Abstract

Contributed Talk - Splinter StarClusters

Tuesday, 08 September 2026, 14:45   (MW-2050)

Runaway Growth and Relativistic Mergers in Black-Hole Clusters

Yi Tan, Rainer Spurzem, Kai Wu
ARIZAH, Univ. of Heidelberg

Dense stellar systems are natural laboratories for black-hole dynamics. Through mass segregation, binary formation, hardening and merger, dense star clusters are thought to be cradle of gravitational-wave sources detected by ground-based observatories and may also provide a dynamical pathway to intermediate-mass black-hole formation. However, whether merger products can be retained inside the birth star cluster after relativistic recoil kicks and continue to grow remains highly uncertain. We study an idealized cluster composed entirely of stellar-mass black holes, using the latest version of NBODY6++GPU with full post-Newtonian (PN) treatment of compact-object dynamics. We employ (1)non-spinning post-Newtonian corrections to the accelerations up to 3.5 PN order;(2) the spin-orbit coupling up to next- to-lowest order and (3) the lowest order spin-spin coupling. The main goal is to test whether repeated mergers can lead to runaway growth of a massive object and to examine how its mass and spin evolve before and after core collapse. We examine how this outcome depends on cluster properties, relativistic prescription and spin assumptions, with particular focus on remnant masses, spins, retention probabilities, and hierarchical-merger signatures. This work also provides a testbed for future applications to realistic nuclear and globular clusters.