Repeated Mergers of Black Hole Binaries: Implications for GW190521

The gravitational wave event GW190521 involves the merger of two black holes of similar to 85 M (circle dot) and similar to 66 M (circle dot) forming an intermediate-mass black hole (IMBH) of mass similar to 142 M (circle dot). 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 (circle dot). 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 less than or similar to p less than or similar to 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.

Automated summary

The study suggests that the gravitational wave event GW190521 may involve the merger of two large black holes to form an intermediate-mass black hole. The researchers propose that this formation pathway could occur through multiple mergers in the core of a globular cluster. These findings contribute to our understanding of the formation pathways for black holes within the upper mass gap.