Application of the Densiity-Wave Theory to the Spiral Structure of the Milky way System. I. Systematic Motion of Neutral Hydrogen

When a density-wave pattern is present in a disk galaxy, the motion of the interstellar gas will system- atically deviate from the mean circular motion. This systematic deviation is studied in this paper in considerable detail from both the theoretical and the observational points of view. The tangential com- ponent of this systematic motion furnishes a completely new method for identifying the spiral structure of the Milky Way system; this method is applied to the determination of the locations of the Sagittarius arm and the Norma-Scutum arm. A fairly detailed discussion is given to compare observational data with a theoretical spiral pattern for the Milky Way system based on the density-wave theory and the 1965 Schmidt model. The spiral pattern is estimated to travel at 13 5 km sec' kpc' for the Milky Way system; this estimation includes the effects of gas and finite thickness. This corresponds to that of an infinitesimally thin stellar disk traveling at 11.5 km sec' kpc'