Abstract
We present a new suite of cosmological zoom-in hydrodynamical ( asymptotic to 20 pc spatial resolution) simulations of Milky-Way mass galaxies to study how a varying mass ratio for a Gaia-Sausage-Enceladus (GSE) progenitor impacts the z = 0 chemodynamics of halo stars. Using the genetic modification approach, we create five cosmological histories for a Milky-Way-mass dark matter halo ( M-200 asymptotic to 10(12) M-?), incrementally increasing the stellar mass ratio of a z asymptotic to 2 merger from 1:25 to 1:2, while fixing the galaxy's final dynamical, stellar mass, and large-scale environment. We find markedly different morphologies at z = 0 following this change in early history, with a growing merger resulting in increasingly compact and bulge-dominated galaxies. Despite this structural diversity, all galaxies show a radially biased population of inner halo stars like the Milky-Way's GSE which, surprisingly, has a similar magnitude, age, [Fe / H], and [ alpha/ Fe] distribution whether the z asymptotic to 2 merger is more minor or major. This arises because a smaller ex-situ population at z asymptotic to 2 is compensated by a larger population formed in an earlier merger-driven starb urst whose contrib ution to the GES can grow dynamically o v er time, and with both populations strongly o v erlapping in the [Fe / H] - [ alpha/ Fe] plane. Our study demonstrates that multiple high-redshift histories can lead to similar z = 0 chemodynamical features in the halo, highlighting the need for additional constraints to distinguish them, and the importance of considering the full spectrum of progenitors when interpreting z = 0 data to reconstruct our Galaxy's past.