2026ApJS..282...39J

We have carried out unprecedentedly deep, nearly confusion-limited JCMT SCUBA-2 mapping observations on the nearest spiral galaxy, M31 (Andromeda). The 850 μm image with a ∼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 interarm GMCs in M31, which are mostly unresolved or marginally resolved. The masses of all these GMCs are in the range of 2 × 10⁴-6 × 10⁶M_⊙; the sizes are in the range of 30-130 pc. They follow a mass-size correlation, M ∝ R_c^2.5. The interarm GMCs are systematically less massive, more diffuse, colder, and have lower star formation 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 β in the M31 galaxy by combining our new JCMT observations with archival Herschel and Planck data and found that the radial variation in β may not be as large as has been proposed by previous studies.