CO ABSORPTION IN THE OUTER GALAXY - ABUNDANT COLD MOLECULAR GAS

We have detected -absorption in the CO(1-0) and CO(2-1) spectra of two very-low-latitude radio continuum sources. Little CO emission is foun d in the immediate regions of these sources. The results, obtained wit h the 30m IRAM radiotelescope, have been confirmed with the IRAM inter ferometer for one of the sources in CO(1-0) absorption. There is only one absorption feature from the local gas, but at least 3 features are detected from gas at about 12 kpc from the Galactic center. This is a n unbiased survey, whereas present CO emission surveys preferentially detect warm (greater than or similar to 10 K) molecular gas associated with recent massive star formation. We conclude that we have found ev idence for the existence of significant amounts of cold molecular gas in the outer Galaxy; there may be 4 times more molecular gas than atom ic gas at 12 kpc radius. The fraction of molecular gas can only increa se at larger Galacocentric distances, and we suggest that cold molecul ar gas may make a significant contribution to the dark matter required by dynamical studies of spiral and irregular galaxies. Another conseq uence of our observations is that both the electron and proton compone nts of cosmic rays must decrease strongly at large radii, pointing to a Galactic origin for GeV cosmic rays.