The nature of proximate damped Lyman α systems☆

We present high-resolution echelle spectra of seven proximate damped Lyman alpha (PDLA) systems. The relative velocity separation of each PDLA from the background quasar is delta V < 3000 km s-1. Combining our sample with a further nine PDLAs from the literature we compare the chemical properties of the proximate systems with a control sample of intervening DLAs. The PDLAs are usually excluded from statistical studies of absorption-selected galaxies and this sample constitutes the first systematic study of their chemical and ionization properties. Taken at face value, the sample of 16 PDLAs exhibits a wide range of metallicities, ranging from Z similar to 1/3 to similar to 1/1000 Z circle dot, including the DLA with the lowest N(Si ii)/N(H i) yet reported in the literature. However, some of these abundances may require ionization corrections. We find several pieces of evidence that indicate enhanced ionization and the presence of a hard ionizing spectrum in PDLAs which lead to properties that contrast with the intervening DLAs, particularly when the N(H i) is low. The abundances of Zn, Si and S in PDLAs with log N(H i) > 21, where ionization corrections are minimized, are systematically higher than the intervening population by a factor of around 3. We also find possible evidence for a higher fraction of N v absorbers amongst the PDLAs, although the statistics are still modest. 6/7 of our echelle sample show high ionization species (Si iv, C iv, O vi or N v) offset by > 100 km s-1 from the main low ion absorption. We analyse fine-structure transitions of C ii star and Si ii star to constrain the PDLA distance from the quasi-stellar object (QSO). Lower limits range from tens of kpc to > 160 kpc for the most stringent limit. We conclude that (at least some) PDLAs do exhibit different characteristics relative to the intervening population out to 3000 km s-1 (and possibly beyond). None the less, the PDLAs appear distinct from lower column density associated systems, and the inferred QSO-absorber separations mean they are unlikely to be associated with the QSO host. No trends with delta V are found, although this requires a larger sample with better emission redshifts to confirm. We speculate that the PDLAs preferentially sample more massive galaxies in more highly clustered regions of the high-redshift Universe.