Build-up and survival of the disc: from numerical models of galaxy formation to the Milky Way

We study the build-up and survival of angular momentum in the stellar disc using a statistical suite of cosmological simulations of Milky Way-mass galaxies. Our results show that stellar kinematics at $z=0$ rarely recover the true times of disc spin-up, due to the disruptive impact of massive radial merger events. The proto-disc (i.e. Aurora) and kicked-up disc stars (the Splash) become indistinguishable at low metallicities, and the local fraction of kicked-up disc stars remains $< 20$ per cent even after major mergers. In contrast, observations from Gaia and legacy surveys reveal that Galactic α-rich populations as old as τ =13.5\, Gyr show significant rotation, with median η > 0.75. This places strong constraints on the total merger ratio between the proto-Milky Way and its last significant merger (Gaia-Sausage-Enceladus, GSE), favouring minor mergers with mass ratios $< 1:4$. We present the age–metallicity relation for the stellar halo and estimate the interaction epoch at τ _ spin-up τ GSE~ 11\, Gyr. We note an abrupt dearth of halo and Splash stars after a lookback time of 10\, Gyr, marking the end of the merger interaction. Finally, we show that Globular Clusters in the metallicity range -0.8<[Fe/H]< -0.3 share a formation time of τ starburst~ 11\, Gyr, which we interpret as a signature of a starburst triggered by the first pericentric interaction of the GSE. This is remarkable corroboration between our GSE interaction and starburst times of τ GSE=τ starburst ~ 11\, Gyr.