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arXiv 2211.04484astro-ph.GA
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自旋晕中的恒星棒:延迟屈曲与无减速现象

Stellar Bars in Spinning Halos: Delayed Buckling and Absence of Slowdown

Xingchen Li, Isaac Shlosman, Clayton Heller, Daniel Pfenniger

AI总结:

本研究通过高分辨率数值模拟分析自旋暗物质晕中恒星棒的演化,发现改变DM密度对恒星棒的振幅、图案速度和屈曲时间等有深远影响,特别是当$λ\gtrsim0.04$时屈曲不稳定性被延迟,恒星棒在平台期保持强健且图案速度恒定,从而解决了观测与N体模拟间的长期差异。

摘要:

我们使用高分辨率数值模拟来分析自旋暗物质(DM)晕中的恒星棒演化。先前的工作表明,晕自旋对恒星棒演化有显著影响,并可能导致恒星棒在经历垂直屈曲不稳定性后消解。在此,我们引入DM自旋序列$λ=0-0.09$,并通过改变DM晕的紧密度来研究沿此$λ$序列DM密度的影响。我们发现:(1) 改变DM密度对沿$λ$序列的恒星棒演化有深远影响,即影响其振幅、图案速度、屈曲时间等;(2) 对于$λ\gtrsim 0.04$,屈曲不稳定性被逐渐延迟,且当恒星棒达到最大强度时并未发生屈曲;(3) 相反,恒星棒保持接近最大强度,其振幅平台期延续约$\sim 1-7$ Gyr,最终以屈曲不稳定性结束;(4) 尽管恒星棒在平台期保持强健,其图案速度却几乎保持恒定。这种恒星棒异常行为的原因在于,DM棒与恒星棒对齐导致恒星棒受到的引力扭矩大幅减小。轨道分析表明,延迟屈曲源于恒星沿棒长轴和垂直轴的缓慢振荡演化——从而推迟了垂直2:1共振的作用,该共振会将旋转能量泵入垂直运动中;(5) 花生/盒状凸起在平台期开始时形成并随时间增长;(6) 自旋晕中的强恒星棒可避免快速制动,从而解决了观测与$N$体模拟之间长期存在的差异。恒星棒沿$λ$-和DM密度序列的这种行为揭示了丰富的恒星棒特性,需要进一步研究。

英文摘要:

We use high resolution numerical simulations in order to analyze the stellar bar evolution in spinning dark matter (DM) halos. Previous works have shown that the halo spin has a substantial effect on the bar evolution and can lead to bar dissolution following the vertical buckling instability. Here, we invoke the DM spin sequence, $λ=0-0.09$, and study the effect of DM density along this $λ$-sequence by varying the compactness of DM halo. We find that (1) varying the DM density has a profound effect on the stellar bar evolution along the $λ$-sequence, namely, on its amplitude, pattern speed, buckling time, etc.; (2) For $λ\gtrsim 0.04$, the buckling instability has been delayed progressively, and does not occur when the bar has reached its maximal strength; (3) Instead, stellar bars remain near maximal strength, and their amplitude plateau stage extends over $\sim 1-7$ Gyr, terminating with the buckling instability; (4) Although stellar bars remain strong during the plateau, their pattern speed stays nearly constant. The reason for this unusual behavior of stellar bars follows from the highly reduced gravitational torques which they experience due to the DM bar being aligned with the stellar bar. The performed orbital analysis shows that the delayed buckling results from a slow evolution of stellar oscillations along the bar major and vertical axes -- thus postponing the action of the vertical 2:1 resonance which pumps the rotational energy into vertical motions; (5) Peanut/boxy shaped bulges form at the beginning of the plateau and grow with time; (6) Strong stellar bars in spinning halos can avoid fast braking, resolving the long standing discrepancy between observations and $N$-body simulations. This behavior of stellar bars along the $λ$- and DM density-sequences, reveals a wealth of stellar bar properties which require additional study.

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