A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at <i>z</i> = 3.5

The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam, and MIRI observations from the RUBIES and PRIMER programs of The Cliff : a bright LRD at z = 3.55 with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad hydrogen (H α FWHM ∼ 1500 km s −1 ) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size ( M * ∼ 10 10.5 M ⊙ , r e ∼ 40 pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and H α absorption line are instead most plausibly explained by a black hole star (BH * ) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH * models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-09-01
DOI
https://doi.org/10.1051/0004-6361/202554681
Citations
47
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
54.27

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article

A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5

Ian McConachie, Jorryt Matthee, Rachel Bezanson, Tim B. Miller et al.
47 citations
Astronomy and Astrophysics
Stellar, planetary, and galactic studies
54.27
article

A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5

Ian McConachie, Jorryt Matthee, Rachel Bezanson, Tim B. Miller, Harley Katz, Hans‐Walter Rix, Rohan P. Naidu, Jenny E. Greene, Ivo Labbé, Josephine F. W. Baggen, David J. Setton, Leindert Boogaard, Christina C. Williams, Andy D. Goulding, Irene Shivaei, Joel Leja, Pieter van Dokkum, Erica J. Nelson, Pascal A. Oesch, Gabriel Brammer, Raphael E. Hviding, Michael V. Maseda, Katherine E. Whitaker, Andrea Weibel, Anna de Graaff, Michaela Hirschmann, Bingjie Wang, Pratika Dayal
article en
47 citations

Abstract

The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam, and MIRI observations from the RUBIES and PRIMER programs of The Cliff : a bright LRD at z = 3.55 with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad hydrogen (H α FWHM ∼ 1500 km s −1 ) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size ( M * ∼ 10 10.5 M ⊙ , r e ∼ 40 pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and H α absorption line are instead most plausibly explained by a black hole star (BH * ) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH * models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.

Astronomy and AstrophysicsVol. 701
Northwestern University (US), University of Geneva (CH), Leiden University (NL), University of Copenhagen (DK), Pennsylvania State University (US), University of Wisconsin–Madison (US), Institute of Science and Technology Austria (AT), University of Groningen (NL), University of Pittsburgh (US), University of Colorado Boulder (US), Princeton University (US), University of Massachusetts Amherst (US), Yale University (US), University of Chicago (US), Kavli Institute for Particle Astrophysics and Cosmology (US), Max Planck Institute for Astronomy (DE), Centro de Astrobiología (ES), NSF NOIRLab (US), École Polytechnique Fédérale de Lausanne (CH), Swinburne University of Technology (AU)
National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, National Research Foundation, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Danmarks Grundforskningsfond, Deutsches Zentrum für Luft- und Raumfahrt, Bundesministerium für Wirtschaft und Energie, Staatssekretariat für Bildung, Forschung und Innovation
Openalex Percentile: Top 0%
Stellar, planetary, and galactic studies
54.27
DOIOpen Access PDFOpenAlex
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