Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
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DOI：
10.3847/1538-4357/ac2d2d
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发表时间：
2021-03
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影响因子：
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通讯作者：
R. Naidu;C. Conroy;A. Bonaca;D. Zaritsky;R. Weinberger;Y. Ting 丁;N. Caldwell;S. Tacchella;
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作者：
R. Naidu;C. Conroy;A. Bonaca;D. Zaritsky;R. Weinberger;Y. Ting 丁;N. Caldwell;S. Tacchella;
多项证据表明，银河系在 z ∼ 2 处与盖亚-香肠-土卫二 (GSE) 星系经历了一次重大合并。在这里，我们使用 H3 调查数据来论证 GSE 在逆行轨道上进入银河系，该轨道基于一群高度逆行恒星，其化学性质与大部分径向 GSE 碎片相似。我们提出了第一个针对合并的定制 N 体模拟。从约 500 个模拟的网格中，我们发现 M ⋆ = 5 × 108 M ⊙、M DM = 2 × 1011 M ⊙ 的 GSE 与 H3 数据最匹配。这个模拟表明，逆行恒星在合并初期就从 GSE 的外盘上剥离了。尽管纯粹根据角动量和径向分布进行选择，但该模拟再现并解释了以下现象：（i）内晕的三轴形状，其长轴与平面约 35°，并连接 GSE 的远心； (ii) 大力神-天鹰云和处女座超密度，它们是由于远心堆积而产生的； (iii) GSE 猝灭与原位晕年龄分布截断之间的 2 Gyr 滞后，追踪第一个和最终 GSE 近心之间的滞后。我们做出以下预测：（i）内晕具有“双断裂”密度分布，在 ≈15-18 kpc 和 30 kpc 处都有断裂，与 GSE 远心重合； (ii) 外晕有等待发现的逆行流，其速度 >30 kpc，其中包含约 10% 的 GSE 恒星。逆行（径向）GSE 碎片源自其外（内）盘——利用这一趋势，我们重建了 GSE 的恒星金属丰度梯度（−0.04 ± 0.01 dex r50−1 ）。这些模拟表明，GSE 提供了约 20% 的银河系当今暗物质和约 50% 的恒星晕。
Several lines of evidence suggest that the Milky Way underwent a major merger at z ∼ 2 with the Gaia-Sausage-Enceladus (GSE) galaxy. Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored N-body simulations of the merger. From a grid of ≈500 simulations we find that a GSE with M ⋆ = 5 × 108 M ⊙, M DM = 2 × 1011 M ⊙ best matches the H3 data. This simulation shows that the retrograde stars are stripped from GSE’s outer disk early in the merger. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following phenomena: (i) the triaxial shape of the inner halo, whose major axis is at ≈35° to the plane and connects GSE’s apocenters; (ii) the Hercules-Aquila Cloud and the Virgo Overdensity, which arise due to apocenter pileup; and (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in situ halo, which tracks the lag between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a “double-break” density profile with breaks at both ≈15–18 kpc and 30 kpc, coincident with the GSE apocenters; and (ii) the outer halo has retrograde streams awaiting discovery at >30 kpc that contain ≈10% of GSE’s stars. The retrograde (radial) GSE debris originates from its outer (inner) disk—exploiting this trend, we reconstruct the stellar metallicity gradient of GSE (−0.04 ± 0.01 dex r50−1 ). These simulations imply that GSE delivered ≈20% of the Milky Way’s present-day dark matter and ≈50% of its stellar halo.