The contribution of supermassive black holes in stripped nuclei to the supermassive black hole population of UCDs and galaxy clusters

TL;DR

This study uses simulations to show that many ultra-compact dwarf galaxies host supermassive black holes, significantly contributing to the black hole population in galaxy clusters and explaining observed mass-to-light ratio elevations.

Contribution

It provides the first comprehensive prediction of supermassive black hole presence in stripped galaxy nuclei and their impact on UCDs and galaxy cluster black hole demographics.

Findings

Approximately 50% of stripped nuclei with M > 2×10^6 M⊙ contain supermassive black holes.

Mass elevation ratios in simulated and observed UCDs are consistent, supporting black hole presence.

Stripped nuclei could increase the number of black holes in galaxy clusters by 30-100%.

Abstract

We use the hydrodynamic EAGLE simulation to predict the numbers and masses of supermassive black holes in remnant nuclei of disrupted galaxies (stripped nuclei) and compare these to confirmed measurements of black holes in observed ultra-compact dwarf galaxies (UCDs). We find that black holes in stripped nuclei are consistent with the numbers and masses of those in observed UCDs. Approximately 50 per cent of stripped nuclei with $M > 2 \times 10^6 M_\odot$ should contain supermassive black holes. We further calculate how the presence of a black hole increases the dynamical mass of a stripped nucleus via the mass elevation ratio $\Psi$ defined as the ratio of the kinematically derived mass to the expected mass from stellar population synthesis. We find $\Psi_{sim} = 1.51^{+0.06}_{-0.04}$ for $M > 10^7 M_\odot$ stripped nuclei, consistent with that of observed UCDs which have $\Psi_{obs} = 1.7 \pm 0.2$ above $M > 10^7 M_\odot$. We also find that the mass elevation ratios of stripped nuclei with supermassive black holes can explain the observed number of UCDs with elevated mass-to-light ratios. Finally, we predict the relative number of massive black holes in stripped nuclei and galaxy nuclei and find that stripped nuclei should increase the number of black holes in galaxy clusters by 30 - 100 per cent, depending on the black hole occupation fraction of low-mass galaxies. We conclude that the population of supermassive black holes in UCDs represents a large and unaccounted-for portion of supermassive black holes in galaxy clusters.