The characterization and Galactic distribution of infrared dark clouds

Using (CO)-C-13 J = 1 -> 0 molecular line emission from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS), we have established kinematic distances to 313 infrared dark clouds (IRDCs) by matching the morphology of the molecular line emission in distinct velocity channels to their mid-infrared extinction. The Galactic distribution of IRDCs shows an enhancement toward the Galaxy's most massive and active star-forming structure, the so-called 5 kpc ring. IRDCs have typical sizes of similar to 5 pc, peak column densities of similar to 10(22) cm(-2), LTE masses of similar to 5; 10(3) M-circle dot, and volume-averaged H-2 densities of similar to 2 x 10(3) cm(-3). The similarity of these properties to those of molecular clumps associated with active star formation suggests that IRDCs represent the densest clumps within giant molecular clouds where clusters may eventually form.