Interferometric observations of formaldehyde absorption in front of strong galactic sources

Observations have been made of formaldehyde absorption at 4830 MHz in front of Sgr A, Sgr B2, W3, NGC 2024, W31, W33, W43, W49A, and W51. The Caltech interferometer, consisting of two 90-foot paraboloids, was used to synthesize a beam of 40" x 60" sec (5 - 1), where 5 is the source declination and 1 = 38 . The main absorption features variations of optical depth over the sources, but there is no significant clumping. The previously determined isotopic ratio `2C/13C of 10 for Sgr A and Sgr B2 should be increased to 25 1 5 for Sgr A and >20 for Sgr 132 because of the high optical depths at the center of the absorbing clouds, 2.0 for Sgr A, ,, > 3 for Sgr 132. The +40 km s ' cloud associated with Sgr A appears to be rotating as a solid body. The mass of the cloud is calculated to be 3 x 10 M0. From neutral-hydrogen absorption data only a small proportion of the cloud is composed of atomic hydrogen; hence the cloud consists mainly of molecular hydrogen with a density 10 . Subject headiiigs: abundances, nebular molecules - nebulae - radio sources