The Density and Temperature of Molecular Clouds in M 33


We have observed the CO J=2-1, CO J=2-1, and CO J=3-2 lines in a sample of seven giant molecular clouds in the Local Group spiral galaxy M33 using the James Clerk Maxwell Telescope. The CO/CO J=2-1 line ratio is constant across the entire sample, while the observed CO J=3-2/J=2-1 line ratio has a weak dependence on the star formation environment of the cloud, with large changes in the line ratio seen only for clouds in the immediate vicinity of an extremely luminous HII region. A large velocity gradient analysis indicates that clouds without HII regions have temperatures of 10-20 K, clouds with HII regions have temperatures of 15-100 K, and the cloud in the giant HII region has a temperature of at least 100 K. Interestingly, the giant HII region appears capable of raising the kinetic temperature of the molecular gas only for clouds that are quite nearby (< 100 pc). The continuous change of physical conditions across the observed range of star formation environments suggests that the unusual physical conditions in the cloud in the giant HII region are due to post-star formation changes in the molecular gas, rather than intrinsic properties of the gas related to the formation of the giant HII region. The results from this study of M33 suggest that similar observations of ensembles of giant molecular clouds in more distant normal spiral galaxies are likely to give meaningful measurements of the average physical conditions inside the molecular clouds. These results also imply that clouds with a factor of three difference in metallicity have similar density and temperature, which in turn imply that the differences in the CO-to-H2 conversion factor seen in these clouds can be attributed entirely to metallicity effects. Subject headings: HII regions – galaxies: individual (M33) – galaxies: ISM –

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@inproceedings{Wilson1996TheDA, title={The Density and Temperature of Molecular Clouds in M 33}, author={Christine D . Wilson and Constance E. Walker and Michele D. Thornley}, year={1996} }