The chemical compositions of 10 new sub-DLAs and strong Lyman-limit systems at z < 1.5

TL;DR

This study measures chemical abundances in 10 new sub-DLAs and Lyman-limit systems at z<1.5, revealing higher Fe levels and diverse metallicity patterns, expanding understanding of gas-phase metallicity in low-redshift quasar absorption systems.

Contribution

It provides new chemical abundance data for 10 sub-DLAs and Lyman-limit systems at z<1.5, increasing the Zn measurement sample by about 50% and analyzing their metallicity and element ratios.

Findings

Fe abundances are higher than typical DLA systems at similar redshifts.

High metallicity systems show elevated [Mn/Fe] and [Zn/Fe] ratios.

One system exhibits super-solar [Zn/H], indicating high gas-phase metallicity.

Abstract

We present chemical abundance measurements from medium resolution observations of 8 sub-damped Lyman-alpha absorber and 2 strong Lyman-limit systems at z < 1.5 observed with the MIKE spectrograph on the 6.5m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in z < 1.5 quasar absorption line systems (QSOALS) focusing on sub-DLA systems. These observations increase the sample of Zn measurements in z < 1.5 sub-DLAs by ~50%. Lines of Mg I, Mg II, Al II, Al III, Ca II, 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\pm0.12 and [Zn/H]>+0.86. The latter system is however a weak system with N(H I)<18.8, and therefore may need significant ionisation 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 onto 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 onto grains, unmixed gas, or an intrinsically non-solar abundance pattern. Based on Cloudy modeling, we do not expect ionisation effects to cause this phenomenon.