Super-Eddington accretion in protogalactic cores

¹ Dipartimento di Fisica, Sapienza, Università di Roma, Piazzale Aldo Moro 5, IT-00185 Roma, Italy
² INAF, Osservatorio Astronomico di Roma, Via di Frascati 33, IT-00078 Monte Porzio Catone, Italy
³ INFN, Sezione di Roma I, Piazzale Aldo Moro 2, IT-00185 Roma, Italy
⁴ Department of Astrophysics, University of Zurich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
⁵ Sapienza School for Advanced Studies, Viale Regina Elena 291, IT-00161 Roma, Italy
⁶ Como Lake Center for Astrophysics, DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, IT-22100 Como, Italy
⁷ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, IT-20126 Milano, Italy
⁸ INAF, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, IT-40129 Bologna, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 August 2025
Accepted: 10 February 2026

Abstract

The presence of massive black holes (BHs) exceeding 10⁹ M_⊙ already at redshift z > 6 challenges standard models of BH growth. Super-Eddington (SE) accretion has emerged as a promising mechanism to solve this issue, yet its impact on early BH evolution in tailored numerical experiments remains largely unexplored. In this work, we investigate the growth of BH seeds embedded in a gas-rich, metal-poor protogalaxy at z ∼ 15 using a suite of high-resolution hydrodynamical simulations that implement a slim-disc-based SE accretion model. We explored a broad parameter space, varying the initial BH mass, feedback efficiency, and spin. We find that SE accretion enables rapid growth in all cases, allowing BHs to accrete up to 10⁵ M_⊙ within a few 10³–10⁴ years, independent of seed properties. Feedback regulates this process, both by depleting central gas and altering BH dynamics via star formation-driven potential fluctuations, yet even the strongest feedback regimes permit significantly greater growth than the Eddington-limited case. Growth stalls after less than ∼1 Myr due to local gas exhaustion, as no large-scale inflows are present in the adopted numerical setup. Our results show that SE accretion naturally leads to BHs that are overmassive relative to their host galaxy stellar content, consistent with JWST observations. We conclude that short low-duty-cycle SE episodes represent a viable pathway for assembling the most massive BHs observed at early cosmic times, even when starting from light seeds.