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

TL;DR

This work leverages a four-epoch SMA survey of 80 Herschel-identified GMAs in M31 to obtain resolved dust continuum emission and CO(2–1) isotopologue data at ≲15 pc. By extracting dust cores within GMC boundaries and pairing them with CO line measurements, the authors compute dust-based CO conversion factors α'_{12CO} and α'_{13CO}, derive molecular gas masses independent of the gas-to-dust ratio, and perform virial analyses on dense dust structures. They find mean values α'_{12CO} ≈ 0.070 and α'_{13CO} ≈ 0.37 M_{⊙} (K km s^{-1} pc^{2})^{-1}, corresponding to α_{12CO} ≈ 9.5 M_{⊙} (K km s^{-1} pc^{2})^{-1} for a MW-like gas-to-dust ratio, with about 80% of the dense cores bound and near virial equilibrium. Across a metallicity range of 8.45 < O/H < 8.65 in M31, α'_{12CO} shows no clear trend with metallicity, challenging simple metallicity-driven expectations for CO-to-H2 conversion and suggesting compensating effects between dust shielding and CO abundance within this regime.

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, $\rm α^{\prime}_{^{12}CO}$, and $\rm α^{\prime}_{^{13}CO}$. We derive $<α^{\prime}_{\rm ^{12}\rm CO}>~=~0.070~\pm~0.031~M_{\odot}\,(\rm K~km~s^{-1}~pc^{2})^{-1}$ and $<α^{\prime}_{\rm ^{13}\rm CO}>~=~0.37~\pm~0.20~M_{\odot}\,(\rm K~km~s^{-1}~pc^{2})^{-1}$ 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 $\rm α^{\prime}_{\rm CO}$ by combining SMA observations with existing MMT/Hectospec optical spectroscopy toward H II regions. We find no trend in $\rm α^{\prime}_{\rm CO}$ with metallicity, supporting the previous findings.

Figures (11)

• Figure 1: Locations of SMA-observed GMAs Kirk_2015 over-plotted onto the Herschel Spectral and Photometric Imaging Receiver (SPIRE) 500-micron image Fritz_2012, with GMAs for which we obtained dust detections displayed as open circles, and GMAs observed but with no dust detections displays as filled circles. The 1 kpc scalebar assumes the distance to M31 to be 780 kpc; Stanek_1998.
• Figure 2: CO line profiles for six individual dust cores. The top panels show three dust cores from the 1G sample, while the bottom panels show three dust cores with multiple-component CO line profiles. Both $^{12}\rm CO$ and $^{13}\rm CO$ spectral lines are displayed with the corresponding Gaussian fits.
• Figure 3: Left, SMA maps of dust continuum of individual Kirk_2015 GMAs in M31, for fields which contain one or more dust cores. Blue contours display dust continuum emission at 2.5$\sigma$, 3.5$\sigma$ and 4.5$\sigma$, where $\sigma$ is the continuum image background rms, the values given in Table \ref{['tab:props']}. Green contours display $\rm ^{12}CO$ at 3, 6, 12, 24 and 48$\sigma$. Red contours display $\rm ^{13}CO$ at 3, 6 and 10$\sigma$. The corresponding center of mass (by area) of individual GMCs as traced by $\rm ^{12}CO$ at 3$\sigma$ are marked by black 'x's', and by white 'x's' for individual dust cores as traced by dust continuum at 2.5$\sigma$. Right, Corresponding CO line profiles extracted from within individual dust cores, for both $\rm ^{12}CO$ and $\rm ^{13}CO$. Hectospec fibre positions corresponding to H ii regions identified from optical spectroscopy Bosomworth_2025 and associated with GMCs hosting a 1G dust core are shown as yellow diamonds (see Section \ref{['Sec: met']}). Gaussian fits to line profiles are displayed for the 1G sample. The complete figure set (51 images) is available in the online journal.
• Figure 4: Histogram of dust masses for left, our full sample of dust cores, of which 26 are resolved and 45 unresolved and right, our 1G sample of dust cores, of which 20 are resolved and 27 unresolved.
• Figure 5: $^{12}\rm CO$ conversion factors, $\alpha^{\prime}_{^{12}\rm CO}$, for individual 1G dust cores for: top, resolved sources and bottom, unresolved sources. We display the mean $\alpha^{\prime}_{^{12}\rm CO}$ calculated for resolved and unresolved sources as a dotted line, with the 1$\sigma$ standard deviation represented by the shaded region.