Investigating variations in the dust emissivity index in the Andromeda galaxy

TL;DR

This study investigates radial variations in the dust emissivity index ($\beta$) in the Andromeda galaxy using CO and dust maps, finding consistent radial variations but little difference inside versus outside molecular clouds.

Contribution

It provides new high-resolution analysis of $\beta$ variations in M31 and compares dust and CO emission to identify molecular clouds and their properties.

Findings

Radial variations of $\beta$ are confirmed in M31.

Little difference in $\beta$ inside versus outside molecular clouds.

Identification of clouds with little CO, possibly atomic or CO-poor molecular clouds.

Abstract

Over the past decade, studies of dust in the Andromeda galaxy (M31) have shown radial variations in the dust emissivity index ($\beta$). 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 $^{12}$CO(J=1-0) observations taken by the Combined Array for Research in Millimeter Astronomy (CARMA) and dust maps derived from \textit{Herschel} images, both with an angular resolution of 8" and spatial resolution of 30 pc, to study variations in $\beta$ across an area of $\approx$ 18.6 kpc$^2$ in M31. We extract sources, which we identify as molecular clouds, by applying the astrodendro algorithm to the $^{12}$CO 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 $\beta$ is different inside and outside molecular clouds. Our results confirm the radial variations of $\beta$ seen in previous studies. However, we find little difference between the average $\beta$ inside molecular clouds compared to outside molecular clouds, in disagreement with models which predict an increase of $\beta$ 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.