The Shadowy World of Cold Interstellar Clouds

Conventional detection methods miss a significant fraction of the interstellar gas in the disk of the Milky Way. New high-resolution 21cm radio synthesis maps from the Canadian Galactic Plane Survey show a striking population of cold hydrogen clouds as intricate absorption ``shadows'' against warmer background emission. These clouds, which are not visible in previous, lower-resolution surveys, contain substantial quantities of cold atomic gas missed by 21cm emission studies and may also include molecular gas untraced by the standard CO proxy. I will discuss an ongoing investigation at the University of Calgary into the nature of these elusive objects and their possible role in the evolution of matter in spiral arms, where molecules coalesce and new stars form.