除了加热的厚圆盘外，Milky Way没有原位晕圈。恒星晕的组成和盖亚DR2和APOGEE最后一次重大合并的年龄测定
The Milky Way has no in-situ halo other than the heated thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE
摘要：
基于对盖亚DR2数据的分析，以前的研究表明，吸积星，可能来自一个单一的祖先卫星，是我们银河系晕的重要组成部分，可能构成了$\rm [Fe/H] < -1$的大部分晕星，距离太阳几千秒差距以内。在本文中，我们耦合的天体测量数据从盖亚DR 2与APOGEE DR 14的元素丰度的运动学和化学的特点，在原位和增生的人口高达$\rm [Fe/H] \sim -2$。吸积恒星似乎显着影响银河系的化学运动学关系，不仅在$\rm [Fe/H] < -1$，而且在金属丰度典型的厚和金属贫薄盘。它们占$\rm [Fe/H] < -1$所有恒星的60%，剩下的40%是由（弱金属）厚盘星组成。我们发现，恒星运动化石记录显示了这个吸积事件留下的印记，加热了旧的银河系盘。我们能够确定这个运动学印记的年龄，表明吸积发生在9到11 Gyr之前，它导致了银河系圆盘的最后一次显着加热。在星系晕中，发现了一个重要部分的恒星，其丰度是典型的（富含金属的）厚盘，并被这种相互作用加热。事实上，在距离太阳几千秒差距的运动学定义的晕中，大约有一半是由富含金属的厚盘星组成的。此外，我们认为，这种富含金属的厚盘组件占主导地位的恒星晕的内银河系。从这项研究中出现的新图片是标准的非旋转原位晕人口，崩溃的晕，似乎比以往任何时候都更加难以捉摸。
英文摘要：
Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at $\rm [Fe/H] < -1$ within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterize the kinematics and chemistry of in-situ and accreted populations up to $\rm [Fe/H] \sim -2$. Accreted stars appear to significantly impact the Galactic chemo-kinematic relations, not only at $\rm [Fe/H] < -1$, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at $\rm [Fe/H] < -1$, the remaining 40% being made of (metal-weak) thick disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event which heated the old Galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between 9 and 11 Gyr ago, and that it led to the last significant heating of the Galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the Galactic halo. Indeed about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick disc stars. Moreover, we suggest that this metal-rich thick disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.
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