Resolved Dust Emission and CO Isotopologues in Giant Molecular Clouds of the Andromeda Galaxy

TL;DR

This study uses submillimeter observations of giant molecular clouds in Andromeda to measure dust and CO isotopologue emissions, deriving key properties like dust mass ratios and virial states, with implications for understanding galaxy molecular gas.

Contribution

It provides the first resolved dust emission measurements in GMCs of M31 and updates the CO-to-H2 conversion factor analysis with a larger sample and metallicity considerations.

Findings

Dust emission detected in 71 cloud cores, 26 resolved.

Derived CO-to-H2 conversion factors consistent with previous studies.

Most GMC regions are gravitationally bound and near virial equilibrium.

Abstract

Dust emission at submillimeter wavelengths can be used to reliably trace the basic properties of molecular clouds. Early results from a recent Submillimeter Array (SMA) survey of the Andromeda Galaxy (M31) include the first detections of resolved dust continuum emission from individual giant molecular clouds (GMCs) in an external spiral galaxy. This paper updates on the now-complete SMA survey of 80 Herschel-identified giant molecular associations (GMAs) in M31. The SMA survey simultaneously probes dust continuum emission at 230 GHz and the $J = 2 \rightarrow 1$ transitions of the CO isotopologues, $^{12}\rm CO$, $^{13}\rm CO$, and $\rm C^{18}O$ at a spatial resolution of $\lesssim 15~\mathrm{pc}$. Dust continuum emission was detected in 71 cloud cores, of which 26 were resolved. This more than doubles the size of the previous sample. By comparing dust and CO observations with identical astrometry, we directly measure the dust mass to-light ratios, $\rm \alpha^{\prime}_{^{12}CO}$, and $\rm \alpha^{\prime}_{^{13}CO}$. We derive $<\alpha^{\prime}_{\rm ^{12}\rm CO}>~=~0.070~\pm~0.031~M_{\odot}\,(\rm K~km~s^{-1}~pc^{2})^{-1}$ and $<\alpha^{\prime}_{\rm ^{13}\rm CO}>~=~0.37~\pm~0.20~M_{\odot}\,(\rm K~km~s^{-1}~pc^{2})^{-1}$ for the increased sample, which are in agreement with previously reported values. From virial analysis, we find that 80% of the GMC regions traced by resolved dust emission are bound and close to virial equilibrium. Finally, we update our analysis on the metallicity dependence of $\rm \alpha^{\prime}_{\rm CO}$ by combining SMA observations with existing MMT/Hectospec optical spectroscopy toward H II regions. We find no trend in $\rm \alpha^{\prime}_{\rm CO}$ with metallicity, supporting the previous findings.