COSMOS-3D: Spectro-photometric Confirmation of Shabgard, a Cool, Metal-poor Brown Dwarf in the COSMOS Field
Abstract
Brown dwarfs are unique laboratories for studying cool atmospheres, the low-mass end of star formation, and the transition between stars and giant planets. Identifying and characterizing the coldest brown dwarfs remains challenging, particularly at faint magnitudes in deep extragalactic surveys. We investigate COSMOS2025 ID639188, hereafter Shabgard, a compact source previously selected as a brown-dwarf candidate from COSMOS-Web photometry. We use serendipitous JWST/NIRCam F444W wide-field slitless spectroscopy from COSMOS-3D, combined with COSMOS2025 NIRCam photometry and multi-epoch imaging. We extract a local-background-subtracted, profile-weighted $4.05−4.85μ{\rm m}$ spectrum and compare the data with six atmosphere-model families. The LOWZ atmosphere-grid fit, using PCA interpolation and MCMC sampling, gives Teff=718.6+10.6−10.2 K and [M/H]=−1.00+0.07−0.08, favoring a cool, subsolar-metallicity atmosphere near the late-T/Y transition. A fixed-atmosphere molecular-opacity test strongly favors a contribution from CH4 to the observed $4−5 μ{\rm m}$ spectral shape, although the restricted wavelength coverage prevents a molecule-by-molecule abundance determination. Model-dependent physical estimates span approximately $23−52 {\rm M_{\rm Jup}}$ and $140−180$ pc. We measure a proper motion of |μ|=89.2±0.4 mas yr−1, while the LOWZ MCMC flux scaling gives a distance of 176.2+18.4−17.8pc. Together, these results support the classification of Shabgard as a cool, metal-poor Galactic brown dwarf and demonstrate the ability of JWST deep-field surveys to discover and characterize faint substellar objects.
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BibTeX
@misc{salmasi2026cosmos3d,
title = {{COSMOS-3D: Spectro-photometric Confirmation of Shabgard, a Cool, Metal-poor Brown Dwarf in the COSMOS Field}},
author = {Bahareh S. Salmasi and Priyanka Jalan and Tomotsugu Goto and Tetsuya Hashimoto and Feige Wang and Tayyaba Zafar and Koki Kakiichi and Ece Kilerci and Srivardini Ayyappan and Romain A. Meyer and Anilkumar Mailvaganam and Tamal Mukherjee and Xuanyi Lyu and Imam Uzma Aalia and Arihant Raidani and Samuel Carlson and Jose Luis Carrillo Martinez and Andreas L. Faisst and Marko Shuntov and Hollis B. Akins and Zihao Li and the COSMOS-3D Collaboration},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07861},
eprint = {2610.07861},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07861},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - COSMOS-3D: Spectro-photometric Confirmation of Shabgard, a Cool, Metal-poor Brown Dwarf in the COSMOS Field
AU - Bahareh S. Salmasi
AU - Priyanka Jalan
AU - Tomotsugu Goto
AU - Tetsuya Hashimoto
AU - Feige Wang
AU - Tayyaba Zafar
AU - Koki Kakiichi
AU - Ece Kilerci
AU - Srivardini Ayyappan
AU - Romain A. Meyer
AU - Anilkumar Mailvaganam
AU - Tamal Mukherjee
AU - Xuanyi Lyu
AU - Imam Uzma Aalia
AU - Arihant Raidani
AU - Samuel Carlson
AU - Jose Luis Carrillo Martinez
AU - Andreas L. Faisst
AU - Marko Shuntov
AU - Hollis B. Akins
AU - Zihao Li
AU - the COSMOS-3D Collaboration
PY - 2026
DA - 2026/10/06
PB - arXiv
DO - 10.48550/arxiv.2610.07861
UR - https://arxiv.org/abs/2610.07861
AB - Brown dwarfs are unique laboratories for studying cool atmospheres, the low-mass end of star formation, and the transition between stars and giant planets. Identifying and characterizing the coldest brown dwarfs remains challenging, particularly at faint magnitudes in deep extragalactic surveys. We investigate COSMOS2025 ID639188, hereafter Shabgard, a compact source previously selected as a brown-dwarf candidate from COSMOS-Web photometry. We use serendipitous JWST/NIRCam F444W wide-field slitless spectroscopy from COSMOS-3D, combined with COSMOS2025 NIRCam photometry and multi-epoch imaging. We extract a local-background-subtracted, profile-weighted $4.05−4.85μ{\rm m}$ spectrum and compare the data with six atmosphere-model families. The LOWZ atmosphere-grid fit, using PCA interpolation and MCMC sampling, gives T_eff=718.6^+10.6_−10.2 K and [M/H]=−1.00^+0.07_−0.08, favoring a cool, subsolar-metallicity atmosphere near the late-T/Y transition. A fixed-atmosphere molecular-opacity test strongly favors a contribution from CH_4 to the observed $4−5 μ{\rm m}$ spectral shape, although the restricted wavelength coverage prevents a molecule-by-molecule abundance determination. Model-dependent physical estimates span approximately $23−52 {\rm M_{\rm Jup}}$ and $140−180$ pc. We measure a proper motion of |μ|=89.2±0.4 mas yr^−1, while the LOWZ MCMC flux scaling gives a distance of 176.2^+18.4_−17.8pc. Together, these results support the classification of Shabgard as a cool, metal-poor Galactic brown dwarf and demonstrate the ability of JWST deep-field surveys to discover and characterize faint substellar objects.
ER -