The Cool Phase of the Interstellar Medium: Atomic Gas

Abstract

High-resolution H I observations of a sample of nearby galaxies are reviewed to elucidate the composition, structure, and energy balance of the ISM. What is known about the ISM of the Galaxy is summarized, and an overview of current theoretical models is presented. It is shown that it has become feasible to extend studies of the local ISM to nearby spirals, thus avoiding the well-known problems associated with Galactic studies such as distance ambiguities and line-of-sight confusion. Individual galaxies are discussed; both the global properties of the ISM and its detailed structure are addressed, with the emphasis placed on the properties of H I shells and supershells.