Observing Cold Clouds in the Milky Way
Steven J. Gibson
Western Kentucky University
Cold atomic gas, observable in the neutral hydrogen (HI) 21cm line, plays a crucial role in the evolution of interstellar matter in galaxies. The gas in galactic disks (including our own) occurs in a wide range of temperatures and densities, most of which are unsuitable for star formation. Somehow, the warmer, diffuse atomic clouds are collected into colder, denser molecular clouds that can collapse under their own gravity. Compared to other phase transitions like photodissociation or recombination, molecular condensation is not easy to observe. However, 21cm HI self-absorption (HISA) appears to trace precisely this stage of cloud evolution by revealing the coldest atomic gas as 3-dimensional shadows against warmer background HI emission. The HISA phenomenon has been known for decades, but it has been brought into sharp focus in recent years by new surveys with interferometers and large single-dish telescopes. These offer the exciting dual prospect of detailed mapping of the cold gas across the Galactic disk and direct comparison with numerical simulations of the interstellar medium. I will review the findings of these recent efforts and the promise of new surveys with future instrumentation.
Date: Tuesday, 17 May 2011 Time: 11:00 Where: Université Laval Pavillon Alexandre-Vachon, local 1661 Contact: Serge Pinault