We derive the physical properties of 580 molecular clouds based on their ^ { 12 } CO and ^ { 13 } CO line emission detected in the University of Massachusetts-Stony Brook ( UMSB ) and Galactic Ring surveys .
We provide a range of values of the physical properties of molecular clouds , and find a power-law correlation between their radii and masses , suggesting that the fractal dimension of the ISM is around 2.36 .
This relation , M = ( 228 \pm 18 ) R ^ { 2.36 \pm 0.04 } , allows us to derive masses for an additional 170 GRS molecular clouds not covered by the UMSB survey .
We derive the Galactic surface mass density of molecular gas and examine its spatial variations throughout the Galaxy .
We find that the azimuthally averaged Galactic surface density of molecular gas peaks between Galactocentric radii of 4 and 5 kpc .
Although the Perseus arm is not detected in molecular gas , the Galactic surface density of molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms .
This may indicate that molecular clouds form in spiral arms and are disrupted in the inter-arm space .
Last , we find that the CO excitation temperature of molecular clouds decreases away from the Galactic center , suggesting a possible decline in the star formation rate with Galactocentric radius .
There is a marginally significant enhancement in the CO excitation temperature of molecular clouds at a Galactocentric radius of about 6 kpc , which in the longitude range of the GRS corresponds to the Sagittarius arm .
This temperature increase could be associated with massive star formation in the Sagittarius spiral arm .