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

Abstract

With close pairs of quasars at different redshifts, a background quasar sightline 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 19 cm −2 selected from 149 projected quasar pair sightlines, 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 r 0 = 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 > 10 19 cm −2 absorber within a velocity window of ∆v < 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 cm −3 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.

Active galactic nucleus outflows in galaxy discs

“…This appears in nice agreement with the observational results of Hennawi & Prochaska (2007) and Prochaska et al (2013), who find a low incidence of gas absorbers along the line of sight to quasars ("down-the-barrel"), and they argue that gas in this direction is photoionized, while the incidence of absorbers in the transverse direction is larger towards smaller scales, implying an increasing density and covering fraction of neutral gas towards the quasar.…”

Probing the circumgalactic medium of active galactic nuclei with background quasars

“…They suggest that the background quasar sight-lines intercept gas that is shadowed from the ionizing radiation of the quasar jet. Similar anistropy has been observed in Hi (Hennawi & Prochaska 2007;Prochaska & Hennawi 2009) and Mgii (Bowen et al 2006;Farina et al 2013Farina et al , 2014. The distribution of the CGM of AGNs, like Seyfert galaxies, has yet to be explored.…”

Metal-enriched plasma in protogalactic halos

“…In the current sample there are only a handful of cases at < ∼ 2000 km s −1 from the QSO. Indeed, Hennawi & Prochaska (2007) predict that absorbers with n H ≤ 0.1 cm −3 will be photo-evaporated by the background quasar at d ≤ 1 Mpc, or only < ∼ 250 km s −1 at z = 3 (see also ). This picture is supported by the detection of strong N v absorption in two highly-proximate DLAs: one at 100 ± 100 km s −1 from the quasar ), and one extending over a few hundred km s −1 around z qso (Rix et al 2007).…”