A HST Spectroscopic study of QSOs with intermediate redshift damped Lyman-alpha systems

We present HST spectra for a sample of six QSOs with intermediate redshift (z_a < 1) damped Ly-alpha systems. These observations aim at measuring the HI column density and detect metal lines in order to investigate the metal enrichment of the gas, as well as the presence of neutral species, molecules and dust. All systems selected on the basis of 21 cm absorption and/or strong FeII lines relative to MgII ones turn out to have N(HI) larger than 10^20 cm-2. It appears that although the scatter of metallicities is as large at z_a<1 as at high redshift, an increasing proportion of systems with metallicities ~ 30% solar are found when going at lower redshifts. Our results suggest that available observations may be biased against dust-rich absorbers. Further, when all available measurements of N(HI) and [Zn/H] are considered, a clear deficiency of systems with large N(HI) and high metallicity is apparent. We conclude that dust extinction causes a preferential selection of QSOs with intervening gas relatively poor in metals, dust and molecules. As a consequence, the high end of the HI column density distribution (and hence Omega_g, the contribution of neutral gas to the cosmological mass density) is probably more heavily underestimated than previously thought, especially at low redshift. Such a bias could also explain the high incidence of non-spiral morphologies in our sample. We stress that observation of a larger sample of low z damped Ly-alpha systems as well as surveys of damped Ly-alpha systems in fainter QSOs would give a more representative view of the true diversity of absorber properties and should help to probe the denser phases of the interstellar medium in distant galaxies.