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

TL;DR

The paper investigates whether a massive primordial black hole seed with $M_{ m BH}=5\times10^7\,M_\\odot$ can jointly drive early structure formation and regulate star formation to explain Abell 2744--QSO1. It couples PBH accretion/feedback with Population III/II star formation and stellar feedback in cosmological simulations from $z\sim1100$ to $z\sim7$ using the GIZMO code and a 1 Mpc box. The results show BH accretion at $\sim$1–10% of the Eddington, growth to $M_{ m BH}\approx6\times10^7\,M_\odot$ by $z=7$, delayed and bursty star formation, and subsolar metallicities ($Z/Z_\odot\lesssim10^{-2}$), reproducing Abell 2744--QSO1's observed properties and MBH/Mstar extremeness. This work supports a PBH-seeded pathway for the most extreme high-redshift systems and highlights the need to incorporate radiative transfer and PBH clustering in future studies.

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".

Figures (3)

• Figure 1: Matter density and metallicity profiles around the central BH at $z=7$. Shown are the spherically averaged density and metallicity profiles from the final snapshot of the PBH_SF_M5e7_fd005 run. Top: Density of total matter (black), gas (green), dark matter (purple dashed), total stellar content (gray), Pop III stars (blue dashed–dotted), and Pop II stars (red dotted) vs. radius. Dashed black lines illustrate select power-law forms that match the simulated density profiles at certain parts. The inset shows the projected spatial distribution of Pop III (blue) and Pop II (red) stars relative to the central PBH (black circle), with a scale as indicated. Bottom: Radial metallicity profile for gas (green) and stars (grey), expressed in units of solar metallicity $Z_\odot$. Shaded regions indicate the observed metallicity constraints for Abell 2744--QSO1 within $\sim150$ pc (yellow) and $\sim300$ pc (orange) apertures Maiolino2025arXiv250522567M. The simulated system exhibits a steep inner density profile and maintains subsolar gas metallicity throughout the central $\sim300$ pc, in agreement with the properties inferred for QSO1.
• Figure 2: Combined BH accretion and star formation history. Shown are the BH accretion rates and star formation rates for the PBH_M5e7_fd005 and PBH_SF_M5e7_fd005 runs. Top: Accretion rate (green), expressed as a fraction of the Eddington limit, $\dot{m}_{\rm BH}/\dot{m}_{\rm edd}$, together with the total star formation rate (gray) within the vicinity of the PBH for the PBH_M5e7_fd005 run. Both quantities are shown as a function of cosmic time, with the corresponding redshift indicated on the upper axis. The shaded region (yellow) represents the accretion rate of Abell 2744-QSO1 inferred from observations as $\dot{m}_{\rm BH} / \dot{m}_{\rm edd}~\sim 1-3\%$QSO1Direct2025arXiv250821748J. Bottom: Same as the top panel, but for the PBH_SF_M5e7_fd005 run. In this case, the total SFR (gray) is decomposed into its Pop III (blue dash-dotted) and Pop II (red dotted) components. In both simulations, BH accretion remains at $\sim 1-10\%$ of the Eddington level, while star formation occurs in short, bursty episodes regulated by BH feedback.
• Figure 3: Metallicity evolution versus black-hole-to-stellar mass ratio in PBH-seeded galaxies. Shown are evolutionary tracks from two simulations with different feedback and spatial sampling: uniform (triangles with shades) from the PBH_M5e7_fd005 run, as well as central (circles, upper track) and 300 pc (squares, lower track) from the PBH_SF_M5e7_fd005 run. The color along each curve denotes redshift, from red ($z\sim10$) to blue ($z\sim7$), tracing metal enrichment over time. The green shaded region indicate model dispersion from analytical post-processing of the PBH_M5e7_fd005 run, while separate tracks show metallicities averaged over $0$–$150$ pc (central) and $150$–$300$ pc (300 pc) apertures. The yellow shaded region marks the allowed parameter space inferred for Abell 2744--QSO1, with its central ($\sim150$ pc) and extended ($\sim300$ pc) metallicity measurements shown as yellow and orange points Maiolino2025arXiv250522567M. The simulations naturally reproduce subsolar metallicities and the observed high $M_{\rm BH}/M_\star$ at early epochs.