The effects of super-Eddington accretion and feedback on the growth of early supermassive black holes and galaxies

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We present results of cosmological zoom-in simulations of a massive protocluster down to redshift z≈ 4 (when the halo mass is ≈ 10¹³ M_) using the swift code and the eagle galaxy formation model, focusing on supermassive black hole (BH) physics. The BH was seeded with a mass of 10⁴ M_ at redshift z≈ 17. We compare the base model that uses an Eddington limit on the BH accretion rate and thermal isotropic feedback by the active galactic nuclei, with one where super-Eddington accretion is allowed, as well as two other models with BH spin and jets. In the base model, the BH grows at the Eddington limit from $z=9$ to $z=5.5$, when it becomes massive enough to halt its own and its host galaxy’s growth through feedback. We find that allowing super-Eddington accretion leads to drastic differences, with the BH going through an intense but short super-Eddington growth burst around z≈ 7.5, during which it increases its mass by orders of magnitude, before feedback stops further growth (of both the BH and the galaxy). By z≈ 4 the galaxy is only half as massive in the super-Eddington cases, and an order of magnitude more extended, with the half-mass radius reaching values of a few physical kpc instead of a few hundred pc. The BH masses in our simulations are consistent with the intrinsic BH mass–stellar mass relation inferred from high-redshift observations by JWST. This shows that galaxy formation models using the Λcold dark matter cosmology are capable of reproducing the observed massive BHs at high redshift. Allowing jets, either at super- or sub-Eddington rates, has little impact on the host galaxy properties, but leads to lower BH masses as a consequence of higher feedback efficiencies.