Nature and Properties of Sub-DLAs (Absorbers with 10¹⁹ ≤ N(HI) ≤ 2 * 10²⁰ CM⁻²)

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, 90% of the HI mass is in damped Lyα systems (DLAs). But it has been suggested that 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.