S. J. Gibson (Arecibo Obs.), A. R. Taylor (U. of Calgary), J. M. Stil (U. of Calgary), C. M. Brunt (U. of Exeter), D. W. Kavars (U. of Minnesota), & J. M. Dickey (U. of Tasmania)
Talk presented to the International Astronomical Union at its 26th General Assembly in Symposium 237, Triggered Star Formation in a Turbulent ISM, in Prague, Czech Republic, on Friday, August 18, 2006
Cold atomic gas contains a large fraction of the mass of the interstellar medium (ISM) in clouds occupying only a few percent of the ISM volume. These clouds can be mapped in 21cm-line H I self-absorption (HISA) against warmer background 21cm-line emission at the same radial velocity, allowing a simultaneous probe of the temperature and velocity structure of H I in the Galactic disk. HISA maps made from the Canadian and VLA Galactic Plane Surveys in the first and second quadrants show a wealth of intricate cold H I structure from sub-parsec to kiloparsec scales. Weak HISA is very widespread, implying a similarly widespread froth of cold atomic gas in the ISM that may be made visible by turbulent eddies. Stronger absorption is concentrated into discrete complexes, many of which appear to trace the velocity reversals of spiral density waves, where the gas may be cooling rapidly to form molecular clouds downstream of the spiral shock. Although strong HISA implies very cold H I, the strong HISA in the outer Galaxy corresponds poorly with CO emission, perhaps because the gas is evolving in the spiral arms prior to star formation.