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

TL;DR

This study measures and compares the masses of supermassive black holes and nuclear star clusters in various galaxies, revealing how their ratios change with galaxy size and providing insights into galaxy evolution.

Contribution

It provides new measurements of coexisting black holes and star clusters in a dozen galaxies, doubling known cases, and analyzes their mass ratios across galaxy sizes.

Findings

Nuclear star cluster fluxes fit Sersic profiles with indices 0.5 to 3.

The nucleus-to-spheroid mass ratio decreases with galaxy mass.

Once nuclear clusters disappear, the ratio stabilizes at a constant value.

Abstract

In large spheroidal stellar systems, such as elliptical galaxies, one invariably finds a 10^6-10^9 M_Sun supermassive black hole at their centre. In contrast, within dwarf elliptical galaxies one predominantly observes a 10^5-10^7 M_Sun 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 from 10^8 to 10^11 M_Sun, 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[(M_BH+M_NC)/M_sph] = -(0.39+/-0.07)log[M_sph/10^10 M_Sun] -(2.18+/-0.07). Once dry merging has commenced by M_sph ~ 10^11 M_Sun 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 Sersic functions having a range of indices from ~0.5 to ~3, the latter index describing the Milky Way's nuclear star cluster.