A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages

Abstract

The spatial distribution of Galactic disk stars of different ages and metallicities encodes its dynamical evolution history and formation pathways. The lack of precise ages for large, representative samples of stars has traditionally been a drawback for securely differentiating among formation scenarios proposed to explain the vertical properties of the Milky Way disk (e.g. upside-down formation versus secular dynamical heating). In this letter, we use color–magnitude diagram fitting of Gaia data to derive very precise age–metallicity distributions for complete stellar samples in the solar neighborhood. We identify quasi-mono-age–metallicity (qMAM) stellar populations with an age resolution better than 5% (≃0.5 Gyr at old age), and study their absolute vertical density profile. We observe a clear correlation between scale height and stellar age, with younger populations consistently more confined to the plane. We find evidence that the scale height decrease rate changed drastically 10.85−0.12+0.13 Gyr ago: at older ages, the evolution is very steep, with hZ dropping from over 1.26 to ∼0.5 kpc in merely 2 Gyr; subsequently, the thinning is very gradual, with hZ ∼ 0.1 kpc for the youngest populations. The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed. We thus strongly support an evolutionary scenario defined by two distinct temporal phases: an initial, highly turbulent epoch that gave birth to the thick disk, and a subsequent more quiescent period of secular evolution and gradual gas settling, which formed the thin disk.