Neutral hydrogen in the Milky Way

Abstract

Since its prediction and first detection just over 50 years ago,the 21-cm line of neutral atomic hydrogen has been a key probe of thestructure and dynamics of the Milky Way Galaxy and of the astrophysics ofthe Galactic interstellar medium (ISM). Emission from Galactic atomichydrogen is seen in all directions and the structures revealed reflect theprocesses relevant to the evolution of the Galaxy and phase transitions ofthe ISM. As in other areas of astronomy, our knowledge of the neutral ISMfrom HI studies has progressed in lock step with radio telescope technology.Over the last several years interferometric surveys have providedarcminute-scale HI images that allow the atomic hydrogen component of theISM to be viewed in the context of the other major ISM components and revealdynamic structures in HI related to the interaction of ISM with the stellarcomponent of the Galaxy. The International Galactic Plane Survey, acollaboration to combined interferometric surveys from Canada, the US and Australia, is imaging over 90% of the stellar disk of the Milky Way in HI at arcminute resolution. When it is built in the post 2010 era, the prodigious survey speed and angular resolution of the Square Kilometre Array will affect another seminal advance in Galactic HI studies.