2023A&A...679A..30F

We report on further observations of the field of the quasar Q 1218+0832. Geier et al. (2019, A&A, 625, L9) presented the discovery of the quasar resulting from a search for quasars reddened and dimmed by dust in foreground damped Lyman-α absorbers (DLAs). The DLA is remarkable by having a very large H I column density close to 10²² cm^–2. Its dust extinction curve shows the 2175 Å bump known from the Local Group. It also shows absorption from cold gas exemplified by C I and CO molecules. For this paper, we present narrow-band observations of the field of Q 1218+0832 and also use an archival Hubble Space Telescope (HST) image to search for the galaxy counterpart of the DLA. No emission from the DLA galaxy is found in either the narrow-band imaging or in the HST image. In the HST image, we could probe down to an impact parameter of 0.3 arcsec and a 3-σ detection limit of 26.8 mag per arcsec². In the narrow-band image, we probed down to a 0 arcsec impact parameter and detected nothing down to a 3-σ detection limit of about 3 × 10^–17 erg s^–1 cm^–2. We did detect a bright Lyman-α emitter 59 arcsec south of Q 1218+0832 with a flux of 3 × 10^–16 erg s^–1 cm^–2. We conclude that the DLA galaxy must be located at a very small impact parameter (< 0.3 arcsec, 2.5 kpc) or it is optically dark. Also, the DLA galaxy most likely is part of a galaxy group.