Cold Massive Molecular Clouds in the Inner Disk of M31

We present new interferometric $^{12}CO$(1-0) and single-dish CO(3-2)observations of the central parts of D478, a large (> 200 pc) dark dust cloudlocated in a quiescent region of the inner disk of M31 where single-dishCO(1-0) and CO(2-1) observations were previously obtained. Only a smallfraction (< 15%) of the CO(1-0) flux previously detected in this region withthe single-dish telescope is recorded by the interferometer. Most of theCO(1-0) emission must therefore have the appearance of a smooth surface withvery little structure on scales smaller than approx 25'' (85 pc). Together withthe earlier CO(1-0) and CO(2-1) single-dish results the new CO(3-2) data are ingood agreement with LTE predictions for optically thick lines at Tex = Tkin =3.5K. These results rule out the conventional model for these clouds consistingof warm clumps with a low filling factor (as would be the case if theyresembled Galactic GMCs) and confirm that large, massive, cold molecular cloudsexist in the inner disk of M31 with kinetic temperatures close to that of thecosmic microwave background. Such extremely low temperatures are likely to be aconsequence of the low heating rate in these particular regions of M31, wherevery little massive star formation is occurring at present. From the CO line profile widths we estimate the Virial mass surface densityof D478 to be 80 -- 177 Msun pc-2. This is a factor 7 -- 16 times larger thanthe value obtained by multiplying the CO(1-0) profile integrals with theconventional ``X-factor''.