Isolated or dynamical? Tracing black hole binary formation through the population of gravitational-wave sources

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Abstract
The population of binary black hole (BBH) mergers observed by the LIGO-Virgo-KAGRA (LVK) collaboration offers a window into the cosmic evolution of compact binaries and the physical mechanisms that drive their formation. We employ the semianalytic population-synthesis code - to model BBHs assembled through isolated binary evolution and dynamical interactions in young, globular, and nuclear star clusters. Our framework self-consistently incorporates star formation history, metallicity evolution, and single and binary stellar evolution to quantify their impact on the observable properties of the BBH population and on the relative contribution of distinct formation channels. Our models are characterized by a merger rate, R = 17.5 – 24.1 Gpc − 3 yr − 1 , broadly consistent with LVK constraints. Moreover, the predicted distributions of primary mass, mass ratio, and effective inspiral spin parameter are compatible with those inferred from current LVK observations. Our primary-mass distribution is dominated by isolated binaries at m 1 ≲ 20 M ⊙ , while dynamically assembled first- and higher-generation mergers dominate at larger masses. As a consequence, the subpopulation of mergers with m 1 ≳ 45 M ⊙ exhibits a nearly flat mass-ratio distribution and distinctive spin properties. We leverage our models to explore how: (i) the fraction of stars in isolated binaries and the fraction of stellar mass bound in clusters regulate the overall merger rate; (ii) common-envelope physics shapes the primary-mass distribution and its redshift evolution; (iii) the inclusion of stellar-collision products enhances the formation of higher-generation mergers; and (iv) the natal spin distribution of stellar-mass black holes influences the effective spin of merging BBHs. Using our models to assess possible origins of selected gravitational waves events, we illustrate how the complexity of the underlying astrophysical processes can hinder the possibility to draw definitive conclusions.
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Funding Information
  • European Commission
  • Horizon 2020 Framework Programme
  • H2020 Marie Skłodowska-Curie Actions (101025436)
  • Fondation Merac
  • HORIZON EUROPE Framework Programme (101131928)
  • National Science Foundation (PHY-0757058)
  • National Science Foundation (PHY-0823459)

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