[论文解读] The formation of two of the major structural components of the Milky Way

A. Helmi, C. Babusiaux+4 more|arXiv (Cornell University)|Jun 15, 2018

Stellar, planetary, and galactic studies被引用 1

一句话总结

本文识别出盖亚-恩克勒杜斯（Gaia-Enceladus）——一个现今已被吸积的矮星系，质量略大于大犬座矮星系（Small Magellanic Cloud）——作为约100亿年前一次重大并合事件的主因，驱动了银河系厚盘与内星系晕的形成。通过分析太阳附近恒星的运动学、化学成分与年龄，本文表明盖亚-恩克勒杜斯的残余物质遍布几乎整个天空，具有明显的逆行轨道、RR Lyr型变星以及遵循一致年龄-金属量关系的球状星团。

ABSTRACT

The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. Chemo-dynamical studies of the nearby halo have long hinted at the presence of multiple components such as streams, clumps, duality and correlations between the stars' chemical abundances and orbital parameters. More recently, the analysis of two large stellar surveys have revealed the presence of a well-populated chemical elemental abundance sequence, of two distinct sequences in the colour-magnitude diagram, and of a prominent slightly retrograde kinematic structure all in the nearby halo, which may trace an important accretion event experienced by the Galaxy. Here report an analysis of the kinematics, chemistry, age and spatial distribution of stars in a relatively large volume around the Sun that are mainly linked to two major Galactic components, the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object which at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia-Enceladus. The stars originating in Gaia-Enceladus cover nearly the full sky, their motions reveal the presence of streams and slightly retrograde and elongated trajectories. Hundreds of RR Lyrae stars and thirteen globular clusters following a consistent age-metallicity relation can be associated to Gaia-Enceladus on the basis of their orbits. With an estimated 4:1 mass-ratio, the merger with Gaia-Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk and therefore contributed to the formation of this component approximately 10 Gyr ago. These findings are in line with simulations of galaxy formation, which predict that the inner stellar halo should be dominated by debris from just a few massive progenitors.

研究动机与目标

• 通过分析恒星运动学与化学成分，确定银河系厚盘与内星系晕的起源。
• 识别出主导银河系前体厚盘动力加热过程的吸积事件。
• 将观测到的恒星族（如RR Lyr型变星与球状星团）与特定的、大质量的前身星系联系起来。
• 通过将观测到的恒星结构与大质量并合事件的预测进行比较，检验星系形成模拟。

提出的方法

• 利用大规模恒星巡天的运动学数据，绘制轨道参数并探测星系晕中一致的结构。
• 通过化学丰度分析，识别出具有不同金属量与元素比值的独立恒星族。
• 利用颜色-星等图序列与恒星演化模型，测定恒星年龄。
• 对恒星轨道进行积分，追溯其起源于同一前身系统。
• 将RR Lyr型变星与球状星团中观测到的年龄-金属量关系，与并合事件预期结果进行比较。
• 利用盖亚-恩克勒杜斯与银河系前体星系之间的质量比估计（4:1），推断动力加热效应。

实验结果

研究问题

• RQ1 银河系厚盘与内星系晕的起源是什么？
• RQ2 哪一颗被吸积的前身星系主导了内晕恒星族的绝大部分？
• RQ3 RR Lyr型变星与球状星团的运动学、化学成分与年龄如何支持其源于单一重大并合事件？
• RQ4 观测到的星系晕结构在多大程度上符合星系形成模拟的预测？

主要发现

• 内晕主要由盖亚-恩克勒杜斯的残余物质主导，其前身星系质量略大于大犬座矮星系。
• 盖亚-恩克勒杜斯恒星表现出一致的、略微逆行的、拉长的轨道，表明曾发生重大并合事件。
• 基于一致的轨道与化学特性，数百颗RR Lyr型变星与十三个球状星团与盖亚-恩克勒杜斯相关。
• 与盖亚-恩克勒杜斯的并合（质量比为4:1）导致前体盘的动力加热，约100亿年前形成了厚盘。
• 与盖亚-恩克勒杜斯相关的恒星所观测到的年龄-金属量关系，在RR Lyr型变星与球状星团中保持一致。
• 盖亚-恩克勒杜斯残余物质的空间分布覆盖了几乎整个天空，证实其在内晕中的主导地位。