2020MNRAS.494.2937D

The larger-than-expected scatter in the opacity of the Ly α forest suggests that the metagalactic ionizing background is strongly fluctuating at z > 5.5. Models for ionizing background fluctuations predict a strong positive bias on large scales, so the environments of massive > 10¹² M_☉ dark matter haloes, e.g. z ∼ 6 quasar hosts, would be ideal laboratories to constrain the sources of ionizing photons. While the quasars themselves should overwhelm any plausible ionizing photon contribution from neighbouring galaxies, proximate damped Ly α absorbers (DLAs) have recently been discovered in the foreground of z ∼ 6 quasars, and the Ly α forest in the shadow of these DLAs could probe the local ionization environment. Using Gpc³ simulations of z = 6 ionizing background fluctuations, we show that while the Ly α forest signal from ionization bias around a quasar host halo should be strong, it is likely suppressed by the associated intergalactic matter overdensity. We also show that the quasar itself may still overwhelm the clustering signal via a 'ghost' of the proximity effect from the quasar radiation, causing a large-scale bias in the ionizing photon mean free path. This ghost proximity effect is sensitive to the lifetime and geometry of quasar emission, potentially unlocking a new avenue for constraining these fundamental quasar properties. Finally, we present observations of a z ∼ 6 quasar with a proximate DLA, which shows a strong excess in Ly α forest transmission at the predicted location of the ghost proximity effect.