Lyman Radiation Hydrodynamics of Turbulent H II Regions in Molecular Clouds: A Physical Origin of LyC Leakage and the Associated Ly$\alpha$ Spectra

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, spectral hardness, and the emergent Lyman-$\alpha$ (Ly$\alpha$) 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 which are evacuated efficiently by radiative feedback via photoheating-induced shocks across the D-type ionization fronts. Ly$\alpha$ photons funnel through the photoionized channels along the paths of LyC escape, resulting in a diverse Ly$\alpha$ spectral morphology including narrow double-peaked profiles. The Ly$\alpha$ 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 Ly$\alpha$ spectral characteristics of some of LyC-leaking galaxies. This RHD turbulence-origin provides an appealing hypothesis to explain high LyC leakage from very young ($\sim 3$ Myr) star-forming galaxies found in the local Universe without need of extreme galactic outflows nor 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.

23 pages, 17 figures, 2 tables, submitted to ApJ

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