GW190521 formation via three-body encounters in young massive star clusters

Abstract

\n GW190521 is the most massive binary black hole (BBH) merger observed to date, and its primary component lies in the pair-instability (PI) mass gap. Here, we investigate the formation of GW190521-like systems via three-body encounters in young massive star clusters. We performed 2× 105 simulations of binary-single interactions between a BBH and a massive ≥60\u2009M⊙ black hole (BH), including post-Newtonian terms up to the 2.5 order and a prescription for relativistic kicks. In our initial conditions, we take into account the possibility of forming BHs in the PI mass gap via stellar collisions. If we assume that first-generation BHs have low spins, $\\sim {0.17}{{\\ \\rm per\\ cent}}$ of all the simulated BBH mergers have component masses, effective and precessing spin, and remnant mass and spin inside the $90{{\\ \\rm per\\ cent}}$ credible intervals of GW190521. Seven of these systems are first-generation exchanged binaries, while five are second-generation BBHs. We estimate a merger rate density $\\mathcal {R}_{\\rm GW190521}\\sim {0.03}\\,$Gpc−3\u2009yr−1 for GW190521-like binaries formed via binary-single interactions in young star clusters. This rate is extremely sensitive to the spin distribution of first-generation BBHs. Stellar collisions, second-generation mergers and dynamical exchanges are the key ingredients to produce GW190521-like systems in young star clusters.