  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.
