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

With close pairs of quasars at different redshifts, a background quasarsightline 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 > 10^19cm^-2 selected from 149 projected quasar pair sightlines, to investigate theclustering pattern of optically thick absorbers around luminous quasars at z ~2.5. Specifically, we measured the quasar-absorber correlation function in thetransverse direction, and found a comoving correlation length ofr_0=9.2_{+1.5}_{-1.7} Mpc/h (comoving) assuming a power law correlationfunction, with gamma=1.6. Applying this transverse clustering strength to theline-of-sight, would predict that ~ 15-50% of all quasars should show a N_HI >10^19 cm^-2 absorber within a velocity window of v < 3000 km/s. Thisoverpredicts the number of absorbers along the line-of-sight by a large factor,providing compelling evidence that the clustering pattern of optically thickabsorbers around quasars is highly anisotropic. The most plausibleexplanationfor the anisotropy is that the transverse direction is less likelyto be illuminated by ionizing photons than the line-of-sight, and thatabsorbers along the line-of-sight are being photoevaporated. A simple model forthe photoevaporation of absorbers subject to the ionizing flux of a quasar ispresented, and it is shown that absorbers with volume densities n_H < 0.1 cm^-3will be photoevaporated if they lie within ~ 1 Mpc (proper) of a luminousquasar. Using this simple model, we illustrate how comparisons of thetransverse and line-of-sight clustering around quasars can ultimately be usedto constrain the distribution of gas in optically thick absorption linesystems.Comment: 14 pages of emulateapj, 7 figures, submitted to Ap