A new view of cold HI clouds in the milky way

We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21 cm synthesis maps, taken from the Canadian Galactic Plane Survey, s how a numerous and diverse population of H I self-absorption (HISA) feature s in gas outside the solar circle. These objects vary in size, shape, and c ontrast against the background H I. All display a high level of angular and velocity structure, and most would appear significantly diluted, if not in visible, in lower resolution H I surveys. A number of Perseus arm features remain unresolved by the 1' beam of our survey, with apparent diameters les s than 0.6 pc at 2 kpc distance. The majority of HISA features we detect ha ve no obvious (CO)-C-12 emission counterparts. This suggests that either HI SA is not found predominantly in molecular clouds, as has often been presum ed in the past, or that CO is not a good tracer of H-2. Some HISA lacking C O shows far-infrared dust emission, though whether this arises from shielde d molecular gas or from diffuse atomic clouds is not clear. Constraining th e gas properties of HISA remains a difficult problem, but we introduce a ne w method that aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships and should prove pow erful if the input variables are sufficiently well known. We explore the cu rrent allowed parameter ranges for three sample features of very different appearance. We find spin temperatures less than or similar to 50 K and dens ities greater than or similar to 10(2) cm(-3).