A High Resolution Investigation of 21 cm Absorption Spectra

Abstract

21 cm absorption spectra of Cassiopeia A, Cygnus A and Taurus A have been observed using the Mark I telescope and a six-channel receiver with 3 kc/s bandwidth crystal filters. Profiles of optical depth versus frequency were plotted, and an attempt has been made to resolve them into gaussian components. Assuming these components represent individual clouds the results indicate that H I clouds in the Orion arm have kinetic temperatures in the range 25 °K to 120 °K and densities in the range 1–16 atoms cm ⁻³ . The harmonic mean kinetic temperature of 21 cloud components is 56 °K and, even allowing for the fact that absorption measurements tend to discriminate against the observation of high temperature clouds, the harmonic mean temperature is not likely to be greater than 70 °K. A suggested explanation for the difference between this value and the value 126 °K visually adopted, which is based upon the Leiden emission measurements, is that it is not justifiable to equate the harmonic mean kinetic temperature with the peak brightness temperature seen in emission.