Resolving Black Hole Family Issues among the Massive Ancestors of Very High-spin Gravitational-wave Events like GW231123

Abstract The latest detection of GW231123, a black hole (BH) merger with exceptionally large component masses and high spins, has been suggested as a smoking gun for hierarchical formation. In this scenario, a first generation of BHs form from collapsing stars in dense environments such as star clusters, where they assemble dynamically and undergo subsequent mergers. We discuss three challenges for forming GW231123-like events in this scenario: (1) The high masses of the incoming BHs appear to be in the predicted pair-instability mass gap, suggesting that higher-order generation BHs are involved. (2) Very high spins ( χ f ≳ 0.8) are unlikely for dynamically assembled BHs because of the isotropic spin distribution. (3) Hierarchically formed BHs can receive large recoils that kick them out of their cluster and prohibit subsequent mergers. We simulate this scenario and show that only a few percent of mergers recover remnants within GW231123’s primary spin estimate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.19</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and are retained inside typical star clusters. A large fraction of very rapidly spinning second-generation BHs (including χ f ≳ 0.9) can form if the first-generation BHs merge 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 receive low recoil kicks ( v k ≲ 100 km s −1 ) and would therefore likely be retained inside a cluster. GW231123-like events, if formed in a star cluster, could require first-generation BHs with aligned spins formed from interacting stellar binaries, followed by the dynamical assembly for a subsequent merger.