The hydrocarbon ring C3H2 is ubiquitous in the Galaxy.

We report the discovery of a strong microwave (lambda 1.6 cm) spectral line, the carrier of which is common and widespread throughout the Galaxy. A survey of a large number of sources shows that the line appears in emission in cold dust clouds, in absorption in the direction of the Galactic center, and exhibits complex profiles toward H II regions. Toward Cas A and distant H II regions, intervening "spiral arm" clouds produce absorption. For almost all cases, the absorption features show a striking 1 : 1 radial velocity correspondence with those seen, e.g., in H2CO spectra of the same objects. The data indicate that the line arises between low-lying energy states of rather polar molecule. Recent work by Thaddeus, Vrtilek, and Gottlieb, reported in 1985 incorporating the present data, shows that the line in question is the 1(10)-1(01) transition of the small hydrocarbon ring C3H2.