[论文解读] Bars & boxy/peanut bulges in thin & thick discs: I. Morphology and $los$ velocities of a fiducial model

Francesca Fragkoudi, P. Di Matteo+5 more|arXiv (Cornell University)|Apr 3, 2017

Stellar, planetary, and galactic studies参考文献 2被引用 6

一句话总结

本研究利用N体模拟，探究了旋涡星系中薄盘与厚盘组分在棒状星系的棒-盒状/花生（b/p）星暴中的形态与视线（los）速度差异。研究发现，薄盘恒星形成更强、更拉长的棒结构，并伴随显著的X形b/p星暴，而厚盘恒星则形成较弱、更方正的b/p结构；关键的是，由于轨道动力学与角动量损失的差异，厚盘b/p区域的视线速度可比薄盘高出最多40%。

ABSTRACT

We explore trends in the morphology and line-of-sight (los) velocity of stellar populations in the inner regions of disc galaxies, using N-body simulations with both a thin (kinematically cold) and a thick (kinematically hot) disc which form a bar and boxy/peanut (b/p) bulge. The bar in the thin disc component is $\sim$50\% stronger than the thick disc bar and is more elongated, with an axis ratio almost half that of the thick disc bar. The thin disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker with a rather boxy shape. This leads to the signature of the b/p bulge in the thick disc to be weaker and further away from the plane than in the thin disc. Regarding the kinematics, we find that the los velocity of thick disc stars in the outer parts of the b/p bulge can be \emph{larger} than that of thin disc stars, by up to 40\% and 20\% for side-on and Milky Way-like orientations of the bar respectively. This is due to the different orbits followed by thin and thick disc stars in the bar-b/p region, which are affected by the fact that: i) thin disc stars are trapped more efficiently in the bar - b/p instability and thus lose more angular momentum than their thick disc counterparts and ii) thick disc stars have large radial excursions and therefore stars from large radii with high angular momenta can be found in the bar region. We also find that the difference between the los velocities of the thin and thick disc in the b/p bulge ($\Delta v_{los}$) correlates with the initial difference between the radial velocity dispersions of the two discs ($\Delta \sigma$) . We therefore conclude that stars in the bar - b/p bulge will have considerably different morphologies and kinematics depending on the kinematic properties of the disc population they originate from.

研究动机与目标

• 理解星族盘的运动学特性如何影响旋涡星系中棒-盒状/花生（b/p）星暴的形态与运动学特征。
• 研究薄盘（运动学冷态）与厚盘（运动学热态）组分在棒强度、形状及b/p结构上的差异。
• 分析b/p星暴区域中薄盘与厚盘恒星的视线（los）速度差异。
• 探讨轨道动力学与角动量演化在形成观测到的运动学差异中的作用。

提出的方法

• 采用N体模拟构建一个同时包含薄盘（运动学冷态）与厚盘（运动学热态）的星系，以实现棒与b/p星暴的形成。
• 追踪内区恒星组分的演化，重点关注形态与视线（los）速度分布。
• 测量薄盘与厚盘组分的棒强度与轴比，以比较其结构特性。
• 分别对每种盘组分量化b/p星暴的X形与方正形态。
• 计算b/p区域恒星的视线速度，考虑不同视角（侧视与银河系类比视角）。
• 分析初始径向速度弥散度差异（Δσ）与视线速度差异（Δvlos）之间的相关性，以评估运动学依赖性。

实验结果

研究问题

• RQ1 在模拟的棒状星系中，薄盘与厚盘组分的棒-盒状/花生（b/p）星暴形态有何不同？
• RQ2 是什么原因导致厚盘恒星在b/p星暴区域的视线（los）速度超过薄盘恒星？超出幅度是多少？
• RQ3 薄盘与厚盘恒星的轨道动力学——特别是角动量演化——如何促成b/p区域观测到的运动学差异？
• RQ4 两组盘初始速度弥散度差异（Δσ）在多大程度上与b/p星暴区域观测到的视线速度差异（Δvlos）相关？

主要发现

• 薄盘棒的强度约为厚盘棒的50%更强，且更拉长，其轴比接近厚盘棒的一半。
• 薄盘b/p星暴表现出显著的X形结构，而厚盘b/p星暴形态较弱且更方正。
• 厚盘b/p特征较弱，且位于远离星系平面的位置，相较于薄盘b/p星暴更靠外。
• 在b/p星暴外区，侧视方向下厚盘恒星的视线速度可比薄盘恒星高出最多40%，而在银河系类比视角下高出约20%。
• 两组盘之间的视线速度差异（Δvlos）与初始径向速度弥散度差异（Δσ）存在相关性。
• 棒-b/p区域的恒星表现出不同的轨道行为：薄盘恒星损失更多角动量，更易被有效捕获于棒结构中，而厚盘恒星具有更大的径向振荡，且来自大半径区域的高角动量恒星更可能抵达棒区域。