[论文解读] Galactic Archaeology with RAVE: Clues to the Formation of the Thick Disk

Indranil Banik|arXiv (Cornell University)|Jun 17, 2014

Stellar, planetary, and galactic studies被引用 8

一句话总结

该论文提出，银河系的厚盘在70亿至110亿年前通过仙女座星系的近距离飞掠事件迅速形成，其依据是较高的[Mg/Fe]比值（约0.2–0.3）和较高的速度弥散度。该情景比内部加热或直接吸积更能解释厚盘的快速旋转和金属丰度，且基于MOND的轨道动力学与时间估计一致。

ABSTRACT

I present an analysis of ~13000 stars from RAVE Data Release 4 (DR4) to better understand the formation of the thick disk. The stars I consider are mostly within 1 kpc of the Sun. Based on a Monte-Carlo analysis of the data and comparison with a toy model, I suggest the thick disk formed fairly rapidly when [Mg/Fe] ${\approx}$0.2-0.3. We consider an origin via direct accretion unlikely as the thick disk is fast-rotating and fairly metal-rich for its age. As internal disk heating can not easily reach the high observed velocity dispersions and would likely be more gradual, it also appears implausible. A very early formation of the thick disk (e.g. from star formation in the collapsing primordial gas cloud of the Milky Way) appears unlikely given it would require [Mg/Fe] ${\geq}$ 0.4 at the time the thick disk formed. An origin via tidal interaction with another galaxy is one of the few remaining possibilities. My proposed explanation for the nature of the event which formed the thick disk is a close flyby of Andromeda, whose orbital dynamics in MOND are consistent with my estimate that the event occurred 7-11 Gyr ago. This leads to reasonable thick disk velocity dispersions. It will be important to determine whether the event heated the outer regions of the thin disk uniformly or if substantial portions of it were left largely unaffected. This may distinguish between models (like mine) involving a single close encounter and models with multiple encounters (as common in mergers). The main limiting factor at present seems to be the accuracy of elemental abundances and lack of direct observations at different galactocentric radii to the Sun.

研究动机与目标

• 利用高精度恒星数据理解银河系厚盘的形成机制。
• 解决厚盘的高弥散速度、金属丰度与[Mg/Fe]丰度之间的矛盾。
• 将直接吸积、内部加热、早期坍缩和潮汐相互作用等不同形成情景与观测约束进行对比检验。
• 评估单次仙女座星系近距离飞掠是否能重现观测到的厚盘运动学与化学特性。
• 识别关键的观测检验方法，以区分单次相遇模型与多次相遇并合情景。

提出的方法

• 分析RAVE数据发布4.0中约13,000颗恒星的数据，主要位于太阳周围1 kpc范围内。
• 采用蒙特卡洛统计建模方法评估运动学与丰度测量中的不确定性。
• 将观测到的[Mg/Fe]比值与速度弥散度与一个简化的盘形成玩具模型进行比较。
• 利用基于MOND的轨道动力学估算过去仙女座星系飞掠事件的时间与能量。
• 评估单次遭遇引起的潮汐加热是否能产生观测到的厚盘特性。
• 评估薄盘中加热的空间均匀性，以区分单次与多次遭遇模型。

实验结果

研究问题

• RQ1 单次仙女座星系近距离飞掠能否解释厚盘的高弥散速度与[Mg/Fe]丰度？
• RQ2 为何厚盘在年龄较老的情况下仍表现出快速旋转与相对富金属的特性，这与标准形成模型相矛盾？
• RQ3 内部盘加热或原始气体云早期坍缩是否与观测到的[Mg/Fe]丰度和运动学一致？
• RQ4 厚盘形成时间（70亿至110亿年前）是否与基于MOND的仙女座星系飞掠轨道预测相符？
• RQ5 单次飞掠是否会使外侧薄盘均匀加热，还是可能留下部分区域未受影响——这能否作为与并合模型相区别的可检验特征？

主要发现

• 厚盘很可能在[Mg/Fe]约为0.2–0.3时迅速形成，这与早期坍缩要求[Mg/Fe]≥0.4的假设不一致。
• 直接吸积的可能性较低，因为厚盘表现出快速旋转，且其年龄对应的金属丰度相对较高。
• 内部盘加热机制不可行，因为它会导致速度弥散度渐进式增加，而非观测到的高值。
• 约70亿至110亿年前仙女座星系的近距离飞掠事件，若与MOND动力学一致，可重现观测到的厚盘速度弥散度。
• 该事件可能均匀加热了外侧薄盘，为区分单次相遇模型与多并合情景提供了潜在的可检验观测特征。
• 主要局限性仍在于当前元素丰度测量的精度不足，且缺乏在不同银心半径处的直接观测。