[论文解读] Wrinkles in the Gaia data unveil a dynamically young and perturbed Milky Way disk

T. Antoja, A. Helmi+11 more|arXiv (Cornell University)|Jan 1, 2017

Stellar, planetary, and galactic studies参考文献 50被引用 18

一句话总结

利用盖亚DR2数据，本研究揭示了银河系盘星中广泛存在的相空间子结构——如拱形、壳层、螺旋壳状结构和脊状结构——表明银河系盘目前处于动力学年轻状态且尚未达到平衡。研究结果暗示了近期的扰动（约300至900百万年前），极有可能源自人马座矮星系，挑战了长期以来认为银河系盘处于动力学平衡与轴对称状态的假设。

ABSTRACT

Most of the stars in our Galaxy including our Sun, move in a disk-like component and give the Milky Way its characteristic appearance on the night sky. As in all fields in science, motions can be used to reveal the underlying forces, and in the case of disk stars they provide important diagnostics on the structure and history of the Galaxy. But because of the challenges involved in measuring stellar motions, samples have so far remained limited in their number of stars, precision and spatial extent. This has changed dramatically with the second Data Release of the Gaia mission which has just become available. Here we report that the phase space distribution of stars in the disk of the Milky Way is full of substructure with a variety of morphologies, most of which have never been observed before. This includes shapes such as arches and shells in velocity space, and snail shells and ridges when spatial and velocity coordinates are combined. The nature of these substructures implies that the disk is phase mixing from an out of equilibrium state, and that the Galactic bar and/or spiral structure are strongly affecting the orbits of disk stars. Our analysis of the features leads us to infer that the disk was perturbed between 300 and 900 Myr ago, which matches current estimations of the previous pericentric passage of the Sagittarius dwarf galaxy. The Gaia data challenge the most basic premise of stellar dynamics of dynamical equilibrium, and show that modelling the Galactic disk as a time-independent axisymmetric component is definitively incorrect. These findings mark the start of a new era when, by modelling the richness of phase space substructures, we can determine the gravitational potential of the Galaxy, its time evolution and the characteristics of the perturbers that have most influenced our home in the Universe.

研究动机与目标

• 利用高精度盖亚数据研究盘星的相空间分布，以揭示隐藏的动力学子结构。
• 确定银河系盘是否处于动力学平衡状态，或是否经历过近期扰动。
• 评估银河系棒状结构与旋臂结构对恒星轨道演化的影响。
• 将观测到的子结构与过去的引力相互作用（特别是与人马座矮星系的相互作用）联系起来。
• 挑战银河系动力学中时间独立、轴对称势模型的假设。

提出的方法

• 分析盖亚数据释放2中恒星的完整相空间分布（空间坐标与速度坐标）。
• 识别速度空间中的非均匀形态特征，如拱形、壳层、脊状结构及类似螺旋壳的图案。
• 将相空间子结构用作持续动力学过程与非平衡状态的诊断工具。
• 将观测到的子结构形态与银河系棒和旋臂引起的长期扰动的理论预期进行比较。
• 通过将子结构特征与已知事件（如人马座矮星系的近星点通过）匹配，推断扰动发生的时间。
• 拒绝在建模银河系盘时假设其处于动力学平衡与轴对称状态。

实验结果

研究问题

• RQ1 基于盖亚DR2数据，银河系盘中存在哪些类型的相空间子结构？
• RQ2 这些子结构的形态揭示了银河系盘当前的动力学状态的哪些信息？
• RQ3 银河系盘的最后一次主要扰动是什么时候，可能由什么原因引起？
• RQ4 银河系棒状结构与旋臂结构如何影响盘星的轨道演化？
• RQ5 子结构的存在在多大程度上使星系动力学中时间独立、轴对称势的常规假设失效？

主要发现

• 银河系盘呈现出大量此前未被观测到的相空间子结构，包括拱形、壳层、螺旋壳状结构和脊状结构。
• 这些子结构表明盘未处于动力学平衡状态，目前正从非平衡态经历相空间混合。
• 子结构的形态表明银河系棒和/或旋臂结构造成了强烈扰动。
• 盘很可能在3至9亿年前间受到扰动，与人马座矮星系此前近星点通过的时间一致。
• 研究结果直接挑战了星动理论中的基础假设，即银河系盘处于时间独立、轴对称平衡状态。
• 观测到的子结构为重建银河系引力势及其随时间演化的扰动源开辟了新途径。