High-Metallicity Mg II Absorbers in the z < 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 W_r(Mg II) <= 0.3 Å 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 W<sub>r</sub>(λ2796) detection limit of 0.02 Å. The two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have W<sub>r</sub>(Lyα) = 0.33 and 0.15 Å, 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<sup>-2</sup>. 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.

Based in part on observations obtained at the W. M. Keck Observatory, which is jointly operated by the University of California and the California Institute of Technology.

Based in part on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the STScI for the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.