Heavy seeds and the first black holes: Insights from the BRAHMA simulations

Aklant K. Bhowmick, Laura Blecha, Paul Torrey, Luke Zoltan Kelley, Priyamvada Natarajan, Rachel S. Somerville, Rainer Weinberger, Alex M. Garcia, Lars Hernquist, Tiziana Di Matteo, Jonathan Kho, Mark Vogelsberger

First listed 2025-10-01 | Last updated 2025-10-01

Abstract

From the luminous quasars at $z \sim 6$ to the recent $z \sim 9-11$ 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 ($\sim 10^4-10^5 M_{\odot})$ 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 $z \gtrsim 9$, leaving mergers as the dominant growth channel. Gas accretion dominates at $z \lesssim 9$, where permissive models (super-Eddington or low radiative efficiency) build $\sim 10^9\ M_{\odot}$ BHs powering quasars by $z \sim 6$, while stricter IllustrisTNG-based prescriptions yield much lower BH masses ($\sim 10^6-10^8\ M_{\odot}$). Our seed models strongly affect merger-driven growth at $z \gtrsim 9$: only the most lenient models (with $\sim 10^5\ M_{\odot}$ seeds) produce enough BH mergers to reach $\gtrsim 10^6\ M_{\odot}$ by $z \sim 10$, consistent with current estimates for GN-z11. Our dynamical friction model gives low merger efficiencies, hindering the buildup of $\gtrsim 10^7\ M_{\odot}$ BHs by $z \sim 9-10$, 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.

Short digest

BRAHMA runs constrained-IC hydrodynamic simulations of 5σ overdensities with heavy seeds (∼10^4–10^5 Msun) planted in dense, metal-poor, LW-irradiated gas, tracking growth via Bondi accretion and subgrid dynamical friction. With IllustrisTNG-like feedback, accretion is suppressed at z ≳ 9 so mergers dominate; at z ≲ 9 accretion takes over, and permissive models (super-Eddington/low radiative efficiency) can build ∼10^9 Msun black holes by z ∼ 6 while stricter prescriptions stall at 10^6–10^8 Msun. Only the most lenient seeding (∼10^5 Msun) yields enough early BH-BH coalescences to reach ≳10^6 Msun by z ∼ 10 (GN‑z11-like), but low merger efficiencies from the DF scheme hinder ≳10^7 Msun by z ∼ 9–10 as inferred for GHZ9, UHZ1, and CAPERS‑LRD‑z9. If those extreme BH-to-stellar mass ratios hold, they require very short merger timescales or reduced AGN thermal feedback; weaker stellar feedback alone boosts SFR and accretion but not the ratios.

Key figures to inspect

• Figure 1: Compare 5SIGMA_COMPACT vs 5SIGMA_TYPICAL density–metallicity maps to see how compactness, metallicity, and tidal field set up heavy-seed formation sites in an extreme peak.
• Figure 2: Track the main halo’s mass and stellar-mass histories to verify it reaches quasar-host scales by z ∼ 6 and that host stellar masses align with JWST AGN estimates—establishing the environment where early BH assembly is evaluated.
• Figure 3: Inspect the redshift-dependent seeding rate for different LW thresholds and seed masses; quantify how a ∼10^5 Msun seed and lower J_crit inflate the seed abundance (including the higher-resolution runs).
• Figure 4: Read across seed and accretion models to see the merger-dominated regime at z > 9 vs the accretion-dominated phase later; contrast subgrid-DF with BH repositioning and identify which combinations reach ∼10^9 Msun by z ∼ 6 and approach GN‑z11-like masses by z ∼ 10 while matching MBH/M⋆ trends.