Observations of Interstellar Formaldehyde

The 140-foot telescope of the National Radio Astronomy Observatory was used to make extensive observations of the i H 1 i rotational transition of interstellar formaldehyde (H2CO). This 4830-MHx transition has been detected in absorption in the direction of approximately half of the forty-three galactic or extragalactic objects examined. (These "objects" were not all 6-cm continuum radio sources.) 4830- MHx emission, as well as the H2CO transitions 6's H 624, 133 H 133 55, 214 H 214 17, was also searched for but not detected. Circular and linear polarixations of some features were sought, but neither was detected. H2CO radial velocities have been compared with those of other atomic and molecular constituents of the interstellar medium. The H2CO velocities often correspond to OH and H I absorption features. In such cases the H2CO generally appears to be more localized than the H I. The astronomically determined H2CO rest frequency is in excellent agreement ( 1 kHx) with the laboratory value. Projected densities of observed H2CO features lie between 10 and 10 for an assumed excitation temperature of 3 K. With this assumption and the assumption that the H2CO uniformly covers the continuum source, the ratio of H2CO to H I projected density is typically I0- .