Physical Conditions and Spatial Structure in the Molecular Interstellar Medium of M 33

Early studies of molecular gas in M33 used the CO J=1-0 transition to map out the total gas content in the inner kiloparsec of the galaxy and to study the properties of the giant molecular cloud population. In this review, I discuss recent detailed studies of individual molecular clouds in M33 in higher rotational transitions of CO, CO, and the fine structure line of atomic carbon. These data reveal that the temperature and density of the molecular gas is correlated with the presence of nearby massive star formation, with clouds with more intense star formation having higher temperatures and lower densities. This effect is most likely due to post-star formation processing of the molecular gas. A comparison of CO observations for M33 and individual molecular clouds in the Milky Way suggests that a significant fraction of the molecular interstellar medium resides in low column density regions and accounts for 30-60% of the CO luminosity. Finally, a detailed map of a single molecular cloud in the giant H II region NGC 604 reveals an offset between the atomic carbon and CO emission, which indicates that the dominant source of atomic carbon in this cloud is likely photodissociation of CO by ultraviolet photons from the massive stars. A complete map of the CO emission in M33 would be extremely helpful to guide further detailed studies.