Cold Hydrogen Clouds in the Milky Way: An Evolutionary Missing Link?

The Dominion Radio Astrophysical Observatory in Canada is nearing completion of an 8-year observing project to image the radio emission from northern Galactic Plane as part of the Canadian Galactic Plane Survey (CGPS). This survey forms part of an international collaboration to create a database of arcminute-scale resolution images of the outer Galaxy in all major components of the interstellar medium. The high angular resolution CGPS images of atomic hydrogen emission reveal a wide-spread, cold atomic hydrogen component of the interstellar medium in the form of dark self-absorption cloud complexes. These clouds resemble molecular cloud complexes in structure and trace the spiral arm structure of the Galaxy. While they occupy an (l,b,v) space similar to that of CO clouds, there is no strict correlation between cold atomic hydrogen clouds and CO emitting gas. Nevertheless, continuum absorption studies show that some of the cold atomic hydrogen clouds have temperatures as low as 15 - 20 K --- similar to dense molecular clouds. We examine the hypothesis that cold atomic hydrogen revealed by these observations represents a phase in the evolution of the ISM marking the transformation of warm diffuse atomic hydrogen gas to molecular clouds, perhaps triggered by compression from the passage of spiral arm density waves.