Molecules in Galaxies. VI. Diffuse and Dense Cloud Contributions to the Large-Scale CO Emission of the Galaxy.
01 January 1988
Maps of large areas of the Galactic plane made in the sup (12) CO J=1->0 and sup (13) CO J=1->0 lines are studied to determine mean emissivities. The average ratio of line intensities. I (sup (12) CO) / I (sup (13) CO) is found to be 6.7+-0.7. higher than the values of 3-5 found for Giant Molecular Clouds (GMCs). This suggests that a significant contribution to the large-scale sup (12) CO emission of the Galaxy is made by lower optical depth (diffuse) molecular gas. Observational determinations of the sup (12) CO / sup (13) CO intensity ratios and the sup (12) CO emission to total column density ratio for both GMCs and diffuse clouds are used to evaluate the relative amounts of CO emission from, and mass in, these two components.