The nature of Damped Lyman-{\alpha} and MgII absorbers explored with their dust contents

We estimate the abundance of dust in damped Lyman-{\alpha} absorbers (DLAs) by statistically measuring the excess reddening they induce on their background quasars. We detect systematic reddening behind DLA consistent with the SMC type reddening curve, but it is inconsistent with the Milky Way type reddening. We find that the derived dust-to-gas ratio is, on average, inversely proportional to the column density of neutral hydrogen, implying that the amount of dust is constant, irrespective of the column density of hydrogen. It means that the average metallicity is inversely proportional to the column density of hydrogen, unless the average dust-to-metal ratio varies with the hydrogen column density. This indicates that the prime origin of metals seen in DLAs is not by in situ star formation, with which Z ~ N_{HI}^{0.4} is expected from the empirical star formation law, contrary to our observation. We interpret the metals observed in absorbers being deposited dominantly from nearby galaxies by galactic winds ubiquitous in intergalactic space. When extrapolating the relation between dust-to-gas ratio and HI column density to lower column density, we find a value which is consistent with what is observed for Mg II absorbers.