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Arxiv: UNCOVER: JWST Spectroscopy of Three Cold Brown Dwarfs at Kiloparsec-scale Distances Published: 8/22/2023 8:46:06 PM Updated: 2/8/2024 5:00:51 AM

Paper abstract: We report JWST/NIRSpec spectra of three distant T-type brown dwarfsidentified in the Ultradeep NIRSpec and NIRCam ObserVations before the Epoch ofReionization (UNCOVER) survey of the Abell 2744 lensing field. One source waspreviously reported as a candidate T dwarf on the basis of NIRCam photometry,while two sources were initially identified as candidate active galacticnuclei. Low-resolution 1--5 \mum spectra confirm the presence of molecularfeatures consistent with T dwarf atmospheres, and comparison to spectralstandards infers classifications of sdT1, T6, and T8--T9. The warmest source,UNCOVER-BD-1, shows evidence of subsolar metallicity, and atmosphere model fitsindicates T_{eff} = 1300 K and [M/H] ~ -1.0, making this one of thefew spectroscopically-confirmed T subdwarfs known. The coldest source,UNCOVER-BD-3, is near the T/Y dwarf boundary with T_{eff} = 550 K, and ouranalysis indicates the presence of PH_3 in the 3--5~\mum region, favoredover CO_2 and a possible indicator of subsolar metallicity. We estimatedistances of 0.9--4.5 kpc from the Galactic midplane, making these the mostdistant brown dwarfs with spectroscopic confirmation. Population simulationsindicate high probabilities of membership in the Galactic thick disk for two ofthese brown dwarfs, and potential halo membership for UNCOVER-BD-1. Oursimulations indicate that there are approximately 5 T dwarfs and 1--2 L dwarfsin the Abell 2744 field down to F444W = 30 AB mag, roughly one-third of whichare thick disk members. These results highlight the utility of deepJWST/NIRSpec spectroscopy for identifying and characterizing the oldestmetal-poor brown dwarfs in the Milky Way.