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The Andromeda Galaxy (M31) is one of a few galaxies that has sufficientangular size on the sky to be resolved by the Planck satellite. Planck hasdetected M31 in all of its frequency bands, and has mapped out the dustemission with the High Frequency Instrument, clearly resolving multiple spiralarms and sub-features. We examine the morphology of this long-wavelength dustemission as seen by Planck, including a study of its outermost spiral arms, andinvestigate the dust heating mechanism across M31. We find that dust dominatingthe longer wavelength emission ($\gtrsim 0.3\,$mm) is heated by the diffusestellar population (as traced by 3.6$\,\mu$m emission), with the dustdominating the shorter wavelength emission heated by a mix of the old stellarpopulation and star-forming regions (as traced by 24$\,\mu$m emission). We alsofit spectral energy distributions (SEDs) for individual 5' pixels and quantifythe dust properties across the galaxy, taking into account these differentheating mechanisms, finding that there is a linear decrease in temperature withgalactocentric distance for dust heated by the old stellar population, as wouldbe expected, with temperatures ranging from around 22$\,$K in the nucleus to14$\,$K outside of the 10$\,$kpc ring. Finally, we measure the integratedspectrum of the whole galaxy, which we find to be well-fitted with a globaldust temperature of ($18.2\pm1.0$)$\,$K with a spectral index of $1.62\pm0.11$(assuming a single modified blackbody), and a significant amount of free-freeemission at intermediate frequencies of 20-60$\,$GHz, which corresponds to astar formation rate of around $0.12$M$_\odot\,$yr$^{-1}$. We find a$2.3\,\sigma$ detection of the presence of spinning dust emission, with a30$\,$GHz amplitude of $0.7\pm0.3\,$Jy, which is in line with expectations fromour Galaxy.

Planck intermediate results. XXV. The Andromeda Galaxy as seen by Planck