Molecular line observations of infrared dark clouds: Seeking the precursors to intermediate and massive star formation

We have identified 41 infrared dark clouds from the 8 mu m maps of the Midcourse Space Experiment (MSX), selected to be found within 1 deg(2) areas centered on known ultracompact H II regions. We have mapped these infrared dark clouds in N2H+ 1 -> 0, CS 2 -> 1, and (CO)-O-18 1 -> 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 N2H+ column densities of approximate to 2500M(circle dot), a value comparable to that found in previous studies of high-mass star-forming cores using other mass tracers. The line widths of these clouds are typically similar to 2.0-2.9 km s(-1). Based on the fact that they are dark at 8 mu m, compact, and massive, and have large velocity dispersions, we suggest that these clouds may be the precursor sites of intermediate- and high-mass star formation.