21) Interpreting chemo-dynamical features of the Milky Way with the Auriga simulation

Abstract: The Milky Way possesses several striking features, such as the accretion debris known as the Gaia-Enceladus-Sausage (GES), its bimodal chemical disc structure, and its X-shaped peanut bulge/bar. Using the large suite of state-of-the-art Auriga cosmological-zoom simulations for the formation of Milky Way-mass galaxies, we explore the origin and prevalence of these features. We find that roughly a third of haloes experience a GES merger on the appropriate radial orbit. Our simulations indicate that the GES progenitor was gas-rich and that the merger created the rotationally-dominated alpha-rich part of the thick disc, which smoothly connects to the inner halo, from a combination of an intense starburst and dynamical heating of the Galactic proto-disc. Interestingly, we find that the most quiescent merger histories (since a look-back time of about 12 Gyr) best reproduce the chemo-dynamical properties of stellar populations in the MW’s boxy bulge, including its boxy structure, suggesting an upper limit of a few percent for the mass ratio of the GES. We find the formation of the chemical disc bimodality to be a rather rare occurrence, requiring first an intense starburst, followed by a temporary lull in gas accretion before a second, prolonged period of star formation ensues after the accretion of metal-poor gas that arrives with a minor merger.

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