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

¹ Helmholtz-Instituts für Strahlen- und Kernphysik, Nussallee 14-16, 53115 Bonn, Germany
² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: sambaran@astro.uni-bonn.de

Received: 29 September 2021
Accepted: 14 June 2022

Abstract

Context. The recent discovery of the binary black hole (BBH) merger event GW190521, between two black holes (BHs) of ≈100 M_samp, 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 M_cl = 7.5 × 10⁴ M_samp (N ≈ 1.28 × 10⁵) and r_h = 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, χ_eff, of the events from the latest gravitational-wave transient catalogue (GWTC). In particular, GW190521-like, that is to say ≈200 M_samp, low χ_eff 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 χ_eff 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.