Neutral Hydrogen 21cm line emission is widely used to chart the warm and cool diffuse components of the interstellar medium. Less studied is the colder constituent revealed by HI self-absorption against background HI emission, though where detected, it has often shown a significant correlation with dense clouds of molecular gas and dust. While there is some evidence that HI self-absorption may be a more sensitive tracer of cold gas than CO emission, detailed examination of the exact relationships between these and other ISM constituents requires both high angular resolution and an unbiased sample.
The ongoing Canadian Galactic Plane Survey (CGPS) meets these criteria to an unprecedented degree, permitting the comparison of HI, CO, and dust features at arcminute scales over hundreds of square of degrees of the Milky Way. We will present the first results of an investigation of neutral Hydrogen self-absorption features in the CGPS dataset.