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

TL;DR

This study uses the TNG50 simulation to analyze how the escape fraction of ionizing photons varies with galaxy properties during reionization, revealing complex dependencies on mass, dust, and photon energy.

Contribution

It provides a detailed, multi-frequency analysis of ionizing photon escape fractions in simulated galaxies, highlighting their dependence on galaxy mass, dust, and photon energy.

Findings

Escape fraction increases with galaxy mass up to a certain point

Dust reduces escape fractions by up to 10% in larger halos

Escape fraction varies with photon energy, decreasing above 54.4 eV

Abstract

In this work we investigate the dependence of the escape fraction of ionizing photons, $f_{\rm esc}$, on various galaxy and host halo properties during the epoch of reionization. We post-process the TNG50 magneto-hydrodynamical simulation from the IllustrisTNG project using the 3D multi-frequency radiative transfer code CRASH. Our work covers the stellar mass range $10^6 \lesssim M_\star/{\rm M_\odot} \lesssim 10^8$ at redshifts $6 < z < 10$. Adopting an unresolved, cloud-scale escape fraction parameter of unity, the halo escape fraction $f_{\rm esc}$ increases with mass from $\sim 0.3$ at $M_\star = 10^6$M$_\odot$ to $\sim 0.6$ at $M_\star = 10^{7.5}$M$_\odot$, 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 $f_{\rm esc}$ on the adopted sub-grid escape fraction. In addition, $f_{\rm esc}$ 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 halos with a stellar mass $\lesssim 10^{7.5}$M$_\odot$ contribute the majority of ionizing photons at all redshifts. Incorporating dust reduces $f_{\rm esc}$ by a few percent at $M_\star \lesssim 10^{6.5}$M$_\odot$, and up to 10\% for larger halos. Our multi-frequency approach shows that $f_{\rm esc}$ 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.