为了揭示我们银河系的集会历史 ，我们需要了解诞生多少恒星，当时是从什么材料和什么轨道上出生的 。这需要大量恒星样本延伸至最古老的年龄（约14 Gyr）9,12的精确年龄。亚巨星是由氢壳融合所维持的恒星
，可以是出于这些目的的独特示踪剂，因为它们存在于允许最精确和直接年龄确定的短暂恒星进化阶段 ，因为它们的亮度是对年龄的直接度量
。此外
，从其光球表面的光谱确定的化学元素组成准确地反映了其出生物质成分数十亿年前。这使得次级构成了银河考古学的最佳实用示踪剂，即使与主要的关闭恒星相比 ，其表面丰度可能会因原子扩散效应而改变 。但是
，由于其进化阶段的寿命短，次巨星相对较少 ，并且大型调查对于建立具有良好光谱的这些物体的大量样本至关重要
，而这些对象过去曾在过去尚未可用
。 　　随着Gaia Mission14,15的最新数据释放（EDR3）以及Lamost光谱调查的最新数据释放（DR7）16,17，我们根据其在有效温度（TEFF） - 少量（MK）图（MK）图（图1A）中的位置确定了一组约250,000个亚巨星。这些子巨星的年龄（τ）是通过与贝叶斯方法拟合拟合的Yessei -yale（yy）恒星等异孔估计的 ，它采用了贝叶斯方法
，该方法借鉴了天体距离（视差），明显的幅度（波动） ，光谱化学丰度（[fe/h]，[fe/h]
，[fe/h]，α ，α
，α，α ，α
，m 。Ti），Teff和Mk
。如图1B所示 ，样本星的中位相对年龄不确定性在整个年龄范围从1.5 GYR到宇宙的年龄范围仅为7.5％（13.8 GYR；参考文献19）。我们样本的较低年龄限制是我们方法固有的：因此
，年轻的和更多的发光子巨人可以与不同的恒星进化阶段混淆，这是较老的恒星的水平分支相 ，这将导致严重的样品污染 。该样本构成了恒星的样本量100倍
，年龄相对精确且一致的年龄为20,21
。此外，它是一个大型样本 ，涵盖了银河系中的大量空间体积（图1C）和大多数相关范围的年龄和金属性（1.5 Gyr）< τ < 13.8 Gyr, and −2.5 < [Fe/H] < 0.4). The sample also has a straightforward spatial selection function that allows us to estimate the space density of the tracers. These ingredients enable an alternative view of the Milky Way’s assembly history, especially the early formation history. 　　The photospheric metallicity of any subgiant star of age τ reflects the element composition of the gas from which it formed at the epoch τ Gyr ago. The overall distribution of these stellar metallicities at different epochs, p(τ, [Fe/H]), thus encodes the chemical enrichment history of our Milky Way galaxy. Figure 2a presents this distribution for our data. It shows that the age–metallicity distribution exhibits a number of prominent and distinct sequences, including at least two age-separated sequences with [Fe/H] >-1，以及一系列仅在低金属性的旧恒星
，[fe/h]< −1. The density of p(τ, [Fe/H]) may change with stellar orbit or Galactocentric radius, in the range our sample covers (6–14 kpc; Fig. 1). Yet, the ‘morphology’ of the distribution varies only slightly, enabling us to focus on the radially averaged distribution p(τ, [Fe/H]) here. 　　It turns out that the complexity of p(τ, [Fe/H]) (Fig. 2a) can be unravelled by dividing the sample into two subsamples using stellar quantities that are neither τ nor [Fe/H]: the angular momentum Jϕ (also denoted as LZ) and the ‘α-enhancement’, [α/Fe]. Extensive observations indicate that the majority of stars in the Milky Way formed from gradually enriched gas on high-angular momentum orbits, or the extended (‘thin’) disk4,22, at high Jϕ and low [α/Fe]. It is also well established that the distribution of Galactic stars in the [α/Fe]–[Fe/H] plane is bimodal, with a high-α sequence reflecting rapid enrichment and a low-α sequence reflecting gradual enrichment, which indicates a natural way to divide any sample in the [α/Fe]–[Fe/H] plane8. This inspired our approach to divide our sample into two, separating the dominant sample portion of gradually enriched disk stars with high angular momentum from the rest. Specifically, we used the cut 　　which is illustrated as a yellow shaded area in Fig. 2b, c. The resulting subsamples in the τ–[Fe/H] plane are shown in Fig. 2d, e, where it is crucial to recall that the sample split involved neither of the quantities on the two axes, τ and [Fe/H]. As we want to focus first on the Milky Way’s elemental enrichment history, rather than its star-formation history, we normalize the distribution p(τ, [Fe/H]) at each [Fe/H] to yield p(τ | [Fe/H]), the age distribution at a given [Fe/H]. 　　Figure 2d, e shows that this cut in angular momentum and [α/Fe] separates the Milky Way’s enrichment history neatly into two distinct age regimes, with a rather sharp transition at τ  8 Gyr. We will therefore refer to these two portions, not clearly apparent in earlier data, as and . The distribution of clearly exhibits a V-shape23. This shape is presumably a consequence of the secular evolution of the dynamically quiescent disk; the metal-rich ([Fe/H]  −0.1) branch arises from stars that have migrated from the inner disk to near the Solar radius. The slope of that branch in then results from the (negative) radial metallicity gradient in the disk1 and the fact that the stars that have migrated more needed more time to do so, and are hence older. Analogously, we presume the lower branch of at [Fe/H]  −0.1 to arise from stars that were born further out and have migrated inwards6. A quantitative comparison with secular evolution models of the Galactic disk4,22 is part of separate ongoing work. 　　The older stars, reflected in , show two prominent sequences with distinct [Fe/H](τ) relations. The stars with −2.5 < [Fe/H] < −1.0 reflect the well-established stellar halo population of our Milky Way, whereas the more metal-rich sequence ([Fe/H]  −1) reflects the Milky Way’s inner, high-α (thick) disk24; this designation as an old disk component is also justified by the stars’ angular momentum, as we will show below. 　　