Intermediate-mass black hole binary evolution in nuclear star clusters: The effect of the stellar-mass black hole population

¹ New York University Abu Dhabi PO Box 129188 Abu Dhabi, United Arab Emirates
² Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi Abu Dhabi, United Arab Emirates
³ Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences ul. Bartycka 18 00-716 Warsaw, Poland
⁴ Szechenyi Istvan University, Space Technology and Space Law Research Center, H-9026 Gyor Egyetem ter 1., Hungary
⁵ Main Astronomical Observatory, National Academy of Sciences of Ukraine 27 Akademika Zabolotnoho St 03143 Kyiv, Ukraine
⁶ Astronomisches Rechen-Institut, Zentrum für Astronomie, University of Heidelberg Mönchhofstrasse 12-14 69120 Heidelberg, Germany
⁷ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Rd. Chaoyang District 100101 Beijing, China
⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yi He Yuan Road Haidian District Beijing 100871, P.R. China
⁹ Department of Physics and Astronomy, Vanderbilt University Nashville TN 37240, USA
¹⁰ Department of Physics, Fisk University Nashville TN 37208, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 September 2025
Accepted: 7 December 2025

Abstract

Aims. In this study, we investigate the dynamics of intermediate-mass black hole (IMBH) binaries within nuclear star clusters (NSCs) that contain a population of stellar-mass black holes (BHs). We examine how these stellar and BH populations influence the dynamics of the IMBH binary and, in turn, how the evolving IMBH binary affects the surrounding stellar and BH populations.

Methods. We conducted high-resolution N-body simulations of NSCs constructed based on observational parameters from two local dwarf galaxies: NGC205 and NGC404. For the first time, we achieved a star particle mass resolution of 1 M_⊙ and a BH mass resolution of 10 M_⊙. This level of resolution is crucial for accurately modeling the collisional dynamics of these dense systems.

Results. Including stellar-mass BHs within the stellar population significantly influences the IMBH binary dynamics, nearly doubling the sinking rate and halving the merger time. During the initial phase of the inspiral, the IMBH binary disrupts both the stellar and BH cusps. However, the BH cusp quickly regains its steep slope due to its shorter relaxation time and continues to dominate the evolution of the IMBH binary, despite being much less massive than the stellar component. We uncover an interesting mechanism in which BHs first efficiently extract energy from the IMBH binary and then transfer this energy to the surrounding stars, allowing the BHs to spiral back toward the center of the system and restart the process. Our results imply that although stellar-mass BHs are a minor component of a stellar population they can significantly facilitate IMBH growth within NSCs via mergers. We also notice that these dense systems can potentially boost intermediate-mass ratio inspirals (IMRIs) predominantly on radial orbits.