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The Andromeda galaxy is the best-known large galaxy besides our own MilkyWay. Several images and studies exist at all wavelengths from radio to hardX-ray. Nevertheless, only a few observations are available in the microwaverange where its average radio emission reaches the minimum. In this paper, wewant to study the radio morphology of the galaxy, decouple thermal fromnonthermal emission, and extract the star formation rate. We also aim to derivea complete catalog of radio sources for the mapped patch of sky. We observedthe Andromeda galaxy with the Sardinia Radio Telescope at 6.6 GHz with veryhigh sensitivity and angular resolution, and an unprecedented sky coverage.Using new 6.6 GHz data and Effelsberg radio telescope ancillary data, weconfirm that, globally, the spectral index is $\sim 0.7-0.8$, while in the starforming regions it decreases to $\sim 0.5$. By disentangling (gas) thermal andnonthermal emission, we find that at 6.6 GHz, thermal emission follows thedistribution of HII regions around the ring. Nonthermal emission within thering appears smoother and more uniform than thermal emission because ofdiffusion of the cosmic ray electrons away from their birthplaces. This causesthe magnetic fields to appear almost constant in intensity. Furthermore, wecalculated a map of the star formation rate based on the map of thermalemission. Integrating within a radius of $R_{max}=15$ kpc, we obtained a totalstar formation rate of $0.19 \pm 0.01$ $M_{\odot}$/yr in agreement withprevious results in the literature. Finally, we correlated our radio data withinfrared images of the Andromeda galaxy. We find an unexpectedly highcorrelation between nonthermal and mid-infrared data in the central region,with a correlation parameter $r=0.93$.

Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz