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.

我们的银河系何时首次形成了一个恒星盘组件，该组件在随后的星系合并中幸存下来并保持了其盘状结构，这仍然是一个谜。我们提出了一项关于银河系盘的年龄依赖性结构和恒星形成速率的研究，这些恒星使用具有大量轨道角动量的高α恒星，这些恒星具有精确的年龄确定。我们的结果表明，径向尺度长度几乎与年龄无关，而垂直尺度高度经历了戏剧性的演变。即使对于 13 Gir 以上的种群也会出现圆盘状几何形状，对于 12.5 Gir 以下的种群，鳞片高长比降至 0.5 以下。我们将保持盘状几何形状的最古老的种群称为盘古——显然是在 13 Gyr 之前形成的。盘古现在的恒星质量估计约为 2 × 10⁹M_⊙，据推测是最早时代我们银河系的主要恒星组成部分。整个高α盘的现代恒星总质量为 2 ×^{10 10}M_⊙，这主要是在 11 Gyr 之前每年 11 M_⊙ 的明显恒星形成速率峰值期间形成的。与 TNG50 模拟中的银河系类似物进行比较意味着，我们的银河系甚至在盖亚-土卫二合并之前就经历了异常静止的动力学历史。