STScI Preprint #1206
We present new interferometric 12CO(1-0) and single-dish 12CO(3-2) observations of the central parts of D478, a large (> 200 pc) dark dust cloud located in a quiescent region in the inner disk of M31 where single-dish 12CO(1-0) and 12CO(2-1) observations were previously obtained. Only a small fraction (< 15%) of the 12CO(1-0) flux previously detected in this region with the single-dish telescope is recorded by the interferometer. Most of the 12CO(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 12CO(1-0) and 12CO(2-1) single-dish results the new 12CO(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 12CO line profile widths we estimate the Virial mass surface density of D478 to be 80 - 177 M pc-2. This is a factor 7 - 16 times larger than the value obtained by multiplying the 12CO profile integrals with the conventional "X-factor".