Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the "thin" and "thick" discs and the halo. We show that in the Solar neighborhood, there exists a large population of metal-rich ([Fe/H]>-0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics and stellar ages in the plane of azimuthal velocity v_phi and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the alpha-rich (or "thick") disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the Milky Way's proto-disc prior to the massive ancient accretion event which drastically altered their orbits. We can not, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting-edge numerical simulations of the Milky Way's formation.
Universidade de São Paulo（圣保罗大学）；Shanghai Jiao Tong University（上海交通大学）；Observatório Nacional, MCTI（巴西国家天文台）；University of Lancashire（兰开夏大学）；Queen’s University（女王大学）
The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue -- II. Were Splash stars heated or already born hot?
Research School of Astronomy and Astrophysics, Australian National University（澳大利亚国立大学天文与天体物理研究学院）；Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen（海德堡大学跨学科科学计算中心）；Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik（海德堡大学天文学中心理论天体物理研究所）；Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze（佛罗伦萨大学物理与天文学系）；The Observatories of the Carnegie Institution for Science（卡内基科学机构天文台）；Institute of Astronomy, University of Cambridge（剑桥大学天文学研究所）；Department of Astronomy, New Mexico State University（新墨西哥州立大学天文学系）
Models of Multi-component Splash Bridges in Face-on Galaxy Disc Collisions
我们使用一个非弹性粒子代码与冲击和冷却计算的亚网格水平上研究气体的星系盘之间的直接碰撞。星际介质（ISM）的磁盘建模与连续的热相。这些模型产生了许多独特的结构，统称为飞溅桥。它们的范围从中央的桥盘到漩涡状的薄片，类似于在相互作用的星系中观察到的。这些形貌对旋转、相对质量、圆盘偏移和圆盘中的气体结构敏感。在太菲星系- NGC 12914/15的情况下，广泛的观测揭示了无线电连续发射气体，HI气体，热X射线从热扩散气体和更多的$H_2$比存在于银河系共存的桥梁。$H_2$的起源和ISM的大的不对称分布尚不清楚。我们发现，对于小光盘碰撞参量，多个阶段的ISM与密度超过许多数量级，可以从他们的宿主星系到一个Taffy一样的桥梁。盘初始重叠的方向对ISM各相的分布有很大的影响。在某些情况下，这些模型还预测了一个可能的“暗星系”的形成，这是一个远离任何一个星系的星盘潜力的致密ISM的大平坦区域。
Build-up and survival of the disc: From numerical models of galaxy formation to the Milky Way
Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona（巴塞罗那大学宇宙科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚太空研究院）；LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS（巴黎天文台拉格朗日天体物理学实验室（隶属于巴黎文理研究大学、索邦大学、巴黎西岱大学、塞吉-蓬图瓦兹大学及法国国家科学研究中心））；Kavli IPMU (WPI), UTIAS, The University of Tokyo（东京大学 Kavli宇宙起源与演化研究中心）
