On the Origins of the High-Latitude H-alpha Background

TL;DR

This paper challenges the traditional view that high-latitude H-alpha emission is mainly from local ionized gas, showing instead that dust scattering significantly contributes to the observed background.

Contribution

It demonstrates that dust scattering accounts for a large fraction of high-latitude H-alpha emission and provides an empirical relation linking H-alpha intensity to IRAS 100um background.

Findings

Dust scattering explains much of the high-latitude H-alpha background.

An empirical relation links H-alpha intensity to IRAS 100um background.

Implications for CMB foreground modeling are significant.

Abstract

The diffuse high-latitude H-alpha background is widely believed to be predominantly the result of in-situ recombination of ionized hydrogen in the warm interstellar medium of the Galaxy. Instead, we show that both a substantial fraction of the diffuse high-latitude H-alpha intensity in regions dominated by Galactic cirrus dust and much of the variance in the high-latitude H-alpha background are the result of scattering by interstellar dust of H-alpha photons originating elsewhere in the Galaxy. We provide an empirical relation, which relates the expected scattered H-alpha intensity to the IRAS 100um diffuse background intensity, applicable to about 81% of the entire sky. The assumption commonly made in reductions of CMB observations, namely that the observed all-sky map of diffuse H-alpha light is a suitable template for Galactic free-free foreground emission, is found to be in need of reexamination.