Quasars Probing Quasars. II. The Anisotropic Clustering of Optically Thick Absorbers around Quasars

• Published in: The Astrophysical journal Vol. 655; no. 2; pp. 735 - 748
• Main Authors: Hennawi, Joseph F, Prochaska, Jason X
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 01.02.2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Quasars; Intergalactic absorption; Intergalactic matter; Absorption line; intergalactic medium- quasars: absorption lines -quasars: general -surveys
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/509770

With close pairs of quasars at different redshifts, a background quasar sight line can be used to study a foreground quasar's environment in absorption. We used a sample of 17 Lyman limit systems with column density N super(Hl) > 10 super(19) cm super(-2) selected from 149 projected quasar pair sight lines to investigate the clustering pattern of optically thick absorbers around luminous quasars at z62.5. Specifically, we measured the quasar-absorber correlation function in the transverse direction and found a comoving correlation length of r sub(O) = 9.2 super(+) sub(-) super(1) sub(1) super(.) sub(.) super(5) sub(7) h super(-1) Mpc (comoving), assuming a power-law correlation function y8 r super(-g), with g = 1.6. Applying this transverse clustering strength to the line of sight, would predict that 615%-50% of all quasars should show a N super(HI) > 10 super(19) cm super(-2) absorber within a velocity window of upsilon < 3000 km s super(-1). This overpredicts the number of absorbers along the line of sight by a large factor, providing compelling evidence that the clustering pattern of optically thick absorbers around quasars is highly anisotropic. The most plausible explanation for the anisotropy is that the transverse direction is less likely to be illuminated by ionizing photons than the line of sight, and that absorbers along the line of sight are being photoevaporated. A simple model for the photoevaporation of absorbers subject to the ionizing flux of a quasar is presented, and it is shown that absorbers with volume densities n sub(H) 0.1 will be photoevaporated if they lie within 61 Mpc (proper) of a luminous quasar. Using this simple model, we illustrate how comparisons of the transverse and line-of-sight clustering around quasars can ultimately be used to constrain the distribution of gas in optically thick absorption-line systems.