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

TL;DR

This paper introduces a new chemo-dynamical method combining metallicity distribution functions and orbital parameters to identify and characterize Galactic halo substructures, including the discovery of a new stream.

Contribution

The novel approach effectively links MDF peaks with progenitor signatures and uncovers new substructures, enhancing understanding of Galactic halo assembly.

Findings

Identified four low-inclination retrograde substructures with distinct MDF peaks.

Discovered a new stream, LRS 3, through MDF analysis.

Linked some substructures to known streams and progenitors.

Abstract

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\"ackel and McMillan) and find consistent results. Our sample consists of retrograde halo stars with low orbital inclinations and intermediate eccentricities ($0.5 < e \leq 0.7$), drawn from SDSS and LAMOST 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; LRS3 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 to I'itoi. Comparison with the ED-2 stream suggests 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 chemo-dynamical 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.