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

The formation of our Milky Way can be split up qualitatively into different phases that resulted in its structurally different stellar populations:the halo and the disk components(1-3). Revealing a quantitative overall picture of our Galaxy's assembly requires a large sample of stars with very precise ages. Here we report an analysis of such a sample using subgiant stars. We find that the stellar age-metallicity distribution p(tau, [Fe/H]) splits into two almost disjoint parts, separated at age tau similar or equal to 8 Gyr. The younger part reflects a late phase of dynamically quiescent Galactic disk formation with manifest evidence for stellar radial orbit migration(4-6); the other part reflectsthe earlier phase, when the stellar halo(7) and the old alpha-process-enhanced (thick) disk(8,9) formed. Our results indicate that the formation of the Galaxy's old (thick) disk started approximately 13 Gyr ago, only 0.8 Gyr afterthe Big Bang, and 2 Gyr earlier than the final assembly ofthe inner Galactic halo. Most ofthese stars formed around 11 Gyr ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy(10,11). Overthe next 5-6 Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately by a factor of 10, while the star-forminggas managed to stay well mixed.

Automated summary

The formation of our Milky Way can be divided into different phases resulting in different stellar populations. A large sample of stars with precise ages was analyzed. The age-metallicity distribution of the stars was found to split into two parts, indicating different formation phases of the galactic disk and halo. The results suggest that the old thick disk of the Galaxy formed approximately 13 billion years ago, shortly after the Big Bang, and 2 billion years earlier than the inner Galactic halo. The merging of the Gaia-Sausage-Enceladus satellite with the Galaxy played a significant role in the formation of these stars. The Galaxy experienced continuous chemical enrichment over the next 5-6 billion years.