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

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, ${\bar{x}}_{\mathrm{H}\mathrm{I}}$. The reported excess Ly $\alpha$ forest transmission on scales r ~ 10 cMpc at $⟨z⟩\approx 5.2$ – 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 $M_{\mathrm{h}}\ge {10}^{10}{h}^{-1}{\mathrm{M}}_{\odot }$. However, this redshift mismatch is equivalent to requiring ${\bar{x}}_{\mathrm{H}\mathrm{I}}\sim 0.1$ 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 $M_{\mathrm{h}}={10}^{10}{h}^{-1}{\mathrm{M}}_{\odot }$. 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 $4\le z\le 5$. Constraining the redshift evolution of the cross-correlation over this period may therefore provide further insight into the timing of reionisation.