Early results from GLASS-JWST. XIII. A faint, distant, and cold brown dwarf

M. Nonino, K. Glazebrook, A. J. Burgasser, G. Polenta, T. Morishita, M. Lepinzan, M. Castellano, A. Fontana, E. Merlin, A. Bonchi, D. Paris, T. Treu, B. Vulcani, X. Wang, P. Santini, E. Vanzella, T. Nanayakkara, A. Mercurio, P. Rosati, C. Grillo, M. Bradac

Published 2022-07-29Version 1

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.