ABSTRACT 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 -rich populations as old as =13. 5\, Gyr show significant rotation, with median 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 ₒ₈₍-ₔ ₆ₒ₄ 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 ₒₓ₀ₑ₁ₔₑₒₓ 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 ₆ₒ₄= ₒₓ₀ₑ₁ₔₑₒₓ 11\, Gyr.
Orkney et al. (Thu,) studied this question.