The mean free path of hydrogen ionizing photons during the epoch of reionization

We use the Aurora radiation-hydrodynamical simulations to study the mean free path (MFP) for hydrogen ionizing photons during the epoch of reionization. We directly measure the MFP by averaging the distance 1 Ry photons travel before reaching an optical depth of unity along random lines of sight. During reionization the free paths tend to end in neutral gas with densities near the cosmic mean, while after reionization the end points tend to be overdense but highly ionized. Despite the increasing importance of discrete, overdense systems, the cumulative contribution of systems with N_HI ≲ 10^16.5cm^-2 suffices to drive the MFP at z ≈ 6, while at earlier times higher column densities are more important. After reionization the typical size of H I systems is close to the local Jeans length, but during reionization it is much larger. The MFP for photons originating close to galaxies, {MFP_{gal}}, is much smaller than the cosmic MFP. After reionization this enhancement can remain significant up to starting distances of ∼1 comoving Mpc. During reionization, however, {MFP_{gal}} for distances ∼10²-10³ comoving kpc typically exceeds the cosmic MFP. These findings have important consequences for models that interpret the intergalactic MFP as the distance escaped ionizing photons can travel from galaxies before being absorbed and may cause them to underestimate the required escape fraction from galaxies, and/or the required emissivity of ionizing photons after reionization.