The Metallicity - Kinematics Relation in Large-N(HI) Absorbers

Original · EN

Sloan Digital Sky Survey (SDSS) quasar spectroscopy is yielding a database of strong low-ionization MgII absorbers over the redshift interval 0.36<z<2.28 which is over two orders of magnitude larger than anything previously assembled. HST UV spectroscopy has been used to measure neutral hydrogen column densities for a small subset of them. These data empirically show that MgII absorbers with rest equivalent widths Wλ²⁷⁹⁶₀ ≥ 0.6 Åhave a mean neutral hydrogen column density that is roughly constant at N(HI) ≈ 4 × 10²⁰ atoms cm⁻², with individual systems lying in the damped Ly-alpha (DLA) and sub-DLA regimes. Since the MgII doublets generally exhibit saturation, the Wλ²⁷⁹⁶₀ values are an indication of the absorbers' velocity spreads. Thus, we can study neutral-gas-phase metallicities as a function of kinematics by forming SDSS composite spectra and measuring weak unsaturated metal lines that form in neutral gas (e.g., CrII, FeII, MnII, SiII, ZnII) as a function of Wλ²⁷⁹⁶₀. We use this method on SDSS composite spectra to show how metallicity and kinematics are positively correlated for large N(HI) absorbers, including trends related to dust depletion and the enhancement of alpha-elements. We also discuss the need to account for selection effects in DLA surveys, and we make inferences about models for DLA absorption and their contribution to cosmic star formation.