Mysteries of <i>Capotauro</i> : Investigating the puzzling nature of an extreme F356W-dropout

Context. The James Webb Space Telescope (JWST) has uncovered a diverse population of extreme near-infrared dropouts, including ultra-high-redshift ( z &gt; 15) galaxy candidates, dust-obscured galaxies challenging theories of dust production, sources with strong Balmer breaks (possibly compact active galactic nuclei in dense gas-rich environments), and cold, substellar Galactic objects. Aims. This work presents Capotauro , a F356W-dropout identified in the CEERS survey with a F444W AB magnitude of ∼27.68 and exhibiting a sharp flux drop by &gt; 3 mag between 3.5 and 4.5 μm, being nondetected below 3.5 μm. We investigated its nature and constrained its properties, paving the way for follow-up observations. Methods. We combined JWST/NIRCam, MIRI, and NIRSpec/MSA data with HST/ACS and WFC3 observations to perform a spectrophotometric analysis of Capotauro using multiple SED-fitting codes. Our setup is tailored to test z ≥ 15 as well as z &lt; 10 dusty, Balmer-break or strong-line emitter galaxy solutions, and the possibility of Capotauro being a Milky Way substellar object. Results. Among extragalactic options, our analysis favors interpreting the sharp flux drop of Capotauro as a bright ( M UV ∼ −21.5) Lyman break at z ∼ 32, consistent with the formation epoch of the first stars and black holes, with only ∼0.5% of the redshift posterior volume lying at z &lt; 25. Lower-redshift solutions struggle to reproduce the extreme break, suggesting that if Capotauro resides at z &lt; 10, it must show a nonstandard combination of high dust attenuation and/or prominent Balmer breaks, making it a peculiar interloper. Finally, our analysis indicates that Capotauro ’s properties could be consistent with it being a very cold (i.e., Y2-Y3 type) brown dwarf or a free-floating exoplanet with a record-breaking combination of low temperature and large distance ( T eff ≤ 300 K; d ≳ 130 pc, up to ∼2 kpc). Conclusions. While present observations cannot determine Capotauro ’s nature, our analysis points to a remarkably unique object in all plausible scenarios. This makes Capotauro stand out as a compelling target for follow-up observations.

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Journal
Astronomy and Astrophysics
Published
2026-01-09
DOI
https://doi.org/10.1051/0004-6361/202557061
Primary Topic
Galaxies: Formation, Evolution, Phenomena
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article
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article

Mysteries of Capotauro : Investigating the puzzling nature of an extreme F356W-dropout

Laura Bisigello, G. Rodighiero, Giovanni Gandolfi, Bren E. Backhaus et al.
Astronomy and Astrophysics
Galaxies: Formation, Evolution, Phenomena
article

Mysteries of Capotauro : Investigating the puzzling nature of an extreme F356W-dropout

Laura Bisigello, G. Rodighiero, Giovanni Gandolfi, Bren E. Backhaus, E. Merlin, Stephen M. Wilkins, Marco Castellano, Allison Kirkpatrick, Benedetta Vulcani, A. Grazian, L. Pentericci, Pablo Arrabal Haro, Henry C. Ferguson, Anton M. Koekemoer, Anthony J. Taylor, Antonello Calabrò, L. Y. Aaron Yung, Marika Giulietti, Nikko J. Cleri, Mark Dickinson, Emanuele Daddi, Micaela B. Bagley, Jeyhan S. Kartaltepe, A. Fontana, Benne W. Holwerda, Pablo G. Pérez-González, Lorenzo Napolitano, Paola Santini, Steven Finkelstein, Michele Catone
article en

Abstract

Context. The James Webb Space Telescope (JWST) has uncovered a diverse population of extreme near-infrared dropouts, including ultra-high-redshift ( z &gt; 15) galaxy candidates, dust-obscured galaxies challenging theories of dust production, sources with strong Balmer breaks (possibly compact active galactic nuclei in dense gas-rich environments), and cold, substellar Galactic objects. Aims. This work presents Capotauro , a F356W-dropout identified in the CEERS survey with a F444W AB magnitude of ∼27.68 and exhibiting a sharp flux drop by &gt; 3 mag between 3.5 and 4.5 μm, being nondetected below 3.5 μm. We investigated its nature and constrained its properties, paving the way for follow-up observations. Methods. We combined JWST/NIRCam, MIRI, and NIRSpec/MSA data with HST/ACS and WFC3 observations to perform a spectrophotometric analysis of Capotauro using multiple SED-fitting codes. Our setup is tailored to test z ≥ 15 as well as z &lt; 10 dusty, Balmer-break or strong-line emitter galaxy solutions, and the possibility of Capotauro being a Milky Way substellar object. Results. Among extragalactic options, our analysis favors interpreting the sharp flux drop of Capotauro as a bright ( M UV ∼ −21.5) Lyman break at z ∼ 32, consistent with the formation epoch of the first stars and black holes, with only ∼0.5% of the redshift posterior volume lying at z &lt; 25. Lower-redshift solutions struggle to reproduce the extreme break, suggesting that if Capotauro resides at z &lt; 10, it must show a nonstandard combination of high dust attenuation and/or prominent Balmer breaks, making it a peculiar interloper. Finally, our analysis indicates that Capotauro ’s properties could be consistent with it being a very cold (i.e., Y2-Y3 type) brown dwarf or a free-floating exoplanet with a record-breaking combination of low temperature and large distance ( T eff ≤ 300 K; d ≳ 130 pc, up to ∼2 kpc). Conclusions. While present observations cannot determine Capotauro ’s nature, our analysis points to a remarkably unique object in all plausible scenarios. This makes Capotauro stand out as a compelling target for follow-up observations.

Astronomy and AstrophysicsVol. 706
Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), Goddard Space Flight Center (US), Pennsylvania State University (US), Woodwell Climate Research Center (US), Scuola Internazionale Superiore di Studi Avanzati (IT), University of Padua (IT), University of Louisville (US), University of Kansas (US), Rochester Institute of Technology (US), University of Sussex (GB), Université Paris Cité (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris-Saclay (FR), Astronomical Observatory of Rome (IT), Mitchell Institute (US), European Space Agency (FR), Astrophysique, Instrumentation et Modélisation (FR), Sorbonne Paris Cité (FR), European Space Astronomy Centre (ES), Istituto di Radioastronomia di Bologna (IT), Center for Astrophysics Harvard & Smithsonian (US), CEA Paris-Saclay (FR), Centro de Astrobiología (ES), NSF NOIRLab (US), Astronomy and Space (AU), Osservatorio Astronomico di Padova (IT), Institut de Recherche sur les Lois Fondamentales de l'Univers (FR), Deleted Institution, École Polytechnique Fédérale de Lausanne (CH), Sapienza University of Rome (IT), The University of Texas at Austin (US), National Institute for Astrophysics (IT), Texas A&M University (US)
Openalex Percentile: Top 99%
Galaxies: Formation, Evolution, Phenomena
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