THE KINEMATICS OF FACE-ON DISK GALAXIES, AND THE NATURE OF THE GALACTIC H-I LAYER

The line-of-sight velocity distribution of a face-on disc galaxy provides an extremely useful diagnostic of its three-dimensional structure. Credible models for the density distribution perpendicular to the plane of the galaxy result in significantly varying forms for the observable velocity distribution. If the gravitational potential is externally specified or if the system is self-gravitating, then the density distribution can be inferred directly from the line-of-sight velocity distribution. Application of these methods to high-latitude 21-cm observations of the Milky Way shows that the atomic hydrogen layer in our own Galaxy cannot be supported purely by its own turbulent motions.