The hydrogen line in absorption

Abstract

21-cm Absorption spectra have been obtained for twelve radio sources, using conventional techniques. Six of these sources were also studied with a hydrogen-line interferometer. Differences in the profiles obtained by the two profiles give information on the structure and temparature of the H₁ clouds seen in absorption. Estimation of the angular sizes of four clouds based on these diffferences lead to an average diameter of 13 parsecs. In some cases it has been possible to find an upper limit to the temparature of the clouds somewhat lower than the generally assumed 125°K. A temparature of 60° K is suggested, hgiving densities of 20 atoms/cc and masses of 10³ M ⊙ for the absorbing clouds.