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 the structure and dynamics of the Milky Way Galaxy and of the astrophysics of the Galactic interstellar medium (ISM). Emission from Galactic atomic hydrogen is seen in all directions and the structures revealed reflect the processes relevant to the evolution of the Galaxy and phase transitions of the ISM. As in other areas of astronomy, our knowledge of the neutral ISM from HI studies has progressed in lock step with radio telescope technology. Over the last several years interferometric surveys have provided arcminute-scale HI images that allow the atomic hydrogen component of the ISM to be viewed in the context of the other major ISM components and reveal dynamic structures in HI related to the interaction of ISM with the stellar component of the Galaxy. The International Galactic Plane Survey, a collaboration 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.