Constraints on the End of Reionization from the Density Fields Surrounding Two Highly Opaque Quasar Sightlines

TL;DR

This study investigates the large-scale fluctuations in Lyman alpha opacity at redshift 5.7 by analyzing galaxy distributions around two high-opacity quasar sightlines, supporting models of late or fluctuating reionization.

Contribution

It provides new observational evidence linking galaxy underdensities to high-opacity regions, testing models of reionization fluctuations at z=5.7.

Findings

Deficit of Lyα-emitting galaxies near quasars

Supports models of large-scale UV background fluctuations

Suggests reionization may not have fully concluded by z=5.7

Abstract

The observed large-scale scatter in Lyman $\alpha$ opacity of the intergalactic medium at $z<6$ implies large fluctuations in the neutral hydrogen fraction that are unexpected long after reionization has ended. A number of models have emerged to explain these fluctuations that make testable predictions for the relationship between Ly$\alpha$ opacity and density. We present selections of $z=5.7$ Ly$\alpha$-emitting galaxies (LAEs) in the fields surrounding two highly opaque quasar sightlines with long Ly$\alpha$ troughs. The fields lie towards the $z=6.0$ quasar ULAS J0148+0600, for which we re-analyze previously published results using improved photometric selection, and towards the $z=6.15$ quasar SDSS J1250+3130, for which results are presented here for the first time. In both fields, we report a deficit of LAEs within 20 $h^{-1}$ Mpc of the quasar. The association of highly opaque sightlines with galaxy underdensities in these two fields is consistent with models in which the scatter in Ly$\alpha$ opacity is driven by large-scale fluctuations in the ionizing UV background, or by an ultra-late reionization that has not yet concluded at $z=5.7$.