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$\pm$0.15 and F444W = 25.84$\pm$0.01 (AB), and its F115W$-$F444W and F356W$-$F444W colors match those expected for other, known T dwarfs. We can exclude it as a reddened background star, high redshift quasar, or very high redshift galaxy. Comparison with stellar atmospheric models indicates a temperature of $T_{eff}$ $\approx$ 600~K and surface gravity $\log{g}$ $\approx$ 5, implying a mass of 0.03~M$_{\odot}$ 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.