Evidence for a highly opaque large-scale galaxy void at the end of reionization

TL;DR

This study provides evidence of a large, highly opaque galaxy void at the end of reionization, linking high Lyα optical depth regions to underdense galaxy distributions and implications for reionization models.

Contribution

It presents the first observational evidence connecting a high Lyα optical depth region to a large-scale galaxy underdensity at the end of reionization.

Findings

Identified a significant galaxy underdensity near a high-opacity Lyα trough.

Found the void's significance level to be 99%.

Supported models with large optical depth fluctuations due to UV background or residual neutral islands.

Abstract

We present evidence that a region of high effective Ly$\alpha$ optical depth at $z\sim5.7$ is associated with an underdense region at the tail end of cosmic reionization. We carried out a survey of Lyman-break Galaxies (LBGs) using Subaru Hyper Suprime-Cam in the field of the $z=5.98$ quasar J0148+0600, whose spectrum presents an unusually long ($\sim160 \mathrm{cMpc}$) and opaque ($\tau \gtrsim7$) Ly$\alpha$ trough at $5.5\le z\le 5.9$. LBG candidates were selected to lie within the redshift range of the trough, and the projected number densities were measured within 90~cMpc of the quasar sightline. The region within $8'$ (or $\approx 19~\mathrm{cMpc}$) of the quasar position is the most underdense of the whole field. The significance of the presence of the void is estimated to be $99\%$. This is consistent with the significant deficit of Ly$\alpha$ emitters (LAEs) at $z=5.72$ reported by Becker et al. and suggests that the paucity of LAEs is not purely due to the removal of the Ly$\alpha$ emission by the high opacity but reflects a real coherent underdensity of galaxies across the entire redshift range of the trough. These observations are consistent with scenarios in which large optical depth fluctuations arise due to fluctuations in the galaxy-dominant UV background or due to residual neutral islands that are expected from reionization that is completed at redshifts as low as $z\lesssim5.5$.