Substructures of the Milky Way's Retrograde Halo: Evidence for Multiple Accretion Events

TL;DR

This study reveals that the Milky Way's retrograde halo is composed of multiple accreted dwarf galaxies, identified through chemical and dynamical analysis of stellar substructures, indicating a complex, multi-progenitor origin.

Contribution

It demonstrates that metallicity distribution functions alone cannot distinguish progenitors, emphasizing the need for combined chemical and dynamical data to understand halo assembly.

Findings

Identified four substructures with distinct MDF peaks at low eccentricities.

Found five substructures spanning a range of metallicities at intermediate eccentricities.

Showed that similar MDF peaks can belong to either coherent or separate dynamical structures.

Abstract

We investigate the progenitors of low-inclination retrograde substructures in the Milky Way (MW) halo, which are remnants of accreted dwarf galaxies on retrograde orbits. Our sample consists of halo stars with low orbital inclinations and eccentricities ($0 \le e \le 0.5$), constructed by combining spectroscopic data with $\it Gaia$ astrometry. We identify substructures using metallicity distribution functions (MDFs) in apogalactic distance-orbital phase space. In the low-eccentricity range ($0 \le e \le 0.3$), we find four substructures with MDF peaks at [Fe/H] $\approx -1.5$, $-1.9$, $-2.1$, and $-2.3$. In the intermediate-eccentricity range ($0.3 < e \le 0.5$), we identify five substructures that span [Fe/H] $\approx -1.5$ to $-2.3$. By combining chemical and dynamical information, we show that substructures with identical MDF peaks in the two eccentricity regions can either form coherent structures or remain dynamically distinct. This shows that MDF similarity alone is insufficient to uniquely identify progenitor systems and must be combined with dynamical information. We find that the retrograde halo was assembled through multiple accretion events rather than a single progenitor. The dominant contribution arises from a primary progenitor whose debris traces a coherent metallicity-energy sequence, consistent with hierarchical tidal stripping and core bifurcation. In addition, we identify independent progenitors that contribute to other substructures. In particular, the components with [Fe/H] $\approx -1.7$ are interpreted as a dual-origin population, likely associated with systems accreted at different epochs. These results highlight the complex, multi-progenitor origin of the retrograde stellar halo of the MW.