Identification of Spinning Dust in Halpha-Correlated Microwave Emission

TL;DR

This paper identifies a broad bump in the microwave emission spectrum correlated with Halpha, attributed to spinning dust grains, which challenges the assumption of pure free-free emission in CMB foreground modeling.

Contribution

It demonstrates that Halpha-correlated microwave emission includes a significant spinning dust component, deviating from the standard free-free spectrum assumption.

Findings

Spinning dust emission is about 5 times weaker than free-free at 50 GHz.

The Halpha-correlated spectrum significantly deviates from pure free-free emission.

Spinning dust model fits the observed spectrum across different regions.

Abstract

CMB experiments commonly use maps of Halpha intensity as a spatial template for Galactic free-free emission, assuming a power law I_nu \propto nu^-0.15 for the spectrum. Any departure from the assumed free-free spectrum could have a detrimental effect on determination of the primary CMB anisotropy. We show that the Halpha-correlated emission spectrum in the diffuse warm ionized medium (WIM) is not the expected free-free spectrum at WMAP frequencies. Instead, there is a broad bump in the spectrum at ~50 GHz which is consistent with emission from spinning dust grains. Spectra from both the full sky and smaller regions of interest are well fit by a superposition of a free-free and WIM Draine & Lazarian (1998) spinning dust model, shifted in frequency. The spinning dust emission is ~5 times weaker than the free-free component at 50 GHz, with the null hypothesis that the Halpha-correlated spectrum is pure free-free, ruled out at >8 sigma in all regions and >100 sigma for the full sky fit.