Ultra-compact dwarf galaxies are fascinating objects. They lie in a middle ground between smaller, less massive globular clusters and larger, more massive dwarf galaxies. Globular clusters and dwarf galaxies have very distinct formation scenarios: clusters are formed similar to other star clusters, in especially dense star formation episodes. Dwarf galaxies form in their own distinct dark matter halos. Which process contributes to ultra-compact dwarfs is an open question: are these objects the largest star clusters in the universe, or the smallest galaxies?
I recently published a paper confirming that, at least in one case, the latter is true. We acquired a deep image of the nearby galaxy NGC 3628, focusing on a stellar stream to the east of the galaxy (pictured above). The stellar stream is an indication of a recent galaxy accretion event, a smaller dwarf galaxy has recently fallen into the halo of NGC 3628 and is being disrupted by the tidal forces of the galaxy.
Inside the stream, we identified a cluster of typical size for an ultra compact dwarf. We hypothesized that it is the nucleus of the accreted galaxy, and confirmed that it is located directly in the middle of the stream. NGC-3628 UCD1 offers direct confirmation that at least some of the UCD population are born through tidal stripping of larger galaxies.
Relevant Papers: Jennings et al. 2015