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

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_HI>10¹⁹ cm^-2 selected from 149 projected quasar pair sight lines to investigate the clustering pattern of optically thick absorbers around luminous quasars at z~2.5. Specifically, we measured the quasar-absorber correlation function in the transverse direction and found a comoving correlation length of r₀=9.2^+1.5_-1.7 h^-1 Mpc (comoving), assuming a power-law correlation function ξ~r^-γ, with γ=1.6. Applying this transverse clustering strength to the line of sight, would predict that ~15%-50% of all quasars should show a N_HI>10¹⁹ cm^-2 absorber within a velocity window of Δv<3000 km s^-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_H<~0.1 will be photoevaporated if they lie within ~1 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.