Binary black hole merger rate from young massive clusters, open clusters, and isolated binaries

We study the roles of cluster dynamics and massive binary evolution in producing stellar-mass
binary black hole (BBH) mergers over the cosmic time. Dynamical BBH mergers are obtained from
long-term direct N-body evolutionary models of ~10^4M_sun young massive clusters (YMC) evolving
into open clusters. Fast evolutionary models of massive isolated binaries yield merging BBHs
from binary evolution. A cosmological population synthesis is performed to obtain BBH-merger
yields from these two channels, observable at the present day and over cosmic time.
The merging BBH populations from the two channels are combined by applying a Bayesian
regression chain, taking into account the observed differential BBH merger rate densities
from the GW events of the LIGO-Virgo-KAGRA (LVK). The analysis estimates an OB-star binary
fraction of >90% and a YMC formation efficiency of ~10^−2, being consistent with recent optical
observations and structure-formation simulations. The corresponding combined present-day
differential BBH merger rate density and the cosmic evolution of BBH merger rate density both agree
well with those from LVK. We also demonstrate that the interplay between cluster dynamics and
binary evolution in YMCs can potentially lead to an asymmetric distribution of effective spin
parameter of the merging BBH population, as hinted by the observed GW events.

References:

Banerjee, S., 2022, PRD, Vol 105, id.023004
Banerjee, S., Olejak, A., Belczynski, K., 2023, ApJ, 953, 80