Repeated Mergers of Black Hole Binaries: Implications for GW190521

Abstract

The gravitational wave event GW190521 involves the merger of two black holes of ∼85 M ⊙ and ∼66 M ⊙ forming an intermediate-mass black hole (IMBH) of mass ∼142 M ⊙. Both progenitors are challenging to explain within standard stellar evolution as they are within the upper black hole mass gap. We propose a dynamical formation pathway for this IMBH based on multiple mergers in the core of a globular cluster. We identify such scenarios from analysis of a set of 58 N-body simulations using NBODY6-gpu. In one of our simulations, we observe a stellar black hole undergoing a chain of seven binary mergers within 6 Gyr, attaining a final mass of 97.8 M ⊙. We discuss the dynamical interactions that lead to the final IMBH product, as well as the evolution of the black hole population in that simulation. We explore statistically the effects of gravitational recoil on the viability of such hierarchical mergers. From the analysis of all 58 simulations we observe additional smaller chains, tentatively inferring that an IMBH formation through hierarchical mergers is expected in the lifetime of a median-mass globular cluster with probability 0.01 ≲ p ≲ 0.1 without gravitational merger recoil. Using this order-of-magnitude estimate we show that our results are broadly consistent with the rate implied by GW190521, assuming that gravitational recoil ejection of progenitors has a low probability. We discuss implications for future gravitational wave detections, emphasizing the importance of studying such formation pathways for black holes within the upper mass gap as a means to constrain such modeling.