H I Column Densities, Metallicities, and Dust Extinction of Metal-Strong Damped Lyα Systems

With the Blue Channel Spectrograph (BCS) on the MMT telescope, we have obtained spectra to the atmospheric cutoff of quasars previously known to show at least one absorption system at z > 1.6 with very strong metal lines. We refer to these absorbers as candidate metal-strong damped Ly alpha systems (cMSDLAs), the majority of which were culled from the Sloan Digital Sky Survey. The BCS/MMT spectra yield precise estimates of the H I column densities (N(HI)) of the systems through Voigt profile analysis of their Ly alpha transitions. Nearly all of the cMSDLAs (41/43) satisfy the NHI criterion of DLAs, 10(20.3) atoms cm(-2). As a population, these systems have systematically higher NHI values than DLAs chosen randomly from quasar sightlines. Combining our N(HI) measurements with previously measured metal column densities, we estimate metallicities for the MSDLAs. These systems have significantly higher values than randomly selected DLAs; at z approximate to 2, the MSDLAs show a median metallicity [M/H] approximate to -0.67 that is 0.6 dex higher than a corresponding control sample. This establishes MSDLAs as having among the most metal-rich gas in the high z universe. Our measurements extend the observed correlation between Si II 1526 equivalent width and the gas metallicity to higher values. If interpreted as a mass-metallicity relation, this implies the MSDLAs are the high-mass subset of the DLA population. We demonstrate that dust in the MSDLAs reddens their background quasars, with a median shift in the spectral slope of delta alpha = 0.29. Assuming an SMC extinction law, this implies a median reddening E(B-V) approximate to 0.025 mag and visual extinction A(V) approximate to 0.076 mag. The latter quantity yields a dust-to-gas ratio of log(A(V)/N(HI)) approximate to -22.0, very similar to estimation for the SMC. Future studies of MSDLAs offer the opportunity to study the extinction, nucleosynthesis, and kinematics of the most chemically evolved, gas-rich galaxies at high z.