We have identified 41 infrared dark clouds from the 8 \mu m maps of the Midcourse Space Experiment ( MSX ) , selected to be found within one square degree areas centered on known ultracompact HII regions .
We have mapped these infrared dark clouds in N _ { 2 } H ^ { + } 1 \rightarrow 0 , CS 2 \rightarrow 1 and C ^ { 18 } O 1 \rightarrow 0 emission using the Five College Radio Astronomy Observatory .
The maps of the different species often show striking differences in morphologies , indicating differences in evolutionary state and/or the presence of undetected , deeply embedded protostars .
We derive an average mass for these clouds using N _ { 2 } H ^ { + } column densities of \approx 2500 M _ { \odot } , a value comparable to that found in previous studies of high mass star forming cores using other mass tracers .
The linewidths of these clouds are typically \sim 2.0 - 2.9 km s ^ { -1 } .
Based on the fact that they are dark at 8 \mu m , compact , massive , and have large velocity dispersions , we suggest that these clouds may be the precursor sites of intermediate and high mass star formation .