Nature and Properties of sub-DLAs (absorbers with 1019 ≤ N(HI) ≤ 2 * 1020 cm-2)

Quasar absorbers provide an alternative approach to the study of galaxies. In particular, the detection of high-column density systems allow to measure the cosmological evolution of the neutral gas mass and metals in the Universe. At z <3.5, 90% of the HI mass is in damped Lyα systems (DLAs). But it has been suggested that atz > 3.5, 45% of the HI lies in systems below the traditional DLA definition (2 x 10²⁰ cm^-2), in ‘sub-DLAs’ with 10¹⁹ < N(HI) < 2 x 10²⁰ cm^-2. Here we present a sample of sub-DLAs issued from UVES archive quasar spectra. We have measured the sub-DLA number density redshift evolution and directly establish their column density distribution function. We have also undertaken a detailed metal abundance analysis of this class of absorbers to compare with the characteristics of the well studied DLAs. This revised version was published online in September 2006 with corrections to the Cover Date.