Description
Historic merger events are expected to leave imprints of their collisions in the stellar halo’s phase space and chemodynamics. However, interpreting such imprints is complicated by the complex hierarchical history expected in LCDM (Rey M. P.et al. , 2023, MNRAS, 521, 995 10.1093/mnras/stad513). Here, we explore the impact of merging within the PARADIGM simulations (Joshi G. D. et al , 2025,MNRAS, 537, 3792 10.1093/mnras/staf276), comprising a series of Milky-Way-like galaxies with systematically varying accretion histories, each one simulated with two complementary codes (RAMSES/Vintergatan and TNG50). We have post-processed these simulations to keep track of accreted and insitu stellar populations. We then use this to track how the impact of mergers effects the ability of metallicity and phase-space observations to disentangle hierarchical history. In particular, we show how early formation history gives rise to tightly bound halo stars, and how a major merger can disrupt the present-day galaxy to such extremes that the early in-situ and ex-situ stellar populations become indistinguishable. This work provides insights into whether modern observations like Gaia can accurately disentangle the merger history of the Milky Way, and to within what epochs, answering the question ‘how well does the Milky Way remember its merger past?’
