Physically motivated modeling of LyC escape fraction during reionization

Abstract

Aims: We present an analysis of the Lyman continuum (LyC) escape fraction of high redshift galaxies using an analytic post-processing approach. Methods: We apply the model to 600,000 galaxies of the Illustris TNG50 simulation in the redshift range z = 5.2 - 20. Results: Our study reveals a bimodal nature of LyC escape, which is associated either to (a) high metallicity (1e-3.5 < Z < 1e-2), low mass (Mstar < 1e7Msun) galaxies which due to their efficient cooling exhibiting extended star formation, with photons escaping primarily from the outer regions of the galactic plane (ext-mode), or (b) low metallicity (Z < 1e-3), moderately massive galaxies (Mstar < 1e8Msun) in which star-formation can only take place in small high density regions with localized 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, once sufficient metal enrichment has occurred. Building on these findings, we develop 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.