Galactic Velocity Models and the Interpretation of 21-cm Surveys

Abstract

A detailed comparison has been made between the Leiden and Sydney 21-cm surveys for the two halves of the Galaxy. Use of the Leiden velocity model leads to an implausible spiral structure diagram on the southern side, and alternative models have been considered, with a basic assumption that the structure and motions of the Galaxy are symmetrical on a large scale. One way to reconcile the results on the two sides of the Galaxy is to introduce an outward velocity component of 7 km/sec for the Sun and the “local centre of rest”. Other evidence suggests that the gas at the Sun's distance from the centre may also be moving outwards with a similar velocity. A spiral diagram, which takes both rotation and expansion into account, has a more reasonable appearance than is given by earlier interpretations. Some of the other implications of such a velocity picture are discussed, and an observational programme is set out which should provide a critical test of its plausibility.