DIFFUSE MOLECULAR CLOUDS AND THE MOLECULAR INTERSTELLAR-MEDIUM FROM (CO)-C-13 OBSERVATIONS OF M33

We have obtained (CO)-C-12 and (CO)-C-13 J = 1-0 observations of the nearby spiral galaxy M33 to try to resolve the long-standing discrepancy between (CO)-C-12/(CO)-C-13 line ratios measured in Galactic giant molecular clouds and external galaxies. Interferometer maps of the molecular cloud MC 20 give a (CO)-C-12/(CO)-C-13 line ratio of 7.5 +/- 2.1, which agrees reasonably well with the line ratio measured in Galactic giant molecular clouds. In contrast, the (CO)-C-12/(CO)-C-13 line ratio obtained from single-dish data is 10.0 +/- 0.9, significantly higher than Galactic values but in good agreement with line ratios measured in other galaxies. The interferometer map of MC 20 reveals that the cloud has similar spatial and velocity extents in the two lines, and thus the high single-dish line ratio cannot be due to different filling factors in the two lines. In addition, the single-dish data show no evidence for significant variations in the line ratio with metallicity, which eliminates abundance changes as the explanation for the high single-dish line ratio. We conclude that the high (CO)-C-12/(CO)-C-13 line ratios observed in M33, and in the disks of spiral galaxies in general, are due to the presence of a population of diffuse molecular clouds. We obtain a lower limit to the (CO)-C-12/(CO)-C-13 line ratio in the diffuse molecular clouds of 13 +/- 5. The lower limit to the fraction of the total (CO)-C-12 emission from M33 that originates in the diffuse clouds is 30 +/- 30%, while the upper limit is -60%. We have combined our line ratios with published measurements of the (CO)-C-12 J = 2-1 to J = 1-0 line ratio to determine the (CO)-C-12 and (CO)-C-13 column densities to within an order of magnitude, but the density and the temperature of the gas are not well constrained by these measurements. We estimate that differences in the physical conditions in diffuse and giant molecular clouds are unlikely to cause the overall molecular gas mass calculated for the central kiloparsec of M33 to be in error by more than a factor of 2.