Primordial Black Holes as Seeds for Extremely Overmassive AGN Observed by JWST

Saiyang Zhang, Boyuan Liu, Volker Bromm, Florian Kühnel

First listed 2025-12-16 | Last updated 2026-03-12

Abstract

The James Webb Space Telescope (JWST) has recently identified Abell 2744-QSO1 as a compact, metal-poor, black hole (BH) dominated galaxy at $z\simeq 7$. This system exhibits an extreme black-hole-to-stellar mass ratio and unusually low metallicity, posing significant challenges to BH seeding models. Motivated by these discoveries, we perform high-resolution cosmological simulations with a massive primordial black hole (PBH; $M_{\rm BH}=5\times10^7\,M_\odot$) seed, incorporating for the first time a fully coupled treatment of PBH accretion, BH feedback, and Population~III/II star formation and stellar feedback. Although PBHs accelerate structure formation through the seed effect, the associated strong thermal feedback from the accretion delays the onset of star formation to $z\lesssim 10$, producing short, bursty episodes throughout the subsequent evolution. PBH-driven outflows expel enriched gas from the nucleus, while sustained inflows from the intergalactic medium continuously replenish pristine material. This feedback-regulated cycle naturally yields low accretion rates ($\dot{m}_{\rm BH}/\dot{m}_{\rm edd} \sim 1-10\%$), subsolar metallicities ($Z/Z_\odot\lesssim10^{-2}$) and extreme $M_{\rm BH}/M_\star$ ratios during both the initial star-forming phase and the subsequent quenching phases, in excellent agreement with JWST observations. Our results demonstrate that massive PBHs offer a viable pathway for forming the most extreme high-redshift systems, providing a physically motivated explanation for the extraordinary properties of Abell 2744-QSO1, as a sub-class of the broader population of JWST-discovered "little red dots".

Short digest

High-resolution cosmological simulations with a massive PBH seed (MBH=5×10^7 M⊙) couple accretion/feedback to Pop III/II star formation to test LRD-like systems such as Abell 2744–QSO1 at z≈7. The PBH accelerates halo assembly but its thermal feedback delays first stars to z≲10 and drives short, bursty star-formation episodes while outflows purge metals and inflows resupply pristine gas. This cycle sustains low accretion (∼1–10% Eddington), subsolar metallicities (Z/Z⊙≲10^−2), and extreme MBH/M⋆ during both early star-forming and quenched phases. The tracks match A2744–QSO1’s low-Z and overmassive-BH constraints, pointing to massive PBHs as a viable channel for the most extreme LRDs.

Key figures to inspect

• Figure 1: Inspect the radial density and metallicity profiles at the final snapshot to see the steep inner density cusp and persistently subsolar gas metallicity across the central 100–300 pc, and compare the Pop III vs. Pop II spatial distribution relative to the PBH.
• Figure 2: Compare BH accretion histories (as Eddington fractions) to the observed range for A2744–QSO1 and note how star formation occurs in short, feedback-regulated bursts; check the impact of including full stellar feedback and the Pop III/Pop II decomposition.
• Figure 3: Follow the redshift-colored tracks of metallicity versus MBH/M⋆ for central and 300 pc apertures to verify that the simulations enter the observed A2744–QSO1 zone of low Z and extreme mass ratio at early times.