Models of diffuse Hα in the interstellar medium: the relative contributions from in situ ionization and dust scattering

Using three-dimensional Monte Carlo radiation transfer models of photoionization and dust scattering, we explore different components of the widespread diffuse Hα emission observed in the interstellar medium of the Milky Way and other galaxies. We investigate the relative contributions of Hα from recombination emission in ionized gas and Hα that originates in HII regions near the Galactic mid-plane and scatters off high-altitude dust in the diffuse interstellar medium. For the radiation transfer simulations, we consider two geometries for the interstellar medium: a three-dimensional fractal geometry that reproduces the average density structure inferred for hydrogen in the Milky Way, and a density structure from a magnetohydrodynamic simulation of a supernova-driven turbulent interstellar medium. Although some sight lines that are close to HII regions can be dominated by scattered light, overall we find that less than ~20 per cent of the total Hα intensity in our simulations can be attributed to dust scattering. Our findings on the relative contribution of scattered Hα are consistent with previous observational and theoretical analyses. We also investigate the relative contributions of dust scattering and in situ ionization of high-density dust clouds in the diffuse gas. Dust scattering in these partially ionized clouds contribute ~40 per cent to the total intensity of Hα.