GIANT MOLECULAR CLOUDS IN THE OUTER ARM OF THE GALAXY

Giant molecular clouds in the outer arm of the Galaxy comparable in size and mass to the largest clouds in the inner Galaxy between R ≈ 3 kpc and the solar circle have been detected in a new CO survey along the Galactic plane from l = 65° to 71° centered about b = + 1°. These objects at R ≈ 12 kpc (for R⊙ = 8.5 kpc) are underluminous in CO; the N(H2)/WCO ratio determined from either virial masses or the CO luminosityline width relation is 4 ± 2 times that in the inner Galaxy. When extrapolated to the entire Galaxy beyond R = 11 kpc, the present survey and related data yield a total molecular mass in the range (1-7) × 108 M⊙. The outer arm complexes surveyed contain the two most luminous H II regions known beyond R ≈ 10 kpc; one is the distant component of W58, and the other is a previously unidentified distant component of S98.