标题： 层次聚类中的剧烈松弛 显示英文标题

标题： Violent Relaxation in Hierarchical Clustering

摘要： “暴力弛豫”这一术语由唐纳德·林登-贝尔创造，用作一个令人印象深刻的矛盾修辞，描述了一个恒星动力学系统如何从混沌的初始状态松弛到准平衡状态。他的分析表明，即使对于包含许多恒星的系统，这个过程也是迅速的，并且它会导致可能与有界等温球相关的平衡态。我回顾了过去三十年数值模拟如何改善了我们对暴力弛豫的理解。很明显，这一过程导致的平衡态强烈依赖于初始状态，但它们仍然具有一些共同特征。一个特别有趣的情况涉及在膨胀宇宙中通过无耗散的层次聚类形成的物体，这些物体可能对应于星系团或星系的暗晕。尽管这些物体表现出各种形状和自旋，但这些属性的分布仅弱依赖于宇宙环境和初始密度涨落谱。晕的密度剖面似乎具有普遍的形式，其中心结构奇异，且特征密度仅取决于形成时期。低质量晕通常形成时间较早，因此其特征密度高于高质量晕。 显示英文摘要

摘要： The term ``violent relaxation'' was coined by Donald Lynden-Bell as a memorable oxymoron describing how a stellar dynamical system relaxes from a chaotic initial state to a quasi-equilibrium. His analysis showed that this process is rapid, even for systems with many stars, and that it leads to equilibria which may plausibly be related to bounded isothermal spheres. I review how numerical simulations have improved our understanding of violent relaxation over the last thirty years. It is clear that the process leads to equilibria which depend strongly on the initial state, but which nevertheless have certain common features. A particularly interesting case concerns objects formed in an expanding universe through dissipationless hierarchical clustering from gaussian initial conditions; these may correspond to galaxy clusters or to the dark halos of galaxies. While such objects display a wide range of shapes and spins, the distributions of these properties depend only weakly on the cosmological context and on the initial spectrum of density fluctuations. Halo density profiles appear to have a universal form with a singular central structure and a characteristic density which depends only on formation epoch. Low mass halos typically have earlier formation times and thus higher characteristic densities than high mass halos.