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 (τ 0 ∼ 106-109). Scattering of Lyα photons is already significant on cloud scales, leading to double-peaked profiles with peak separations of vsep ∼ 400 km s-1 during the initial stage of the cloud evolution, while it becomes narrower than vsep ≲ 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.
Bibliographical noteFunding Information:
We thank Julien Devriendt, Kwang-Il Seon, and an anonymous referee for helpful comments. This work was supported by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2017-C3-0073) and in part by the ERC Advanced Grant 320596 ‘The Emergence of Structure during the Epoch of Reionization’. This study was performed under the umbrella of the joint collaboration between Yonsei University Observatory and the Korean Astronomy and Space Science Institute. TK was supported in part by the Yonsei University Future-leading Research Initiative (RMS2-2018-22-0183) and in part by the National Research Foundation of Korea (No. 2017R1A5A1070354 and No. 2018036146). HK thanks Brasenose College and acknowledges the support of the Nicholas Kurti Junior Fellowship as well as the Beecroft Fellowship. JR and JB acknowledge the support from the ORAGE project from the Agence Nationale de la Recherche under grand ANR-14-CE33-0016-03. TG is grateful to the LABEX Lyon Institute of Origins (ANR-10-LABX-0066) of the Université de Lyon for its financial support within the programme ‘Investissements d’Avenir’ (ANR-11-IDEX-0007) of the French government operated by the National Research Agency (ANR). TG acknowledges the support from the European Research Council under grant agreement ERC-stg-757258 (TRIPLE). This work made use of the following DiRAC facilities (www.dirac.ac.uk): the Data Analytic system at the University of Cambridge (funded by BIS National E-infrastructure capital grant ST/K001590/1, STFC capital grants ST/H008861/1 and ST/H00887X/1, and STFC DiRAC Operations grant ST/K00333X/1) and the Complexity system at the University of Leicester (funded by BIS National E-infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1). DiRAC is part of the National E-Infrastructure.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science