Radial Outflow Explains the Rotation Curves of Disk Galaxies

Abstract

The circular velocities of the inner region of disk galaxies are predicted by standard physics but velocities beyond the stellar disks are not consistent with Newtonian physics if the material there is in stable circular orbits. However, this material is not gravitationally bound and so does not trace the gravitational field in the way that is usually assumed. The gravitational attraction near the edge of a flattened mass distribution is significantly greater than that of an equal mass in a spherical distribution. The size of the effect depends on the specifics of the mass distribution but is greater than a factor of two for reasonable models. In fact, the circular velocity can exceed the escape velocity so that these galaxies are gravitationally unstable in way not previously considered and disk material is lost due to thermal escape, bars or other disturbances. The nearly constant velocity observed in the outer disk region has been interpreted to mean that the dynamical mass of galaxies is much larger than the observed mass. In fact, there is no great discrepancy and no need to invoke dark matter at these scales. The gravitational field of a disk galaxy is determined at all radii by the observed mass. In the region of the stellar disk, stars and gas move in nearly circular orbits at velocities consistent with the gravitational field. In the outer regions the gravitational force drops rapidly so that stars and gas move outward almost unaffected by the attraction of the host galaxy.