The accretion of quasars at the epoch of reionisation: $JWST$ catches the primeval monsters slowly feasting

B. Trefoloni, E. Nardini, S. Carniani, E. Lusso, A. Marconi, E. Parlanti, A. Sacchi, A. Shlentsova, M. Signorini, G. Risaliti, S. Zamora

First listed 2025-12-18 | Last updated 2025-12-18

Abstract

Quasars (QSOs) emit an enormous amount of light as a result of the accretion of gas onto supermassive black holes (SMBHs). Thanks to their luminosity, the most distant known QSOs allow us to trace the growth of SMBHs deep into the epoch of reionisation. In this work, we employed $JWST$/NIRSpec observations of eight luminous (log$(L_{3000\,A^{\circ}}/(erg \, s^{-1}))>$45.7) QSOs at $z\geq$5.9 to constrain their accretion properties, namely black hole mass, accretion disc (AD) luminosity, and Eddington ratio ($M_{BH}$, $L_{AD}$, $λ_{Edd}$), by fitting the rest-frame UV and optical emission with different AD models. This method provided self-consistent measurements of both $M_{BH}$ and $L_{AD}$. The uncertainties on $M_{BH}$ and $L_{AD}$, obtained within the AD-modelling framework ($σ^{AD}_{M_{BH}}\sim$0.2 dex; $σ^{AD}_{L_{AD}}\sim$0.1 dex), are significantly smaller than the systematic uncertainties associated with single-epoch $M_{BH}$ ($\sim$0.4 dex) and $L_{AD}$ derived via bolometric corrections ($\sim$0.2 dex). Based on these results, in our sample we found an average Eddington ratio of $\langle \log(λ_{Edd}) \rangle=-0.9$, with a dispersion of $\sim$0.2 dex. Assuming that our high-z QSOs are representative of optically-selected bright blue QSOs, we derive a fraction of systems accreting above the Eddington limit of $\sim$0.2%. In conclusion, this work i) demonstrates the suitability of $JWST$ to test AD models on high-redshift ($z\gtrsim$4) QSOs, thanks to the large NIRSpec spectral coverage; ii) shows that AD modelling can yield robust $M_{\rm BH}$ and $L_{\rm AD}$ measurements, with smaller uncertainties than the typical calibrations; and iii) provides compelling evidence for sub-Eddington accretion in bright high-$z$ QSOs, challenging the widespread paradigm of near- or super-Eddington accretion occurring in these sources.

Short digest

Using NIRSpec’s 0.6–5.3 μm coverage, the authors fit accretion-disc models to rest-UV/optical spectra of eight luminous z≥5.9 quasars to obtain self-consistent MBH and LAD with tightened uncertainties (~0.2 and ~0.1 dex). They find a mean Eddington ratio ⟨log λEdd⟩ = −0.9 with ~0.2 dex scatter, indicating these bright EoR quasars are typically sub‑Eddington. Compared to single‑epoch recipes, the AD approach avoids C IV biases by anchoring to Hβ/Mg II and continuum windows while shrinking systematics. Assuming the sample represents optically selected blue QSOs, only ~0.2% appear super‑Eddington, challenging the prevailing near/super‑Eddington picture.

Key figures to inspect

• NIRSpec PRISM spectra for each QSO across rest 1200–6700 Å: mark continuum windows and show line fits to Mg II and Hβ–[O III]; verify line widths used for SE comparisons and the clean avoidance of C IV.
• AD SED fits overlaid on the spectra: locate the SED peak and show best‑fit/posterior ranges for MBH and LAD; note any assumptions on efficiency/spin and their impact on the fit.
• AD‑based vs single‑epoch MBH and LAD comparison plots: check offsets, scatter, and any luminosity/redshift trends that could bias SE calibrations at z≈6–8.
• Distribution of λEdd for the eight QSOs: confirm the mean −0.9 dex, ~0.2 dex dispersion, and quantify the tiny super‑Eddington tail (~0.2%).
• Sample overview figure (z–L3000 plane): place these eight objects among other surveys to see why NIRSpec coverage captures both Mg II and Hβ at z≥5.9.