The Leftover Nucleus of a Dwarf Galaxy

NGC 3628-UCD1 is an ultra-compact dwarf that is embedded in a stream of stars around the spiral galaxy NGC 3628. UCD1 is made up of a compact cluster of stars with a total luminosity of approximately 1.4 million times the Sun’s luminosity and a half-light radius of roughly 30 light years. UCD1 is likely to be associated with the stream of stars it is embedded in. This is because UCD1 is located in the brightest region of the stream of stars and the spatial density of stars in the stream appears to fall off gradually in all directions away from UCD1.

The size and luminosity of UCD1 is remarkably similar to Omega Centauri, the most luminous Milky Way globular cluster. Omega Centauri is believed to be the leftover nucleus of a dwarf galaxy that was tidally stripped as it got accreted by the Milky Way. UCD1 and the stream of stars it is embedded in were probably once a dwarf galaxy before it got tidally stripped and accreted by the much larger spiral galaxy NGC 3628. Measurements of the total brightness of the stream of stars UCD1 is embedded in suggest it was once a dwarf galaxy with approximately 40 million times the Sun’s luminosity.

Reference:
Jennings et al. (2015), “NGC 3628-UCD1: A possible ω Cen Analog Embedded in a Stellar Stream”, arXiv:1509.04710 [astro-ph.GA]