Massive dwarf galaxies that merge with the Milky Way on prograde orbits can be dragged into the disk plane before being completely disrupted. Such mergers can contribute to an accreted stellar disk and a dark matter disk. Here we present Nyx, a vast stellar stream in the vicinity of the Sun, which provides the first indication that such an event occurred in the Milky Way. We identify about 200 stars that have coherent radial and prograde motion in this stream using a catalogue of accreted stars built by applying deep learning methods to the Gaia data. Taken together with chemical abundance and orbital information, these results strongly favour the interpretation that Nyx is the remnant of a disrupted dwarf galaxy. Further justified by FIRE hydrodynamic simulations, we demonstrate that prograde streams like Nyx can be found in the disk plane of galaxies and identified using our methods.

在前进轨道上与银河系合并的大规模矮星系可以在完全破坏之前被拖入磁盘平面。此类合并可能会增加星盘和暗物质盘的积聚。在这里，我们展示了Nyx，它是太阳附近的巨大恒星流，这首次表明此类事件发生在银河系中。我们使用通过对Gaia数据应用深度学习方法构建的增生恒星目录，识别出这条流中约有200颗径向和前进运动连贯的恒星。结合化学丰度和轨道信息，这些结果强烈支持以下解释，即Nyx是被破坏的矮星系的残余。FIRE水动力仿真进一步证明了这一点，