Density waves are thought to be at the basis of the explanation of spiral structures in galaxies, especially of the so-called grand-design structure whose extent is on the global scale. A physical discussion of the problem of spiral structure in galaxies will be given in Chapter 18. In this and Chapters 16 and 17 we focus instead on some relevant dynamical mechanisms. As anticipated earlier (Chapter 9), density waves are one of the two main classes of natural perturbations that are expected in a disk (the other class, bending waves, will be addressed in Chapter 19). They leave the equatorial symmetry of the galaxy disk unchanged and are associated with density enhancements and rarefactions that usually break the axisymmetry of the basic state. A special class of density waves (m=0) leaves the disk axisymmetric.

The concept of density waves is a general one because the phenomenon simply reflects the oscillatory character of the disk that has been described in terms of single-star orbits. In practice, the detailed properties of density waves depend on the model considered. In early investigations, the galaxy disk was thought of mainly as a stellar disk, and thus the studies focused on the equations of stellar dynamics.