VLT/UVES observations of peculiar α abundances in a sub-DLA at z ≈ 1.8 towards the quasar B1101−26

Institut für Physik und Astronomie, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
e-mail: annfox@astro.physik.uni-potsdam.de

Received: 17 March 2014
Accepted: 4 September 2014

Abstract

We present a detailed analysis of chemical abundances in a sub-damped Lyman α absorber at z = 1.839 towards the quasar B1101−26, based on a very-high-resolution (R ~ 75 000) and high-signal-to-noise (S/N> 100) spectrum observed with the UV Visual Echelle spectrograph (UVES) installed on the ESO Very Large Telescope (VLT). The absorption line profiles are resolved into a maximum of eleven velocity components spanning a rest-frame velocity range of ≈200 km s^-1. Detected ions include C ii, C iv, N ii, O i, Mg i, Mg ii, Al ii, Al iii, Si ii, Si iii, Si iv, Fe ii, and possibly S ii. The total neutral hydrogen column density is log N(H i) = 19.48 ± 0.01. From measurements of column densities and Doppler parameters we estimate element abundances of the above-given elements. The overall metallicity, as traced by [O i/H i], is −1.56±0.01. For the nitrogen-to-oxygen ratio we derive an upper limit of [N i/O i] ≤ − 0.65, which suggests a chemically young absorption line system. This is supported by a supersolar α/Fe ratio of [Si ii/Fe ii] ≈ 0.5. The most striking feature in the observed abundance pattern is an unusually high sulphur-to-oxygen ratio of 0.69 ≤ [S ii/O i] ≤ 1.26. We calculate detailed photoionisation models for two subcomponents with Cloudy, and can rule out that ionisation effects alone are responsible for the high S/O ratio. We instead speculate that the high S/O ratio is caused by the combination of several effects, such as specific ionisation conditions in multi-phase gas, unusual relative abundances of heavy elements, and/or dust depletion in a local gas environment that is not well mixed and/or that might be related to star-formation activity in the host galaxy. We discuss the implications of our findings for the interpretation of α-element abundances in metal absorbers at high redshift.