Seeds to success: Growing heavy black holes in dense star clusters

¹ Gran Sasso Science Institute (GSSI), Viale Francesco Crispi 7, L’Aquila, Italy
² INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi, Italy
³ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ Universität Heidelberg, Zentrum für Astronomie (ZAH), Institut für Theoretische Astrophysik, Albert Ueberle Str. 2, 69120 Heidelberg, Germany

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

Received: 12 September 2025
Accepted: 30 January 2026

Abstract

Context. The observational dearth of black holes (BHs) with masses between ∼100 and 100 000 M_⊙ raises questions about the nature of intermediate-mass black holes (IMBHs). The proposed formation channels for IMBHs include runaway stellar collisions and repeated binary BH (BBH) mergers driven by dynamical interactions in stellar clusters, but the formation efficiency of these processes and the associated IMBH occupation fraction are largely unconstrained.

Aims. For this work we studied IMBH formation via both mechanisms in young (YCs), globular (GCs), and nuclear star clusters (NSCs). We carried out a comprehensive investigation of IMBH formation efficiency by exploring the impact of different seeding models and star cluster formation histories.

Methods. We employed a new version of the B-POP population synthesis code, able to model several seeding mechanisms as well as hierarchical BBH mergers.

Results. We quantify the efficiency of IMBH production across different cluster families and demonstrate that stellar collisions are essential for sustaining IMBH formation. Furthermore, a comparison with low-redshift IMBH candidates indicates that, depending on the seeding mechanism, they may play a pivotal role in explaining the presence of IMBHs in local GCs.

Conclusions. Our simulations highlight stellar collisions as the primary IMBH formation channel across a wide range of cluster types. They further suggest that wandering IMBHs may populate Milky Way-like galaxies, and that correlations between cluster and IMBH masses can help distinguish the origins of Galactic GCs.