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

TL;DR

This study uses simulations to measure the mean free path of hydrogen ionizing photons during reionization, revealing how it varies with density, ionization state, and proximity to galaxies, impacting models of cosmic reionization.

Contribution

It provides the first direct measurements of the mean free path during reionization using radiation-hydrodynamical simulations, highlighting the role of different gas densities and galaxy proximity.

Findings

Mean free path ends mostly in neutral gas near cosmic mean density during reionization.

Post-reionization, the typical size of HI systems is close to the local Jeans length.

Photon mean free path near galaxies is significantly smaller than the cosmic average, especially during reionization.

Abstract

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 reionizaton the end points tend to be overdense but highly ionized. Despite the increasing importance of discrete, over-dense systems, the cumulative contribution of systems with $N_{\rm{HI}} \lesssim 10^{16.5}~{\rm cm^{-2}}$ suffices to drive the MFP at $z \approx 6$, while at earlier times higher column densities are more important. After reionization the typical size of HI systems is close to the local Jeans length, but during reionization it is much larger. The mean free path for photons originating close to galaxies, $\rm{MFP_{gal}}$, is much smaller than the cosmic MFP. After reionization this enhancement can remain significant up to starting distances of $\sim 1$ comoving Mpc. During reionization, however, $\rm{MFP_{gal}}$ for distances $\sim 10^2 - 10^3$ 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 under-estimate the required escape fraction from galaxies, and/or the required emissivity of ionizing photons after reionization.