Simultaneous Deep Measurements of CO Isotopologues and Dust Emission in Giant Molecular Clouds in the Andromeda Galaxy

Abstract

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 × 10−6 and 4.4 × 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 α 12 = 8.7 ± 3.9, α 13 = 48.9 ± 20.4, and M ⊙ (K km s−1 pc2)−1 for the J = 2–1 transitions of 12CO, 13CO, and C18O, respectively.