From the luminous quasars at z 6 to the recent z 9-11 AGNs revealed by JWST, observations of the earliest black hole (BH) populations can provide unique constraints on BH formation and growth models. We use the BRAHMA simulations with constrained initial conditions to investigate BH assembly in extreme overdense regions. The simulations implement heavy seeds (10⁴-10⁵ M_) forming in dense, metal-poor gas exposed to sufficient Lyman-Werner flux. With gas accretion modeled via Bondi-Hoyle formalism and BH dynamics and mergers using a subgrid dynamical friction scheme, we isolate the impact of seeding, dynamics, accretion, and feedback on early BH growth. With fiducial stellar and AGN feedback inherited from IllustrisTNG, accretion is strongly 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 10⁹\ M_ BHs powering quasars by z 6, while stricter IllustrisTNG-based prescriptions yield much lower BH masses (10⁶-10⁸\ M_). Our seed models strongly affect merger-driven growth at z 9: only the most lenient models (with 10⁵\ M_ seeds) produce enough BH mergers to reach 10⁶\ M_ by z 10, consistent with current estimates for GN-z11. Our dynamical friction model gives low merger efficiencies, hindering the buildup of 10⁷\ M_ 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 as extreme as currently inferred, 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.
Bhowmick et al. (Wed,) studied this question.