按时间顺序确定Milky Way早期组装的年代
Chronologically dating the early assembly of the Milky Way
摘要：
标准宇宙学模型（$Λ$ -CDM）预言星系是通过宇宙学时间尺度$^{1,2}$上的等级组装而形成的.银河系，像其他盘状星系一样，在其早期历史中经历了小卫星星系的剧烈合并和吸积。多亏了Gaia-DR 2 $^3$和光谱巡天$^4$，这种合并的恒星残骸已经被确定为$^{5-7}$。这些事件的年代测定对于揭示高红移星系的形成和演化至关重要，但由于难以获得这些最古老恒星的精确年龄，迄今为止一直具有挑战性。在这里，我们结合星震学-恒星振荡的研究-与运动学和化学丰度，估计精确的恒星年龄（$\sim$ 11%）的样本观测到的恒星的$\mathit{Kepler}$空间任务$^8$。至关重要的是，这个样本不仅包括一些在银河系内部形成的最古老的恒星，而且还包括在外部形成并随后吸积到银河系上的恒星。利用这一年龄分辨率，我们提供了令人信服的证据，支持银河系在大约100亿年前的卫星星系盖亚-土卫二/香肠$^{5,6}$落入之前已经形成了大量恒星（现在主要居住在其厚盘中）的模型
英文摘要：
The standard cosmological model ($Λ$-CDM) predicts that galaxies are built through hierarchical assembly on cosmological timescales$^{1,2}$. The Milky Way, like other disc galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Thanks to Gaia-DR2$^3$ and spectroscopic surveys$^4$, the stellar remnants of such mergers have been identified$^{5-7}$. The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging owing to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology -- the study of stellar oscillations -- with kinematics and chemical abundances, to estimate precise stellar ages ($\sim$ 11%) for a sample of stars observed by the $\mathit{Kepler}$ space mission$^8$. Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy, but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disc) before the in-fall of the satellite galaxy Gaia-Enceladus/Sausage$^{5,6}$ around 10 billions years ago
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