2024ApJ...963...95C

The stellar stream connected to the Sagittarius (Sgr) dwarf galaxy is the most massive tidal stream that has been mapped in the Galaxy, and is the dominant contributor to the outer stellar halo of the Milky Way (MW). We present metallicity maps of the Sgr stream, using 34,240 red giant branch stars with inferred metallicities from Gaia BP/RP spectra. This sample is larger than previous samples of Sgr stream members with chemical abundances by an order of magnitude. We measure metallicity gradients with respect to Sgr stream coordinates (Λ, B), and highlight the gradient in metallicity with respect to stream latitude coordinate B, which has not been observed before. Including the core, we find ∇[M/H] = -2.48 ± 0.08 × 10^–2 dex deg^–1 above the stream track (B > B₀, where B₀ = 1.5° is the latitude of the Sgr remnant) and ∇[M/H] = -2.02 ± 0.08 × 10^–2 dex deg^–1 below the stream track (B < B₀). By painting metallicity gradients onto a tailored N-body simulation of the Sgr stream, we find that the observed metallicities in the stream are consistent with an initial radial metallicity gradient in the Sgr dwarf galaxy of ∼-0.1 to -0.2 dex kpc^–1, well within the range of observed metallicity gradients in Local Group dwarf galaxies. Our results provide novel observational constraints for the internal structure of the dwarf galaxy progenitor of the Sgr stream. Leveraging new large data sets in conjunction with tailored simulations, we can connect the present-day properties of disrupted dwarfs in the MW to their initial conditions.