The Dark Side of the Galaxy: A Skiagraphic Panorama of Cold H I

A skiagram is an image made up of shadows or outlines. Much of the cold atomic hydrogen in the interstellar medium can only be studied skiagraphically as I self-absorption (HISA) shadows against background I 21cm emission. The Canadian Galactic Plane Survey (CGPS) has captured a multitude of HISA features, many of which are invisible in prior surveys at lower resolution. We have developed automated methods of detecting and analyzing HISA which allow us to study the cloud population systematically. We present the first panoramic view of HISA over the entire 73 x 9 degree area of the CGPS. Our images reveal a complex froth of cold I clouds from 74 to 147 degrees Galactic longitude at both Local and Perseus arm velocities. At low levels, HISA is ubiquitous. Stronger features have more concentrated and organized distributions, some of which may be related to the Perseus arm spiral shock. Traditionally, HISA has been identified with the small fraction of atomic gas in molecular clouds. However, the CGPS HISA and CO line strengths do not correlate, suggesting either that cold I and H₂ are mostly separate, or that they are not completely traced by available methods.

This work is supported by a grant from the Natural Sciences and Engineering Research Council of Canada.