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

¹ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova Vicolo dell’Osservatorio 3 35122 Padova, Italy
² INAF, Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5 35122 Padova, Italy
³ INAF, Osservatorio Astronomico di Roma Via Frascati 33 00078 Monteporzio Catone Roma, Italy
⁴ NSF’s National Optical-Infrared Astronomy Research Laboratory 950 N. Cherry Ave. Tucson AZ 85719, USA
⁵ Department of Astronomy, The University of Texas at Austin Austin TX, USA
⁶ Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro 2 00185 Roma, Italy
⁷ Astrophysics Science Division, NASA Goddard Space Flight Center 8800 Greenbelt Rd Greenbelt MD 20771, USA
⁸ Department of Physics and Astronomy, University of Kansas Lawrence KS 66045, USA
⁹ Department of Physics & Astronomy, University of Louisville Natural Science Building 102 Louisville KY 40292, USA
¹⁰ INAF, Istituto di Radioastronomia Via Piero Gobetti 101 40129 Bologna, Italy
¹¹ Department of Astronomy and Astrophysics, The Pennsylvania State University University Park PA 16802, USA
¹² Institute for Computational and Data Sciences, The Pennsylvania State University University Park PA 16802, USA
¹³ Institute for Gravitation and the Cosmos, The Pennsylvania State University University Park PA 16802, USA
¹⁴ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université Paris Cité, Sorbonne Paris Cité, CNRS 91191 Gif-sur-Yvette, France
¹⁵ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
¹⁶ Institute of Physics, Laboratory of Galaxy Evolution, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny 1290 Versoix, Switzerland
¹⁷ SISSA Via Bonomea 265 34136 Trieste, Italy
¹⁸ Center for Astrophysics | Harvard & Smithsonian 60 Garden St Cambridge MA 02138, USA
¹⁹ Black Hole Initiative, Harvard University 20 Garden St Cambridge MA 02138, USA
²⁰ Centro de Astrobiología (CAB), CSIC-INTA Ctra. de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain
²¹ Department of Physics and Astronomy, University of California 900 University Ave Riverside CA 92521, USA
²² Astronomy Centre, University of Sussex Falmer Brighton BN1 9QH, UK
²³ European Space Agency (ESA), European Space Astronomy Centre (ESAC) Camino Bajo del Castillo s/n 28692 Villanueva de la Cañada Madrid, Spain
²⁴ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology 84 Lomb Memorial Drive Rochester NY 14623, USA
²⁵ Department of Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
²⁶ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
²⁷ ESA/AURA Space Telescope Science Institute Baltimore MD, USA
²⁸ University of Louisville, Department of Physics and Astronomy 102 Natural Science Building 40292 KY Louisville, USA

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Received: 1 September 2025
Accepted: 6 December 2025

Abstract

Context. The James Webb Space Telescope (JWST) has uncovered a diverse population of extreme near-infrared dropouts, including ultra-high-redshift (z > 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 > 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 < 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 < 25. Lower-redshift solutions struggle to reproduce the extreme break, suggesting that if Capotauro resides at z < 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.