Binary black hole mergers from young massive clusters in the pair-instability supernova mass gap

Abstract: The recent discovery of the binary black hole (BBH) merger event GW190521, between two black holes (BHs) of $\approx100M_\odot$, and as well as other massive BBH merger events involving BHs within the pair-instability supernova (PSN) mass gap have sparked widespread debate on the origin of such extreme gravitational-wave (GW) events. In this study, I investigate whether dynamical interactions in young massive clusters (YMCs) serves as a viable scenario for assembling PSN-gap BBH mergers. To that end, I explore a grid of 40 new evolutionary models of a representative YMC of initial mass $M_{\rm cl}=7.5\times10^4M_\odot$ ($N\approx1.28\times10^5$) and size $r_h=2$ pc, with all BH progenitor stars being initially in primordial binaries. All cluster models are evolved with the direct, relativistic N-body code NBODY7 incorporating up to date remnant formation, BH natal spin, and general-relativistic (GR) merger recoil schemes. The BBH mergers from these model cluster computations agree well with the masses and effective spin parameters of the GW events in the latest GW transient catalogue (GWTC). In particular, GW190521-like, i.e., $\approx200M_\odot$, low aligned spin events are produced via dynamical merger among BHs derived from star-star merger products. GW190403-like, i.e., PSN-gap, highly asymmetric and aligned events result from mergers involving BHs that are spun up via matter accretion or binary interaction. The present YMC models yield a present day, intrinsic merger rate density of $0-3.8\times10^{-2}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$ for GW190521-type events. They produce GW190403-like events at a rate within $0-1.6\times10^{-1}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$ and their total BBH-merger yield within the PSN gap is $0-8.4\times10^{-1}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$.