The 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 \approx - 3.0 , with a mean overall metallicity of \langle [ Fe/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 } < 20 \mathrm { km } \mathrm { 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 , compared to the colder metal-rich component .
We speculate that this kinematically diffuse , low metallicity , population is the stellar halo of the Sagittarius progenitor system .