The physics of Lyman-alpha escape from disc-like galaxies

Hydrogen emission lines can provide extensive information about star-forming galaxies in both the local and high-redshift Universe. We present a detailed Lyman continuum (LyC), Lyman-alpha (Ly alpha), and Balmer line (H alpha and H beta) radiative transfer study of a high-resolution isolated Milky Way simulation using the state-of-the-art Arepo-RT radiation hydrodynamics code with the SMUGGLE galaxy formation model. The realistic framework includes stellar feedback, non-equilibrium thermochemistry accounting for molecular hydrogen, and dust grain evolution in the interstellar medium (ISM). We extend our publicly available Cosmic Ly alpha Transfer (COLT) code with photoionization equilibrium Monte Carlo radiative transfer and various methodology improvements for self-consistent end-to-end (non-)resonant line predictions. Accurate LyC reprocessing to recombination emission requires modelling pre-absorption by dust (f(abs) approximate to 27.5 per cent), helium ionization (f(He) approximate to 8.7 per cent), and anisotropic escape fractions (f(esc) approximate to 7.9 per cent), as these reduce the available budget for hydrogen line emission (f(H) approximate to 55.9 per cent). We investigate the role of the multiphase dusty ISM, disc geometry, gas kinematics, and star formation activity in governing the physics of emission and escape, focusing on the time variability, gas-phase structure, and spatial spectral, and viewing angle dependence of the emergent photons. Isolated disc simulations are well-suited for comprehensive observational comparisons with local H alpha surveys, but would require a proper cosmological circumgalactic medium (CGM) environment as well as less dust absorption and rotational broadening to serve as analogs for high-redshift Ly alpha emitting galaxies. Future applications of our framework to next-generation cosmological simulations of galaxy formation including radiation-hydrodynamics that resolve less than or similar to 10 pc multiphase ISM and less than or similar to 1 kpc CGM structures will provide crucial insights and predictions for current and upcoming Ly alpha observations.