Dark Hydrogen in the Galactic Plane

S. J. Gibson, A. R. Taylor, & J. M. Stil (University of Calgary);
L. A. Higgs, P. E. Dewdney, & C. M. Brunt (Herzberg Institute of Astrophysics)

H I 21cm line emission is excellent for charting the mixture of warm and cold neutral atomic gas in the Galaxy, but mapping these phases separately in emission is difficult. Fortunately, cold H I appears readily as H I self-absorption (HISA) shadows against warmer background H I. Though the HISA phenomenon was first observed in the 1950s, a detailed, systematic study of HISA clouds has only recently become possible with the advent of large-area synthesis surveys like the Canadian and VLA Galactic Plane Surveys. The CGPS and VGPS images of the first and second quadrants reveal a complex froth of HISA features, many of which are undetectable at lower angular resolution. Faint HISA is visible almost everywhere that backgrounds are bright enough to produce self-absorption. Stronger HISA appears concentrated in discrete cloud complexes associated with spiral arm structure. The properties and distribution of the HISA cloud population will be discussed, including the degree of correlation between HISA and molecular gas in the inner and outer Galaxy. While some HISA is found with CO emission as expected by theory, a considerable amount of HISA is found without CO.

This work is supported by a grant from the Natural Sciences and Engineering Research Council of Canada.