Johnson, Benjamin DConroy, CharlieNaidu, RohanBonaca, AnaZaritsky, DTing, Yuan-SenCargile, Phillip AHan, Jiwon JesseSpeagle, Joshua S2022-08-012022-08-010004-637Xhttp://hdl.handle.net/1885/270073The tidal disruption of the Sagittarius dwarf galaxy has generated a spectacular stream of stars wrapping around the entire Galaxy. We use data from Gaia and the H3 Stellar Spectroscopic Survey to identify $823$ high-quality Sagittarius members based on their angular momenta. The H3 Survey is largely unbiased in metallicity, and so our sample of Sagittarius members is similarly unbiased. Stream stars span a wide range in [Fe/H] from −0.2 to ≈−3.0, with a mean overall metallicity of $\langle [\mathrm{Fe}/{\rm{H}}]\rangle =-0.99$. We identify a strong metallicity dependence to the kinematics of the stream members. At [Fe/H] > −0.8 nearly all members belong to the well-known cold (${\sigma }_{v}\lt 20\,\mathrm{km}\,{{\rm{s}}}^{-1}$) leading and trailing arms. At intermediate metallicities (−1.9 < [Fe/H] < −0.8) a significant population (24%) emerges of stars that are kinematically offset from the cold arms. These stars also appear to have hotter kinematics. At the lowest metallicities ([Fe/H] lesssim −2), the majority of stars (69%) belong to this kinematically offset diffuse population. Comparison to simulations suggests that the diffuse component was stripped from the Sagittarius progenitor at earlier epochs, and therefore resided at larger radius on average than the colder metal-rich component. We speculate that this kinematically diffuse, low-metallicity population is the stellar halo of the Sagittarius progenitor system.Y.S.T. is supported by the NASA Hubble Fellowship grant HST-HF2-51425.001 awarded by the Space Telescope Science Institute. C.C. acknowledges support from the Packard Foundationapplication/pdfen-AU© 2020. The American Astronomical SocietyMilky Way GalaxyDwarf galaxiesTidal disruptionSagittarius dwarf spheroidal galaxy202010.3847/1538-4357/abab082021-08-01