  We have observed the Andromeda galaxy, Messier 31 (M31), at 6.7GHz 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+0.17-0.19)Jy at the
peaking frequency of ~25GHz. 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 10GHz, by
synchrotron emission with a spectral index of -1.10+0.10-0.08, with subdominant
free-free emission. At frequencies >10GHz, AME has a similar intensity to that
of synchrotron and free-free emission, overtaking them between 20GHz and 50GHz,
whereas thermal dust emission dominates the emission budget at frequencies
above 60GHz, as expected.
