2025/09/11 by Orkney, Matthew D. A., Laporte, Chervin F. P.
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2509.09576
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 in the case of major mergers. In contrast, observations from Gaia and legacy surveys reveal that Galactic α-rich populations as old as τ=13.5 \rmGyr show significant rotation, with median η> 0.75. This places strong constraints on the merger ratio between the proto-Milky Way and its last significant merger (Gaia-Sausage Enceladus, GSE), favouring values of \lesssim 1:7. We present the age-metallicity relation for the stellar halo and estimate the interaction epoch at τ_\rmspin-up≃τ_\rmGSE∼11 \rmGyr. We note an abrupt dearth of halo and Splash stars after a lookback time of 10 \rmGyr, marking the end of the merger interaction. Finally, we show that Globular Clusters in the metallicity range -0.8<\rm[Fe/H]<-0.3 align with a formation time of τ_\rmstarburst∼11 \rmGyr, 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\rm GSE=τ\rm starburst ∼ 11 Gyr.