Identification of spinning dust in Hα-correlated microwave emission

CMB experiments commonly use maps of H alpha intensity as a spatial template for Galactic free-free emission, assuming a power law I-nu alpha 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 H alpha-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 similar to 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 similar to 5 times weaker than the free-free component at 50 GHz, with the null hypothesis that the H alpha-correlated spectrum is pure free-free ruled out at >= 8 sigma in all regions and > 100 sigma for the full-sky fit.