The connection between high-redshift galaxies and Lyman α transmission in the Sherwood–Relics simulations of patchy reionization

ABSTRACT Recent work has suggested that, during reionization, spatial variations in the ionizing 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, $\bar{x}_{\rm H\,I}$. The reported excess Ly $\alpha$ forest transmission on scales $r \sim 10~{\rm cMpc}$ 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 ionizing photons are hosted in haloes with mass $M_{\rm h}\ge 10^{10}~h^{-1}\, {\rm M}_\odot$. However, this redshift mismatch is equivalent to requiring $\bar{x}_{\rm H\,I}\sim 0.1$ 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 $M_{\rm h}=10^{10}~h^{-1}\, {\rm M}_\odot$. After reionization 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 reionization.