Parkes observations for project P974 semester 2018OCTS

A significant fraction of the neutral ISM is now known to be "dark" -- that is, invisible to the standard spectral line tracers of HI and CO. This material is thought to be dominated diffuse molecular gas in which hydrogen is in the form of H2 but densities are insufficient to form and shield CO molecules. Open questions abound: How much dark ISM exists in the Milky Way? How does it participate in the gas-to-stars cycle? How does environment affect the dark gas fraction? What are its characteristic physical and chemical conditions? While major advances have been made using gamma-ray and infrared excess maps to infer the dark gas distribution in the local ISM, spectral line tracers are needed to probe the full volume of the Galactic Disk and to access information on the physical and chemical state of the dark phase. This project will use the new spectral line capabilities of the UWL to simultaneously observe 11 hyperfine lines of the dark gas tracers CH and OH in a pristine, high-altitude cloud in order to benchmark these tracers against IR and gamma-ray based maps, model for the first time the temperature, density and IR field of the dark phase, and definitively establish the regimes most effectively probed by both the OH and CH lines -- the latter of which in particular are not well known. In addition to these science goals, the observations will provide an important demonstration of the UWL's spectral line mode.