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UNCOVER: JWST Spectroscopy of Three Cold Brown Dwarfs at Kiloparsec-scale Distances Published: 8/22/2023 8:46:06 PM Updated: 8/22/2023 8:46:06 PM

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 the two other sources were initially identified as candidate activegalactic nuclei. Ultradeep, low-resolution 1-5 \mum spectra confirm thepresence of molecular features consistent with T dwarf atmospheres, andcomparison to near-infrared spectral standards infers spectral classificationsof sdT1, T6, and T8-T9. The spectrum of the warmest source, UNCOVER-BD-1, showsevidence of subsolar metallicity, and a fit to atmosphere models indicates Teff= 1300 K and [M/H] ~ -1.0, making this one of the few examples of aspectroscopically-confirmed T subdwarf. The spectrum of the coldest source,UNCOVER-BD-3, places it near the T/Y dwarf boundary with Teff = 550 K, and weidentify features of CH_4, CO, H_2O, and likely PH_3 in the 3.5-5.0\mum band. Our analysis suggests that PH3 is favored over CO_2 in thissource, a possible indicator of subsolar metallicity. We estimate distances of0.9-4.5 kpc from the Galactic midplane, and population simulations indicatehigh probabilities of membership in the Galactic thick disk or halo. Oursimulations also indicate that there may be up to 7 T dwarfs and 5-6 L dwarfsin the Abell 2744 field down to F444W = 30 AB mag, roughly half of which arehalo members. These results highlight the utility of deep JWST/NIRSpecspectroscopy for identifying and characterizing the oldest metal-poor browndwarfs in the Milky Way.