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

TL;DR

This paper proposes a new dynamical and secular evolution model in nuclear star clusters that naturally produces second-generation black hole mergers like GW231123, explaining their high masses and spins.

Contribution

It introduces a combined dynamical-secular channel in nuclear star clusters for hierarchical black hole mergers, aligning with observed GW231123 properties.

Findings

Model reproduces GW231123-like merger rates

Predicts abundant 2G BH-star binaries observable as low-frequency GWs

Suggests these binaries could be detected as micro tidal disruption 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 and highly unconstrained. We show that such 2G mergers can be naturally produced in the nuclear star cluster of Milky Way-like galaxy. The dominant channel combines a sequence of binary-single interactions with secular evolution driven by the central supermassive BH. Our model produces a merger rate consistent with GW231123 and further predicts an abundant population of 2G BH-star (or low-mass BH) binaries; these binaries may observationally manifest as micro tidal disruption events or low-frequency gravitational-wave (GW) sources. Detecting these binaries would provide crucial insights into the dynamical pathways of hierarchical BH assembly.