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This study uses cosmological simulations to show that the Milky Way's evolution proceeded through three distinct kinematic phases: an early disordered protogalaxy with bursty star formation, a "spin-up" phase forming a thick stellar disk during peak gas-to-star conversion, and finally a "cooling down" phase creating the thin disk we observe today. The research demonstrates these phases arise naturally from internal galactic processes rather than major merger events, with the key transition to thin disk formation occurring when gas accretion slows enough for angular momentum to become coherent.
Why it matters
Understanding the Milky Way's formation history provides crucial insights into galaxy evolution mechanisms more broadly and helps interpret observational data from current surveys like Gaia and APOGEE. This work suggests that the structural features of disk galaxies can emerge from internal processes without requiring dramatic external disruptions.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: APOGEE and Gaia data suggest the Milky Way’s kinematic structure evolved through three distinct phases: a disordered protogalaxy, which “spun up” into a thick stellar disk, and then “cooled down” to a final cold, thin stellar disk. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas ($T leq 10^4$ K) converting into stars is low, star formation is bursty, and the baryonic mass “sloshes” within the host potential with respect to the center of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disk formation appears to be minimal: a stable center of mass motion. Thin disk formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.
Source: From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?