Anomalous motions of H I clouds

Abstract

In a comparison of H I absorption line velocities and radio recombination line velocities for 38 H II regions of known distance, it is found that the H I absorption extends beyond the recombination line velocity (i.e. corresponds to a greater distance) in the large majority of cases. This is shown to be the result of the overlapping of velocities due to the chaotic motions of two populations of H I clouds. The absorption generally extends all the way to the source and can be used to estimate the velocity corresponding to the distance of the source itself. But it can be misleading if used to resolve the kinematic distance ambiguity when the recombination line velocity is within about 10-20 km/sec of the tangential velocity.