Abstract
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, 45% of the HI lies in systems below the traditional DLA definition (2 × 1020 cm−2), in ‘sub-DLAs’ with 1019 < N(HI) < 2 × 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.
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