2016A&A...588A..94F

We present a detailed analysis of a very unusual sub-damped Lyman α (sub-DLA) system at redshift z=2.304 towards the quasar Q 0453-423, based on high signal-to-noise (S/N), high-resolution spectral data obtained with VLT/UVES. With a neutral hydrogen column density of log N(HI)=19.23 and a metallicity of -1.61 as indicated by [OI/HI] the sub-DLA mimics the properties of many other optically thick absorbers at this redshift. A very unusual feature of this system is, however, the lack of any CIV absorption at the redshift of the neutral hydrogen absorption, although the relevant spectral region is free of line blends and has very high S/N. Instead, we find high-ion absorption from CIV and OVI in another metal absorber at a velocity more than 220km/s redwards of the neutral gas component. We explore the physical conditions in the two different absorption systems using Cloudy photoionisation models. We find that the weakly ionised absorber is dense and metal-poor while the highly ionised system is thin and more metal-rich. The absorber pair towards Q 0453-423 mimics the expected features of a galactic outflow with highly ionised material that moves away with high radial velocities from a (proto)galactic gas disk in which star-formation takes place. We discuss our findings in the context of CIV absorption line statistics at high redshift and compare our results to recent galactic-wind and outflow models.