Title: Self-shielding of hydrogen in the IGM during the epoch of reionization,
Abstract: We investigate self-shielding of intergalactic hydrogen against ionizing
radiation in radiative transfer simulations of cosmic reionization carefully
calibrated with Lyman alpha 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 $\mathrm{n_H \sim 3 \times 10^{-3} cm^{-3}}$, or $\sim$ 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 parameterization of the photoionization rate as a
function of density in self-shielded regions during the epoch of reionization.