The Metallicity Gradient of Sagittarius Dwarf Spheroidal Galaxy Prior to Infall Constrained by S-PLUS Observations of its Tidal Stream
We study the metallicity distribution along the Sagittarius (Sgr) stream using photometric metallicities from S-PLUS DR4, combined with Gaia DR3 kinematics and APOGEE DR17 spectroscopy. Our analysis confirms that the leading arm (Galactic latitude $b > 0$) is systematically more metal-poor than the trailing arm ($b < 0$) by 0.15--0.20 dex, and reveals a clear negative metallicity gradient along the leading arm. The trailing arm shows no significant overall gradient but displays distinct inner (negative) and outer (positive) trends. These features are consistently recovered across different photometric estimators and agree with spectroscopic data. We compare these results with predictions from an $N$-body simulation, in which metallicities were assigned according to a set of imprinted radial gradients in the progenitor. We were able to constrain the original metallicity gradient of the Sgr progenitor to be between $-0.38$ and $-0.24$ dex kpc$^{-1}$ based on photometric data, and $-0.42$ to $-0.10$ dex kpc$^{-1}$ from APOGEE. These values are consistent with gradients observed in other Local Group dwarf galaxies. Our findings demonstrate that metallicity-sensitive photometric surveys such as S-PLUS are powerful tools for reconstructing the chemodynamical evolution of disrupted satellites.