标题： 关于N体模拟中形状的演化 显示英文标题

标题： On the evolution of shape in N-body simulations

摘要： 使用GRAPE和HARP专用计算机生成了一个关于由冷且无耗散塌缩形成的直接N体模型形状演化的数据库。 此类模型对于理解椭圆星系的形成非常重要。 发现了形状演化过程中的三个动态上不同的阶段：首先是快速的动力学塌缩，这导致了径向轨道不稳定性（ROI），并在结束时产生了系统的最大三轴性。 随后，发生了两次剧烈的和两体形状弛豫阶段，这些阶段首先将系统驱动向轴对称，最终达到球对称（然而，最终状态仍然比初始模型集中得多）。 在一系列模型中，检查了数值和物理参数的影响，如粒子数量、软化长度、初始柯尔莫哥洛夫-里雅普诺夫比率、时间步长选择、不同的N体代码等。 我们发现，软化长度和粒子数量的不当组合可能会产生错误的结果。 一些选定的模型被演化到其 secular 时间尺度，直到它们再次变得球对称。 结果显示secular 形状弛豫时间尺度与两体弛豫时间尺度非常一致，如果适当地考虑后者的软化长度。 最后，我们认为，在塌缩后的中间阶段的剧烈形状弛豫是由中心强烈的核振荡引起的，这些振荡导致了电位波动，从而抑制了三轴性。 显示英文摘要

摘要： A database on shape evolution of direct N-body models formed out of cold, dissipationless collapse is generated using GRAPE and HARP special purpose computers. Such models are important to understand the formation of elliptical galaxies. Three dynamically distinct phases of shape evolution were found, first a fast dynamical collapse which gives rise to the radial orbit instability (ROI) and generates at its end the maximal triaxiality of the system. Subsequently, two phases of violent and two-body shape relaxation occur, which drive the system first towards axisymmetry, finally to spherical symmetry (the final state, however, is still much more concentrated than the initial model). In a sequence of models the influence of numerical and physical parameters, like particle number, softening, initial virial ratio, timestep choice, different N-body codes, are examined. We find that an improper combination of softening and particle number can produce erroneous results. Selected models were evolved on the secular timescale until they became spherically symmetric again. The secular shape relaxation time scale is shown to agree very well with the two-body relaxation time, if softening is properly taken into account for the latter. Finally, we argue, that the intermediate phase of violent shape relaxation after collapse is induced by strong core oscillations in the centre, which cause potential fluctuations, dampening out the triaxiality.