Abstract
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 ($0.3$ 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). We also fit
spectral energy distributions (SEDs) for individual 5' pixels and quantify the
dust properties across the galaxy, taking into account these different heating
mechanisms, finding that there is a linear decrease in temperature with
galactocentric distance for dust heated by the old stellar population, as would
be expected, with temperatures ranging from around 22 K in the nucleus to 14 K
outside of the 10 kpc ring. Finally, we measure the integrated spectrum of the
whole galaxy, which we find to be well-fitted with a global dust temperature of
($18.9\pm0.9$) K with a spectral index of $1.61\pm0.11$ (assuming a single
modified blackbody), and a significant amount of free-free emission at
intermediate frequencies, which when converted into a star formation rate
agrees well with the star formation estimate from H$\alpha$ emission of
0.4$M_ødot$ yr$^-1$. We see no evidence for spinning dust emission, with a
3$\sigma$ upper limit of 1.26 Jy in the 20-60 GHz band.
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