Abstract
Determining the astrophysical origin of binary black holes (BBHs) and whether merger remnants are retained in their birth environments is essential for understanding hierarchical mergers and the growth of intermediate-mass black holes.
We identified the gravitational wave (GW) events most consistent with a dense-cluster origin and assessed whether their merger remnants are retained in globular clusters, nuclear star clusters, or galactic potentials.
We considered the 84 events consistent with BBH mergers from the first part of the fourth observing run ( of the LIGO-Virgo-KAGRA (LVK) GW detector network and three selected events from the second part ( We compared parameter-estimation posteriors with synthetic population models for field and cluster binaries using Bayes factors while accounting for the relative abundances of these formation channels in the local Universe. We computed recoil-velocity posteriors for all events using the waveform model, which incorporates multipole asymmetries.
We identified five events whose inferred intrinsic parameters show preference for the adopted dense-cluster population models over the considered field-binary populations, including the most massive , while finding no robust preference for a dense-cluster origin for the high-̧hieff within the adopted population models. The typical recoil velocities of the analyzed events are on the order of a few hundred kilometers per second, with extended high-velocity tails. These kicks suggest that merger remnants are likely ejected from typical globular clusters, while retention in nuclear star clusters remains possible but not guaranteed. event GW231123 event GW241011
Within the adopted models and assumptions, our results suggest that efficient hierarchical growth may be challenging in typical globular clusters, whereas nuclear star clusters remain viable environments for repeated mergers. Although results depend on the adopted astrophysical population models, this analysis highlights the importance of improved and larger population models as well as higher-quality detections that will be enabled by future developments in GW detectors.