Neutral hydrogen and galactic structure

Observations of the 21-cm line of neutral hydrogen give the best method for mapping the spiral structure of our galaxy, but there are still difficulties of interpretation. These arise from the need to infer the distance of any hydrogen concentration from its radial velocity, under circumstances where the velocity field and the structural pattern must both be derived from the same observations. The role of the density-wave theory of spiral structure is discussed. In addition to the mapping problem, this paper also considers the general properties of the galactic disk, the galactic center region, normal and high-velocity hydrogen clouds, and some aspects of hydrogen in other galaxies.