H<scp>I</scp>Column Densities, Metallicities, and Dust Extinction of Metal-Strong Damped Lyα Systems1

Abstract

With the Blue Channel Spectrograph (BCS) on the MMT telescope, we have\nobtained spectra to the atmospheric cutoff of quasars previously known to show\nat least one absorption system at z&gt;1.6 with very strong metal lines (candidate\nmetal-strong damped Lya systems; cMSDLAs). The BCS/MMT spectra yield precise\nestimates of the HI column densities (NHI) of the systems through Voigt profile\nanalysis of their Lya transitions. Nearly all of the cMSDLAs (41/43) satisfy\nthe NHI criterion of DLAs, 10^20.3. As a population, these systems have\nsystematically higher NHI values than DLAs chosen randomly from quasar\nsightlines. Combining our NHI measurements with previously measured metal\ncolumn densities, we estimate metallicities for the MSDLAs. These systems have\nsignificantly higher values than randomly selected DLAs; at z~2, the MSDLAs\nshow a median metallicity [M/H] ~ -0.67 that is 0.6dex higher than a\ncorresponding control sample. This establishes MSDLAs as having amongst the\nmost metal-rich gas in the high z universe. Our measurements extend the\nobserved correlation between SiII 1526 equivalent width and the gas metallicity\nto higher values. If interpreted as a mass-metallicity relation, this implies\nthe MSDLAs are the high mass subset of the DLA population. We demonstrate that\ndust in the MSDLAs reddens their background quasars, with a median shift in the\nspectral slope of Da = 0.29. Assuming an SMC extinction law, this implies a\nmedian reddening E(B-V)=0.025mag and visual extinction A_V=0.076mag. Future\nstudies of MSDLAs offer the opportunity to study the extinction,\nnucleosynthesis, and kinematics of the most chemically evolved, gas-rich\ngalaxies at high z. [abridged]\n