Published online by Cambridge University Press: 25 May 2016
If the CO distribution of the Milky Way is described as a truncated exponential rather than as a molecular ring with some gas at large radii, it becomes easier to understand the evolution of the disk of stars. The star formation rate per unit molecular gas mass is constant as a function of radius, and the H2 depletion time turns out to be only a few percent of the Hubble time. This very short timescale requires that the atomic gas act as a reservoir for the active star forming gas. Because the HI has such a different radial distribution, there must either be infall from outside the Galaxy, an efficient way for the atomic gas in the disk to lose angular momentum, or both, leading to measurable infall or inflow velocities. The truncation radius of CO is probably due to the recently identified stellar bar.