Early Results from GLASS-JWST. XIII. A Faint, Distant, and Cold Brown Dwarf

We present the serendipitous discovery of a late T-type brown dwarf candidate in JWST NIRCam observations of the Early Release Science Abell 2744 parallel field. The discovery was enabled by the sensitivity of JWST at 4 mu m wavelengths and the panchromatic 0.9-4.5 mu m coverage of the spectral energy distribution. The unresolved point source has magnitudes F115W = 27.95 +/- 0.15 and F444W = 25.84 +/- 0.01 (AB), and its F115W-F444W and F356W-F444W colors match those expected for other known T dwarfs. We can exclude it as a reddened background star, high redshift quasar, or a very high redshift galaxy. Comparison with stellar atmospheric models indicates a temperature of T-eff asymptotic to 650 K and surface gravity log g asymptotic to 5.25, implying a mass of 0.03 M-? and age of 5 Gyr. We estimate the distance of this candidate to be 570-720 pc in a direction perpendicular to the Galactic plane, making it a likely thick disk or halo brown dwarf. These observations underscore the power of JWST to probe the very low-mass end of the substellar mass function in the Galactic thick disk and halo.