A black hole in a near-pristine galaxy 700 million years after the Big Bang

TL;DR

This paper analyzes a strongly lensed Little Red Dot at $z=7.04$ (Abell2744-QSO1) using JWST/NIRSpec-IFU to measure extremely low gas-phase metallicity around an accreting black hole. By disentangling narrow and broad Hβ and resolving the narrow-line region out to several hundred parsecs, it finds [OIII]5007/Hβ_N ratios implying $Z o Z_\odot$ values of order $10^{-3}$ to $10^{-2}$, with a central $Z obreak oreak 4.7 imes10^{-3}Z_\odot$ and outer $Z < 3.9 imes10^{-3}Z_\odot$. The black hole mass is directly constrained to $ ext{log}(M_{BH}/M_\odot) = 7.7 \\pm 0.3$, while the host’s dynamical mass is constrained to $M_ ext{star} < 2\times10^7\,M_\\odot$, yielding $M_{BH}/M_ ext{star} > 2$, i.e., a massive BH in a nearly pristine host. These results challenge traditional BH seed models (heavy seeds, super-Eddington growth) and favor primordial or alternative seed scenarios, highlighting that such metal-poor AGN/LRDs may be relatively common at $z>7$ and motivating further JWST studies and model refinements.

Abstract

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$β$ emission indicates extremely low metallicity, about $4\times 10^{-2}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.

Figures (11)

• Figure 1: Spectra of QSO1 around H$\beta$ and [OIII]5007. a) Spectrum extracted from the central region (blue solid line), with the fitted broad component of H$\beta$ (orange dashed line), continuum and H$\beta$ absorption (green dashed line), as in Ji2025, while the violet dashed line shows the total fit. b) The same central spectrum where the broad H$\beta$, continuum and H$\beta$ absorption have been subtracted to highlight the narrow component of H$\beta$ and [OIII]5007. c) Spectrum extracted from a semi-annulus from 0.1$\arcsec$ ($\sim 150$ pc) to 0.2$"$ ($\sim 300$ pc) from the centre -- H$\beta$ is clearly detected while [OIII]5007 is formally undetected (flux mostly on a single pixel). In all panels the dashed gray line shows the 1$\sigma$ error level.
• Figure 2: Map of H$\beta$ narrow (obtained by simply collapsing the three central channel of the continuum-subtracted line) with overlaid the central aperture and annulus used for extracting the spectra in panels a,b and c of Fig.\ref{['fig:spectra']}, respectively; [OIII] falls in the detector gap in the region on the East (i.e. left) of the dashed straight line, hence this portion of the annulus was not used for extracting the spectrum.
• Figure 3: Radial profiles (normalized to the peak) of the narrow component of H$\beta$ (solid blue, the shaded area is 1$\sigma$ error on the mean), the broad component of H$\beta$ (dashed orange), the continuum on the blue side of H$\beta$ (dotted green) and the continuum on the red side of H$\beta$ (dot-dashed red), which trace the PSF. The narrow component has a compact core but it also shows a clear extended component.
• Figure 4: Metallicity constraints on QSO1 inferred from the observed emission line ratios. (a) [OIII]5007/H$\beta$ vs 12+log(O/H); (b) $\hat{R}=0.46~R_2+0.88~R_3$ vs 12+log(O/H) The red solid line shows the calibration for [OIII]/H$\beta$ from Sanders2024 and while the red dot-dashed line shows the calibration for $\hat{R}$ from Laseter2024metallicitycalib, in both cases using high-z star forming galaxies. Small blue points are individual, T$_e$-based measurements at high-z from Cataldi2025Marta. Green squares are other high-z galaxies at low metallicity Willott2025Cullen2025Mowla2024. Purple pentagons are median values at high redshift from the Aurora survey Sanders2025Aurora. The QSO1 line ratios are shown with large circles, yellow and orange for the central region and for the annulus at $\sim$200 pc, respectively. The green long-dashed line in (a) is the photoionization models relation by Nakajima2022 adopted by Morishita2025_lowZ and short-dashed purple line in (b) is the calibration for $\hat{R}$ from Scholte2025; both these alternative calibrations would give even lower metallicities. The shaded area indicate the high metallicity solutions excluded by the low ionization lines (Methods). In both panels errobars are at 1$\sigma$ while upper limits are at 3$\sigma$
• Figure 5: Comparison of the QSO1 properties with Semi-Analytical Models (SAMs) and Hydrodynamical Simulations on the metallicity versus $M_{BH}$ (left) and metallicity versus $M_{BH}/M_{star}$ (right) diagrams. Description of all models are provided in the text and in Appendix \ref{['app:models']}. Contours enclose 1%, 30%, 68%, 84%, 95%, and 99% of the models or simulations. Models and simulations outside the 99% countours are plotted individually. For the PBH hydrodynamical simulations, the red line shows the evolution of a representative case, and the shaded regions indicate the uncertainty on the metallicity. The golden and orange circles show the properties of QSO1 in the central aperture and in the outer annulus ($\sim 200~\rm{pc}$ from the centre), respectively (upper limits are at 3$\sigma$).