Deep Andromeda JCMT-SCUBA2 Observations. The Submillimeter Maps and Giant Molecular Clouds

TL;DR

This study presents deep JCMT-SCUBA2 submillimeter maps of M31, revealing detailed properties of 572 giant molecular clouds, their distribution, physical characteristics, and star formation efficiency variations across the galaxy.

Contribution

It provides the first comprehensive catalog of GMCs in M31 with detailed analysis of their physical properties and star formation activity, including the discovery of inter-arm GMCs and refined dust opacity measurements.

Findings

Identified 383 on-arm and 189 inter-arm GMCs in M31.

GMCs follow a mass-size relation of M ∝ R^2.5.

Inter-arm GMCs are less massive, colder, and less efficient at star formation.

Abstract

We have carried out unprecedentedly deep, nearly confusion-limited JCMT-SCUBA2 mapping observations on the nearest spiral galaxy, M31 (Andromeda). The 850 $\mu$m image with a $\sim$50 pc resolution yields a comprehensive catalog of 383 giant molecular clouds (GMCs) that are associated with the spiral arms. In addition, it unveiled a population of 189 compact inter-arm GMCs in M31, which are mostly unresolved or marginally resolved. The masses of all these GMCs are in the range of 2$\times$10$^4$ -- 6$\times$10$^6$ $M_{\odot}$; the sizes are in the range of 30--130 pc. They follow a mass-size correlation, $M$ $\propto$ $R_{c}$$^{2.5}$. The inter-arm GMCs are systematically less massive, more diffuse, colder, and have lower star-forming efficiency (SFE) than on-arm GMCs. Moreover, within individual spatially resolved on-arm and off-arm M31 GMCs, the SFE is considerably lower than the SFE in molecular clouds in main sequence and green valley galaxies. Follow-up investigations on M31 GMCs may provide clues for how star formation may be quenched in galactic environments. Finally, we reconstrained the dust opacity spectral index $\beta$ in the M31 galaxy by combining our new JCMT observations with archival Herschel and Planck data and found that the radial variation of $\beta$ may not be as large as was proposed by previous studies.