Neutral Hydrogen Self-Absorption Features in the Canadian Galactic Plane Survey

S. J. Gibson, A. R. Taylor (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada)

P. E. Dewdney, L. A. Higgs (Dominion Radio Astrophysical Observatory, Box 248, Penticton, BC, V2A 6K3, Canada)

W. H. McCutcheon (Department of Physics and Astronomy, University of British Columbia, 5224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada)

H. J. Wendker (Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany)

Proceedings Article Abstract

We have begun a systematic study of cold HI within the 73^o x 9^o 21cm coverage of the Canadian Galactic Plane Survey (CGPS; Taylor et al. 1998). The ~ 1' beam of the CGPS reveals considerable substructure within detected clouds. Our investigation has uncovered a wealth of remarkable features in both the Local and Perseus Arms. Some of these have clear counterparts in ¹²CO and far-IR dust emission, while others are only visible in 21cm absorption, often at extremely low contrast. All display a mix of clumpy and filamentary morphologies. We present examples below and discuss their physical properties.

Conference Program Abstract

Neutral Hydrogen 21 cm 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 (HISA) against background HI emission, though where detected, it has often shown a significant correlation with dense clouds of molecular gas and dust. While some evidence suggests HISA may be a more sensitive tracer of cold gas than CO emission, a detailed examination of the relations between these and other ISM constituents is only possible if several conditions are met. First, a multiwavelength dataset showing all relevant ISM phases at a common angular resolution is essential. Second, this resolution must allow HI spectra to be taken close to HISA feature edges, since measuring the properties of the absorbing material depends upon accurate knowledge of the background emission. Finally, a large area should be used to ensure an unbiased sample, rather than, e.g., a preselected set of known molecular clouds. All of these criteria are met by the Canadian Galactic Plane Survey, which is mapping the major phases of the ISM at arcminute resolution over hundreds of square degrees.

Although our investigation of HISA in the CGPS dataset has just begun, we have already found a wealth of remarkable features in the Perseus Arm and in local gas which will require careful study. Some of these have clear counterparts in ¹²CO and far-IR dust emission, while others are only visible in 21 cm absorption, often at extremely low contrast. All display a complex, multiscale structure of interconnected filaments, knots, and sheets. We present examples of various features along with the fruits of our current efforts to analyze their physical nature.