Letter to the Editor

Fast-spinning massive black holes from slowly rotating low-metallicity stars: Implications for GW231123

¹ Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
² Center of Astronomy and Astrophysics, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
³ Astrophysics Research Centre, Lennard-Jones Laboratories, Keele University, Keele ST5 5BG, UK
⁴ Kavli IPMU (WPI), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8583, Japan
⁵ Departament d’Astronomia i Astrofísica, Universitat de València, Av. Vicent Andrés Estellés 19, E-46100 Burjassot, València, Spain
⁶ Observatori Astronòmic, Universitat de València, 46980 Paterna, Spain
⁷ Geneva Observatory, University of Geneva, Chemin Pegasi 51, CH-1290 Sauverny, Switzerland

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Received: 2 April 2026
Accepted: 22 June 2026

Abstract

Context. The origin of massive black holes in the early Universe remains uncertain and is still unexplored. Population III (Pop III; zero-metallicity) stars are among the first stellar sources capable of producing such remnants, but their evolution is very sensitive to rotation.

Aims. We explore how slow initial rotation affects the evolution and black hole formation of very massive Pop III stars and assess their potential to become massive fast-spinning black holes consistent with gravitational-wave events such as GW231123.

Methods. We computed a grid of non-rotating and slowly rotating Pop III stellar models with initial masses of 80, 85, and 90 M_⊙ using the GENEC stellar evolution code. Our models included rotation-induced mixing and angular momentum transport by magnetic torques (Taylor-Spruit dynamo and the magneto-rotational instability). We analysed the CO core masses and their volume-averaged adiabatic index ⟨Γ₁⟩ to assess stability against electron-positron pair creation. From the angular momentum profiles of the stellar models by the end of He-burning, we estimated the resulting black hole masses and dimensionless spins under the assumption of a direct collapse.

Results. Our non-rotating and slowly rotating 80 and 85 M_⊙ models developed carbon-oxygen core masses between 31 and 36 M_⊙ and had an adiabatic index ⟨Γ₁⟩, which remained above 4/3 (i. e. they avoided pair instability). Our models thus predict that Pop III stars can keep most of their mass and collapse directly to form black holes of 80–85 M_⊙ with dimensionless spins up to a_BH ≲ 0.7.

Conclusions. Initially slowly rotating massive Pop III stars can form very massive rapidly spinning black holes just below the pair-instability regime. This supports an interpretation of the lower boundary of the pair-instability supernova mass gap as a smooth, structure-dependent transition and identifies single-star Pop III evolution as a possible component channel for the population of massive fast-spinning black holes observed by gravitational-wave detectors, subject to the uncertain efficiency of internal angular momentum transport and mass-loss prescriptions.