ChandraACIS‐S Observations of Three Quasars with Low‐Redshift Damped Lyα Absorption: Constraints on the Cosmic Neutral Gas‐Phase Metallicity at Redshiftz ≈ 0.4

Abstract

Chandra X-Ray Observatory ACIS-S spectra of three quasars that lie behind three foreground damped Lyα (DLA) absorbers are analyzed in order to attempt to determine the amount of photoelectric absorption due to metals present in their X-ray spectra. These absorbers are the three largest neutral hydrogen column density absorption-line systems known at low redshift (0.313 ≤ zabs ≤ 0.524). They have H I column densities that lie in the range 3 × 1021 NH I 5 × 1021 atoms cm-2. At these redshifts the amount of photoelectric absorption at X-ray energies is primarily an indicator of the oxygen abundance. Since the column densities of these systems are so high, one would expect accurate metallicity measurements of them to yield a robust estimate of the column density-weighted cosmic neutral gas-phase metallicity at z ≈ 0.4. We consider cases in which the DLA gas has solar element abundance ratios and ones with the α group element abundance ratios enhanced. For the adopted assumptions, the column density-weighted cosmic neutral gas-phase metallicity of the nonenhanced elements (e.g., Zn) at z ≈ 0.4 likely lies in the range ≈0.04-0.38 Z☉.