Elemental Abundances in Two High Column Density Damped Lyα Systems at z < 1.5

Abstract

We present Keck HIRES abundance measurements and metal-line kinematic profiles of the damped Lyα systems (DLAs) toward the quasars Q0933+733 (z_abs=1.479) and Q0948+433 (z_abs=1.233). These two DLAs have among the five highest H I column densities at any redshift: N(HI)=4.2×10²¹ cm^-2. The metal-line data, presented here for the first time, reveal that these DLAs are noteworthy for several other reasons as well. (1) The Q0933+733 DLA exhibits a simple kinematic structure with unusually narrow velocity widths as measured from its unsaturated metal lines (Δv=16 km s^-1). At 2.6% solar, it has the second lowest metallicity at z<2. (2) The Q0948+433 DLA has among the strongest metal-line transitions of any known DLA. The saturated Si II λ1808 line implies a high metallicity ([Si/H]>-1) and a significant α-enhancement. The strong metal lines of this DLA have made possible the detection of Ti II λ1910, Co II λ2012, and Mg I λ2026. (3) We find that the relative gas-phase abundances of both DLAs follow the general trend seen at high redshift, e.g., enhanced Zn/Fe and Si/Fe and subsolar Mn/Fe, indicating that there is little evolution in the nucleosynthetic patterns of DLAs down to this epoch. (4) Their high H I column densities imply that these DLAs dominate the column density-weighted cosmic mean metallicity, <Z>, of the universe at z<1.5. Using the 15 DLAs with measured metallicities in the redshift interval 0.4<z<1.5, we find <Z>=-0.89^+0.40_-0.33, where the uncertainties are 95% confidence limits.

Based on data obtained with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.