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R. Simon, J.M. Jackson, T.M. Bania, D.P. Clemens (IAR, Boston University), M.H. Heyer (FCRAO, UMass Amherst), M.P. Egan, S.D. Price (Air Force Research Laboratory, Hanscom AFB)
Recent high resolution, high sensitivity mid-infrared surveys of the Galactic plane with the Midcourse Space Experiment (MSX) and Infrared Space Observatory satellites have revealed a large population of clouds seen in silhouette against the Galactic background. The origin and nature of these IR dark clouds are still poorly understood. Most of these objects are concentrated in the first and fourth Galactic quadrant and at low Galactic latitudes where the infrared background is highest. Recent studies of some of these clouds show that they are dense, cold, and have very high column densities. Combined with the absence of embedded mid- to far-infrared sources, this suggests that the clouds are cores before or in the earliest stages of star-formation (protostellar class 0 or earlier).
Preliminary results for some of the IR dark clouds identified from the MSX data set in the region sampled by the GRS are: (1) For all dark clouds we studied, mid-infrared absorption morphologically matches strong molecular line emission in velocity channels of 13CO. (2) Requiring a significant background to absorb against, IR dark clouds must be located at the near kinematic distance. The 13CO velocities then can be used to solve the kinematic distance ambiguity and to assign unique distances to the IR dark clouds. (3) Our observations confirm high densities, high column densities, and relatively high masses, typically a few thousand solar masses. 12CO spectra towards the IR dark clouds are saturated and show signs of self-absorption. Line ratios of C13CO/C18O even suggest moderate to high opticities in the 13CO line towards the cores. (4) The dark clouds sampled by the GRS are condensations within Giant Molecular Clouds and are not isolated objects.
The Galactic Ring Survey acknowledges support by the NSF via grants AST--9800334 and AST--0098562.
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