Unveiling the Sagittarius Dwarf Spheroidal Galaxy Core with Gaia DR3: A Red Clump Distance Precise to 2%

The Sagittarius dwarf spheroidal galaxy (Sgr dSph) provides us with the unique opportunity to study an ongoing Galactic cannibalistic event between our Milky Way (MW) Galaxy and a satellite dwarf galaxy. Understanding this event crucially requires memberships and high-precision metallicities. Here, we present the first major membership star catalog of the Sgr dwarf core (≈140,000 sources) and Messier 54 (M54; ≈2000 sources) with positions, proper motions, and parallaxes from the third Gaia data release (DR3), supplemented with metallicities from the Apache Point Observatory Galactic Evolution Experiment (or APOGEE). We initially isolate the Sgr dwarf core and M54 spatially from prior literature positions. Using evolutionary subsamples separated within a color–magnitude diagram, we analyze the substructures of the Sgr core and infer its positional relationship with M54 within 5D phase space. A sample of MW stars from a similar Galactic latitude were used to identify contaminants and separate member stars from the core of the Sgr dSph and M54 using a Gaussian mixture model. We present the derived proper motions, parallaxes, and metallicities for these evolutionary subsamples and demonstrate the precision of our sample using red clump (RC) standard candles. We find distance moduli for the Sgr core and M54 of ${(m-M)}_{0}=16.95{8}_{-0.044}^{+0.044}$ mag and ${(m-M)}_{0}=16.9{4}_{-0.056}^{+0.047}$ mag, corresponding to heliocentric distances of $d=24.63{5}_{-0.49}^{+0.49}$ kpc and $d=24.45{2}_{-0.602}^{+0.537}$ kpc, respectively. Using RC distance analysis, our results imply that there is no separation between the Sgr core and M54. Finally, we describe the metallicity distributions of the evolved stars within these two systems, finding evidence for the infall scenario.