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An explanation for the observed difference in radial profiles of the atomic and molecular gas components of disk galaxies is proposed which involves GMC formation in local gravitational potential perturbations such as spiral arms. It is assumed that GMC form through binary cloud-cloud collisions within the potential of spiral density perturbations, with progenitors whose radial profile is that of the atomic gas, the rate of collisions being determined by how often a spiral arms is encountered. This model provides a good fit to the overall shape of the molecular gas distribution for disk galaxies where there is believed to be an underlying stationary density wave, with corotation resonance in the outer regions of the disk, as well as for those disk galaxies where the spiral arms are more probably transient.
Rosemary F. Ġ. Wyse (Mon,) studied this question.