Annu. Rev. Astron. Astrophys. 1982. 20: 517-45 Copyright © 1982 by Annual Reviews. All rights reserved

2. GALAXIES WITH ABUNDANT MOLECULAR GAS

Because of the limitations imposed by present radio receiver sensitivities and by the low spatial resolution of existing telescopes, most galaxies within about 20 Mpc have detectable molecular lines only if their fractional molecular gas content is no less than 10-20% of that of our Galaxy. It follows that if molecules are detected at all in a galaxy, then they must be relatively abundant. Searches have been carried out in radio lines of two molecules: OH in absorption against 18-cm nuclear continuum sources, and CO in emission from both the central and disk regions of galaxies.