The Speed and Origin of the H<scp>i</scp>Spiral Pattern in M81

Abstract

A study of the pattern speeds of disk galaxies is under way using integrated forms of the continuity equation that relate the pattern speed to the velocity and surface density of the pattern tracer. Integration removes the unobservable sky-plane velocities and allows averaging to improve accuracy and spatial resolution. This approach assumes the disk is at and the pattern rigid but makes no assumption about the dynamics. The method relies on the nonaxisymmetric nature of the velocity eld and brightness distribution of a spiral. The speeds of nonrigid patterns may be approximated by the same equations in regions where the shear is small.