Ionizing photon production and escape fractions during cosmic reionization in the TNG50 simulation.

In this work, we investigate the dependence of the escape fraction of ionizing photons, f _esc, on various galaxy and host halo properties during the epoch of reionization. We post-process the TNG50 magnetohydrodynamical simulation from the IllustrisTNG project using the three-dimensional multifrequency radiative transfer code CRASH. Our work covers the stellar mass range of 10⁶ ≲ M _⋆/M_⊙ ≲ 10⁸ at redshifts 6 < z < 10. Adopting an unresolved, cloud-scale escape fraction parameter of unity, the average halo escape fraction f _esc increases with mass from ∼0.3 at M _⋆ = 10⁶ M_⊙ to ∼0.6 at M _⋆ = 10^7.5 M_⊙, after which we find hints of a turnover and decreasing escape fractions for even more massive galaxies. However, we demonstrate a strong and non-linear dependence of f _esc on the adopted subgrid escape fraction, resulting in uncertainties for the absolute value of the escape fraction. In addition, f _esc has significant scatter at fixed mass, driven by diversity in the ionizing photon rate together with a complex relationship between (stellar) source positions and the underling density distribution. The global emissivity is consistent with observations for reasonable cloud-scale absorption values, and haloes with a stellar mass ≲10^7.5 M_⊙ contribute the majority of escaping ionizing photons at all redshifts. Incorporating dust reduces f _esc by a few per cent at M _⋆ ≲ 10^6.5 M_⊙, and up to 10  per cent for larger haloes. Our multifrequency approach shows that f _esc depends on photon energy, and is reduced substantially at E > 54.4 eV versus lower energies. This suggests that the impact of high-energy photons from binary stars is reduced when accounting for an energy-dependent escape fraction. [ABSTRACT FROM AUTHOR]