The Density and Temperature of Molecular Clouds in M33

We have observed the 12CO J = 2-1,13CO J = 2-1, and 12CO 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 12CO/13CO J = 2-1 line ratio is constant across the entire sample, while the observed 12CO 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 H II region. A large velocity gradient analysis indicates that clouds without H II regions have temperatures of 10-20 K, clouds with H II regions have temperatures of 15-100 K, and the cloud in the giant H II region has a temperature of at least 100 K. Interestingly, the giant H II 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 H II 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 H II 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 3 difference in metallicity have similar density and temperature, which in turn implies that the differences in the CO-to-H2 conversion factor seen in these clouds can be attributed to metallicity effects entirely.