BlackTHUNDER – A non-stellar Balmer break in a black hole-dominated little red dot at <i>z</i> = 7.04

ABSTRACT Recent observations from James Webb Space Telescope (JWST) have revealed an abundant population of active galactic nuclei (AGNs) and the so-called ‘Little Red Dots’ (LRDs) at $2\lesssim z \lesssim 11$, many of which are characterized by V-shaped UV-to-optical continua with turnovers around the Balmer limit. The physical nature of these LRDs is unclear, and it remains debated whether the peculiar spectral shape originates from AGN, compact galaxies, or both. We present the analysis of new NIRSpec-IFU data from the BlackTHUNDER JWST Large Programme and archival NIRSpec-MSA data of a lensed LRD at $z=7.04$. The spectra confirm the presence of a smooth Balmer break and a broad H$\beta$ tracing the Broad Line Region (BLR) of an AGN. The small velocity dispersion of the H$\beta$ narrow component indicates a small dynamical mass of the host galaxy of $M_{\rm dyn}&amp;lt; 4 \times 10^8~{\rm M}_{\odot }$, which implies that the stellar population cannot contribute more than 10 per cent to the optical continuum. We show that the Balmer break can be well described by an AGN continuum absorbed by very dense ($n_{\rm H}\sim 10^{10}~{\rm cm^{-3}}$) and nearly dust-free gas along our line of sight (possibly gas in the BLR or its surrounding). The same gas is expected to produce H$\beta$ absorption, at a level consistent with a tentative detection ($3\sigma$) in the high-resolution spectrum. Such a non-stellar origin of the Balmer break may apply to other LRDs, and would alleviate the issue of extremely high stellar mass surface densities inferred in the case of a stellar interpretation of the Balmer break. We note that this is a rare case of a black hole that is overmassive relative to both the host galaxy stellar and dynamical masses. We finally report indications of variability and the first attempt of AGN reverberation mapping at such an early epoch.

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Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-10-29
DOI
https://doi.org/10.1093/mnras/staf1867
Citations
24
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
27.71

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article

BlackTHUNDER – A non-stellar Balmer break in a black hole-dominated little red dot at z = 7.04

Nimisha Kumari, Raffaella Schneider, Eiichi Egami, G. Cresci et al.
24 citations
Monthly Notices of the Royal Astronomical Society
Astrophysical Phenomena and Observations
27.71
article

BlackTHUNDER – A non-stellar Balmer break in a black hole-dominated little red dot at z = 7.04

Nimisha Kumari, Raffaella Schneider, Eiichi Egami, G. Cresci, Callum Witten, Francesco D’Eugenio, Joris Witstok, Santiago Arribas, Bruno Rodríguez Del Pino, Michele Perna, Fengwu Sun, R. Maiolino, Debora Sijacki, A. C. Fabian, Jake S. Bennett, Giovanni Mazzolari, Stefano Carniani, Ignas Juodžbalis, Sandro Tacchella, Chris J. Willott, Gareth C. Jones, Kohei Inayoshi, Alessandro Trinca, Xihan Ji, S. Charlot, Mirko Curti, Jianwei Lyu, Hannah Übler, Hannah C. Turner, Jan Scholtz, Andrew J. Bunker, Brant E. Robertson, Rosa Valiante, Marta Volonteri, Yuki Isobe, Giacomo Venturi, Eleonora Parlanti
article en
24 citations

Abstract

ABSTRACT Recent observations from James Webb Space Telescope (JWST) have revealed an abundant population of active galactic nuclei (AGNs) and the so-called ‘Little Red Dots’ (LRDs) at $2\lesssim z \lesssim 11$, many of which are characterized by V-shaped UV-to-optical continua with turnovers around the Balmer limit. The physical nature of these LRDs is unclear, and it remains debated whether the peculiar spectral shape originates from AGN, compact galaxies, or both. We present the analysis of new NIRSpec-IFU data from the BlackTHUNDER JWST Large Programme and archival NIRSpec-MSA data of a lensed LRD at $z=7.04$. The spectra confirm the presence of a smooth Balmer break and a broad H$\beta$ tracing the Broad Line Region (BLR) of an AGN. The small velocity dispersion of the H$\beta$ narrow component indicates a small dynamical mass of the host galaxy of $M_{\rm dyn}&amp;lt; 4 \times 10^8~{\rm M}_{\odot }$, which implies that the stellar population cannot contribute more than 10 per cent to the optical continuum. We show that the Balmer break can be well described by an AGN continuum absorbed by very dense ($n_{\rm H}\sim 10^{10}~{\rm cm^{-3}}$) and nearly dust-free gas along our line of sight (possibly gas in the BLR or its surrounding). The same gas is expected to produce H$\beta$ absorption, at a level consistent with a tentative detection ($3\sigma$) in the high-resolution spectrum. Such a non-stellar origin of the Balmer break may apply to other LRDs, and would alleviate the issue of extremely high stellar mass surface densities inferred in the case of a stellar interpretation of the Balmer break. We note that this is a rare case of a black hole that is overmassive relative to both the host galaxy stellar and dynamical masses. We finally report indications of variability and the first attempt of AGN reverberation mapping at such an early epoch.

Monthly Notices of the Royal Astronomical SocietyVol. 544(4)
University of Geneva (CH), University of Insubria (IT), Herzberg Institute of Astrophysics (CA), University of Copenhagen (DK), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), European Southern Observatory (DE), University of Arizona (US), Waseda University (JP), Scuola Normale Superiore (IT), University of California, Santa Cruz (US), Peking University (CN), University of Cambridge (GB), Arcetri Astrophysical Observatory (IT), Astronomical Observatory of Rome (IT), Institut d'Astrophysique de Paris (FR), Sorbonne Université (FR), University of Oxford (GB), 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 Roma I (IT), Osservatorio di Astrofisica e Scienza dello Spazio, University College London (GB), Geneva College (US), Sapienza University of Rome (IT), University of Bologna (IT)
National Aeronautics and Space Administration, Space Telescope Science Institute, UK Research and Innovation, European Commission, National Natural Science Foundation of China, Ministerio de Ciencia e Innovación, Science and Technology Facilities Council, Agencia Estatal de Investigación
Openalex Percentile: Top 0%
Astrophysical Phenomena and Observations
27.71
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