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