Are nuclear star clusters the precursors of massive black holes?

TL;DR

This paper investigates the relationship between nuclear star clusters and black holes in galaxies, providing new upper limits on black hole masses and exploring how these components evolve and relate across different galaxy types.

Contribution

It offers new upper limits for black hole and nuclear star cluster masses, and analyzes their relations, suggesting nuclear star clusters may be precursors to massive black holes.

Findings

Black hole masses in late-type spirals are less than 10^6 solar masses.

The M_BH-sigma relation may steepen at low masses.

Nuclear star clusters could be the precursors of massive black holes.

Abstract

We present new upper limits for black hole masses in extremely late type spiral galaxies. We confirm that this class of galaxies has black holes with masses less than 10^6 Msolar, if any. We also derive new upper limits for nuclear star cluster (NC) masses in massive galaxies with previously determined black hole masses. We use the newly derived upper limits and a literature compilation to study the low mass end of the global-to-nucleus relations. We find the following (1) The M_BH-sigma relation cannot flatten at low masses, but may steepen. (2) The M_BH-M_bulge relation may well flatten in contrast. (3) The M_BH-Sersic n relation is able to account for the large scatter in black hole masses in low-mass disk galaxies. Outliers in the M_BH-Sersic n relation seem to be dwarf elliptical galaxies. When plotting M_BH versus M_NC we find three different regimes: (a) nuclear cluster dominated nuclei, (b) a transition region, and (c) black hole-dominated nuclei. This is consistent with the picture, in which black holes form inside nuclear clusters with a very low-mass fraction. They subsequently grow much faster than the nuclear cluster, destroying it when the ratio M_BH/M_NC grows above 100. Nuclear star clusters may thus be the precursors of massive black holes in galaxy nuclei.