A Distant HII Region In The Galaxy

Abstract

It has long been realized that 21 cm absorption studies can provide information about the distance of unidentified radio sources. When the radiation from a continuum source is absorbed by neutral hydrogen clouds in the foreground, an observation of the radial velocity range over which the absorption occurs indicates the position of the source in relation to the various hydrogen features along the line of sight. Whenever absorption effects can be seen, we can immediately tell whether the source concerned is galactic or extragalactic; if it is galactic, we can then place limits on its distance.