Large-Scale Shock Formation in Spiral Galaxies and its Implications on Star Formation

The dynamical problems of shock formation and star formation in normal spiral galaxies are investi- gated. The motion considered is that of the continuum of turbulent gas composing the gaseous disk moving in a gravitational field consisting of a two-armed spiral field superposed on the Schmidt model for the Milky Way system. The possible existence of a stationary two~armed spiral shock pattern is demonstrated. It is suggested that galactic shock waves may very well form the triggering mechanism for the gravitational collapse of gas clouds, leading to star formation. If an upper bound of 30 million years is assumed for the process of formation and evolution of relatively massive stars initiated at the shock, it is shown that the possible locations of the regions of luminous, newly born stars and the H ii regions lie on the inner side of each observable gaseous spiral arm of H i, extending from the sharp H i peak at the shock on the inner edge to approximately the center of the arm, in general agreement with observations. L INTRODUCTIO