2026ApJ...997..187B

From the luminous quasars at z ∼ 6 to the recent z ∼ 9-11 active galactic nuclei (AGN) revealed by JWST, observations of the earliest black hole (BH) populations can provide unique constraints on BH evolution. We use the BRAHMA simulations with constrained initial conditions to investigate BH assembly in extreme overdense regions. The simulations implement heavy ∼10⁴-10⁵M_⊙ seeds forming in dense, metal-poor gas exposed to sufficient Lyman-Werner flux. With gas accretion modeled via the Bondi-Hoyle formalism and BH dynamics with a subgrid dynamical friction scheme, we isolate the impact of seeding, dynamics, accretion, and feedback on BH evolution. With fiducial stellar and AGN feedback inherited from IllustrisTNG, accretion is suppressed at z ≳ 9, leaving mergers as the dominant growth channel. Gas accretion dominates at z ≲ 9, where permissive models (super-Eddington or low radiative efficiency) build ∼10⁹M_⊙ BHs powering quasars by z ∼ 6, while stricter IllustrisTNG-based prescriptions yield much smaller BHs (∼10⁶-10⁸M_⊙). Our seed models strongly affect mergers at z ≳ 9: only the most lenient models (with ∼10⁵M_⊙ seeds) produce enough BH mergers to reach ≳10⁶M_⊙ by z ∼ 10, consistent with current estimates for GN-z11. Our dynamical friction model gives low merger efficiencies. Therefore, even in such extreme regions, we are unable to produce ≳10⁷M_⊙ BHs by z ∼ 9-10, as currently inferred for GHZ9, UHZ1, and CAPERS-LRD-z9. If the BH-to-stellar mass ratios of these sources are indeed so extreme, they would require either very short BH merger timescales or reduced AGN thermal feedback. Weaker stellar feedback boosts both star formation and BH accretion and cannot raise these ratios.