The chemical compositions of 10 new sub-DLAs and strong Lyman-limit systems atz≲ 1.5

Abstract

We present chemical abundance measurements from medium-resolution observations of eight subdamped Lya (sub-DLA) absorber and two strong Lyman-limit systems at z ≤ 1.5 observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in z abs ≤ 1.5 quasar absorption line systems focusing on sub-DLA systems. These observations increase the sample of Zn measurements in z abs ≤ 1.5 sub-DLAs by ∼50 per cent. Lines of Mg i, Mg II, Al II, Al III, Can, Mn II, Fe II and Zn II were detected and column densities were determined. Zn II, a relatively undepleted element and tracer of the gas-phase metallicity is detected in two of these systems, with [Zn/H] = -0.05 ±0.12 and [Zn/H] > +0.86. The latter one is however a weak system with N H I < 18.8, and therefore may need significant ionization corrections to the abundances. Fe II lines were detected in all systems, with an average Fe abundance of ([Fe/H]) = -0.68, higher than typical Fe abundances for DLA systems at these redshifts. This high mean [Fe/H] could be due to less depletion of Fe on to dust grains, or to higher abundances in these systems. We also discuss the relative abundances in these absorbers. The systems with high metallicity show high ratios of [Mn/Fe] and [Zn/Fe], as seen previously in another sub-DLA. These higher values of [Mn/Fe] could be a result of heavy depletion of Fe on to grains, unmixed gas, or an intrinsically non-solar abundance pattern. Based on CLOUDY modelling, we do not expect ionization effects to cause this phenomenon.