SIMBAD references

Recently, Allen & Lequeux (1993) have detected faint CO emission from two massive molecular clouds in the inner disk of M 31. These clouds were first located optically by their dust content. In order to determine the validity of this selection criterion, we have carried out an "unbiased" survey at 26 positions regularly spaced every 3.6' along the major axis of M 31. Faint CO emission was detected for 16 of them. Dark clouds are seen optically at many of the positions where CO is detected, confirming a posteriori the proposition that there is a close correlation between the presence of dust and the presence of faint CO emission. However, we also find detectable CO emission at positions for which there are no catalogued dust clouds; the survey therefore points to a significantly larger area covering factor for the molecular gas in the inner disk of M 31 than previously thought. Our observations suggest that the mass surface density of gas perpendicular to the plane of the disk is as high as 10xM_☉pc^–2 in the inner regions of M 31, which exhibits no central "gas hole" contrary to the common belief. The velocities of the CO(1-0) lines trace a rotation curve which is in general agreement with that of the HI obtained earlier by Brinks & Shane (1984), although the component of the HI attributed by those authors to a warp of the outer disk of M 31 seen in projection on the inner disk does not appear to have a counterpart in CO at the present level of detection. The ratios of the integrated CO(2-1) to CO(1-0) line intensities, when both can be measured, are generally very low (∼0.4), consistent with models of cold (T_K<5K) molecular clouds at low density (∼100cm^–3).