Models of Diffuse H{\alpha} in the Interstellar Medium: The Relative Contributions from In Situ Ionisation and Dust Scattering

TL;DR

This study uses 3D Monte Carlo models to quantify the contributions of in situ ionisation and dust scattering to diffuse Hα emission in the interstellar medium, finding dust scattering accounts for less than 20% overall.

Contribution

It introduces detailed 3D Monte Carlo simulations with fractal and MHD-based geometries to analyze Hα emission sources in the ISM, providing new quantitative insights.

Findings

Less than 20% of Hα is due to dust scattering overall.

Dust scattering contributes about 40% in partially ionised clouds.

Results align with previous observational and theoretical studies.

Abstract

Using three dimensional Monte Carlo radiation transfer models of photoionisation and dust scattering, we explore different components of the widespread diffuse H$\alpha$ emission observed in the interstellar medium of the Milky Way and other galaxies. We investigate the relative contributions of H$\alpha$ from recombination emission in ionised gas and H$\alpha$ that originates in H{\sc ii} regions near the Galactic midplane 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 magneto hydrodynamic simulation of a supernovae driven turbulent interstellar medium. Although some sight lines that are close to H{\sc ii} regions can be dominated by scattered light, overall we find that less than $\sim 20\%$ of the total H$\alpha$ intensity in our simulations can be attributed to dust scattering. Our findings on the relative contribution of scattered H$\alpha$ are consistent with previous observational and theoretical analyses. We also investigate the relative contributions of dust scattering and in situ ionisation of high density dust clouds in the diffuse gas. Dust scattering in these partially ionised clouds contribute $\sim 40\%$ to the total intensity of H$\alpha$.