Radiation Hydrodynamics of Turbulent H ii Regions in Molecular Clouds: A Physical Origin of LyC Leakage and the Associated Lyα Spectra

Abstract

Abstract We examine Lyman continuum (LyC) leakage through H ii regions regulated by turbulence and radiative feedback in a giant molecular cloud in the context of fully coupled radiation hydrodynamics (RHD). The physical relations of the LyC escape with H i covering fraction, kinematics, ionizing photon production efficiency, and emergent Lyα line profiles are studied using a series of RHD turbulence simulations performed with ramses-rt. The turbulence-regulated mechanism allows ionizing photons to leak out at early times before the onset of supernova feedback. The LyC photons escape through turbulence-generated low column density channels that are evacuated efficiently by radiative feedback via photoheating-induced shocks across the D-type ionization fronts. The Lyα photons funnel through the photoionized channels along the paths of LyC escape, resulting in a diverse Lyα spectral morphology including narrow double-peaked profiles. The Lyα peak separation is controlled by the residual H i column density of the channels, and the line asymmetry correlates with the porosity and multiphase structure of the H ii region. This mechanism through the turbulent H ii regions can naturally reproduce the observed Lyα spectral characteristics of some of the LyC-leaking galaxies. This RHD turbulence origin provides an appealing hypothesis to explain high LyC leakage from very young (∼3 Myr) star-forming galaxies found in the local universe without need of extreme galactic outflows or supernova feedback. We discuss the implications of the turbulent H ii regions on other nebular emission lines and a possible observational test with the Magellanic System and local blue compact dwarf galaxies as analogs of reionization-era systems.