2017A&A...606A..13F

It is important to understand the selection effects behind the quasar samples to fully exploit the potential of quasars as probes of cosmic chemical evolution and the internal gas dynamics of galaxies; in particular, it is vital to understand whether the selection criteria exclude foreground galaxies with certain properties, most importantly a high dust content. Here we present spectroscopic follow-up from the 10.4m GTC telescope of a dust-reddened quasar, eHAQ0111+0641, from the extended High A_V Quasar (HAQ) survey. We find that the z=3.21 quasar has a foreground damped Lyman-α absorber (DLA) at z=2.027 along the line of sight. The DLA has very strong metal lines due to a moderately high metallicity with an inferred lower limit of 25% of the solar metallicity, but a very large gas column density along the line of sight in its host galaxy. This discovery is further evidence that there is a dust bias affecting the census of metals, caused by the combined effect of dust obscuration and reddening, in existing samples of z>2 DLAs. The case of eHAQ0111+0641 illustrates that dust bias is not only caused by dust obscuration, but also dust reddening.