H-alpha and Free-Free Emission from the WIM

TL;DR

This paper presents a three-component model explaining the unexpectedly high H-alpha to free-free emission ratio in the WIM, accounting for low electron temperatures and dust reflection effects.

Contribution

It introduces a novel three-component model that successfully reproduces observed emission ratios and explains low electron temperatures in the WIM.

Findings

The model matches observed free-free, H-alpha, and NII line intensities.

Lowering PAH abundance reduces free-free to H-alpha ratio.

Dust reflection accounts for ~20% of diffuse H-alpha.

Abstract

Recent observations have found the ratio of H-alpha to free-free radio continuum to be surprisingly high in the diffuse ionized ISM (the so-called WIM), corresponding to an electron temperature of only ~3000K. Such low temperatures were unexpected in gas that was presumed to be photoionized. We consider a 3-component model for the observed diffuse emission, consisting of a mix of (1) photoionized gas, (2) gas that is recombining and cooling, and (3) cool H I gas. This model can successfully reproduce the observed intensities of free-free continuum, H-alpha, and collisionally-excited lines such as NII 6583. To reproduce the low observed value of free-free to H-alpha, the PAH abundance in the photoionized regions must be lowered by a factor ~3, and ~20% of the diffuse H-alpha must be reflected from dust grains, as suggested by Wood & Reynolds (1999).