The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue I: A golden thread through time and space (2510.11284v1)

Abstract: Understanding how the Milky Way's present-day structure was shaped by past major mergers is a key goal of Galactic archaeology. The chemical and dynamical structure of the Galaxy 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 chemically enriched, while those from the outskirts are less bound and metal-poor. This supports the scenario proposed by Sk\'ulad\'ottir et al. (2025) using idealised simulations, now demonstrated in a cosmological context. The preserved chemodynamical memory is also evident in elemental planes such as [Al/Fe] vs. [Mg/Mn], reflecting gradients in star formation efficiency. The median abundance trends for different orbital energies are broadly similar, though less pronounced and more non-linear than the linear relations reported by Sk\'ulad\'ottir et al. (2025), with relatively good agreement at the highest metallicities. Our results reveal that spatial and temporal memory is retained across the merger -- connecting birth locations to present-day properties like a golden thread. We demonstrate that selection methods in integrals-of-motion space systematically bias progenitor reconstructions by missing its enriched core, and we outline strategies to obtain a more representative census of accreted stars.