Density-wave theory of galactic spirals

Two questionable assumptions made two decades ago have persisted: that because there is no dust between the spiral arms of the Andromeda Galaxy there is no gas, and that the elongated features of cool gas (H i) seen in our Galaxy are parts of twin spiral arms. These have led to the wide acceptance of the belief that spiral arms are gas streamers, to the general abandonment of magnetic theories, and to the wide acceptance of the Lin-Shu density-wave theory. In the present paper these assumptions and their extensions have been tested in our Galaxy and some others. We confirm that the spiral tracers which delineate spiral forms occur in regions of low H I density yet are absent from nearby regions of high density. The density-wave theory must now include an X-factor of star formation, perhaps a shock wave as already suggested; the theory has lost its appealing simplicity and must explain how multiple shocks form and continue to exist and how they account for the creation of most or all stars in spiral systems. There is little evidence, in our own Galaxy or in external galaxies, of gaseous spiral arms. On the contrary, spiral tracers cut across equal-density contours of H I, showing that there is roughly as much gas outside as inside the arms. These and other observations are shown to provide strong evidence against the density-wave theory. This apparent lack of concentration of gas in spiral arms reopens the possibility of a hydromagnetic origin, but an entirely new model is required based essentially on motions perpendicular to the galactic plane. Subject heading: galactic structure