Dynamical double black holes and their host cluster properties

We investigate the relationship between the global properties of star clusters and their double black hole (DBH) populations. We use the code NBODY6 to evolve a suite of star cluster models with an initial mass of O(10(4))M-circle dot and varying initial parameters. We conclude that cluster metallicity plays the most significant role in determining the lifespan of a cluster, while the initial half-mass radius is dominant in setting the rate of BII exchange interactions in the central cluster regions. We find that the mass of interacting BHs, rather than how frequently their interactions with other BHs occur, is more crucial in the thermal expansion and eventual evaporation of the cluster. We formulate a novel approach to easily quantify the degree of BII-BII dynamical activity in each model. We report 12 in-cluster and three out-of-cluster (after ejection from the cluster) DBH mergers, of different types (inspiral, eccentric, and hierarchical) across the 10 N-body models presented. Our DBI I merger efficiency is 3-4 x 10(-5) mergers per M-circle dot. We note the cluster initial density plays the most crucial role in determining the number of DBH mergers, with the potential presence of a transitional density point (between 1.2 and 3.8 x 10(3) M-circle dot pc(-3)) below which the number of in-cluster mergers increases with cluster density and above which the increased stellar density acts to prevent in-cluster BH mergers. The importance of the history of dynamical interactions within the cluster in setting up the pathways to ejected DBH mergers is also discussed.

Automated summary

This study investigates the relationship between star cluster global properties and their double black hole populations. It concludes that cluster metallicity is crucial for determining lifespan, while initial half-mass radius dominates BII exchange interactions. The mass of interacting BHs, rather than interaction frequency, is key in thermal expansion and evaporation. Cluster initial density plays a critical role in determining the number of DBH mergers.