A consistency-test for determining whether ultra-compact dwarf galaxies could be the remnant nuclei of threshed galaxies

TL;DR

This paper introduces a quantitative consistency test based on black hole and nuclear star cluster mass relations to evaluate whether ultra-compact dwarf galaxies are remnants of tidally stripped larger galaxies.

Contribution

It proposes a new test using black hole and nuclear star cluster mass relations to support the galaxy stripping hypothesis for UCDs.

Findings

Most UCDs with massive black holes follow the expected mass relation.

The test supports the stripped galaxy origin for several UCDs.

Predictions for black hole masses in additional UCDs are consistent with the scenario.

Abstract

It has been suggested that ultra-compact 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 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 black hole. The (black hole mass)-(nuclear star cluster mass) relation is then used to predict the central black hole 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.