Formation of massive black holes in ultra-compact dwarf galaxies: migration of primordial intermediate-mass black holes in N-body simulation

TL;DR

This study uses N-body simulations to explore how intermediate-mass black holes can migrate and merge within dwarf galaxy nuclei, potentially forming the massive black holes observed in ultra-compact dwarf galaxies.

Contribution

It demonstrates the conditions under which IMBHs can sink to galaxy centers and form binaries, advancing understanding of MBH formation in UCDs through dynamical processes.

Findings

Massive IMBHs ($10^5 M_ ext{sun}$) sink into dwarf galaxy centers within 1 Gyr.

Lighter IMBHs ($1-30 imes 10^3 M_ ext{sun}$) sink more efficiently in low-mass dwarfs.

IMBH binaries can form and merge, emitting gravitational waves detectable by LISA.

Abstract

Recent observational studies of ultra-compact dwarf galaxies (UCDs) have discovered massive black holes (MBHs) with masses of more than ${10^6~\rm M_\odot}$, in their central regions. We here consider that these MBHs can be formed through merging of intermediate-mass black holes (IMBH), with masses of ${[10^3-10^5]~{\rm M}_{\odot}}$, within the stellar nuclei of dwarf galaxies, which are progenitors of UCDs. We numerically investigate this formation process for a wide range of model parameters using N-body simulations. This means that IMBH growth and feedback is neglected in this study. We find that only massive IMBHs of $10^5~\rm M_\odot$ sink into the central regions of their host dwarf ($\approx 10^{10}~\rm M_\odot$) to be gravitationally trapped by its stellar nucleus within less than 1 Gyr in most dwarf models. We also find that lighter IMBHs with $[1 - 30] \times 10^3~\rm M_\odot$ sink into the centre in low-mass dwarfs ($\approx 10^{9}~\rm M_\odot$) due to more efficient dynamical friction (DF). Additionally, we show that the IMBHs can form binaries in the centre and, rarely, before they reach the centre, which may lead to the IMBHs merging and thus emitting gravitational waves that could be detected by LISA. Finally, we discuss the required number of IMBHs for the MBH formation in UCDs and the physical roles of stellar nuclei in IMBH binaries and mergers.