VLT/UVES observations of peculiarαabundances in a sub-DLA atz≈ 1.8 towards the quasar B1101−26
Astronomy and Astrophysics
We present a detailed analysis of chemical abundances in a sub-damped Lyman alpha 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. Detected ions include CII, CIV, NII,
... lude CII, CIV, NII, OI, MgI, MgII, AlII, AlIII, SiII, SiIII, SiIV, FeII, and possibly SII. The total neutral hydrogen column density is log N(HI) = 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 [OI/HI], is -1.56 +/- 0.01. For the nitrogen-to-oxygen ratio we derive an upper limit of [NI/OI] < -0.65, which suggests a chemically young absorption line system. This is supported by a supersolar alpha/Fe ratio of [SiII/FeII] ~ 0.5. The most striking feature in the observed abundance pattern is an unusually high sulphur-to-oxygen ratio of 0.69 < [SII/OI] < 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 alpha-element abundances in metal absorbers at high redshift.