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

TL;DR

This study uses precise ages of subgiant stars to reconstruct the Milky Way's early formation, revealing distinct phases of halo and disk development and timing of key events like the thick disk formation and satellite mergers.

Contribution

It provides a quantitative timeline of the Milky Way's formation phases based on stellar age-metallicity distributions, highlighting the early thick disk formation and subsequent chemical enrichment.

Findings

The old (thick) disk started forming 13 Gyr ago.

Most stars formed around 11 Gyr ago during a major merger.

The Galaxy's chemical enrichment increased tenfold over 5-6 Gyr.

Abstract

The formation of our Milky Way can be parsed qualitatively into different phases that resulted in its structurally different stellar populations: the halo and the disk components. Revealing a quantitative overall picture of the Galactic assembly awaits 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(age, metallicity) splits into two almost disjoint parts, separated at 8 Gyr. The younger reflecting a late phase of quiescent Galactic disk formation with manifest evidence for stellar radial orbit migration; the other reflecting the earlier phase, when the stellar halo and the old alpha-process-enhanced (thick) disk formed. Our results indicate that the formation of the Galactic old (thick) disk started 13 Gyr ago, only 0.8 Gyr after the Big Bang, and two Gigayears earlier than the final assembly of the inner Galactic halo. Most of these stars formed 11 Gyr ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy. Over the next 5--6 Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately by a factor of 10, while the star-forming gas managed to stay well-mixed.