THE PHYSICAL-PROPERTIES OF GIANT MOLECULAR CLOUD COMPLEXES IN THE OUTER GALAXY - IMPLICATIONS FOR THE RATIO OF H2 COLUMN DENSITY TO (CO)-C-12 INTENSITY

The Goddard-Columbia 12CO surveys of the Galactic plane region (|b|<10°) are used to derive the physical properties of 35 giant molecular cloud complexes in the outer Galaxy (60°<l<300°). Distances to the complexes are derived kinematically by assuming a Sun-to-Galactic center distance of 8.5 kpc and a flat rotation curve for galactocentric distances greater than 8.5 sin 60° ( = 7.4) kpc. Most (∼80%) of the complexes lie between 8.5 and 10.5 kpc from the Galactic center. The outer Galaxy giant molecular cloud complexes are very similar to their inner Galaxy counterparts. The 12CO line width of the complexes varies proportionally to the 0.5±0.1 power of their radius, and the mean molecular density of the complexes varies inversely as the 0.9±0.1 power of their radius, suggesting that the complexes are in approximate virial equilibrium. The derived cloud complex size and mass distributions in the outer Galaxy are consistent with the corresponding distributions in the inner Galaxy, where 85% of the total mass is contained within complexes having radii greater than 10 pc and individual masses greater than 105 M⊙. From a statistical analysis of their virial masses and 12CO luminosities, the ratio of H2 column density to 12CO intensity for the outer Galaxy complexes is derived. The best estimate of this ratio is 6.0×1020 molecules cm-2 K-1 km-1 s, which is a factor of 2-3 greater than the value derived by other authors for the ratio of H2 column density to 12CO intensity for inner Galaxy complexes. It is shown that a rapid increase in the Galactic ratio of H2 column density to 12CO intensity with increasing galactocentric distance, consistent with the above results, is expected on the basis of the optically thick cloudlet model of giant molecular cloud complexes, and the estimated variations in cloud complex heating and CO abundance with galactocentric distance.