Investigating variations in the dust emissivity index in the Andromeda Galaxy

Abstract

Over the past decade, studies of dust in the Andromeda Galaxy (M31) have shown radial variations in the dust emissivity index (β). Understanding the astrophysical reasons behind these radial variations may give clues about the chemical composition of dust grains, their physical structure, and the evolution of dust. We use ¹²CO(J = 1⟶⟶0) observations taken by the Combined Array for Research in Millimeter-wave Astronomy and dust maps derived from Herschel images, both with an angular resolution of 8 arcsec and with spatial resolution of 30 pc, to study variations in β across an area of ≈18.6 kpc² in M31. We extract sources, which we identify as molecular clouds, by applying the ASTRODENDRO algorithm to the ¹²CO and dust maps, which as a byproduct allows us to compare continuum emission from dust and CO emission as alternative ways of finding molecular clouds. We then use these catalogues to investigate whether there is evidence that β is different inside and outside molecular clouds. Our results confirm the radial variations of β seen in previous studies. However, we find little difference between the average β inside molecular clouds compared with that outside molecular clouds, in disagreement with models that predict an increase of β in dense environments. Finally, we find some clouds traced by dust with very little CO, which may be either clouds dominated by atomic gas or clouds of molecular gas that contain little CO.

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