On the HI content, dust-to-gas ratio and nature of MgII absorbers
Abstract
We estimate the mean dust-to-gas ratio of MgII absorbers as a function of rest equivalent width W0 and redshift over the range 0.5<z<1.4. Using the expanded SDSS/HST sample of low-redshift Lyman-alpha absorbers we first show the existence of a 8-sigma correlation between the mean hydrogen column density <NHI> and W0, an indicator of gas velocity dispersion. By combining these results with recent dust-reddening measurements we show that the mean dust-to-gas ratio of MgII absorbers does not appreciably depend on rest equivalent width. Assuming that, on average, dust-to-gas ratio is proportional to metallicity, we find its redshift evolution to be consistent with that of Lstar galaxies from z=0.5 to 1.4 and we show that our constraints disfavor dwarf galaxies as the origin of such absorbers. We discuss other scenarii and favor galactic outflows from ~Lstar galaxies as the origin of the majority of strong MgII absorbers. Finally, we show that, once evolutionary effects are taken into account, the Bohlin et al. relation between AV and NH is also satisfied by strong MgII systems down to lower column densities than those probed in our Galaxy.