A homogeneous sample of sub-damped Lyman α systems -: III.: Total gas mass Ω H I+He II at z > 2

Absorbers seen in the spectrum of background quasars are a unique tool with which to select HI-rich galaxies at all redshifts. In turns, these galaxies allow us to determine the cosmological evolution of the HI gas Omega(HI+HeII), which is a possible indicator of gas consumption as star formation proceeds. The damped Lyman alpha (Ly alpha) systems (DLAs with N (HI) >= 10(20.3) cm(-2)), in particular, are believed to contain a large fraction of the HI gas but there are also indications that lower column-density systems, called 'sub-damped Ly alpha' systems, play a role at high redshift. Here we present the discovery of high-redshift sub-DLAs based on 17 z > 4 quasar spectra observed with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT). This sample is composed of 21 new sub-DLAs which, together with another 10 systems from previous European Southern Observatory archive studies, make up a homogeneous sample. The redshift evolution of the number density of several classes of absorbers is derived and shows that all systems seem to be evolving in the redshift range from z = 5 to z similar to 3. These results are further used to estimate the redshift evolution of the characteristic radius of these classes of absorbers, assuming a Holmberg relation and one unique underlying parent population. DLAs are found to have R* similar to 20 h(100)(-1) kpc, while sub-DLAs have R* similar to 40 h(100)(-1) kpc. The redshift evolution of the column density distribution, f( N, z), down to N (HI) = 10(19) cm(-2) is also presented. A departure from a power law due to a flattening of f(N, z) in the sub- DLA regime is present in the data. f(N, z) is further used to determine the HI gas mass contained in sub- DLAs at z > 2. The complete sample shows that sub- DLAs are important at all redshifts from z = 5 to z = 2. Finally, the possibility that sub- DLAs are less affected by the effects of dust obscuration than classical DLAs is discussed.