Identification of Spinning Dust in Hα–Correlated Microwave Emission

CMB experiments commonly use maps of Hα intensity as a spatial template for Galactic free-free emission, assuming a power law I ν ∝ ν −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 Hα-correlated emission spectrum in the diffuse 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 "warm ionized medium" Draine & Lazarian 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 Hα-correlated spectrum is pure free-free ruled out at ≥8 σ in all regions and >100 σ for the full-sky fit.