BlackTHUNDER strikes twice: Balmer-line absorption in an overmassive Little Red Dot at <i>z</i> = 7.04

Abstract JWST has revealed a population of ‘Little Red Dots’ (LRDs): compact, red objects at redshifts z = 2–9 with ‘v’-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. We use NIRSpec/IFS data from the BlackTHUNDER survey to study the H α line in the LRD Abell2744-QSO1 at z = 7.04, which is a confirmed AGN due to time-variable equivalent width (EW) in its broad emission lines. The H α spectral profile is non-Gaussian, requiring at least two Gaussian components. We also detect a narrow-line Gaussian component, and strong H α absorption (EW relative to the continuum $\sim 22_{+12}^{-7}\mathring{\rm A}$), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H α, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $\sigma _abs = 110^{+20}_{-10}$ km s−1, consistent with the value inferred from the analysis of the Balmer break. Based on H α, we infer a black hole mass of log (M•/M⊙) = 7.2, smaller but close to the previous estimates based on H β. The Eddington ratio is 0.09. Combining the high signal-to-noise ratio of the narrow H α line with the spectral resolution R = 3, 700 of the G395H grating, we infer a narrow-line intrinsic dispersion $\sigma _\mathrm{n}=22_{-6}^{+5}$ km s−1, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be M•/Mdyn = 0.15–1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.

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Journal
Monthly Notices of the Royal Astronomical Society
Published
2026-02-25
DOI
https://doi.org/10.1093/mnras/stag401
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
0.00

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article

BlackTHUNDER strikes twice: Balmer-line absorption in an overmassive Little Red Dot at z = 7.04

Callum Witten, S. Tacchella, G. C. Jones, William McClymont et al.
Monthly Notices of the Royal Astronomical Society
Astrophysical Phenomena and Observations
article

BlackTHUNDER strikes twice: Balmer-line absorption in an overmassive Little Red Dot at z = 7.04

Callum Witten, S. Tacchella, G. C. Jones, William McClymont, Sophie Koudmani, М. Н. Ачасов, A. Bunker, E. Curtis-Lake, Eleonora Parlanti, R. Maiolino, Xihan Ji, Giovanni Cresci, Elena Dalla Bontà, Kohei Inayoshi, Hannah Übler, Giacomo Venturi, Debora Sijacki, Stefano Carniani, Francesco D’Eugenio, Brant E Robertson, Raffaella Schneider, Charlotte Simmonds, Chris Willott, Jianwei Lyu, Michele Perna, Stéphane Charlot, Jan Scholtz, Ignas Juodžbalis, Erica J Nelson, Giovanni Mazzolari
article en

Abstract

Abstract JWST has revealed a population of ‘Little Red Dots’ (LRDs): compact, red objects at redshifts z = 2–9 with ‘v’-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. We use NIRSpec/IFS data from the BlackTHUNDER survey to study the H α line in the LRD Abell2744-QSO1 at z = 7.04, which is a confirmed AGN due to time-variable equivalent width (EW) in its broad emission lines. The H α spectral profile is non-Gaussian, requiring at least two Gaussian components. We also detect a narrow-line Gaussian component, and strong H α absorption (EW relative to the continuum $\sim 22_{+12}^{-7}\mathring{\rm A}$), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H α, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $\sigma _abs = 110^{+20}_{-10}$ km s−1, consistent with the value inferred from the analysis of the Balmer break. Based on H α, we infer a black hole mass of log (M•/M⊙) = 7.2, smaller but close to the previous estimates based on H β. The Eddington ratio is 0.09. Combining the high signal-to-noise ratio of the narrow H α line with the spectral resolution R = 3, 700 of the G395H grating, we infer a narrow-line intrinsic dispersion $\sigma _\mathrm{n}=22_{-6}^{+5}$ km s−1, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be M•/Mdyn = 0.15–1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.

Monthly Notices of the Royal Astronomical Society
Herzberg Institute of Astrophysics (CA), University of Copenhagen (DK), Centre National de la Recherche Scientifique (FR), Planetary Science Institute (US), Joint Institute for Laboratory Astrophysics (US), University of Arizona (US), University of Hertfordshire (GB), Scuola Normale Superiore (IT), University of California, Santa Cruz (US), Peking University (CN), University of Cambridge (GB), Arcetri Astrophysical Observatory (IT), Institut d'Astrophysique de Paris (FR), Sorbonne Université (FR), University of Oxford (GB), Osservatorio astronomico di Bologna (IT), Max Planck Institute for Extraterrestrial Physics (DE), Kavli Institute for Theoretical Sciences (CN), DAWN Center for Independent Living (US), Institute of Astronomy (RU), Center for Astrophysics Harvard & Smithsonian (US), Centro de Astrobiología (ES), Istituto Nazionale di Fisica Nucleare, Sezione di Firenze (IT), Osservatorio Astronomico di Padova (IT), University of Florence (IT), University College London (GB), Geneva College (US), Sapienza University of Rome (IT), University of Bologna (IT)
National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, UK Research and Innovation, European Commission, Ministero dell’Istruzione, dell’Università e della Ricerca, Ministerio de Ciencia e Innovación, Science and Technology Facilities Council, Agencia Estatal de Investigación
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