Strong Evidence of Anomalous Microwave Emission from the Flux Density Spectrum of M31

Abstract

We have observed the Andromeda galaxy, Messier 31 (M31), at 6.7 GHz with the Sardinia Radio Telescope. We mapped the radio emission in the C-band, re-analyzed WMAP and Planck maps, as well as other ancillary data, and we have derived an overall integrated flux density spectrum from the radio to the infrared. This allowed us to estimate the emission budget from M31. Integrating over the whole galaxy, we found strong and highly significant evidence for anomalous microwave emission (AME), at the level of 1.45 Jy at the peaking frequency of ≃25 GHz. Decomposing the spectrum into known emission mechanisms such as free-free, synchrotron, thermal dust, and AME arising from electric dipole emission from rapidly rotating dust grains, we found that the overall emission from M31 is dominated, at frequencies below 10 GHz, by synchrotron emission with a spectral index of - 1.1, with subdominant free-free emission. At frequencies ≳10 GHz, AME has a similar intensity to that of synchrotron and free-free emission, overtaking them between 20 and 50 GHz, whereas thermal dust emission dominates the emission budget at frequencies above 60 GHz, as expected.