Mass, luminosity, and line width relations of Galactic molecular clouds

The authors present measurements of the velocity line width, size, virial mass, and CO luminosity for 273 molecular clouds in the Galactic disk between longitudes of 8° and 90°. These are obtained from three-dimensional data in the Massachusetts-Stony Brook CO Galactic Plane Survey. It is shown that the molecular clouds are in or near virial equilibrium and are not confined by pressure equilibrium with a warm or hot phase of interstellar matter. The velocity line width is proportional to the 0.5 power of the size, σ<SUB>v</SUB> ∝ S<SUP>0.5</SUP>. A tight relationship, over four orders of magnitude, is found between the cloud dynamical mass, as measured by the virial theorem, and the CO luminosity M ∝ (L<SUB>CO</SUB>)<SUP>0.81</SUP>. The cloud CO luminosity is L<SUB>CO</SUB>∝ σ<SUB>v</SUB><SUP>5</SUP>.