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S. J. Gibson, A. R. Taylor (U. of Calgary)
HI Self-Absorption (HISA) against warmer background HI emission reveals cold atomic gas along sightlines of sufficient column density. New 1' resolution 21cm maps from the ongoing Canadian Galactic Plane Survey have uncovered a large population of HISA features beyond the Solar circle. The majority of these objects are found in the velocity range predicted for shocked gas encountering the spiral density wave of the nearby Perseus Arm. Their complex knotted and filamentary forms exhibit morphological aspects of both HI emission wisps and molecular cloud clumps. Many HISA features also appear associated with CO emission, though the large range of CO brightness to HISA opacity precludes a simple relationship between the two. One possible explanation for this is that we may be seeing the actual phase transition from atomic to molecular gas brought about by the shock environment. We present examples of this Perseus HISA population and discuss constraints on their geometries, physical properties, and circumstances.
This work is supported by a grant from the Natural Sciences and Engineering Research Council of Canada.
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The author(s) of this abstract have provided an email address for comments about the abstract: gibson@ras.ucalgary.ca