The connection between high-redshift galaxies and Lyman ${\alpha}$ transmission in the Sherwood-Relics simulations of patchy reionisation

TL;DR

This study uses Sherwood-Relics simulations to explore how high-redshift galaxies influence Lyman alpha transmission during reionisation, revealing the impact of galaxy mass and neutral hydrogen fraction on observed correlations.

Contribution

It demonstrates that Sherwood-Relics simulations can reproduce observed Lyman alpha transmission enhancements and explores the implications of galaxy halo mass and neutral hydrogen fraction during reionisation.

Findings

Simulations match observed Ly α transmission around high-redshift galaxies.

Galaxy halo mass M_h ≥ 10^10 h^{-1} M_⊙ reproduces transmission at z ≈ 6.

Tension between model predictions and observed optical depth suggests larger host halo masses at z > 5.

Abstract

Recent work has suggested that, during reionisation, spatial variations in the ionising radiation field should produce enhanced Ly ${\alpha}$ forest transmission at distances of tens of comoving Mpc from high-redshift galaxies. We demonstrate that the Sherwood-Relics suite of hybrid radiation-hydrodynamical simulations are qualitatively consistent with this interpretation. The shape of the galaxy--Ly ${\alpha}$ transmission cross-correlation is sensitive to both the mass of the haloes hosting the galaxies and the volume averaged fraction of neutral hydrogen in the IGM, $\bar{x}_{\rm HI}$. The reported excess Ly ${\alpha}$ forest transmission on scales r ~ 10 cMpc at $\langle z \rangle \approx 5.2$ -- as measured using C IV absorbers as proxies for high-redshift galaxies -- is quantitatively reproduced by Sherwood-Relics at z = 6 if we assume the galaxies that produce ionising photons are hosted in haloes with mass $M_{\rm h}\geq 10^{10}~h^{-1}\,{\rm M}_\odot$. However, this redshift mismatch is equivalent to requiring $\bar{x}_{\rm HI}\sim 0.1$ at $z\simeq 5.2$, which is inconsistent with the observed Ly ${\alpha}$ forest effective optical depth distribution. We suggest this tension may be partly resolved if the minimum C IV absorber host halo mass at z > 5 is larger than $M_{\rm h}=10^{10}~h^{-1}\,{\rm M}_\odot$. After reionisation completes, relic IGM temperature fluctuations will continue to influence the shape of the cross-correlation on scales of a few comoving Mpc at $4 \leq z \leq 5$. Constraining the redshift evolution of the cross-correlation over this period may therefore provide further insight into the timing of reionisation.