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

TL;DR

This paper investigates how the Milky Way’s last major merger leaves chemodynamical memory in present-day stars using a high-resolution NIHAO-UHD cosmological zoom-in Milky Way analogue. By classifying accreted stars into four orbital-energy zones and analyzing birth radii, ages, and current orbits, the study demonstrates that stars born in the progenitor core are more metal-rich and more tightly bound, while outskirts-born stars are metal-poorer and less bound, evidencing chemodynamical memory across the merger. The authors also reveal non-linear abundance trends in multiple element ratios and show that selection in integrals-of-motion space biases against core remnants, emphasizing the need for inner-Galaxy, chemistry-driven strategies to recover a representative accreted-star census. These findings validate and extend prior idealized results to a cosmological context, offering guidance for future observational surveys to trace the GSE and refine the inferred mass-metallicity relation of accreted systems.

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 Skúladóttir 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 Skúladóttir 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.

Figures (10)

• Figure 6: Histograms of ${\mathrm{[Fe/H]}}$ distributions of the whole galaxy (grey), and its previously accreted stars with different orbit energies as given in Table \ref{['tab:energy_selection']} (more negative energies from red to yellow, as shown in the inset panel of $L_Z$ vs. $E$). The percentiles of each distribution are plotted at the top right of the figure and show an increase of average [Fe/H] with more negative energies. See https://github.com/svenbuder/golden_thread_I/tree/main/figures for individual figures showing the build-up of this plot.
• Figure 7: Abundance distributions of (past) accreted stars in blue, while the whole simulation is shown in the background (black). Yellow, orange, light and dark red dashed lines show the median abundances for 5-percent-bins if the x-axis of accreted stars for the different energy quantiles (same as in Fig. \ref{['fig:fe_h_histograms']}). Narrow contours show the distribution of 68% of each sample. Squares show the median abundances of each sample https://github.com/svenbuder/golden_thread_I/tree/main/figures.
• Figure 8: Present-day spatial distribution of accreted stars in the Galactocentric $X$-$Y$ plane. Panels a-d) show the density distribution of accreted stars with highest to lowest orbit energy quartiles, where the dashed circle shows a solar-analogue $R_\mathrm{2D}=8.2$ kpc. Panel legends list the 50th and 16th-84th percentiles of galactocentric cylindrical radii $R_\mathrm{2D}$. Panel e) shows the $R_\mathrm{2D}$ distributions, with an inset visualising the $L_Z$ vs. $E$ selection. See Fig. \ref{['fig:xz_distribution_ezones']} for a similar figure showing the $X$-$Z$ plane https://github.com/svenbuder/golden_thread_I/tree/main/figures.
• Figure 9: Birth positions (determined in batches of $100\,\mathrm{Myr}$) coloured by present-day orbit energy, both before the major merger (top, for star formation between 11.65 and $10.25\,\mathrm{Gyr}$ ago) and just before and during the merger (bottom, for star formation between 10.15 and $8.55\,\mathrm{Gyr}$ ago), with the main galaxy's birth positions visible in greyscale https://github.com/svenbuder/golden_thread_I/tree/main/figures.
• Figure 10: Histograms of [Fe/H] distribution for accreted stars with different present-day galactocentric radii $R_\mathrm{2D}$ with median [Fe/H] indicated in the legend for a each region https://github.com/svenbuder/golden_thread_I/tree/main/figures.