The Lyα-LyC connection: evidence for an enhanced contribution of UV-faint galaxies to cosmic reionization

Abstract

The escape of ionizing Lyman continuum (LyC) photons requires the existence of low-NH I sightlines, which also promote escape of Lyα. We use a suite of 2500 Lyα Monte-Carlo radiative transfer simulations through models of dusty, clumpy interstellar (“multiphase”) media from Gronke & Dijkstra, and compare the escape fractions of Lyα ( a f esc Ly ) and LyC radiation (f esc ion ). We find that f esc ion and a f esc Ly are correlated: galaxies with a low a f esc Ly consistently have a low f esc ion , while galaxies with a high a f esc Ly exhibit a large dispersion in f esc ion . We argue that there is increasing observational evidence that Lyα escapes more easily from UV-faint galaxies. The correlation between f esc ion and a f esc Ly 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, fcl, which implies that f esc ion is closely connected to the observed Lyα spectral line shape. Specifically, LyC-emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for “shell models.”