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Kick matters: The impact of a new recoil model on the retention of hierarchical black-hole remnants in dense star clusters

Tousif Islam1,*, Digvijay Wadekar2,3, and Konstantinos Kritos3

  • *Contact author: tousifislam@ucsb.edu
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Phys. Rev. D 114, 043041 – Published 17 August, 2026

DOI: https://doi.org/10.1103/tchv-vdv5

Abstract

In a dense star cluster, hierarchical mergers are among the most promising pathways to forming massive black holes such as GW231123. A key factor determining whether a merger-remnant black hole will be retained in these environments and thus participate in subsequent hierarchical mergers is the recoil kick velocity. Analytic models for the recoil velocity are currently employed in nearly all population-synthesis frameworks. We instead use a state-of-the-art recoil-kick model gwmodel_flow_prec [T. Islam and D. Wadekar, arXiv:2511.11536] developed from a combination of numerical-relativity and black-hole–perturbation-theory data, together with data-driven techniques such as normalizing flows and the post-Newtonian structure of the kick. Employing both back-of-the-envelope estimates and detailed N-body as well as semianalytical cluster simulations, we show that gwmodel_flow_prec leads to a noticeable increase in the retention probability of hierarchical-merger remnants compared to the previously used analytic model and changes the mass and spin distribution of the black holes formed through hierarchical mergers. Additionally, we discuss the implications of our results in the context of massive binaries such as GW231123.

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Phys. Rev. D 114, 043041– Published 17 August, 2026
  • Received 10 April 2026
  • Accepted 22 July 2026
Article part of CHORUS
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