Low surface brightness galaxy

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A low surface brightness galaxy or LSB galaxy is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky.

Most LSBs are dwarf galaxies, in which most of their baryonic matter is still neutral gaseous hydrogen not locked up in stars. They appear to have over 95% of their mass as non-baryonic dark matter. There appears to be no supernova activity in these galaxies. It is estimated that 90% of all galaxies are LSB galaxies, not high-surface-brightness types. Currently around 3000 are catalogued (as of 2005).

Rotation curve measurements indicate an extremely high mass-to-light ratio, meaning that stars and luminous gas contribute only very little to the overall mass balance of a LSB. The center of LSBs show no large overdensity in stars, unlike e.g. the bulge of normal spiral galaxies. Therefore they seem to be dark matter dominated even in their center which makes them an excellent laboratory to study dark matter systems.

In comparison to the more well-studied high-surface brightness galaxies, LSBs are mainly isolated field galaxies, found in regions devoid of other galaxies. In their past, they had fewer tidal interactions or mergers with other galaxies, which could have triggered enhanced star formation. This is an explanation for the small stellar content.