Physically motivated modelling of LyC escape fraction during reionisation

Aims. We present an analysis of the Lyman continuum (LyC) escape fraction of high-redshift galaxies using an analytic post-processing approach. Methods. We applied the model to ∼6 × 105 galaxies of the Illustris TNG50 simulation in the redshift range z = 5.2 − 20. Results. Our study reveals a bimodal nature of LyC escape that is associated with either (a) high-metallicity (10−3.5 < Z < 10−2) low-mass (M⋆ < 107 M⊙) galaxies, which due to their efficient cooling exhibit extended star formation, with photons escaping primarily from the outer regions of the galactic plane (ext-mode), or (b) low-metallicity (Z < 10−3) moderately massive galaxies (M⋆ < 108 M⊙) in which star formation can only take place in small high-density regions, with localised LyC escape originating from these regions (loc-mode). While the loc-mode is present at all redshifts under investigation, the ext-mode becomes prominent in small galaxies at later cosmic times, that is, once sufficient metal enrichment has occurred. Building on these findings, we have developed an analytical fitting formula to determine the escape fraction of galaxies based on their stellar and gas mass as well as redshift, providing a valuable subgrid modelling tool for future studies.