Resolving Black Hole Family Issues Among the Massive Ancestors of Very High-Spin Gravitational-Wave Events Like GW231123

TL;DR

GW231123 challenges standard black hole formation models due to its very high masses near the proposed mass gap and a primary spin near $\chi_1\approx0.9$. The authors explore hierarchical merger scenarios in dense star clusters, including purely dynamical sequences and channels where the first-generation BHs have aligned spins from binary-star evolution, finding that random orientations yield only a small retention fraction, while aligned spins can produce remnants near $\chi_f\approx0.9$ and modest kicks. The chemically homogeneous binary evolution channel emerges as the most promising pathway to reproduce GW231123-like properties, suggesting that massive, aligned-spin BHs formed in binaries could subsequently merge dynamically. These results have implications for the location of the upper black hole mass gap and motivate further study of nuclear reaction-rate uncertainties and super-Eddington accretion as potential routes to higher-mass, high-spin BHs.

Abstract

The latest detection of GW231123, a binary black hole (BH) merger with exceptionally large masses and high spins for the incoming components, has been suggested as a smoking gun for hierarchical formation. In this scenario, a first generation of BHs resulting from collapsing stars form in a dense environment. Here they can assemble dynamically and undergo subsequent mergers. We discuss three challenges for the formation of a GW231123-like event inside a star cluster: 1) The high masses of the incoming BHs appear to be in the predicted pair-instability mass gap and thus suggest that second-generation or higher-order generation BHs are involved. 2) Very high spins ($χ_f \gtrsim 0.8$) are very unlikely for dynamically assembled BHs because of the isotropic distribution of spin vectors. 3) Hierarchically formed BHs are susceptible to receive large recoils, which could kick them out of their cluster. We simulate this scenario and show that only a few percent of mergers recover remnants within GW231123's primary spin estimate $χ_1=0.9^{+0.10}_{-0.19}$ and are retained inside typical star clusters. A large fraction of very rapidly spinning second-generation BHs (including $χ_f>0.9$) can only form if the first-generation BHs merges with aligned spins. This is a natural outcome of massive binary star evolution scenarios, such as a chemically homogeneous evolution. This scenario also predicts equal masses for the components, implying that the resulting BHs tend to receive very low recoil kicks and would therefore likely be retained inside a cluster. We conclude that GW231123-like events, if formed in a star cluster, could require first-generation BHs with large aligned spins that evolved through stellar binary interaction, followed by the dynamical assembly for a subsequent merger. We discuss the implications for the uncertain lower edge of the putative mass gap 60-130 $\rm M_\odot$.

Figures (3)

• Figure 1: Cartoons of formation scenarios considered in this work. For purely dynamical scenarios we assume that the primary black hole is formed through a hierarchical sequence 1g+1g (red), 2g+2g (blue), 2g+1g (turquoise), and 3g+1g (orange), and subsequently merges with a secondary black hole in a GW231123-like event. For these dynamical formation sequences we assume the 1st-generation black holes (1g) to be slowly spinning GWTC3 the remnants to obtain a large spin. At each merger, the directions of the spin and orbital angular momentum vectors are fully randomised. Alternatively, we consider a scenario where the primary black hole forms from massive binary star evolution (green), e.g., of primordial stellar binaries in a star cluster, before it dynamically assembles and merges with the secondary black hole. Here, we assume the 1g black holes to obtain aligned spins (exploring different assumptions about their magnitudes) through binary star evolution which leads to much higher primary spins $\chi_f\approx0.9$.
• Figure 2: Recoil kick velocity distribution (left panel) and remnant spin distribution (right panel) of black holes formed through 1g+1g (red), 2g+1g (turquoise), 2g+2g (blue), and 3g+1g (orange) merger sequences. For all mergers, it is assumed that the black hole spin and orbital angular momentum directions are randomly distributed. For the coloured solid lines, we assume all mergers result from incoming black holes of the same mass. The dashed turquoise and orange line assume $q=1/2$ and $q=1/3$ for 2g+1g and 3g+1g, respectively. The vertical dashed line in the left panel indicates the adopted ejection velocity $v_{\rm ej}=100\,\rm km\,s^{-1}$, beyond which remnant black holes are removed from participating in further mergers (see text). In the right panel, we show GW231123's Combined posterior distributions for the primary (black) and secondary spin (grey) from the public LVK data release GW231123Zenodo. Vertical dashed and dotted lines indicate their $0.1$, $0.5$, and $0.9$-quantiles, respectively.
• Figure 3: Same as Figure \ref{['fig:isotropic']} for our primordial binary star scenario, where the ancestral black holes of GW231123's are assumed to have equal masses and spins aligned with the orbital angular momentum. For the green line we assume both black holes to be slowly spinning, for the purple line we assume just one to be highly spinning, and for the gold line we assume both to be highly spinning (see text for details).