Merger rate density of stellar-mass binary black holes from young massive clusters, open clusters, and isolated binaries: Comparisons with LIGO-Virgo-KAGRA results

I investigate the roles of cluster dynamics and massive binary evolution in producing stellar-remnant binary black hole (BBH) mergers over the cosmic time. To that end, dynamical BBH mergers are obtained from long-term direct N-body evolutionary models of similar to 10(4) M-circle dot, pc-scale young massive clusters (YMC) evolving into moderate-mass open clusters. Fast evolutionary models of massive isolated binaries yield BBHs from binary evolution. Population synthesis in a Model Universe is then performed, taking into account observed cosmic star formation and enrichment histories, 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 proof-of-concept Bayesian regression chain, taking into account observed differential intrinsic BBH merger rate densities from the second gravitational-wave transient catalog (GWTC-2). The analysis estimates an OB-star binary fraction of f(Obin) greater than or similar to 90% and a YMC formation efficiency of f(YMC) similar to 10(-2), being consistent with recent optical observations and large scale structure formation simulations. The corresponding combined Model Universe present-day, differential intrinsic BBH merger rate density and the cosmic evolution of BBH merger rate density both agree well with those from GWTC-2. The analysis also suggests that despite significant dynamical mixing at low redshifts, BBH mergers at high redshifts (z(event) greater than or similar to 1) could still be predominantly determined by binary evolution physics. Caveats in the present approach and future improvements are discussed.

Automated summary

This study investigates the roles of cluster dynamics and massive binary evolution in the formation of binary black hole (BBH) mergers over cosmic time. The results show that dynamical BBH mergers can be obtained from long-term models of young massive clusters evolving into open clusters. In addition, population synthesis models and observed cosmic star formation histories are used to estimate BBH merger yields from both dynamical and binary evolution channels. The combined results are consistent with gravitational wave observations and suggest that BBH mergers at high redshifts may be predominately determined by binary evolution physics. However, there are some limitations in the present approach that need to be considered in future research.