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 similar or equal to 0.2) we find a substantial population of low-mass galaxies (< 10(9) M-circle dot) composed of faint blue galaxies (M-I-M-K similar or equal to 0.3). In general the stellar mass function evolves slowly up to z similar to 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 similar or equal to 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-circle dot 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 similar to 0.7 for massive galaxies (> 6 x 10(10) M-circle dot). 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 similar to 1, ruling out "dry mergers" as the major mechanism of their assembly history below z similar or equal to 1. This fraction decreases to similar to 33% at z similar to 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.