[论文解读] The origin of the Galaxy's system of globular clusters

D. Massari, Helmer H. Koppelman+1 more|arXiv (Cornell University)|Jun 19, 2019

Stellar, planetary, and galactic studies被引用 1

一句话总结

本研究利用盖亚星历表的运动学数据与经过校正的大样本年龄数据，将银河系球状星团与星系并合历史联系起来。研究发现，约40%的星团在原位形成，35%与已知的并合事件相关，尤其是盖亚-土卫二（19%）、人马座（5%）、赫尔米前身星系（6%）和红杉（5%）；其余星团则显示出多源起源的特征，而紧密的年龄-金属量关系支持了这些关联。

ABSTRACT

$Context$. The assembly history experienced by the Milky Way is currently being unveiled thanks to the data provided by the $Gaia$ mission. It is likely that the globular cluster system of our Galaxy has followed a similarly intricate formation path. $Aims$. To constrain this formation path, we explore the link between the globular clusters and the known merging events that the Milky Way has experienced. $Methods$. To this end, we combined the kinematic information provided by $Gaia$ for almost all Galactic clusters, with the largest sample of cluster ages available after carefully correcting for systematic errors. To identify clusters with a common origin we analysed their dynamical properties, particularly in the space of integrals of motion. $Results$. We find that about 40% of the clusters likely formed in situ. A similarly large fraction, 35%, appear to be possibly associated to known merger events, in particular to $Gaia$-Enceladus (19%), the Sagittarius dwarf galaxy (5%), the progenitor of the Helmi streams (6%), and to the Sequoia galaxy (5%), although some uncertainty remains due to the degree of overlap in their dynamical characteristics. Of the remaining clusters, 16% are tentatively associated to a group with high binding energy, while the rest are all on loosely bound orbits and likely have a more heterogeneous origin. The resulting age-metallicity relations are remarkably tight and differ in their detailed properties depending on the progenitor, providing further confidence on the associations made. $Conclusions$. We provide a table listing the likely associations. Improved kinematic data by future Gaia data releases and especially a larger, systematic error-free sample of cluster ages would help to further solidify our conclusions.

研究动机与目标

• 利用运动学与年龄数据，约束银河系球状星团系统的形成历史。
• 识别球状星团与银河系形成历史中已知并合事件之间的关联。
• 通过校正年龄测量中的系统性误差，减少星团起源归属的不确定性。
• 通过星团关联，增进对银河系晕动力学与化学演化过程的理解。

提出的方法

• 结合盖亚提供的几乎所有银河系球状星团的运动学数据，与目前最大的星团年龄样本。
• 对星团年龄进行系统性误差校正，以提高基于年龄的关联的可靠性。
• 利用运动积分分析动力学特性，以识别具有共同起源的星团群。
• 利用动力学相似性与轨道特征，将星团与特定并合事件关联。
• 评估每组星团的年龄-金属量关系，以验证其关联性。
• 基于统计与动力学一致性，构建星团-前身星系关联的表格。

实验结果

研究问题

• RQ1 银河系中的哪些球状星团可能与已知的并合事件（如盖亚-土卫二和人马座）相关？
• RQ2 银河系晕中，有多少球状星团是在原位形成，又有多少是外源形成？
• RQ3 年龄-金属量关系在多大程度上支持所提出的星团-前身星系关联？
• RQ4 有多少比例的星团轨道表明其起源于非单一前身星系，而是具有异质性起源？
• RQ5 动力学特性（如运动积分）如何帮助识别来自不同前身星系的星团家族？

主要发现

• 银河系约40%的球状星团在原位形成，表明存在显著的原位形成成分。
• 约35%的星团可能与已知的并合事件相关，其中盖亚-土卫二贡献19%，人马座贡献5%，赫尔米前身星系贡献6%，红杉贡献5%。
• 其余16%的星团暂且与高束缚能星团群相关，暗示其具有独特的动力学起源。
• 其余星团处于松散束缚轨道，表明其形成历史更具异质性，且可能源自多个来源。
• 各星团组内的年龄-金属量关系极为紧密，且其详细特征因前身星系而异，支持了关联关系的有效性。
• 未来盖亚数据的改进以及更大、无误差的星团年龄样本，有望进一步优化星团-前身星系的关联。