The Hierarchical Merger Scenario for GW231123

Abstract: GW231123 exhibits exceedingly massive components and high spins, which challenges the formation of first-generation (1G) black holes from stellar collapse and implies that this event might originate from hierarchical mergers. Here we show that the \texttt{2G+2G} merger scenario for GW231123 is favored over a \texttt{2G+1G} (or \texttt{3G+2G}) merger, with odds ratios of $>$$\mathcal{O}(10^3)$. The primary (secondary) black hole is consistent with merging binary black holes with masses of $75^{+7}{-7}\,M\odot$ and $66^{+4}{-10}\,M\odot$ ($64^{+8}{-10}\,M\odot$ and $59^{+7}{-9}\,M\odot$; 90\% credible intervals), respectively. Our results reveal that the treatment of spin priors from the population level and waveform model choice are decisive in interpreting potential hierarchical gravitational-wave signals.