Mg ii Absorbers: Metallicity Evolution and Cloud Morphology

Metal abundance and its evolution are studied for Mg ii quasar absorption line systems from their weak, unsaturated spectral lines using stacked spectra from the archived data of the Sloan Digital Sky Survey. They show an abundance pattern that resembles that of the Galactic halo or Small Magellanic Cloud, with metallicity [Z/H] showing an evolution from redshift z = 2 to 0.5: metallicity becomes approximately solar or even larger at . We show that the evolution of the metal abundance traces the cumulative amount of the hydrogen fuel consumed in star formation in galaxies. With the aid of a spectroscopic simulation code, we infer the median gas density of the cloud to be roughly 0.3 , with which the elemental abundance in various ionization stages, in particular C i, is consistently explained. This gas density implies that the size of the Mg ii clouds is of the order of 0.03 kpc, which suggests that individual Mg ii clouds around a galaxy are of a baryonic mass typically . This means that Mg ii clouds are numerous and "foamy," rather than a large entity that covers a sizable fraction of galaxies with a single cloud.