Carbon Monoxide as an Extragalactic Mass Tracer

The validity of integrated CO emission intensity as a tracer of molecular cloud mass in external galaxies is examined critically. By modeling extragalactic CO emission with an ensemble of independently emitting clouds, each of which obeys the virial theorem, it is demonstrated that, on average, there exists a linear relationship between integral CO intensity and the mass surface density of emitting cloud material lying within a radio telescope's antenna beam. Using molecular cloud parameters typical of the Milky Way, a ratio of mass surface density to integrated CO intensity is found which is within a factor of 2 of those frequently used to interpret extragalactic carbon monoxide observations.