Formation mechanisms of massive binary black holes and intermediate-mass black holes

Determine the astrophysical mechanisms that produce massive binary black holes (MBBHs) and intermediate-mass black holes (IMBHs), and identify their dominant formation channels.

Background

Gravitational-wave observations include events with black hole masses that challenge the predicted pair-instability mass gap, making the origin of very massive black holes uncertain.

This work couples the GAMESH galaxy formation model with cluster population synthesis codes (RAPSTER and FASTCLUSTER) to study hierarchical mergers in globular clusters, showing that such mergers can form very massive black holes. However, different synthesis codes yield divergent predictions for masses and rates, underscoring the difficulty of pinpointing the dominant channels.

Resolving which environments and processes—such as isolated binaries, dense star clusters, or active galactic nucleus disks—primarily produce MBBHs and IMBHs is central to interpreting current and future gravitational-wave populations.

References

The formation of MBBHs and IMBHs remains an open problem in astrophysics.

— Milky Way Globular Clusters: Nurseries for Dynamically-Formed Binary Black Holes  (2512.11059 - Angeloni et al., 11 Dec 2025) in Section 6 (Conclusions)

For the purposes of this study, the formation of the IMBH is not of great importance and is instead treated as the initial conditions for the model. The task of understanding the creation of the initial IMBH can be conducted in future work.

— Formation of Accretion Disks around Intermediate-Mass Black Holes in Ultra Compact Dwarf Galaxies and Nuclear Stellar Clusters  (2608.25856 - Darr et al., 26 Aug 2026) in Section 2, Model

However, placing this feature at such a low mass requires the ${12}\mathrm{C}(\alpha,\gamma){16}\mathrm{O}$ reaction rate to exceed its laboratory-measured value, so its astrophysical interpretation remains open.

— Probing the Mass--Redshift Dependence of Binary Black Holes and its Implications for $H_0$ with GWTC-5.0  (2609.25662 - Liu et al., 22 Sep 2026) in Section 1, Introduction

Whether it represents regime IV of Section~\ref{sec:PISN}, repeated hierarchical growth, or something else entirely is unresolved.

— The School on Gravity: from motion to commotion  (2610.13514 - Cardoso et al., 9 Oct 2026) in Chapter 1, Section 1.4.5, subsection “The pair-instability gap and the highest masses”