An ejection theory of spiral structure

The density wave theory of spiral structure is apparently a viable description of the physical state in our Galaxy between 6 < R < 16 kpc (adopting the standard R0 = 10 kpc). In this region the velocity variations due to the d.w. exceed those due to radial expansion, Δv(d.w.) > Δv(ej.). There are, however, two basic problems the d.w. theory has to cope with: the importance of the d.w. in the presence of the very hot phase of the interstellar gas, and the excitation and maintenance of the d.w. in the presence of strong dissipative forces. These problems are discussed. (Auth.)