Discovery of a Broad-Line Active Galactic Nucleus at z = 9.98

Abstract

The James Webb Space Telescope (JWST) has uncovered a previously unexplored population of faint active galactic nuclei (AGNs) in the early Universe. We present MoM-BL-z10 at z=9.9837±0.0009, identified through a systematic search for broad-line AGNs at z≥6 using public JWST/NIRSpec PRISM spectra from the DAWN JWST Archive. The PRISM spectrum exhibits a broad component in Hβ at 5.35 microns. A narrow+broad Gaussian fit yields a broad Hβ detection at S/N = 3.87, with FWHM = 3480⁺²⁰⁴⁰₋₉₅₀ km s⁻¹ and L_Hβ,broad=1.25^+0.33_−0.31×10⁴² erg s⁻¹. The detection of [Ne III] λ3870 and the high [Ne III]/[O II] and [O III]/Hβ ratios further support the AGN interpretation. Its rest-frame UV and optical continuum slopes, compact NIRCam morphology, and a tentative MIRI F770W detection suggest similarities to Little Red Dots (LRDs). The broad Hβ detection enables the first virial SMBH mass estimate at z∼10. We derive M_BH=3.6^+5.8_−1.7×10⁷ M_⊙ and λ_Edd=0.11^+0.10_−0.07 using the standard single-epoch virial calibration and quasar bolometric correction. We also consider an alternative scenario adopting the LRD-specific bolometric correction and assuming λ_Edd=1, which yields M_BH=2.1^+0.6_−0.5×10⁶ M_⊙. Under either scenario, the inferred SMBH mass at a cosmic age of only 470 Myr highlights the challenge of rapid BH growth in the early Universe, suggesting super-Eddington accretion from a stellar-mass seed or a more massive seed. As the highest-redshift broad-line AGN identified to date, MoM-BL-z10 provides a new observational benchmark for understanding the early growth of supermassive black holes.

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BibTeX RIS

BibTeX

@misc{cai2026discovery,
  title = {{Discovery of a Broad-Line Active Galactic Nucleus at $z = 9.98$}},
  author = {Xingyao Cai and Akio K. Inoue and Masafusa Onoue and Suzuka Arai and Yoshinobu Fudamoto and Rohan P. Naidu and Pascal A. Oesch and Jenny E. Greene and Ivo Labbe and Jorryt Matthee and Anna de Graaff and Yuma Sugahara and Ken Mawatari and Pratika Dayal and Xi Kang},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.28735},
  eprint = {2610.28735},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.28735},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Discovery of a Broad-Line Active Galactic Nucleus at z = 9.98
AU  - Xingyao Cai
AU  - Akio K. Inoue
AU  - Masafusa Onoue
AU  - Suzuka Arai
AU  - Yoshinobu Fudamoto
AU  - Rohan P. Naidu
AU  - Pascal A. Oesch
AU  - Jenny E. Greene
AU  - Ivo Labbe
AU  - Jorryt Matthee
AU  - Anna de Graaff
AU  - Yuma Sugahara
AU  - Ken Mawatari
AU  - Pratika Dayal
AU  - Xi Kang
PY  - 2026
DA  - 2026/10/20
PB  - arXiv
DO  - 10.48550/arxiv.2610.28735
UR  - https://arxiv.org/abs/2610.28735
AB  - The James Webb Space Telescope (JWST) has uncovered a previously unexplored population of faint active galactic nuclei (AGNs) in the early Universe. We present MoM-BL-z10 at z=9.9837±0.0009, identified through a systematic search for broad-line AGNs at z≥6 using public JWST/NIRSpec PRISM spectra from the DAWN JWST Archive. The PRISM spectrum exhibits a broad component in Hβ at 5.35 microns. A narrow+broad Gaussian fit yields a broad Hβ detection at S/N = 3.87, with FWHM = 3480^+2040_−950 km s^−1 and L_Hβ,broad=1.25^+0.33_−0.31×10^42 erg s^−1. The detection of [Ne III] λ3870 and the high [Ne III]/[O II] and [O III]/Hβ ratios further support the AGN interpretation. Its rest-frame UV and optical continuum slopes, compact NIRCam morphology, and a tentative MIRI F770W detection suggest similarities to Little Red Dots (LRDs). The broad Hβ detection enables the first virial SMBH mass estimate at z∼10. We derive M_BH=3.6^+5.8_−1.7×10^7 M_⊙ and λ_Edd=0.11^+0.10_−0.07 using the standard single-epoch virial calibration and quasar bolometric correction. We also consider an alternative scenario adopting the LRD-specific bolometric correction and assuming λ_Edd=1, which yields M_BH=2.1^+0.6_−0.5×10^6 M_⊙. Under either scenario, the inferred SMBH mass at a cosmic age of only 470 Myr highlights the challenge of rapid BH growth in the early Universe, suggesting super-Eddington accretion from a stellar-mass seed or a more massive seed. As the highest-redshift broad-line AGN identified to date, MoM-BL-z10 provides a new observational benchmark for understanding the early growth of supermassive black holes.
ER  -

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