Numerical Simulations of the Milky Way's Accretion History

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The second data release of the Gaia satellite has revealed much complexity in the structure and kinematics of stars in the Milky Way than previously appreciated. In the disc, Gaia has shown that our Galaxy is still enduring the effects of a collision that set millions of stars moving like ripples on a pond. In the stellar halo, the data uncovered a large single debris structure pointing to a massive accretion event 10 billion years ago, at a time when the disc was in its infancy. Our basic assumptions of dynamical equilibrium and axisymmetry at the basis of nearly all mathematical models of the Galaxy are now falling short to make further progress on our inference on the Galaxy’s formation or the distribution of dark matter. Understanding the detailed time-dependent non-axisymmetric phase-space structure of the Galaxy would open new pathways to understand its detailed accretion history, potentially dating its most major perturbations. This proposal aims to explore the deep coupling between the stellar halo and the Milky Way disc and bulge, to gain new insights on the formation history of the Milky Way through its most major accretion events through a number of state-of-the-art computing techniques. Study 1 will look into studying the formation of the inner-halo through a combination of cosmological genetically modified (constrained) simulations and idealised simulations to constrain the mass and accretion time of the Gaia-Sausage progenitor galaxy (and its potential satellite population which came with it) as well as its impact on the formation of the ``thick disc'' and growth of the Galaxy past z~3-2. Study 2 will look into the impact of known satellites on the dynamical and chemical and age populations’ evolution of the Milky Way using both cosmological/isolated hydrodynamical simulations and idealised numerical N-body simulations, particularly focusing on the role of the Sagittarius dwarf in seeding the perturbations in the disc we see today.