Damped Lyman Alpha Gas Metallicities at Redshifts z=0.9-2.0 from SDSS Spectra

Using Sloan Digital Sky Survey (SDSS) early data release spectra, we have identified 370 Mg II absorption systems with Mg II λ 2796 rest equivalent widths ≥ 1 ˚A and redshifts z = 0 . 9 − 2 . 2. From our previous and ongoing HST UV spectroscopic studies, we estimate that the mean neutral hydrogen column density of a system selected in this manner is N HI = 3 . 6 ± 1 . 3 × 10 20 atoms cm − 2 , which corresponds to the damped Ly α (DLA) regime. We have formed high signal-to-noise ratio composite spectra using 223 of these systems with z = 0 . 9 − 2 . 0 in order to study the strength of the Zn II and Cr II absorption lines corresponding to this mean neutral hydrogen column density. After making a correction for missed DLAs, overall we find that [Zn/H] = − 1 . 13 ± 0 . 19. We find [Cr/Zn] = − 0 . 45 ± 0 . 13, which indicates that ≈ 65% of the Cr is depleted on to grains, but this does not correct for the missed DLAs. We have also derived Zn and Cr abundances in two kinematic regimes, and within each regime we consider two redshift intervals. We find trends which indicate that metallicities are higher in the composites where the absorption has larger velocity spreads as measured by Mg II λ 2796 rest equivalent width. Larger velocity spreads may correspond to deeper gravitational potential wells which represent more massive and chemically evolved structures, and/or regions associated with winds from starbursting galaxies, also leading to kinematically broad structures of chemically enriched gas. Within the large velocity spread regime, we find that at lower redshifts the Zn metallicity is larger and more Cr is depleted on to grains.