The formation and survival of the Milky Way’s oldest stellar disk

It remains a mystery when our Milky Way first formed a stellar disk component that survived and maintained its disk structure from subsequent galaxy mergers. We present a study of the age-dependent structure and star formation rate of the Milky Way’s disk using high-α stars with substantial orbital angular momentum that have precise age determinations. Our results show that the radial scale length is nearly independent of age, whereas the vertical scale height experienced dramatic evolution. A disk-like geometry presents even for populations older than 13 Gyr, with the scale height-to-length ratio dropping below 0.5 for populations younger than 12.5 Gyr. We dub the oldest population that has maintained a disk geometry—apparently formed over 13 Gyr ago—PanGu. With an estimated present-day stellar mass of approximately 2 × 109 M⊙, PanGu is presumed to be a major stellar component of our Galaxy in the earliest epoch. The total present-day stellar mass of the whole high-α disk is 2 × 1010 M⊙, which was mostly formed during a distinct star formation rate peak of 11 M⊙ per year around 11 Gyr ago. A comparison with Milky Way analogues in the TNG50 simulations implies that our Galaxy has experienced an exceptionally quiescent dynamical history, even before the Gaia–Enceladus merger. The Milky Way contains an extremely long-lived stellar disk component that formed, according to this study, more than 13 billion years ago. Such longevity is due to the exceptionally quiescent dynamical history of the Galaxy.