A 12CO, 13CO, and C18O Survey of Infrared Dark Clouds
  InfraRed Dark Clouds (IRDCs) are extinction features against the Galactic infrared background, mainly in mid-infrared band. Recently they were proposed to be potential sites of massive star formation. In this work we have made 12CO, 13CO, and C18O (J=1-0) survey of 61 IRDCs, 52 of which are in the first Galactic quadrant, selected from a catalog given by Simon et al. (2006), while the others are in the outer Galaxy, selected by visually inspecting the MSX images. Detection rates in the three CO lines are 90%, 71%, and 62% respectively. The distribution of IRDCs in the first Galactic quadrant is consistent with the 5 kpc molecular ring picture, while slight trace of spiral pattern can also be noticed, which needs to be further examined. The IRDCs have typical excitation temperature of 10 K and typical column density of several 10^{22} cm^{-2}. Their typical physical size is estimated to be several pcs using angular sizes from the Simon catalog. Typical volume density and typical LTE mass are ~5000 cm^{-3} and ~5000 M_{sun} respectively. The IRDCs are in or near virial equilibrium. The properties of IRDCs are similar to those of star forming molecular clumps, and they seem to be intermediate between giant molecular clouds and Bok globules, thus they may represent early stages of massive star formation. 
Fujun Du and Ji Yang