On the transition from nuclear-cluster to black-hole dominated galaxy cores

TL;DR

This paper investigates how merging star clusters with seed black holes evolve, revealing a mechanism that explains the transition from nuclear star clusters to black-hole dominated galaxy cores in hierarchical galaxy formation.

Contribution

It introduces a numerical model showing how black hole binary coalescence during mergers heats and expands star clusters, leading to their eventual destruction and the dominance of black holes in galaxy centers.

Findings

Black hole binaries heat and expand merging star clusters.

This process explains the disappearance of nuclear star clusters in larger galaxies.

The mechanism accounts for the decreasing nucleus-to-galaxy stellar mass ratio.

Abstract

Giant elliptical galaxies, believed to be built from the merger of lesser galaxies, are known to house a massive black hole at their center rather than a compact star cluster. If low- and intermediate-mass galaxies do indeed partake in the hierarchical merger scenario, then one needs to explain why their dense nuclear star clusters are not preserved in merger events. A valuable clue may the recent revelation that nuclear star clusters and massive black holes frequently co-exist in intermediate mass bulges and elliptical galaxies. In an effort to understand the physical mechanism responsible for the disappearance of nuclear star clusters, we have numerically investigated the evolution of merging star clusters with seed black holes. Using black holes that are 1-5% of their host nuclear cluster mass, we reveal how their binary coalescence during a merger dynamically heats the newly wed star cluster, expanding it, significantly lowering its central stellar density, and thus making it susceptible to tidal destruction during galaxy merging. Moreover, this mechanism provides a pathway to explain the observed reduction in the nucleus-to-galaxy stellar mass ratio as one proceeds from dwarf to giant elliptical galaxies.