The Sagittarius dwarf galaxy has a disk with a Λ-like metallicity profile

Unveiling the properties of the progenitors that formed the tidal structures of the Milky Way (MW) is essential for exploring the MW's assembly history. Such studies further afford a unique opportunity to dissect the formation and evolution of dwarf galaxies by contrasting surviving and disrupted systems. However, reconstructing the intrinsic structure of progenitors solely from the positions and velocities of their tidal debris remains a formidable challenge. Here, we decode the chemo-kinematical signatures embedded in the stellar streams of the Sagittarius (Sgr) dwarf galaxy through N-body simulations. For the first time, we reproduce a vertical metallicity gradient in the Sgr streams, which we attribute to a disk-dominated progenitor with a Λ-like metallicity profile across the disk. Remarkably, similar metallicity profiles are identified in MaNGA survey data for several field dwarf galaxies, suggesting this feature is not unique to Sgr. The vertical metallicity gradient of stellar streams preserves extra detailed galactic fossil records, enabling quantitative reconstruction of the peculiar structure of the satellite galaxy. The similar metallicity profile of Sgr to MW opens a question on a possible special coevolution.