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

Understand

The Andromeda Galaxy (M31) is one of a few galaxies that has sufficient angular size on the sky to be resolved by the Planck satellite.

• Planck has detected M31 in all of its frequency bands, and has mapped out the dust emission with the High Frequency Instrument, clearly resolving multiple spiral arms and sub-features.
• We examine the morphology of this long-wavelength dust emission as seen by Planck, including a study of its outermost spiral arms, and investigate the dust heating mechanism across M31.
• We find that dust dominating the longer wavelength emission ($\gtrsim 0.3\,$mm) is heated by the diffuse stellar population (as traced by 3.6$\,\mu$m emission), with the dust dominating the shorter wavelength emission heated by a mix of the old stellar population and star-forming regions (as traced by 24$\,\mu$m emission).