Mergers and ejections of black holes in globular clusters

TL;DR

This study uses advanced N-body simulations to explore black hole dynamics in globular clusters, revealing processes of formation, ejection, and mergers, including relativistic coalescence and high-velocity ejections.

Contribution

First to incorporate general relativistic coalescence in realistic globular cluster simulations with detailed dynamical processes.

Findings

Black holes form dense cores and dominant binaries.

High-velocity ejections of stars and black holes occur.

Black hole growth in clusters is limited.

Abstract

We report on results of fully consistent N-body simulations of globular cluster models with N = 100 000 members containing neutron stars and black holes. Using the improved `algorithmic regularization' method of Hellstrom and Mikkola for compact subsystems, the new code NBODY7 enables for the first time general relativistic coalescence to be achieved for post-Newtonian terms and realistic parameters. Following an early stage of mass segregation, a few black holes form a small dense core which usually leads to the formation of one dominant binary. The subsequent evolution by dynamical shrinkage involves the competing processes of ejection and mergers by radiation energy loss. Unless the binary is ejected, long-lived triple systems often exhibit Kozai cycles with extremely high inner eccentricity (e > 0.999) which may terminate in coalescence at a few Schwarzschild radii. A characteristic feature is that ordinary stars as well as black holes and even BH binaries are ejected with high velocities. On the basis of the models studied so far, the results suggest a limited growth of a few remaining stellar mass black holes in globular clusters.