The nature of proximate damped Lyman α systems

Sara L. Ellison, J. Xavier Prochaska, Joseph Hennawi, Sebastian Lopez, Christopher Usher, Arthur M. Wolfe, David M. Russell, Chris R. Benn

Research output: Contribution to journalArticlepeer-review


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 Δ 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 ∼ 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{black star} and Si ii{black 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 Δ 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.

Original languageEnglish (US)
Pages (from-to)1435-1459
Number of pages25
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Aug 2010


  • Galaxies: ISM
  • Galaxies: abundances
  • Galaxies: high redshift
  • Quasers: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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