The cold substellar frontier in JWST Extragalactic surveys: a robust parallax measurement yields the most distant known ultra-cold Y dwarf
Abstract
We present an analysis of JADES-GS-BD-5, an extremely cold Y dwarf identified in the JADES/GOODS-South field. Using multi-epoch JWST/NIRCam imaging spanning a ≃2-yr baseline, we calibrate the JWST astrometry and obtain the first robust parallax measurement for a brown dwarf discovered in a JWST extragalactic deep field. We measure a proper motion of μtot=49.8+0.3−0.3 mas yr−1 and a parallax of ϖ=15.7+1.4−1.3 mas, corresponding to a geometric distance of d=64+6−5 pc, ∼3× further than that of the previously known most distant Y dwarfs with parallax-measured distances. We hence determine a tangential velocity of vtan=15.0+1.4−1.3 km s−1, consistent with a Galactic thin-disk object. We find no evidence for binary acceleration, significant photometric variability, or a plausible host. The J-band spectral morphology of JADES-GS-BD-5 is consistent with existing Y-dwarf templates, while its extremely faint luminosity and overall near-infrared spectral profile confirm that it is one of the coldest dwarfs discovered to date. We therefore adopt a classification of >Y1, consistent with a NIRCam colour-based proxy of cY1.5. The robust parallax-determined distance allows us to determine its bolometric luminosity, log(Lbol/L⊙)=−7.08+0.09−0.09. Evolutionary modelling places JADES-GS-BD-5 in the planetary-mass regime and makes it a rare benchmark for the study of cold giant-planet atmospheres. Current atmospheric models fail to reproduce its SED in detail, pointing to the importance of disequilibrium chemistry and vertical mixing. Finally, we also report additional proper motion measurements for eight of the other nine known JADES Y-dwarf candidates, highlighting JWST extragalactic surveys as a powerful route to building a statistically useful, parallax-calibrated sample of Y dwarfs.
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BibTeX
@misc{liu2026cold,
title = {{The cold substellar frontier in JWST Extragalactic surveys: a robust parallax measurement yields the most distant known ultra-cold Y dwarf}},
author = {Feng-Yuan Frey Liu and Derek J. McLeod and Xueqing Chen and Ben J. Sutlieff and Ho-Hin Leung and Clémence Fontanive and James S. Dunlop and Ross J. McLure and Trent J. Dupuy and Adam C. Carnall and Thomas M. Stanton and Beth A. Biller and Isaac J. B. Holst and Callum T. Donnan and Ryan Begley},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.08346},
eprint = {2611.08346},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.08346},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The cold substellar frontier in JWST Extragalactic surveys: a robust parallax measurement yields the most distant known ultra-cold Y dwarf
AU - Feng-Yuan Frey Liu
AU - Derek J. McLeod
AU - Xueqing Chen
AU - Ben J. Sutlieff
AU - Ho-Hin Leung
AU - Clémence Fontanive
AU - James S. Dunlop
AU - Ross J. McLure
AU - Trent J. Dupuy
AU - Adam C. Carnall
AU - Thomas M. Stanton
AU - Beth A. Biller
AU - Isaac J. B. Holst
AU - Callum T. Donnan
AU - Ryan Begley
PY - 2026
DA - 2026/11/05
PB - arXiv
DO - 10.48550/arxiv.2611.08346
UR - https://arxiv.org/abs/2611.08346
AB - We present an analysis of JADES-GS-BD-5, an extremely cold Y dwarf identified in the JADES/GOODS-South field. Using multi-epoch JWST/NIRCam imaging spanning a ≃2-yr baseline, we calibrate the JWST astrometry and obtain the first robust parallax measurement for a brown dwarf discovered in a JWST extragalactic deep field. We measure a proper motion of μ_tot=49.8^+0.3_−0.3 mas yr^−1 and a parallax of ϖ=15.7^+1.4_−1.3 mas, corresponding to a geometric distance of d=64^+6_−5 pc, ∼3× further than that of the previously known most distant Y dwarfs with parallax-measured distances. We hence determine a tangential velocity of v_tan=15.0^+1.4_−1.3 km s^−1, consistent with a Galactic thin-disk object. We find no evidence for binary acceleration, significant photometric variability, or a plausible host. The J-band spectral morphology of JADES-GS-BD-5 is consistent with existing Y-dwarf templates, while its extremely faint luminosity and overall near-infrared spectral profile confirm that it is one of the coldest dwarfs discovered to date. We therefore adopt a classification of >Y1, consistent with a NIRCam colour-based proxy of cY1.5. The robust parallax-determined distance allows us to determine its bolometric luminosity, log(L_bol/L_⊙)=−7.08^+0.09_−0.09. Evolutionary modelling places JADES-GS-BD-5 in the planetary-mass regime and makes it a rare benchmark for the study of cold giant-planet atmospheres. Current atmospheric models fail to reproduce its SED in detail, pointing to the importance of disequilibrium chemistry and vertical mixing. Finally, we also report additional proper motion measurements for eight of the other nine known JADES Y-dwarf candidates, highlighting JWST extragalactic surveys as a powerful route to building a statistically useful, parallax-calibrated sample of Y dwarfs.
ER -