On the HI content, dust-to-gas ratio and nature of MgII absorbers

We estimate the mean dust-to-gas ratio of MgII absorbers as a function of rest equivalent width W-0 and redshift over the range 0.5 < z < 1.4. Using the expanded Sloan Digital Sky Survey/Hubble Space Telescope sample of low-redshift Lyman-alpha absorbers, we first show the existence of a 8 sigma correlation between the mean hydrogen column density < N-HI > and W-0, 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 L-star galaxies from z = 0.5 to 1.4, and we show that our constraints disfavour dwarf galaxies as the origin of such absorbers. We discuss other scenarios and favour galactic outflows from similar to L-star 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 A(V) and N-H is also satisfied by strong MgII systems down to lower column densities than those probed in our Galaxy.