JADES: Spectroscopic Confirmation and Proper Motion for a T-Dwarf at 2 kpc

Large area observations of extragalactic deep fields with the James Webb Space Telescope (JWST) have provided a wealth of candidate low-mass L- and T-class brown dwarfs. The existence of these sources, which are at derived distances of hundreds of parsecs to several kiloparsecs from the Sun, has strong implications for the low-mass end of the stellar initial mass function, and the link between stars and planets at low metallicities. In this letter, we present a JWST/NIRSpec PRISM spectrum of brown dwarf JADES-GS-BD-9, confirming its photometric selection from observations taken as part of the JWST Advanced Deep Extragalactic Survey (JADES) program. Fits to this spectrum indicate that the brown dwarf has an effective temperature of $800-900\ \mathrm{K}$ (T5–T6) at a distance of $1.8-2.3\ \mathrm{kpc}$ from the Sun, with evidence of the source being at low metallicity ($\mathrm{[M/H]} \leq -0.5$ ). Finally, because of the cadence of JADES NIRCam observations of this source, we additionally uncover a proper motion between the $2022$ and $2023$ centroids, and we measure a proper motion of $20 \pm 4\ \mathrm{mas/yr}$ (a transverse velocity of $214\ \mathrm{km/s}$ at $2.25\ \mathrm{kpc}$ ). At this predicted metallicity, distance, and transverse velocity, it is likely that this source belongs either to the edge of the Milky Way thick disk or the galactic halo. This spectral confirmation demonstrates the efficacy of photometric selection of these important sources across deep extragalactic JWST imaging.

The Astrophysical Journal, Volume 975, Issue 1, id.31, 8 pages