Zinc and chromium abundances in a third damped Lyman α system at intermediate redshift

We have determined the metallicity of the z_abs = 1.0093 damped Lyman α system in the bright QSO EX 0302-223; this is only the third such measurement at redshifts z ≲ 1. Unlike the previous two cases, we find that the abundance of Zn is only a factor of ∼ 2 lower than in the Galactic interstellar medium today and is entirely compatible with the typical metallicity of stars in the Milky Way disk at a look-back time of 9.5 Gyrs. Although the galaxy responsible for producing the absorption system has yet to be positively identified, our observations show that galaxies on a chemical evolution path similar to that of the Milky Way do contribute to the damped Lyman a population at intermediate redshifts. Cr is 2.5 times less abundant than Zn, presumably because of depletion onto dust; however, the degree of depletion is less severe than in diffuse interstellar clouds in the disk of our Galaxy and in the Magellanic Clouds. Evidently, the interstellar environment in damped Lyman a galaxies is less conducive to the formation and survival of dust grains (and molecular hydrogen), but the physical processes at the root of this effect have yet to be clarified.