High‐Metallicity MgiiAbsorbers in thez< 1 Lyα Forest of PKS 0454+039: Giant Low Surface Brightness Galaxies?

Abstract

We report the discovery of two iron-group enhanced high-metallicity Mg II absorbers in a search through 28 Lyα forest clouds along the PKS 0454+039 sight line. Based upon our survey and the measured redshift number densities of Wr(Mg II) ≤ 0.3 A absorbers and Lyα absorbers at z ~ 1, we suggest that roughly 5% of Lyα absorbers at z ≤ 1 will exhibit weak Mg II absorption to a 5 σ Wr(λ2796) detection limit of 0.02 A. The two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have Wr(Lyα) = 0.33 and 0.15 A, respectively. Based upon photoionization modeling, the H I column densities are inferred to be in the range 15.8 ≤ log N(H I) ≤ 16.8 cm-2. For the z = 0.6428 absorber, if the abundance pattern is solar, then the cloud has [Fe/H] > -1; if its gas-phase abundance follows that of depleted clouds in our Galaxy, then [Fe/H] > 0 is inferred. For the z = 0.9315 absorber, the metallicity is [Fe/H] > 0, whether the abundance pattern is solar or suffers depletion. Imaging and spectroscopic studies of the PKS 0454+039 field reveal no candidate luminous objects at these redshifts. We discuss the possibility that these Mg II absorbers may arise in the class of giant low surface brightness galaxies, which have [Fe/H] ≥ -1, and even [Fe/H] ≥ 0, in their extended disks. We tentatively suggest that a substantial fraction of these weak Mg II absorbers may select low surface brightness galaxies out to z ~ 1.