21-Cm Absorption Studies of Galactic Radio Sources

Cm Absorption Studies of Galactic Radio Sources. V. RADHAKRIsHNAN AND J. G. BoLTON, California Institute of Technology. A study is being made at the Cal Tech Radio Observatory of the 21-cm absorption features in the continuum radiation from discrete galactic sources. Earlier investigations [Hagen, J. P., Lilley, A. E., and McClain, E. F., 1957, IA U Symposium No. 4 on Radio Astronomy (Cambridge, England), p. 80; Davies, R. D., and Williams, D. R. W., ibid., p. 71; Muller, C. A., 1958, Proc. URSI-IA U, Radio Astron. Symp. (Paris), Paper 69] have clearly demonstrated the efficacy of absorption measurements in determining the fine structure of the distribution of neutral hydrogen in the galaxy. In addition to the higher resolution obtainable with the absorption method when the angular size of the source is smaller than the antenna beam- width, it is also possible to detect the presence of cold dense concentrations of hydrogen that do not contribute substantially to emission profiles. A frequency-switched radiometer with a 7 kc/sec bandwidth was used with a 90-foot equatorially mounted paraboloid. The sources observed were Cas A, Taurus A, the Orion Nebula, M17, W51, W75, and Vela X. The first three are among those investigated by Muller (1958) and a comparison of our results with his shows excellent agreement in the radial velocities of the main features. The broad feature at +3.5 km/sec in Muller's spectrum for Taurus A, which he suspected of being a blend, shows splitting in our profiles. There are minor discrepancies in the computed optical depths for some of the features, but this may not be significant as the accurafe intercalibration of the line and continuum temperature scales is very difficult, and hence subject to errors. Of the four new sources, M17, W51, and W75 show substantial absorption. Vela X also appears to have absorption features in its spectrum, but the large angular size ( 30) and low antenna temperature of the source make it difficult to separate with certainty absorption features from the variations in the background 21-cm emission. With improved receiver sensitivity, further observations on the hundred or so galactic sources known will increase considerably our knowledge of their distances and of the fine-structure parameters of the interstellar medium. This work was supported by the United States Office of Naval Research under contract.