The nature of proximate damped Lyman α systems


  • Based on observations made with ESO Telescopes at the Paranal Observatories under program 080.A-0014(A).



We present high-resolution echelle spectra of seven proximate damped Lyman α (PDLA) systems. The relative velocity separation of each PDLA from the background quasar is ΔV < 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∼ 1/3 to ∼ 1/1000   Z⊙, 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 and Si ii 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 ΔV 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.