The LyA-LyC Connection: Evidence for an Enhanced Contribution of UV-faint Galaxies to Cosmic Reionization

TL;DR

This study uses radiative transfer simulations to explore the relationship between Lyman-Alpha and Lyman-Continuum escape fractions, revealing that UV-faint galaxies may significantly contribute to cosmic reionization due to their higher escape probabilities.

Contribution

It demonstrates a correlation between LyC and Lya escape fractions in dusty, clumpy interstellar media, highlighting the importance of UV-faint galaxies in reionization.

Findings

f_esc(LyC) and f_esc(Lya) are correlated

UV-faint galaxies likely contribute more to reionization

LyC escape linked to Lya spectral line shape

Abstract

The escape of ionizing Lyman Continuum (LyC) photons requires the existence of low-N_HI sightlines, which also promote escape of Lyman-Alpha (Lya). We use a suite of 2500 Lya Monte-Carlo radiative transfer simulations through models of dusty, clumpy interstellar (`multiphase') media from Gronke & Dijkstra (2016), and compare the escape fractions of Lya [f_esc(Lya)] and LyC radiation [f_esc(LyC)]. We find that f_esc(LyC) and f_esc(Lya) are correlated: galaxies with a low f_esc(Lya) consistently have a low f_esc(LyC), while galaxies with a high f_esc(Lya) exhibit a large dispersion in f_esc(LyC). We argue that there is increasing observational evidence that Lya escapes more easily from UV-faint galaxies. The correlation between f_esc(LyC) and f_esc(Lya) then implies that UV-faint galaxies contribute more to the ionizing background than implied by the faint-end slope of the UV-luminosity function. In multiphase gases, the ionizing escape fraction is most strongly affected by the cloud covering factor, f_cl, which implies that f_esc(LyC) is closely connected to the observed Lya spectral line shape. Specifically, LyC emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for `shell models'.