The Role of Sub–Damped Lyα Absorbers in the Cosmic Evolution of Metals

Observations of low mean metallicity of damped Lyman-alpha (DLA) quasarabsorbers at all redshifts studied appear to contradict the predictions for theglobal mean interstellar metallicity in galaxies from cosmic chemical evolutionmodels. On the other hand, a number of metal-rich sub-DLA systems have beenidentified recently, and the fraction of metal-rich sub-DLAs appears to beconsiderably larger than that of metal-rich DLAs, especially at z < 1.5. Inview of this, here we investigate the evolution of metallicity in sub-DLAs. Wefind that the mean Zn metallicity of the observed sub-DLAs may be higher thanthat of the observed DLAs, especially at low redshifts, reaching a near-solarlevel at z <~ 1. This trend does not appear to be an artifact of sampleselection, the use of Zn, the use of N_{HI}-weighting, or observationalsensitivity. While a bias against very low metallicity could be present in thesub-DLA sample in some situations, this cannot explain the difference betweenthe DLA and sub-DLA metallicities at low z. The primary reason for thedifference between the DLAs and sub-DLAs appears to be the dearth of metal-richDLAs. We estimate the sub-DLA contribution to the total metal budget usingmeasures of their metallicity and comoving gas density. These calculationssuggest that at z <~ 1, the contribution of sub-DLAs to the total metal budgetmay be several times that of DLAs. At higher redshifts also, there areindications that the sub-DLAs may contribute significantly to the cosmic metalbudget.Comment: 9 pages, 2 figures, Accepted for Publication in the Astrophysical Journa