The Parameter Space Occupied by Galaxies: The Optical Window

We discuss the optical parameter space (luminosity and surface brightness) over which galaxies are known to exist and report on the results from three recent datasets: the Two-degree Field Galaxy Redshift Survey, the Hubble Deep Field north, and the Local Group. These data are combined to provide a unique and comprehensive insight into the luminosity distribution of galaxies over the range -24 < M(B) < -8 mags, and the surface brightness distribution over the range 18 < SB(Effective) < 28 mags/sq arcsec. The main conclusions are: (1) luminous low surface brightness galaxies are rare; (2) there exists a universal luminosity-surface brightness relation; (3) the most numerous galaxies are low luminosity low surface brightness dwarfs.