On The Origin of HI in Galaxies: Photodissociation and the ``Schmidt   Law'' for Global Star Formation
  Young stars in the disks of galaxies produce HI from their parent H2 clouds by photodissociation. This process is widespread in late-type galaxies, and follows the distribution of Far-UV photons produced primarily by B-type stars. An estimate of the amount of dissociated gas can be made using observed Far-UV fluxes and simple approximations for the physics of photodissociation. This leads to the startling conclusion that much, and perhaps even all, of the HI in galaxy disks can be produced in this way. This result offers a simple, but inverse, cause-effect explanation for the ``Schmidt Law'' of Global Star Formation in galaxies. 
Ronald J. Allen