Low surface brightness galaxies

A member of a particularly faint population of galaxies with a central surface brightness below the brightness of the background sky. The central regions of many of them resemble a dwarf galaxy, but most of the mass is contained in a large gaseous disk of low density that is observable only with long-exposure optical images or at radio wavelengths. Some are as massive as a large spiral galaxy, for example Malin 1. The proportion of LSBGs relative to normal galaxies is unknown. They may however represent a significant fraction of mass in the Universe. LSBGs are thought to be primitive systems because they have total masses similar to normal galaxies, but have typically converted less than 10% of their gas into stars. Spiral LSBGs do not obey Freeman's law.