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This study challenges the standard method of estimating black hole masses from ultraviolet light in distant galaxies. Analyzing GN-z11, a galaxy at very high redshift, researchers found that the ultraviolet continuum suggests a black hole mass ten times larger than estimates from spectral line measurements. They demonstrate that supercritical accretion (matter falling into the black hole faster than the Eddington limit) can produce a standard-looking ultraviolet spectrum while actually harboring a much smaller black hole, reconciling the discrepancy.
Why it matters
This finding significantly reduces the estimated masses of the earliest supermassive black holes in the universe, easing theoretical constraints on how quickly these objects must have grown in the first billion years after the Big Bang. The results suggest that ultraviolet-based mass estimates should be treated as upper limits rather than direct measurements, affecting interpretations of JWST observations of early galaxies.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: The ultraviolet continuum from a high-redshift accreting black hole is routinely used to infer its mass. GN-z11 offers a sharp test of whether that inference is secure. A thin-disk fit to its continuum gives an Eddington mass of $M_{rm E}=1.12times10^{7},Msun$, an order of magnitude above the $log(Mbh/Msun)=6.2pm0.3$ inferred from broad N~textsc{iv}. We show that this apparent tension is not imposed by the ultraviolet slope alone. We derive a closed-form criterion comparing the radius where observed photons are produced with the radius where a supercritical flow departs from efficient solution. For GN-z11, the criterion places the supercritical transition inside the ultraviolet-emitting region. Composite disks retaining the outer solution confirm this numerically, requiring $Mdot_{rm out}=3.1$–$4.4,Msun,mathrm{yr^{-1}}$ and placing $Rsph=(1.2$–$1.8)times10^{3},rg$ inside $Ruv=(2.6$–$2.9)times10^{3},rg$. Replacing the inner $sim10^{3}rg$ by a supercritical flow shifts the fitted slope by only $DeltabetaUV=+0.08$ to $+0.11$, comparable to uncertainties from standard spectral modelling. Reversing the radial ordering requires the effective transition to move outward by a factor $1.7$–$2.1$. Within our composite models, the measured slope constrains black-hole mass only to around $10^{6},Msun$, an order of magnitude below the continuum Eddington mass. Continuum Eddington masses should therefore be treated as model-dependent bounds that assume global thin-disk efficiency. The same hierarchy appears in three additional JWST sources under the disk-dominated interpretation, where separation masses are only $4$–$6$% of continuum Eddington masses. This distinction can substantially weaken the seed-mass and early-growth demands inferred from ultraviolet continua at cosmic dawn for the first massive black holes.
Source: A Standard Ultraviolet Continuum Can Hide Supercritical Accretion in GN-z11