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

## TL;DR

This paper investigates how ionization bias and the proximity effect near $z
oughly6$ quasars influence the Ly$ ext{alpha}$ forest, revealing a ghost proximity effect that can constrain quasar lifetime and emission geometry.

## Contribution

It introduces the concept of a ghost proximity effect caused by quasar radiation, which can be used to probe quasar properties and ionizing background fluctuations at high redshift.

## Key findings

- Simulations show ionization bias signal is suppressed by matter overdensity.
- A ghost proximity effect can dominate the Ly$ ext{alpha}$} forest bias.
- Observations support the presence of the ghost proximity effect near a $z
oughly6$ quasar.

## Abstract

The larger-than-expected scatter in the opacity of the Ly$\alpha$ 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$\alpha$ absorbers (DLAs) have recently been discovered in the foreground of $z\sim6$ quasars, and the Ly$\alpha$ 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$\alpha$ 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$\alpha$ forest transmission at the predicted location of the ghost proximity effect.