The nature of proximate damped Lyman alpha systems
  We present high resolution echelle spectra of 7 proximate damped Lyman alpha
(PDLA) systems whose relative velocity separation from the background quasar is
Delta V < 3000 km/s. Combining our sample with a further 9 PDLAs from the
literature we compare the chemical properties of the proximate systems with a
control sample of intervening DLAs. Taken at face value, the sample of 16 PDLAs
exhibits a wide range of metallicities, ranging from Z ~ 1/3 Z_sun down to Z ~
1/1000 Z_sun, including the DLA with the lowest N(SiII)/N(HI) yet reported in
the literature. 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(HI)
is low. The abundances of Zn, Si and S in PDLAs with log N(HI) > 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 NV absorbers amongst the PDLAs, although the
statistics are still modest. 6/7 of our echelle sample show high ionization
species (SiIV, CIV, OVI or NV) offset by >100 km/s from the main low ion
absorption. We analyse fine-structure transitions of CII* and SiII* to
constrain the PDLA distance from the QSO. Lower limits range from tens of kpc
up 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 (and possibly beyond). Nonetheless, 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. We speculate that the PDLAs preferentially sample more massive galaxies
in more highly clustered regions of the high redshift universe.
