The physical origin for spatially large scatter of IGM opacity at the end of reionization: The IGM Lyα opacity-galaxy density relation

ABSTRACT The large opacity fluctuations in the z > 5.5 Lyα forest may indicate inhomogeneous progress of reionization. To explain the observed large scatter of the effective Lyα optical depth (τeff) of the intergalactic medium (IGM), fluctuation of UV background (Γ model), or the IGM gas temperature (T model) have been proposed, which predict opposite correlations between τeff and galaxy density. In order to address which model can explain the large scatter of τeff, we search for Lyα emitters (LAEs) around two (J1137+3549 and J1602+4228) quasar sightlines with τeff ∼ 3 and J1630+4012 sightline with τeff ∼ 5.5. Using a narrow-band 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-τeff regions, while a deficit of LAEs is found in the high τeff region. Although the τeff of the three quasar sightlines are neither high nor low enough to clearly distinguish the two models, these observed τeff-galaxy density relations all consistently support the Γ model rather than the T model in the three fields, along with the previous studies. The observed overdensities near the low-τeff 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.