Low surface brightness galaxy

NGC 4395 is a nearby low surface brightness spiral galaxy located about 14 million light-years (or 4.3 Mpc) from Earth in the constellation Canes Venatici. It was discovered by German-British astronomer William Herschel on 2 January 1786. The nucleus of NGC 4395 is active and the galaxy is classified as a Seyfert Type I known for its very low-mass supermassive black hole.

Crater 2 is a low-surface-brightness dwarf satellite galaxy of the Milky Way, located approximately 380,000 ly from Earth. Its discovery in 2016 revealed significant gaps in astronomers' understanding of galaxies possessing relatively small half-light diameters and suggested the possibility of many undiscovered dwarf galaxies orbiting the Milky Way. Crater 2 was identified in imaging data from the VST ATLAS survey.

The galaxy has a half-light radius of ~1100 pc, making it the fourth largest satellite of the Milky Way. It has an angular size about double of that of the moon. Despite the large size, Crater 2 has a surprisingly low surface brightness, implying that it is not very massive. In addition, its velocity dispersion is also low, suggesting it may have formed in a halo of low dark matter density. Alternatively, it may be a result of tidal interactions with it and larger galaxies, such as the Milky Way and the Large Magellanic Cloud, but according to some simulations, this would not explain the relatively large size. This unusually low velocity dispersion was predicted using Modified Newtonian Dynamics, an alternative to the dark matter hypothesis. This prediction was later confirmed by observations.