The SDSS-DR5 Survey for Proximate Damped Lyα Systems

Using the Sloan Digital Sky Survey Data Release 5 (SDSS-DR5), we survey proximate damped Lyα systems (PDLAs): absorption-line systems with H I column density N_HI >= 2×10²⁰ cm^-2 at velocity separation δ v < 3000 km s^-1 from their background quasar. Many of these absorbers may be physically associated with their background quasars, and their statistics allow us to study quasar environments out to z ~ 5. However, the large ionizing flux emitted by a quasar can ionize the neutral gas in a nearby galaxy, possibly giving rise to a "proximity effect," analogous to the similar effect observed in the Lyα forest. From a sample of 108 PDLAs, we measure the HI frequency distribution f(N_HI, X), incidence, and gas mass density of the PDLAs near luminous quasars over the redshift interval z = 2.2-5. The incidence and mass density of PDLAs at z ~ 3 is approximately twice that of intervening DLAs, while at z < 2.5 and >3.5 the f(N_HI, X) distribution is enhanced but statistically consistent with the intervening population. We interpret the observed enhancement of PDLAs around quasars in terms of quasar-galaxy clustering and compare the strength of the clustering signal to the expectation from independent measures of the respective clustering strengths of DLAs and quasars, as well as a complementary analysis of the clustering of absorbers around quasars in the transverse direction. We find that there are a factor of 5-10 fewer PDLAs around quasars than expected and interpret this result as evidence for the hypothesis that the ionizing flux from the quasars photoevaporates H I in nearby DLA galaxies, thus reducing their cross section for DLA absorption. This constitutes the first detection of a "proximity effect" for DLAs.