of a factor 2.3 ± 0.1 at z = 1 and about 4.5± 0.3 at z = 2.5.