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 < \mu _ { e } < 28 mags / \Box ^ { \prime \prime } .
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 .