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2008ApJ...675.1002P - Astrophys. J., 675, 1002-1013 (2008/March-2)

The SDSS-DR5 survey for proximate damped Lyα systems.

PROCHASKA J.X., HENNAWI J.F. and HERBERT-FORT S.

Abstract (from CDS):

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_≥2x1020/cm2 at velocity separation δv<3000 km/s 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 H I 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.

Journal keyword(s): Galaxies: Quasars: Absorption Lines

VizieR on-line data: <Available at CDS (J/ApJ/675/1002): table1.dat table2.dat table3.dat>

Nomenclature: Tables 2, 3: PDLA QSO JHHMM+DDMM abs N.NNNN N=107+40.