Detection of Interstellar OH in Two External Galaxies

Molecular OH lines at 1665 and 1667 MHx have been detected in absorption in the spiral galaxy NGC 253 and the irregular galaxy M82. This is the first time that interstellar molecular lines, either radio or optical, have been observed outside the Galaxy. Observations were made with the Owens Valley interferometer; thus the amplitude and phase of the absorption features could both be measured. The run of phase versus velocity agrees with the sense of rotation of both galaxies. Abundances close to those found in the Galaxy are derived; however, this comparison is not very meaningful because of possible differences in excitation conditions.