A consistency test for determining whether ultracompact dwarf galaxies could be the remnant nuclei of threshed galaxies

It has been suggested that ultracompact dwarf (UCD) galaxies are the ‘threshed’ remains of larger galaxies. Simulations have revealed that extensive tidal-stripping may pare a galaxy back to its tightly bound, compact nuclear star cluster. It has therefore been proposed that the two-component nature of UCD galaxies may reflect the original nuclear star cluster surrounded by the paltry remnants of its host galaxy. A simple quantitative test of this theory is devised and applied here. If the mass of the central black hole (BH) in UCD galaxies, relative to the mass of the UCD galaxies’ inner stellar component, i.e. the suspected nuclear star cluster, matches with the (black hole mass)–(nuclear star cluster mass) relation observed in other galaxies, then it would provide quantitative support for the stripped galaxy scenario. Such consistency is found for four of the five UCD galaxies reported to have a massive BH. This (black hole mass)–(nuclear star cluster mass) relation is then used to predict the central BH mass in two additional UCD galaxies, and to reveal that NGC 205 and possibly NGC 404 (which only has an upper limit to its black hole mass) also follow this scaling relation.