The physical origin for spatially large scatter of IGM opacity at the end of reionization: the IGM Ly$\alpha$ opacity-galaxy density relation

TL;DR

This study investigates the large scatter in IGM opacity at high redshift by analyzing galaxy distributions around quasar sightlines, finding evidence favoring UV background fluctuations over temperature variations as the primary cause.

Contribution

The paper provides observational evidence supporting the UV background fluctuation model for IGM opacity scatter during reionization, using LAE density maps around specific quasar sightlines.

Findings

Overdensities near low $ au_{ m eff}$ sightlines support the $ m extbf{ extit{ extGamma}}$ model.

Deficit of LAEs in high $ au_{ m eff}$ regions aligns with the $ m extbf{ extit{T}}$ model predictions.

Results suggest relic temperature fluctuations have limited impact on reionization.

Abstract

The large opacity fluctuations in the $z > 5.5$ Ly$\alpha$ forest may indicate inhomogeneous progress of reionization. To explain the observed large scatter of the effective Ly$\alpha$ optical depth ($\tau_{\rm eff}$) of the intergalactic medium (IGM), fluctuation of UV background ($\Gamma$ model) or the IGM gas temperature ($T$ model) have been proposed, which predict opposite correlations between $\tau_{\rm eff}$ and galaxy density. In order to address which model can explain the large scatter of $\tau_{\rm eff}$, we search for Ly$\alpha$ emitters (LAEs) around two (J1137+3549 and J1602+4228) quasar sightlines with $\tau_{\rm eff}\sim3$ and J1630+4012 sightline with $\tau_{\rm eff}\sim5.5$. Using a narrowband imaging with Subaru/Hyper Suprime-Cam, we draw LAE density maps to explore their spatial distributions. Overdensities are found within 20 $h^{-1}$Mpc of the quasar sightlines in the low $\tau_{\rm eff}$ regions, while a deficit of LAEs is found in the high $\tau_{\rm eff}$ region. Although the $\tau_{\rm eff}$ of the three quasar sightlines are neither high nor low enough to clearly distinguish the two models, these observed $\tau_{\rm eff}$-galaxy density relations all consistently support the $\Gamma$ model rather than the $T$ model in the three fields, along with the previous studies. The observed overdensities near the low $\tau_{\rm eff}$ sightlines may suggest that the relic temperature fluctuation does not affect reionization that much. Otherwise, these overdensities could be attributed to other factors besides the reionization process, such as the nature of LAEs as poor tracers of underlying large-scale structures.