Great Balls of FIRE: IV. Contribution of massive star clusters to the astrophysical population of merging binary black holes

Context. The detection of over a hundred gravitational wave signals from double compacts objects, reported by the LIGO-Virgo-KAGRA Collaboration, have confirmed the existence of such binaries with tight orbits. Two main formation channels are generally considered to explain the formation of these merging binary black holes (BBHs): the isolated evolution of stellar binaries and the dynamical assembly in dense environments, namely, star clusters. Although their relative contributions remain unclear, several analyses indicate that the detected BBH mergers probably originate from a mixture of these two distinct scenarios. Aims. We study the formation of massive star clusters across time and on a cosmological scale to estimate the contribution of these dense stellar structures to the overall population of BBH mergers. Methods. To this end, we propose three different models of massive star cluster formation based on results obtained with zoom-in simulations of individual galaxies. We applied these models to a large sample of realistic galaxies identified in the (22.1 Mpc)$^{3}$ cosmological volume simulation FIREbox. Each galaxy in this simulation has a unique star formation rate, with its own history of halo mergers and metallicity evolution. Combined with predictions obtained with the Cluster Monte Carlo code for stellar dynamics, we were able to estimate populations of dynamically formed BBHs in a collection of realistic galaxies. Results. Across our three models, we inferred a local merger rate of BBHs formed in massive star clusters consistently in the range 1–10 Gpc$^{−3}$yr$^{−1}$. Compared with the local BBH merger rate inferred by the LIGO-Virgo-KAGRA Collaboration (in the range 17.9–44 Gpc$^{−3}$yr$^{−1}$ at z = 0.2), this could potentially represent up to half of all BBH mergers in the nearby Universe. This shows the importance of this formation channel in the astrophysical production of merging BBHs. We find that these events preferentially take place around cosmic noon and in the most massive galaxies.