Self-shielding of hydrogen in the IGM during the epoch of reionization

We investigate self-shielding of intergalactic hydrogen against ionizing radiation in radiative transfer simulations of cosmic reionization carefully calibrated with Lyα forest data. While self-shielded regions manifest as Lyman limit systems in the post-reionization Universe, here we focus on their evolution during reionization (redshifts z = 6–10). At these redshifts, the spatial distribution of hydrogen-ionizing radiation is highly inhomogeneous, and some regions of the Universe are still neutral. After masking the neutral regions and ionizing sources in the simulation, we find that the hydrogen photoionization rate depends on the local hydrogen density in a manner very similar to that in the post-reionization Universe. The characteristic physical hydrogen density above which self-shielding becomes important at these redshifts is about nH ∼ 3 × 10−3 cm−3, or ∼20 times the mean hydrogen density, reflecting the fact that during reionization photoionization rates are typically low enough that the filaments in the cosmic web are often self-shielded. The value of the typical self-shielding density decreases by a factor of 3 between redshifts z = 3 and 10, and follows the evolution of the average photoionization rate in ionized regions in a simple fashion. We provide a simple parametrization of the photoionization rate as a function of density in self-shielded regions during the epoch of reionization.This work was supported by the ERC Advanced Grant 320596 ‘The Emergence of Structure During the Epoch of Reionization’. The RAMSES simulations presented in this paper were performed on the COSMOS Shared Memory system at DAMTP, University of Cambridge, operated on behalf of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1 and ST/K00333X/1. The ATON radiative transfer simulations in this work were performed using the Wilkes GPU cluster at the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc., NVIDIA, and Mellanox, and part funded by STFC with industrial sponsorship from Rolls Royce and Mitsubishi HeavyIndustries