In Part I we saw that galaxy disks can be thought of as basically comprising two components, Population I and Population II. One component is dominated by cold gas, in atomic or molecular form, but contains significant amounts of stars recently born in the interstellar medium. This component is in a thin layer (at least within the bright optical disk) and is characterized by very low velocity dispersion (often below 10 km s−1) with respect to the circular motions associated with the differential rotation. The other component, Population II, is dominated by relatively old stars, in a thicker layer, and is characterized by higher velocity dispersions; that is, it is warmer from the dynamical point of view. Such separation is only a simplifying tool for dynamical investigations, whereas, in reality, continuous changes of dynamical properties are associated with the many components of a galaxy disk. (The properties of the extraplanar gas were briefly summarized at the end of Chapter 14.)

The gas-dominated component is characterized by small epicycles. Thus, from the dynamical point of view, it is naturally responsible for small-scale features in the disk, which are probably rapidly evolving. (The large-scale spiral structure that will be described in the following chapters then must draw its main support from the stars.) Furthermore, being cold, the gaseous disk is expected to provide an important contribution to the Jeans instability of the disk and, in this sense, to be an important source of excitation also for large-scale spiral modes.