Distant Echoes of the Milky Way’s Last Major Merger
Distant Echoes of the Milky Way’s Last Major Merger
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DOI：
10.3847/1538-4357/accf13
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发表时间：
2022-12
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通讯作者：
V. Chandra;R. Naidu;C. Conroy;A. Ji;H. Rix;A. Bonaca;P. Cargile;Jiwon Jesse Han;Benjamin D. Johnson;Y. Ting 丁;T. Woody;D. Zaritsky
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作者：
V. Chandra;R. Naidu;C. Conroy;A. Ji;H. Rix;A. Bonaca;P. Cargile;Jiwon Jesse Han;Benjamin D. Johnson;Y. Ting 丁;T. Woody;D. Zaritsky
银河系的大部分恒星晕由银河系最后一次大合并的碎片组成，盖亚-香肠-土卫二（GSE）。在过去的几年里，来自GSE的恒星在我们银河系内部30 kpc的区域进行了运动学和化学研究。然而，模拟预测，吸积碎片可能位于更远的距离，形成外晕的子结构。在这里，我们推导出金属丰度和距离使用盖亚DR 3 XP光谱的一个全天空样本的发光红巨星，并映射外晕的运动学和金属丰度到100千秒差距。我们获得后续光谱的恒星在两个强大的overdensity-including先前确定的外部室女座Overdensity-and发现他们是相对丰富的金属和主要逆行轨道，匹配的预测从模拟的GSE合并。我们认为，这些都是远心壳的GSE碎片，形成60-90千秒差距对应的15-20千秒差距的壳，已知占主导地位的内恒星晕。我们用文献径向速度在天空中扩展搜索，我们发现了一个相干的逆行恒星流的证据，这些恒星从50到100千秒差距环绕银河系，与人马座流在同一平面上，但朝相反的方向移动。这是首次发现GSE合并产生的遥远而结构化的印记，巩固了倾斜和逆行碰撞的画面，建立了我们银河系的恒星晕。
The majority of the Milky Way’s stellar halo consists of debris from our galaxy’s last major merger, the Gaia-Sausage-Enceladus (GSE). In the past few years, stars from the GSE have been kinematically and chemically studied in the inner 30 kpc of our galaxy. However, simulations predict that accreted debris could lie at greater distances, forming substructures in the outer halo. Here we derive metallicities and distances using Gaia DR3 XP spectra for an all-sky sample of luminous red giant stars, and map the outer halo with kinematics and metallicities out to 100 kpc. We obtain follow-up spectra of stars in two strong overdensities—including the previously identified outer Virgo Overdensity—and find them to be relatively metal rich and on predominantly retrograde orbits, matching predictions from simulations of the GSE merger. We argue that these are apocentric shells of GSE debris, forming 60–90 kpc counterparts to the 15–20 kpc shells that are known to dominate the inner stellar halo. Extending our search across the sky with literature radial velocities, we find evidence for a coherent stream of retrograde stars encircling the Milky Way from 50 to 100 kpc, in the same plane as the Sagittarius Stream but moving in the opposite direction. These are the first discoveries of distant and structured imprints from the GSE merger, cementing the picture of an inclined and retrograde collision that built up our galaxy’s stellar halo.