2026A&A...706A.364G

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.