Cold Massive Molecular Clouds in the Inner Disk of M31

We present new interferometric CO(1-0) and single-dish CO(3-2) observations of the central parts of D478, a large (> 200 pc) dark dust cloud located in a quiescent region of the inner disk of M31 where single-dish CO(1-0) and CO(2-1) observations were previously obtained. Only a small fraction (< 15%) of the CO(1-0) flux previously detected in this region with the single-dish telescope is recorded by the interferometer. Most of the CO(1-0) emission must therefore have the appearance of a smooth surface with very little structure on scales smaller than ≈ 25 (85 pc). Together with the earlier CO(1-0) and CO(2-1) single-dish results the new CO(3-2) data are in good agreement with LTE predictions for optically thick lines at Tex = Tkin = 3.5K. These results rule out the conventional model for these clouds consisting of warm clumps with a low filling factor (as would be the case if they resembled Galactic GMCs) and confirm that large, massive, cold molecular clouds exist in the inner disk of M31 with kinetic temperatures close to that of the cosmic microwave background. Such extremely low temperatures are likely to be a consequence of the low heating rate in these particular regions of M31, where very little massive star formation is occurring at present. From the CO line profile widths we estimate the Virial mass surface density of D478 to be 80 – 177 M pc . This is a factor 7 – 16 times larger than the value obtained by multiplying the CO profile integrals with the conventional “X-factor”. Subject headings: galaxies: ISM – galaxies: individual: M31 – ISM: molecules – ISM: clouds – radio lines: galaxies 1Observatoire de Grenoble, Laboratoire d’Astrophysique, Université Joseph Fourier, B.P. 53X, F-38041 Grenoble Cedex, France