A clear detection of proper motion confirms that the claimed z ≃ 32 galaxy candidate, “Capotauro”, is a Y-type brown dwarf
7 affiliations
- 1Institute for Astronomy, University of Edinburgh , Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ ,
- 2Centre for Exoplanet Science, University of Edinburgh , Edinburgh, EH9 3HJ ,
- 3Institute of Astronomy , KU Leuven, Celestijnenlaan 200D, B-3001 Leuven ,
- 4Leuven Gravity Institute, KU Leuven , Celestijnenlaan 200D, box 2415, 3001 Leuven ,
- 5Armagh Observatory and Planetarium , College Hill, Armagh, BT61 9DG, N. Ireland ,
- 6NSF’s National Optical-Infrared Astronomy Research Laboratory , 950 N. Cherry Avenue, Tucson, AZ 85719 ,
- 7University College London Department of Physics and Astronomy, , Gower Street, London WC1E 6BT ,
Abstract
The compact red source “Capotauro”, discovered in deep JWST imaging in 2025, has been suggested as a possible galaxy candidate at redshift z ≃ 32 on account of its extreme NIRCam colours. However, given the lack of evidence that this source is spatially resolved, a very cool brown dwarf, at a distance consistent with the scale-height of the Milky Way stellar disc (≃ 500 pc), provides an alternative explanation of the observed photometry. Here we exploit new medium-band NIRCam imaging obtained ≃ 3.5 years after the discovery data to test this hypothesis. Using filter-dependent point-spread-function fitting, a robust local relative astrometric frame defined by 68 compact reference sources, and a joint four-image injection–recovery analysis, we show that Capotauro has moved by 132 ± 20 mas over the 3.5-yr interval span of the observations, ruling out the possibility that it is an extragalactic source (galaxy, AGN, or supernova) at >6σ. The observed apparent proper motion is (μ ξ ,μ η )=(+9.4+6.4−4.8,−35.6+5.1−5.9) mas yr−1, or 37.6+5.5−5.6 mas yr−1 in total. Through comparison of its photometric spectral energy distribution with empirical templates, we find that Capotauro is best described as a brown dwarf of spectral type Y1.0 ± 0.5 (Teff ≈ 350 K) at a distance of 730 ± 110 pc (although the existing data and spectral templates are insufficient to rule out an even colder, later-type brown dwarf). Capotauro is thus one of the most distant Y dwarfs found to date, as expected given its discovery in a deep JWST extragalactic survey field. This result demonstrates the value of multi-epoch imaging for identifying substellar contaminants among the most extreme photometric-redshift candidates.
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BibTeX
@article{liu2026clear,
title = {{A clear detection of proper motion confirms that the claimed z ≃ 32 galaxy candidate, “Capotauro”, is a Y-type brown dwarf}},
author = {F-Y F Liu and D J McLeod and B J Sutlieff and T J Dupuy and J S Dunlop and X Chen and R J McLure and H-H Leung and S Antonogiannaki and C Bondestam and E Laplace and R Begley and A C Carnall and F Cullen and C T Donnan and R S Ellis},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = oct,
doi = {10.1093/mnras/stag2055},
url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2055/9003823},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A clear detection of proper motion confirms that the claimed z ≃ 32 galaxy candidate, “Capotauro”, is a Y-type brown dwarf
AU - F-Y F Liu
AU - D J McLeod
AU - B J Sutlieff
AU - T J Dupuy
AU - J S Dunlop
AU - X Chen
AU - R J McLure
AU - H-H Leung
AU - S Antonogiannaki
AU - C Bondestam
AU - E Laplace
AU - R Begley
AU - A C Carnall
AU - F Cullen
AU - C T Donnan
AU - R S Ellis
PY - 2026
DA - 2026/10/28
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/stag2055
UR - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2055/9003823
AB - The compact red source “Capotauro”, discovered in deep JWST imaging in 2025, has been suggested as a possible galaxy candidate at redshift z ≃ 32 on account of its extreme NIRCam colours. However, given the lack of evidence that this source is spatially resolved, a very cool brown dwarf, at a distance consistent with the scale-height of the Milky Way stellar disc (≃ 500 pc), provides an alternative explanation of the observed photometry. Here we exploit new medium-band NIRCam imaging obtained ≃ 3.5 years after the discovery data to test this hypothesis. Using filter-dependent point-spread-function fitting, a robust local relative astrometric frame defined by 68 compact reference sources, and a joint four-image injection–recovery analysis, we show that Capotauro has moved by 132 ± 20 mas over the 3.5-yr interval span of the observations, ruling out the possibility that it is an extragalactic source (galaxy, AGN, or supernova) at >6σ. The observed apparent proper motion is (μ _ξ ,μ _η )=(+9.4^+6.4_−4.8,−35.6^+5.1_−5.9) mas yr−1, or 37.6^+5.5_−5.6 mas yr−1 in total. Through comparison of its photometric spectral energy distribution with empirical templates, we find that Capotauro is best described as a brown dwarf of spectral type Y1.0 ± 0.5 (T_eff ≈ 350 K) at a distance of 730 ± 110 pc (although the existing data and spectral templates are insufficient to rule out an even colder, later-type brown dwarf). Capotauro is thus one of the most distant Y dwarfs found to date, as expected given its discovery in a deep JWST extragalactic survey field. This result demonstrates the value of multi-epoch imaging for identifying substellar contaminants among the most extreme photometric-redshift candidates.
SN - 0035-8711
SN - 1365-2966
ER -