On the H i content, dust-to-gas ratio and nature of Mg II absorbers

We estimate the mean dust-to-gas ratio of Mg ii absorbers as a function of rest equivalent width W₀ 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-α absorbers, we first show the existence of a 8σ correlation between the mean hydrogen column density 〈N _{H i}〉 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 Mg ii 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^* 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 ∼L ^* galaxies as the origin of the majority of strong Mg ii 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 Mg ii systems down to lower column densities than those probed in our Galaxy.