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Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures

Abstract

While most of the compact-binary mergers detected by LIGO and Virgo are expected to consist of first-generation black holes formed from the collapse of stars, others might instead be of second (or higher) generation, containing the remnants of previous black-hole mergers. We review theoretical findings, astrophysical modelling and current gravitational-wave evidence of hierarchical stellar-mass black-hole mergers. Such a subpopulation of hierarchically assembled black holes presents distinctive gravitational-wave signatures, namely higher masses, possibly within the pair-instability mass gap, and dimensionless spins clustered at the characteristic value of ~0.7. To produce hierarchical mergers, astrophysical environments need to overcome the relativistic recoils imparted to black-hole merger remnants, a condition that prefers hosts with escape speeds of 100 km s−1. Promising locations for efficient production of hierarchical mergers include nuclear star clusters and accretion disks surrounding active galactic nuclei, though environments that are less efficient at retaining merger products such as globular clusters may still contribute significantly to the detectable population of repeated mergers. While GW190521 is the single most promising hierarchical-merger candidate to date, constraints from large population analyses are becoming increasingly more powerful.

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Fig. 1: Masses, spins and recoil velocities of 1g and 2g BHs.
Fig. 2: Interplay between the occurrence of hierarchical mergers and (some of) the proposed formation channels of merging compact binaries.

Data availability

All data generated or analysed during this study are included in this published article (and its supplementary information files). Source data are provided with this paper.

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Acknowledgements

D.G. is supported by the European Union’s H2020 ERC Starting Grant 945155–GWmining, Leverhulme Trust Grant RPG-2019-350 and Royal Society Grant RGS-R2-202004. M.F. is supported by NASA through the NASA Hubble Fellowship Grant HST-HF2-51455.001-A awarded by the Space Telescope Science Institute. Computational work was performed on the University of Birmingham BlueBEAR cluster, the Athena cluster at HPC Midlands+ funded by EPSRC Grant EP/P020232/1, and the Maryland Advanced Research Computing Center (MARCC).

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This paper was written jointly by the two authors. D.G. prepared the figures. M.F. extracted distributions from LIGO/Virgo public data.

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Correspondence to Davide Gerosa.

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Peer review information Nature Astronomy thanks Eric Thrane, Michael Zevin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Gerosa, D., Fishbach, M. Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures. Nat Astron 5, 749–760 (2021). https://doi.org/10.1038/s41550-021-01398-w

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