A Standard Ultraviolet Continuum Can Hide Supercritical Accretion in GN-z11
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 ME=1.12×107 Msun, an order of magnitude above the log(Mbh/Msun)=6.2±0.3 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 Mdotout=3.1--4.4 Msun yr−1 and placing Rsph=(1.2--1.8)×103 rg inside Ruv=(2.6--2.9)×103 rg. Replacing the inner ∼103rg by a supercritical flow shifts the fitted slope by only ΔbetaUV=+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 106 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.
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BibTeX
@misc{mitra2026standard,
title = {{A Standard Ultraviolet Continuum Can Hide Supercritical Accretion in GN-z11}},
author = {Samik Mitra and Ramananda Santra},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.15967},
eprint = {2609.15967},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.15967},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Standard Ultraviolet Continuum Can Hide Supercritical Accretion in GN-z11
AU - Samik Mitra
AU - Ramananda Santra
PY - 2026
DA - 2026/09/14
PB - arXiv
DO - 10.48550/arxiv.2609.15967
UR - https://arxiv.org/abs/2609.15967
AB - 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_E=1.12×10^7 Msun, an order of magnitude above the log(Mbh/Msun)=6.2±0.3 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_out=3.1--4.4 Msun yr^−1 and placing Rsph=(1.2--1.8)×10^3 rg inside Ruv=(2.6--2.9)×10^3 rg. Replacing the inner ∼10^3rg by a supercritical flow shifts the fitted slope by only ΔbetaUV=+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.
ER -