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 $\alpha$-rich populations as old as $\tau =13.5\mathrm{G}\mathrm{y}\mathrm{r}$ show significant rotation, with median $\eta >0.75$. 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 ${\tau }_{\mathrm{s}\mathrm{p}\mathrm{i}\mathrm{n}\text{-}\mathrm{u}\mathrm{p}}\simeq {\tau }_{\mathrm{G}\mathrm{S}\mathrm{E}}\sim 11\mathrm{G}\mathrm{y}\mathrm{r}$. We note an abrupt dearth of halo and Splash stars after a lookback time of $10\mathrm{G}\mathrm{y}\mathrm{r}$, marking the end of the merger interaction. Finally, we show that Globular Clusters in the metallicity range $-0.8<[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-0.3$ share a formation time of ${\tau }_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{u}\mathrm{r}\mathrm{s}\mathrm{t}}\sim 11\mathrm{G}\mathrm{y}\mathrm{r}$, 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 ${\tau }_{\mathrm{G}\mathrm{S}\mathrm{E}}={\tau }_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{u}\mathrm{r}\mathrm{s}\mathrm{t}}\sim 11$Gyr.