GW231123 Formation from Population III Stars: Isolated Binary Evolution

GW231123 is a merger of two black holes (BHs) with estimated masses exceeding $100\;{\rm M}_{\odot}$, 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 $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate is $2\sigma$ 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 $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ 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.