A Novel Approach to Identifying Substructures through Analysis of Metallicity Distribution Functions

We present a new method for identifying Galactic halo substructures accreted from dwarf galaxies by combining metallicity distribution functions (MDFs) with orbital parameters. Using apogalactic distance–orbital phase space, we assume that the MDF peak of a substructure reflects its progenitor’s chemical signature. We test this approach with two Galactic potentials (St a ̈ ckel and McMillan) and find consistent results. Our sample consists of retrograde halo stars with low orbital inclinations and intermediate eccentricities (0.5 < e ≤ 0.7), drawn from Sloan Digital Sky Survey and Large sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy combined with Gaia DR3 astrometry. We identify four distinct low-inclination retrograde substructures (LRS 1, LRS 2, LRS 3, LRS 4) with MDF peaks at [Fe/H] = −1.5, −1.7, −1.9, and −2.1, respectively; LRS 3 is newly discovered. Further analysis reveals an additional stream (LRS 2B) with [Fe/H] = −2.3 embedded within LRS 2; the remaining LRS 2 stars (LRS 2A) are associated with Sequoia. LRS 1 is likely linked to Thamnos 2 and Arjuna, and LRS 4 is likely linked to I’itoi. Comparison with the ED-2 stream suggests that LRS 2B is chemically distinct, but high-resolution spectroscopy is required to confirm whether they originate from separate progenitors. Our MDF-based approach demonstrates the utility of chemodynamical space for uncovering halo substructures, while highlighting caveats such as metallicity gradients and redshift evolution of the mass–metallicity relation, which may blur the mapping between MDF peaks and progenitors.