边缘星系中烹调X结构的轨道成分
Orbital ingredients for cooking X-structures in edge-on galaxies
摘要：
X结构经常在拥有所谓的B/PS（方形/花生状）凸起的星系中观察到，并且从边缘视图中可见。它们是B/PS凸起最显著的特征，表现为从宿主星系盘突出的四条射线，与B/PS凸起的背景相区别。在一些作品中，它们的起源被认为与所谓的香蕉形轨道有关，垂直共振为2：1。在这种轨道上的恒星在棒坐标系中每转一圈，在垂直方向上会做两次振荡。最近的几项研究分析了不同$N$体模型中产生的轨道集合，但没有证实X结构中共振2：1轨道的主导地位。在我们的工作中，我们分析了两个$N$体模型，并显示了如何在我们的模型中的X结构是逐渐组装从中心到周边的轨道小于2：1的频率比。大多数这样的轨道是封闭在一个'farfale'形状（意大利面食）的形式，原来是非周期性的。我们的结论是，X-结构只是由不同类型的轨道在其最高点的交叉或折叠所造成的高密度区域的包络，并没有类似于面内棒的“骨干”。比较两个不同的数值模型的轨道结构，我们表明，一个或另一个家庭的轨道与一定比例的垂直振荡频率的面内频率的主导地位取决于底层星系的参数，并最终决定的X-结构的形态和它的射线的张开角。
英文摘要：
X-structures are often observed in galaxies hosting the so-called B/PS (boxy/peanuts) bulges and are visible from the edge-on view. They are the most notable features of B/PS bulges and appear as four rays protruding from the disk of the host galaxy and distinguishable against the B/PS bulge background. In some works their origin is thought to be connected with the so-called banana-shaped orbits with a vertical resonance 2:1. A star in such an orbit performs two oscillations in the vertical direction per one revolution in the bar frame. Several recent studies that analyzed ensembles of orbits arising in different $N$-body models do not confirm the dominance of the resonant 2:1 orbits in X-structures. In our work we analyze two $N$-body models and show how the X-structure in our models is gradually assembled from the center to the periphery from orbits with less than 2:1 frequency ratio. The most number of such orbits is enclosed in a 'farfalle'-shape (Italian pasta) form and turns out to be non-periodic. We conclude that the X-structure is only the envelope of regions of high density caused by the crossing or folding of different types of orbits at their highest points, and does not have a "backbone" similar to that of the in-plane bar. Comparing the orbital structure of two different numerical models, we show that the dominance of one or another family of orbits with a certain ratio of the vertical oscillations frequency to the in-plane frequency depends on the parameters of the underlying galaxy and ultimately determines the morphology of the X-structure and the opening angle of its rays.
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