Hydrogen Shadows in the Galactic Sky

Hydrogen is the most abundant element in the universe, and in its neutral atomic phase (called ``H I''), it is readily traced by the hyperfine spin-flip transition at a wavelength of 21cm with radio telescopes. 21cm line emission is excellent for charting the mixture of warm and cold atomic gas in the Galaxy, but mapping these phases separately in H I emission is difficult. Fortunately, cold H I can appear 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, high-resolution 21cm maps like those of the International Galactic Plane Survey. IGPS images 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, which appears less than expected by theory. This work is supported by a grant from the Natural Sciences and Engineering Research Council of Canada.