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Ionization bias and the ghost proximity effect near $z\gtrsim6$ quasars in the shadow of proximate absorption systems

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 $>10^{12}\,{\rm M}_\odot$ dark matter halos, e.g. $z\sim6$ 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 neighboring galaxies, proximate damped Ly$α$ absorbers (DLAs) have recently been discovered in the foreground of $z\sim6$ quasars, and the Ly$α$ forest in the shadow of these DLAs could probe the local ionization environment. Using Gpc$^3$ 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 $z\sim6$ quasar with a proximate DLA which shows a strong excess in Ly$α$ forest transmission at the predicted location of the ghost proximity effect.