Gas Content and Star Formation Thresholds in the Evolution of Spiral   Galaxies
  The gas mass fraction (fg) of spiral galaxies is strongly correlated with the central surface brightness of their disks. There exist many dim galaxies with long gas consumption time scales and fg > 0.5. This resolves the gas consumption paradox.   The surface density of gas follows the optical surface brightness, but does not vary by as large a factor. This is the signature of a critical density threshold for star formation. Such a mechanism seems to be responsible for the slow evolution of dim galaxies. 
Stacy McGaugh (DTM, CIW) and Erwin de Blok (Kapteyn Astronomical   Institute)