The morphology of the old disk sequence in is the most striking feature in Fig. 2e; it reveals an exceptionally clear, continuous and tight age–metallicity relation from [Fe/H]  −1 at 13 Gyr ago all the way to [Fe/H]  0.5 at 7 Gyr ago. A simple model for p(τ | [Fe/H]) of this sequence (Supplementary Information) finds an intrinsic age dispersion of less than 0.82  Gyr at a given [Fe/H] across this 6 Gyr interval (Extended Data Fig. 1). Given the sequence’s slope, this implies that the [Fe/H] dispersion at a given age is smaller than 0.22 dex across the 1.5 dex range in [Fe/H]. 　　Both the halo and old disk sequences extend to [Fe/H]  −1. However, at that [Fe/H] value, the old disk sequence is approximately 2 Gyr older than the halo sequence, leading to a Z-shaped structure in . This feature is a second aspect of the distribution that has not, to our knowledge, been seen before21. 　　Tentative hints for some of these features in p(τ | [Fe/H]) have been seen in earlier work24,25 (see the discussion in the Supplementary Information) but these studies lacked the sample size or precision for definitive inferences about the Galactic formation history. Figure 2 shows clearly that the old, high-α ‘thick’ disk of our Milky Way started to form approximately 13 Gyr ago, which is only 0.8 Gyr after the Big Bang19, and extended over 5–6 Gyr, and the interstellar stellar medium (ISM) forming the stars was continually enriched by more than 1 dex, from [Fe/H]  −1 to 0.5. The tightness of this [Fe/H]–age sequence implies that the ISM must have remained spatially mixed thoroughly during this entire period. Had there been any radial (or azimuthal) [Fe/H] variations (or gradients) in excess of 0.2 dex in the star-forming ISM at any time, this would have increased the resulting [Fe/H]–age scatter beyond what is seen. Such gradients, along with orbital migration, are the main reason that the later Galactic disk shows a considerably higher [Fe/H] dispersion at a given age4,26. The results also show that the formation of the Milky Way’s old, α-enhanced disk overlapped in time with the formation of the halo stars: the earliest disk stars are 1–2 Gyr older than the major halo populations at [Fe/H]  −1 (see the Z-shaped structure). 　　In Fig. 3 we examine the distribution more closely by separating stars with at least modest angular momentum, Jϕ >500 kpc km s – 1，来自这些恒星几乎径向甚至逆行轨道< 500 kpc km s–1. This further sample differentiation by angular momentum leads again to two nearly disjoint p(τ | [Fe/H]) distributions. The first (Fig. 3, upper panel), with mostly [Fe/H] >-1 ，由我们已经归因于旧磁盘的紧密p（τ| [fe/h]）序列主导。第二个主要[Fe/H]< −1.2, reflects the halo. 　　Note that Fig. 3, lower panel shows a distinct set of stars with Jϕ < 500 kpc km s–1, for which the p(τ | [Fe/H]) locus indicates that they are the oldest and most metal-poor part of the old disk sequence (see also Extended Data Fig. 2). These stars indicate that some of the oldest members of the old disk sequence were present during an early merger event, by which they were ‘splashed’ to low-angular-momentum orbits27,28. This ancient merger event is presumably the merger with the Gaia-Enceladus satellite galaxy11 (also known as Gaia Sausage10; hereafter Gaia-Sausage-Enceladus), which has contributed most of the Milky Way’s halo stars7,29. The fact that the splashed old disk stars with very little angular momentum are exclusively seen at τ  11 Gyr constitutes strong evidence that the major merger process between the old disk and the Gaia-Sausage-Enceladus satellite galaxy was largely completed 11 Gyr ago. This epoch is 1 Gyr earlier than previous estimates that were based on the lower age limit of the halo stars, 10 Gyr (refs. 11,21,30). 　　Figure 3 shows the volume-corrected two-dimensional distribution p(τ, [Fe/H]) (see the Supplementary Information for the correction of the volume selection effect), rather than the p(τ | [Fe/H]) of Fig. 2. Figure 3 reveals a remarkable feature, namely that the star-formation rate of the old disk reached a prominent maximum at around 11.2 Gyr ago, apparently just when the merger with the Gaia-Sausage-Enceladus satellite galaxy was completed, and then continuously declined with time. The most obvious interpretation of this coincidence is that the perturbation from the Gaia-Sausage-Enceladus satellite galaxy greatly enhanced the star formation of the old disk. Note that this star-formation peak among the old disk stars ~11 Gyr ago is very consistent with earlier indications of such a peak based on abundances only31. 　　To put our results into the bigger picture of galaxy formation and evolution, the multiple assembly phases are seen to be universal among present-day star-forming galaxies. Using the IllustriesTNG simulation, Wang et al.32 showed that galaxy mergers and interactions have played a crucial role in inducing gas inflow, resulting in multiple star formation episodes, intermitted by quiescent phases. Observationally, the best testbed for this theoretical picture would be here at home within our Galaxy. Our study has demonstrated the power of such tests for galactic assembly and enrichment history in the full cosmic timeline, from the very early epoch (τ  13 Gyr or redshift z >10）到当前时间 。
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