Evidence for inverted-spectrum 20 GHz emission in the Galactic plane

A comparison of a 19 GHz full-sky map with the WMAP satellite K-band (23 GHz) map indicates that the bulk of the 20 GHz emission within 7 degrees of the Galactic plane has an inverted ( rising) spectrum with an average spectral index alpha = 0.21 +/- 0.05. While such a spectrum is inconsistent with steep-spectrum synchrotron (alpha similar to -0.7) and flat-spectrum free-free (alpha similar to -0.1) emission, it is consistent with various models of electric dipole emission from thermally excited spinning dust grains, as well as models of magnetic dipole emission from ferromagnetic dust grains. Several regions in the plane, e. g., near the Cygnus arm, have spectra with even larger alpha. While the low signal-to-noise ratio of the 19 GHz data precludes a detailed map of the spectral index, especially off the Galactic plane, it appears that the bulk of the emission in the plane is correlated with the morphology of dust. Regions with higher 23 GHz flux tend to have harder spectra. Off the plane, at Galactic latitudes 7 degrees < vertical bar b vertical bar < 20 degrees, the spectrum steepens to alpha = -0.16 +/- 0.15.