The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue -- I. A golden thread through time and space

Abstract

Understanding how past major mergers shaped the Milky Way's present-day structure is a key goal of Galactic archaeology. The Galaxy's chemical and dynamical structure retains the imprint of such events, including a major accretion episode around 8-10 Gyr ago. Recent findings suggest that present-day orbital energy correlates with stellar chemistry and birth location within the merging progenitor galaxy. Using a high-resolution NIHAO-UHD cosmological zoom-in simulation of a Milky Way analogue, we trace the birth positions, ages, and present-day orbits of stars accreted in its last major merger. We show that stars born in the progenitor's core are more tightly bound to the Milky Way and more chemically enriched, while those from the outskirts are less bound and more metal-poor. This supports the Sk\'ulad\'ottir et al. (2025) scenario that accreted progenitor stars of different chemistry were deposited onto different orbital energies as the galaxy was stripped from the outside in, now in a cosmological context. Quantitatively, we measure a metallicity gradient with progenitor birth radius of $\mathrm{d[Fe/H]}/\mathrm{d}R_\mathrm{birth}^\prime \approx -0.05\,\mathrm{dex\,kpc^{-1}}$, demonstrating that abundance patterns retain measurable memory of formation location within the disrupted satellite. This chemodynamical memory is also evident in elemental planes such as [Al/Fe] vs. [Mg/Mn], consistent with gradients in progenitor star formation efficiency. We further show that common integrals-of-motion selections systematically miss stars from the chemically enriched core, biasing reconstructions toward the metal-poor outskirts. Together, our results demonstrate that chemodynamical memory survives the merger and can reconstruct the accreted galaxy's internal structure, while highlighting biases in current selections of accreted stars.