On The Origin Of HI In Galaxies: The Sizes and Masses of HI   Photodissociation Regions
  Young stars in the disks of galaxies produce HI from their parent H2 clouds by photodissociation. This paper describes the observational evidence for and the morphology of such HI. Simple estimates of the amount of dissociated gas lead to the startling conclusion that much, and perhaps even all, of the HI in galaxy disks can be produced in this way. 
Ronald J. Allen