The Vertical Structure of Galactic Disks

Galactic disks have a nearly universal vertical structure that this paper suggests is substantially a result of internal evolution. Spiral density waves, the collective motions of disk stars and gas, are the dominant large-scale potential perturbations of the disk and are the effective source of heating for random motions in the plane of the galaxy. Spirals couple weakly to the relatively short-period vertical oscillations of stars. However, massive molecular clouds act as scattering centers that redistribute the energy between vertical and horizontal motions. For a fairly wide range of cloud parameters, the scattering is sufficient to produce the observed local velocity ellipsoid. The shape of the velocity ellipsoid is predicted to depend on the ratio of the velocity dispersion to the escape velocity from the molecular clouds, becoming rounder as the stellar velocity dispersion increases.