Aperture synthesis study of neutral hydrogen in the galaxy M101: II. Discussion.

The observations described in paper I (Rogstad and Shostak, 1971) are discussed with the following conclusions: There is a close correspondence between features in the surface density of both light and H I in the galaxy M 101. The H I velocity field contains streaming motions which are in agreement with the predictions of the first-order density wave theory of spiral structure. There is evidence for expansion motions of at least 40 km -1 in the central region of the galaxy. Key word : galaxies - hydrogen