Updated Mass Scaling Relations for Nuclear Star Clusters and a Comparison to Supermassive Black Holes

TL;DR

This study compares the mass scaling relations of nuclear star clusters and supermassive black holes, revealing distinct correlations with host galaxy properties and concluding they are not a single class of central massive objects.

Contribution

It provides the first comprehensive comparison of mass scaling relations for nuclear star clusters and black holes, highlighting their different behaviors and relationships with host galaxy properties.

Findings

Nuclear star cluster mass correlates most tightly with galaxy velocity dispersion.

The slope of the nuclear star cluster relation is shallower than that of black holes.

Nuclear star cluster mass is not a constant fraction of host galaxy mass.

Abstract

We investigate whether nuclear star clusters and supermassive black holes follow a common set of mass scaling relations with their host galaxy's properties, and hence can be considered to form a single class of central massive object. We have compiled a large sample of galaxies with measured nuclear star cluster masses and host galaxy properties from the literature and fit log-linear scaling relations. We find that nuclear star cluster mass, M_{NC}, correlates most tightly with the host galaxy's velocity dispersion: log M_{NC} = (2.11 \pm 0.31) log (\sigma/54) + (6.63 \pm 0.09), but has a slope dramatically shallower than the relation defined by supermassive black holes. We find that the nuclear star cluster mass relations involving host galaxy (and spheroid) luminosity and stellar and dynamical mass, intercept with but are in general shallower than the corresponding black hole scaling relations. In particular M_{NC} \propto {M}_{Gal,dyn}^{0.55 \pm 0.15}; the nuclear cluster mass is not a constant fraction of its host galaxy or spheroid mass. We conclude that nuclear stellar clusters and supermassive black holes do not form a single family of central massive objects.