The Astrophysical Journal Supplement Series · 1971 · 68 citations · 0 references
The interferometer at the Owens Valley Radio Observatory, together with a multichannel receiving system with filters of bandwidth 4 or 6 kHz, has been used to observe ninety-seven sources for absorption at 21-cm wavelength. All but four of the sources are believed to be extragalactic. Measurable absorption was detected in sixty-four cases, and for these the profiles are presented. Optical depths greater than 0.5 are found only within 20 of the galactic plane, except in the region of the Cetus Arc where values up to 1.4 occur near b = - 40 . The highest galactic latitude at which absorption was detected is 75 for Vir A. Upper limits for the optical depths of two intermediate-velocity clouds near b = 40 are 0.02 and 0.06. The mean temperature of the cool absorbing gas is found to be 71 + 9 K and this figure is corrected for the effects of the surrounding hot gas. The highest lower limit for the temperature of the hotter gas is 600 K. There is also some evidence of temperatures of a few hundred degrees Kelvin. On the assumption that the interstellar gas has a two-temperature structure, an analysis has been made to determine the fraction of the neutral atomic hydrogen in the cool state. For a line of sight close to the galactic plane, a figure of about 75 percent is obtained. For the general vicinity of the Sun, the result is percent, depending upon the temperature ratio of the two states of the gas, and this is probably also a good estimate for the whole galaxy. A comparison of 21-cm absorption with published measurements of the free-free absorption for three of the sources results in a mean value for the ionization rate of 2.0 X s- . This figure refers to gas in the cool state and within 2 kpc of the Sun. The distance of the excess absorbing gas in the direction of the Cetus Arc is found to be <120 pc from a comparison of velocities of 21-cm absorption features with those of absorption lines in nearby stars. For the galactic source 3C 58, a lower limit of 8.5 kpc is obtained for the distance.