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The formation of our Milky Way can be parsed qualitatively into differentphases that resulted in its structurally different stellar populations: thehalo and the disk components. Revealing a quantitative overall picture of theGalactic assembly awaits a large sample of stars with very precise ages. Herewe report an analysis of such a sample using subgiant stars. We find that thestellar age-metallicity distribution p(age, metallicity) splits into two almostdisjoint parts, separated at 8 Gyr. The younger reflecting a late phase ofquiescent Galactic disk formation with manifest evidence for stellar radialorbit migration; the other reflecting the earlier phase, when the stellar haloand the old alpha-process-enhanced (thick) disk formed. Our results indicatethat the formation of the Galactic old (thick) disk started 13 Gyr ago, only0.8 Gyr after the Big Bang, and two Gigayears earlier than the final assemblyof the inner Galactic halo. Most of these stars formed 11 Gyr ago, when theGaia-Sausage-Enceladus satellite merged with our Galaxy. Over the next 5--6Gyr, the Galaxy experienced continuous chemical element enrichment, ultimatelyby a factor of 10, while the star-forming gas managed to stay well-mixed.

A time-resolved picture of our Milky Way's early formation history