GW231123 Formation from Population III Stars: Isolated Binary Evolution

Abstract GW231123 is a merger of two black holes (BHs) with estimated masses exceeding 100 M ⊙ , making them the most massive BHs discovered to date via gravitational-wave (GW) observations. We investigate whether GW231123-like events can originate from isolated Population (Pop) III binary stars using binary population synthesis calculations. Our findings indicate that isolated Pop III binaries can produce GW231123-like events at a rate sufficient to explain the discovery of GW231123, provided that three conditions are met: (i) Pop III stars evolve with inefficient convective overshooting, (ii) the 12 C( α , γ ) 16 O rate is 2 σ lower than the standard value, and (iii) Pop III binary stars share the same orbital parameters as Pop I/II binary stars at the initial time. In contrast, GW190521—the most massive BH merger in the Gravitational Wave Transient Catalog 3—can be formed from isolated Pop III binaries even with the standard 12 C( α , γ ) 16 O rate. We demonstrate that the discovery of GW231123 is increasingly constraining the parameter ranges of single-star evolution models, under the assumption that these GW events originate from isolated binary evolution.