[论文解读] The Large Magellanic Cloud stellar content with SMASH: I. Assessing the stability of the Magellanic spiral arms

T. Ruiz-Lara, Carme Gallart+24 more|LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas)|Jun 18, 2020

Stellar, planetary, and galactic studies参考文献 62被引用 5

一句话总结

本研究利用SMASH巡天的深度测光数据与颜色-星等图（CMD）拟合，表明大麦哲伦云（LMC）的旋臂在过去超过20亿年中保持稳定。通过分析外盘区域的恒星族，作者发现旋臂中存在一致的年轻恒星族，表明该结构在差速旋转和潮汐力作用下依然持久存在，其起源可追溯至约20–30亿年前LMC与SMC的相互作用。

ABSTRACT

The Large Magellanic Cloud (LMC) is the closest and most studied example of an irregular galaxy. Among its principal defining morphological features, its off-centred bar and single spiral arm stand out, defining a whole family of galaxies known as the Magellanic spirals (Sm). These structures are thought to be triggered by tidal interactions and possibly maintained via gas accretion. However, it is still unknown whether they are long-lived stable structures. In this work, by combining photometry that reaches down to the oldest main sequence turn-off in the colour-magnitude diagrams (CMD, up to a distance of $\sim$4.4 kpc from the LMC centre) from the SMASH survey and CMD fitting techniques, we find compelling evidence supporting the long-term stability of the LMC spiral arm, dating the origin of this structure to more than 2~Gyr ago. The evidence suggests that the close encounter between the LMC and the Small Magellanic Cloud (SMC) that produced the gaseous Magellanic Stream and its Leading Arm (LA) also triggered the formation of the LMC's spiral arm. Given the mass difference between the Clouds and the notable consequences of this interaction, we can speculate that this should have been one of their closest encounters. These results set important constraints on the timing of LMC-SMC collisions, as well as on the physics behind star formation induced by tidal encounters.

研究动机与目标

• 确定大麦哲伦云（LMC）单旋臂的长期稳定性，该特征是麦哲伦旋涡星系的标志性特征。
• 通过深度测光技术对恒星族进行测龄，探究旋臂是瞬态结构还是长期存在的结构。
• 评估LMC与大麦哲伦云（SMC）之间的潮汐相互作用在触发和维持旋臂中的作用。
• 利用旋臂的恒星形成历史（SFH）约束过去LMC-SMC相遇的时间。
• 为动力学模型提供观测约束，特别是关于麦哲伦系统不对称形态的起源。

提出的方法

• 获取了覆盖LMC外盘区域的Survey of the MAgellanic Stellar History（SMASH）巡天的深度光学测光数据，达到最老主序拐点星等。
• 为LMC多个区域（包括旋臂及周边区域）构建了颜色-星等图（CMDs）。
• 应用CMD拟合技术推导各区域的恒星形成历史（SFH），从而对恒星族的年龄和金属丰度进行约束。
• 将旋臂区域的SFH与周边区域进行比较，识别恒星形成在空间和时间上的相干性。
• 将观测到的SFH与LMC和SMC的轨道历史相关联，特别关注过去近距离相遇的时间点。
• 利用旋臂中年轻恒星族的持续存在，推断其结构在宇宙时空中保持稳定。

Figure 1: Stellar spatial distribution and observed colour-magnitude diagrams corresponding to two visually selected regions representative of the northwest (blue) and southeast (red) regions of the LMC. (Left) LMC-reconstructed stellar density image from SMASH data showing the number density of sta

实验结果

研究问题

• RQ1 LMC的旋臂是长期存在的结构特征，还是瞬态现象？
• RQ2 LMC旋臂中恒星族的年龄是多少？这是否表明其起源于长期过程？
• RQ3 旋臂的形成是否可与LMC与SMC之间的潮汐相互作用相关联？
• RQ4 旋臂的恒星形成历史与LMC盘面其余部分相比有何异同？
• RQ5 观测到的恒星族对过去LMC-SMC相遇的时间和性质设定了哪些约束？

主要发现

• LMC旋臂中存在一个空间上一致的、年龄小于约20亿年的恒星族，表明该结构至少已持续存在如此长时间。
• 旋臂的恒星形成历史在过去约20亿年中表现出显著增强，外盘区域未见明显扰动或再激活的证据。
• 旋臂中观测到的恒星族与形成时间超过20亿年前的起源一致，支持其长期结构稳定性。
• 旋臂形成的时机与约20–30亿年前提出的LMC-SMC近距离相遇时间吻合，表明潮汐相互作用是其触发机制。
• LMC中H I气体分布的不对称性可能由同一长寿命旋臂结构维持，气体相对于棒结构发生偏移，并与恒星旋臂对齐。
• 结果表明，LMC与SMC之间的潮汐相互作用是LMC不对称形态（包括其旋臂、麦哲伦流和领先流）的主要驱动力。

Figure 2: Average star formation histories characteristic of the northwest (blue, arm) and southeast (red) regions of the LMC disc. A magnified inset highlights the youngest burst. Shaded regions represent uncertainties in the SFH recovery computed as described in Hidalgo et al. ( 2011b ) .