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

Abstract

Context. The recent discovery of the binary black hole (BBH) merger event GW190521, between two black holes (BHs) of ≈100 Msamp, in addition to 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. GW190521 simultaneously triggers two critical questions: how BHs can appear within the ‘forbidden’ PSN gap and, if they do, how they get to participate in general-relativistic (GR) mergers. Aims. In this study, I investigate whether dynamical interactions in young massive clusters (YMCs) serve as a viable scenario for assembling PSN-gap BBH mergers. Methods. To that end, I explore a grid of 40 new evolutionary models of a representative YMC of initial mass and size Mcl = 7.5 × 104 Msamp (N ≈ 1.28 × 105) and rh = 2 pc, respectively. The model grid ranges over metallicity 0.0002 ≤ Z ≤ 0.02 and comprises initial cluster configurations of King central concentration parameters W0 = 7 and 9. In each model, all BH progenitor stars are initially in primordial binaries following observationally motivated distributions. All cluster models are evolved with the direct, relativistic N-body code NBODY7, incorporating up-to-date remnant formation, BH natal spin, and GR merger recoil schemes. Results. Binary black hole mergers from these model cluster computations agree well with the masses and effective spin parameters, χeff, of the events from the latest gravitational-wave transient catalogue (GWTC). In particular, GW190521-like, that is to say ≈200 Msamp, low χeff events are produced via a dynamical merger among BHs derived from star-star merger products. GW190403_051519-like, that is PSN-gap, highly asymmetric, high χeff events result from mergers involving BHs that are spun up via matter accretion or a binary interaction. The resulting present-day, differential intrinsic merger rate density, within the PSN gap, accommodates that from GWTC well. Conclusions. This study demonstrates that, subject to model uncertainties, the tandem of massive binary evolution and dynamical interactions in ≲100 Myr-old, low metallicity YMCs in the Universe can plausibly produce GR mergers involving PSN-gap BHs and in rates consistent with that from up-to-date GW observations. Such clusters can produce extreme events similar to GW190521 and GW190403_051519. The upper limit of the models’ GW190521-type event rate is within the corresponding LIGO-Virgo-KAGRA (LVK)-estimated rate limits, although the typical model rate lies below LVK’s lower limit. The present YMC models yield a merger rate density of 0−3.8 × 10−2 yr−1 Gpc−3 for GW190521-type events. They produce GW190403_051519-like events at a rate within 0−1.6 × 10−1 yr−1 Gpc−3 and their total BBH-merger yield within the PSN gap is 0−8.4 × 10−1 yr−1 Gpc−3.

Understanding the high-mass binary black hole population from stable mass transfer and super-Eddington accretion in bpass

“…Without altering the stellar physics, such massive BBH merger could also be explained by non-isolated binary formation pathways (Rodriguez et al 2019 ;Yang et al 2019 ;Arca-Sedda et al 2020 ;Di Carlo et al 2020 ;Santoliquido et al 2020 ;Renzo et al 2020b ;Arca-Sedda et al 2021 ;Bouffanais et al 2021 ;Mapelli et al 2021 ;Zevin et al 2021 ;Ballone et al 2022 ;Banerjee 2022 ;Costa et al 2022 ). Each would leave imprints on the properties of the merging BBH population and could challenge the standard isolated binary evolution formation pathway.…”

Intermediate-mass Black Holes on the Run from Young Star Clusters

“…Several formation channels for BHs in the upper mass gap have been discussed, including hierarchical mergers of lowermass BHs (e.g., Miller & Hamilton 2002;McKernan et al 2012;Rodriguez et al , 2019Antonini et al 2019;Gerosa & Berti 2019;Fragione et al , 2022Fragione & Silk 2020;Zevin & Holz 2022), stellar mergers in dense star clusters (e.g., Portegies Zwart & McMillan 2002;Di Carlo et al 2019;Rizzuto et al 2021;Kremer et al 2020a;Banerjee 2021Banerjee , 2022Banerjee et al 2020;Weatherford et al 2021;González et al 2021), BH growth through gas accretion in star-forming environments (e.g., Roupas & Kazanas 2019), BH formation through gravitational instabilities in the early universe (e.g., Loeb & Rasio 1994;Carr et al 2016), remnants of Population III stars (e.g., Madau & Rees 2001;Bromm & Larson 2004), BH-star collisions in dense star clusters (Giersz et al 2015;Rizzuto et al 2022), and IMBH formation from BH-star collisions in galactic nuclei (e.g., Rose et al 2022).…”