Over the last decade, extensive observations of carbon monoxide emission in the disk of our Galaxy have shown that molecular (H2) gas rather than atomlc (HI) hydrogen is the major actlve component of the interstellar medium. In the Galaxy, virtually all known regions of star formation activity are associated with molecular clouds. In this artlcle, we review the theoretical and empirical basis for using the 2.6 mm CO emission line as a tracer of H2 and summarize the galactic distribution and properties of the molecular clouds.
Molecular clouds are the dominant component of the interstellar medium in the inner half of the galactic disk at R <0.8 R . Most of the molecular gas is in clouds at the hlgh end of the mass spectrum with mass >10^5 (GMC), with over half the mass in clouds with M >4 x 10^5 M^. The longitude and velocity of 'warm' molecular clouds, whose temperatures imply substantial internal heating, are correlated with giant HII regions and may be associated with armlike spiral structure. Cooler clouds are distributed much more widely throughout the disk. The radial distribution of cloud
nuraber denslty is similar to the distribution of CO emlssivity - that is, a ringlike concentratlon at 0.4 - 0.8 Rq with a relatlvely sharp inner edge and a gradual falloff toward large R. Giant molecular clouds do not fit Into a two- or three-phase pressure-equlllbrlum plcture of interstellar matter. They are gravitationally bound and their Internal pressure, domlnated by chaotic raotions, is 2 orders of magnitude greater than the standard Interstellar medium pressure.
A likely orlgin for these clouds is gravitational Instability.