HI Self-Absorption (HISA) against warmer background HI emission reveals cold atomic gas along sightlines of sufficient column density. New arcminute-resolution 21cm maps from the Canadian Galactic Plane Survey (CGPS) have uncovered a large population of HISA features beyond the Solar circle. The sheer number of these and the traditional difficulty of extracting physical properties from such elusive objects pose considerable challenges for analysis.
However the rewards for such a study are considerable. Little is known about HISA, apart from its frequent association with molecular gas. The CGPS data bear this relationship out in part, but display a wide range of CO brightness for a given HISA opacity, with many clouds having no detected CO emission. Furthermore, the complex CGPS HISA knots and filaments appear morphologically intermediate between HI emission wisps and cold molecular cloud clumps. Finally, many of the darker HISA features are found at velocities predicted for shocked gas encountering the spiral density wave of the Perseus Spiral Arm. It is tantalizing to consider whether much of the CGPS HISA may thus represent the ISM phase transition from atomic to molecular gas brought about by compression in the shock environment.