Understanding the escape of LyC and Lyα photons from turbulent clouds

Understanding the escape of Lyman continuum (LyC) and Lyman alpha (Lyα) photons from molecular clouds is one of the keys to constraining the reionization history of the Universe. Using a set of radiation-hydrodynamic simulations, we investigate how photons propagate and escape from turbulent clouds with different masses, star formation efficiencies (SFEs), and metallicities, as well as with different models of stellar spectra and supernova feedback. We find that the escape fractions in both LyC and Lyα are generally increasing with time if the cloud is efficiently dispersed by radiation and supernova feedback. When the total SFE is low (1 per cent of the cloud mass), 0. 1-5{\ per\ cent} of LyC photons leave the metal-poor cloud, whereas the fractions increase to 20-70{\ per\ cent} in clouds with a 10 per cent SFE. LyC photons escape more efficiently if gas metallicity is lower, if the upper mass limit in the stellar initial mass function is higher, if binary interactions are allowed in the evolution of stars, or if additional strong radiation pressure, such as Lyα pressure, is present. The escape fractions of Lyα photons are systemically higher (60-80{\ per\ cent}) than those of LyC photons, despite large optical depths at line centre (τ0 ∼ 106–109). Scattering of Lyα photons is already significant on cloud scales, leading to double-peaked profiles with peak separations of v ₒ₄₏ 400\, { km\, s^{-1}} during the initial stage of the cloud evolution, while it becomes narrower than v ₒ₄₏ 150 \, { km\, s^{-1}} in the LyC bright phase. Comparisons with observations of low-redshift galaxies suggest that Lyα photons require further interactions with neutral hydrogen to reproduce their velocity offset for a given LyC escape fraction.