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S. J. Gibson (NAIC), J. M. Stil, A. R. Taylor (U. of Calgary), C. M. Brunt (U. of Exeter), D. W. Kavars (U. of Minnesota), J. M. Dickey (U. of Tasmania)
Cold atomic gas contains about 1/3 of the mass of the interstellar medium (ISM) in clouds occupying only a few percent of the ISM volume. These clouds can be imaged as 21cm-line HI self-absorption (HISA) shadows against warmer background HI emission at the same radial velocity, allowing a simultaneous probe of the temperature and velocity structure of HI across the Galactic disk. A recent census of HISA in the Canadian Galactic Plane Survey (CGPS) found a wealth of intricate features over 71 degrees of longitude. The weak CGPS HISA is widely scattered and may arise from turbulent fluctuations of the ambient ISM, while stronger absorption is concentrated into discrete complexes. The CGPS HISA census has now been extended with CGPS-II and VLA Galactic Plane Survey (VGPS) data to cover 157 degrees of longitude in the first two Galactic quadrants. We present and discuss the findings of this composite HISA survey in the context of Galactic structure and spiral density waves. We also compare the HISA distribution with detected CO emission and discuss its relevance to the hypothesized atomic-to-molecular phase change in spiral arms prior to star formation.
This work has been supported by a grant from the Natural Sciences and Engineering Research Council of Canada to the University of Calgary and by the National Astronomy and Ionosphere Center operated by Cornell University under Cooperative Agreement with the National Science Foundation.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.