Hierarchical Black Hole Mergers in Nuclear Star Clusters: A Combined Dynamical-secular Channel for GW231123-like Events

Abstract The recent binary black hole (BH) merger GW231123, with both components likely in the high-mass gap and with high spins, challenges standard BH binary formation models. It is usually thought that the BHs are of second (or higher) generation (2G), resulting from the mergers of smaller BHs. But the physical processes that produce the merging 2G BH binaries are unclear. We suggest that such 2G mergers may be produced in the nuclear star cluster of a Milky Way–like galaxy. A potentially dominant channel combines a sequence of binary–single interactions with secular evolution driven by the central supermassive BH. Our model yields a merger rate that is consistent with the inferred rate of GW231123 within current uncertainties and further predicts an abundant population of 2G BH-star (or low-mass BH) binaries; these binaries may observationally manifest as microtidal disruption events or low-frequency gravitational-wave sources. Detecting these binaries would provide crucial insights into the dynamical pathways of hierarchical BH assembly.