Quantifying the Coexistence of Massive Black Holes and Dense Nuclear Star Clusters

In large spheroidal stellar systems, such as elliptical galaxies, one invariably finds a 10-10M⊙ supermassive black hole at their centre. In contrast, within dwarf elliptical galaxies one predominantly observes a 10-10M⊙ nuclear star cluster. To date, few galaxies have been found with both type of nuclei coexisting and even less have had the masses determined for both central components. Here we identify one dozen galaxies housing nuclear star clusters and supermassive black holes whose masses have been measured. This doubles the known number of such hermaphrodite nuclei — which are expected to be fruitful sources of gravitational radiation. Over the host spheroid (stellar) mass range 10–10M⊙, we find that a galaxy’s nucleus-to-spheroid (baryon) mass ratio is not a constant value but decreases from a few percent to ∼0.3 percent such that log[(MBH+MNC)/Msph] = −(0.39±0.07) log[Msph/10 M⊙]−(2.18±0.07). Once dry merging has commenced by Msph ≈ 10 M⊙ and the nuclear star clusters have disappeared, this ratio is expected to become a constant value. As a byproduct of our investigation, we have found that the projected flux from resolved nuclear star clusters can be well approximated with Sérsic functions having a range of indices from ∼0.5 to ∼3, the latter index describing the Milky Way’s nuclear star cluster.