Dynamical formation of GW190521 in stellar clusters

In 2020, the LIGO/Virgo Collaboration announced the discovery of GW190521 in which a 142 solar mass black hole (BH) formed from the merger of 85 solar mass and 66 solar mass BHs. This discovery provides the first concrete evidence for the existence of an intermediate-mass BH (IMBH) larger than a 100 solar mass. BHs with these masses had not been observed before and theoretical models show that they are difficult to form through isolated stellar/binary evolution of massive stars. Many stars are born in stellar clusters where the density of stars can be up to a million times higher than the density of stars in the solar neighbourhood (e.g., globular clusters (GCs), nuclear stellar clusters (NSCs). Close gravitational encounters between binary stars and BHs are conducive to dynamically forming merging BBHs in these dense stellar environments. If the merged BH is retained in the cluster, then additional dynamical encounters may further increase its mass. Therefore, it is possible that GW190521 type BBHs can be formed in dense stellar clusters. In this talk, I will summarize results from state-of-the-art Monte Carlo and direct N-body simulations of stellar clusters to highlight the different pathways that can lead to the dynamical formation of merging binary BHs similar to GW190521.