  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 800-900K (T5 - T6) at a
distance of $1.8 - 2.3$kpc from the Sun, with evidence of the source being at
low metallicity ([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 $20 \pm
4$ mas yr$^{-1}$ (a transverse velocity of 214 km s$^{-1}$ at 2.25 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.
