The Three-Phase Evolution of the Milky Way

Understand

We illustrate the formation and evolution of the Milky Way over cosmic time, utilizing a sample of 10 million red giant stars with full chemodynamical information, including metallicities and $\alpha$-abundances from low-resolution Gaia XP spectra.

• The evolution of angular momentum as a function of metallicity - a rough proxy for stellar age, particularly for high-[$\alpha$/Fe] stars - displays three distinct phases: the disordered and chaotic protogalaxy, the kinematically-hot old disk, and the kinematically-cold young disk.
• The old high-$\alpha$ disk starts at [Fe/H] $\approx -1.0$, 'spinning up' from the nascent protogalaxy, and then exhibits a smooth 'cooldown' toward more ordered and circular orbits at higher metallicities.
• The young low-$\alpha$ disk is kinematically cold throughout its metallicity range, with its observed properties modulated by a strong radial gradient.