2021ApJ...912...68V

We present simultaneous measurements of emission from dust continuum at 230 GHz and the J = 2-1 ¹²CO, ¹³CO, and C¹⁸O isotopologues at ∼15 pc resolution from individual giant molecular clouds (GMCs) in the Andromeda galaxy (M31). These observations were obtained in an ongoing survey of this galaxy being conducted with the Submillimeter Array. Initial results describing the continuum and ¹²CO emission have been published earlier. Here, we primarily analyze the observations of ¹³CO and C¹⁸O emission and compare them to the measurements of dust continuum and ¹²CO emission. We also report additional dust continuum and CO measurements from newly added GMCs to the M31 sample. We detect spatially resolved ¹³CO emission with high signal-to-noise ratios in 31 objects. We find the extent of the ¹³CO emission to be nearly comparable to that of ¹²CO, typically covering 75% of the area of the ¹²CO emission. We derive ¹³CO and C¹⁸O abundances of 2.9 x 10^–6 and 4.4 x 10^–7 relative to H₂, respectively, by comparison with hydrogen column densities of the same regions derived from the dust continuum observations assuming a Milky Way gas-to-dust ratio. We find the isotopic abundance ratio [¹³CO]/[C¹⁸O] = 6.7 ± 2.9 to be consistent with the Milky Way value (8.1). Finally, we derive the mass-to-light conversion factors for all three CO species to be α₁₂ = 8.7 ± 3.9, α₁₃ = 48.9 ± 20.4, and α₁₈=345_–31⁺²⁵M_☉ (K km s^–1 pc²)^–1 for the J = 2-1 transitions of ¹²CO, ¹³CO, and C¹⁸O, respectively.