Hydrogen Shadows in the Milky Way

Atomic hydrogen (HI) with T < 100 K represents a considerable fraction of the interstellar matter in our Galaxy. Unfortunately, the spatial distribution of this gas is hard to study directly: in 21cm emission, cold and warm HI are difficult to separate, while 21cm absorption toward continuum sources is limited to small solid angles.

An alternative approach is to observe HI self-absorption (HISA) against warmer and more extensive background HI emission. We present results of the first wide-field synthesis imaging study of HISA being carried out as part of the Canadian Galactic Plane Survey (CGPS), which is mapping Galactic HI over 650 square degrees between longitudes of 74 and 147 at arcminute scales. To date, we have identified a large number of dark and intricate HISA features which display a range of shapes and properties. Surprisingly, the CGPS HISA clouds demonstrate considerable independence from 12CO emission. This suggests either the traditional view of HISA arising from small amounts of HI in molecular clouds is wrong, or the common assumption of CO tracing H_2 requires reexamination.