Metallicities and dust content of proximate damped Lyman α systems in the Sloan Digital Sky Survey

Abstract

Composite spectra of 85 proximate absorbers [log N(Hi) ≥ 20 cm-2 and velocity difference between the absorption and emission redshifts, ΔV < 10000 km s-1] in the Sloan Digital Sky Survey are used to investigate the trends of metal-line strengths with velocity separation from the quasi-stellar object (QSO). We construct composites in three velocity bins: ΔV < 3000 km s-1, 3000 < ΔV < 6000 km s-1 and ΔV > 6000 km s-1, with further sub-samples to investigate the metal-line dependence on N(Hi) and QSO luminosity. Low (e.g. Siii and Feii) and high ionization (e.g. Siiv and Civ) species alike have equivalent widths (EWs) that are larger by factors of 1.5-3 in the ΔV < 3000 km s-1 composite, compared to the ΔV > 6000 km s-1 spectrum. The EWs show an even stronger dependence on ΔV if only the highest neutral hydrogen column density [log N(Hi) ≥ 20.7] absorbers are considered. We conclude that proximate damped Lyman α systems (PDLAs) generally have higher metallicities than intervening absorbers, with the enhancement being a function of both ΔV and N(Hi). It is also found that absorbers near QSOs with lower rest-frame UV luminosities have significantly stronger metal lines. We speculate that absorbers near to high luminosity QSOs may have had their star formation prematurely quenched. There is no evidence for Lyα emission in the trough of the composite spectrum. Finally, we search for the signature of dust reddening by the PDLAs, based on an analysis of the QSO continuum slopes relative to a control sample, and determine a limit of E(B-V) < 0.014 for a Small Magellanic Cloud extinction curve. This work provides an empirical motivation for distinguishing between proximate and intervening DLAs and establishes a connection between the QSO environment and galaxy properties at high redshifts.