The VIMOS VLT deep survey* 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 the GSMF using two different samples: (1) an optical (I-selected 17.5 < I _AB < 24) main spectroscopic sample of about 6500 galaxies over 1750 arcmin ² and (2) a near-IR (K-selected K _AB < 22.34 and K _AB < 22.84) sample of about 10 200 galaxies, with photometric redshifts accurately calibrated on the VVDS spectroscopic sample, over 610 arcmin ². We apply and compare two different methods to estimate the stellar mass M _stars from broad-band photometry based on different assumptions about the galaxy star-formation history. We find that the accuracy of the photometric stellar mass is satisfactory overall, 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 (<10 ⁹ M _⊙) composed of faint blue galaxies (M _I - M _K ≃ 0.3). In general the stellar mass function evolves slowly up to z ∼ 0.9 and more rapidly above this redshift, in particular for low mass systems. Conversely, a massive population is present up to z = 2.5 and has extremely red colours (M _I - M _K ≃ 0.7-0.8). We find a decline with redshift of the overall number density of galaxies for all masses (59 ± 5% for M _stars > 10 ⁸ 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 × 10 ¹⁰ M _⊙). For less massive systems the no-evolution scenario is excluded. Specifically, a large fraction (≥50%) of massive galaxies have been assembled and converted most of their gas into stars at z ∼ 1, ruling out "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 of up to 4.1 ± 0.9) from the present age to z = 2, consistent 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 of 2.3 ± 0.1 at z = 1 and about 4.5 ± 0.3 at z = 2.5.