银河系形成与演化的新检验--晕星轨道偏心率分布的预测
A new test for the Galactic formation and evolution -- prediction for the orbital eccentricity distribution of the halo stars
摘要：
我们提出的理论计算的微分分布的恒星轨道偏心率在星系晕，假设恒星构成一个球形的，碰撞系统的动态平衡与暗物质晕。对于给定的能量E和角动量L，为了定义晕星的偏心率e，我们采用了两种引力势，即等时势和Navarro-Frenk-White势，它们可以形成两端，覆盖任何实际的暗物质晕势。基于一个f（E，L）形式的分布函数，该分布函数允许速度弥散中的常数各向异性，并以一个参数β为特征，我们发现，对于高度径向各向异性的速度弥散（β> 0.6），偏心率分布是e的单调递增函数，而对于从径向到切向速度各向异性的情况（β< 0.6），偏心率分布呈现出一个驼峰状。我们还发现，当速度各向异性与银河系晕星（β= 0.5-0.7）的观测结果一致时，当e < 0.7时，N（e）αe的偏心率分布近似为线性，与所采用的势无关。我们的理论偏心率分布将是一个重要的工具，检查多远的晕中的动力学平衡已经实现，通过比较晕星的运动学采样在更大的距离。鉴于SEGUE和盖亚项目的大型调查正在进行中，我们讨论了我们的结果将如何作为探索银河系晕形成和演化的新指南。
英文摘要：
We present theoretical calculations for the differential distribution of stellar orbital eccentricity in a galaxy halo, assuming that the stars constitute a spherical, collisionless system in dynamical equilibrium with a dark matter halo. In order to define the eccentricity e of a halo star for given energy E and angular momentum L, we adopt two types of gravitational potential, such as an isochrone potential and a Navarro-Frenk-White potential, that could form two ends covering in-between any realistic potential of dark matter halo. Based on a distribution function of the form f(E,L) that allows constant anisotropy in velocity dispersions characterized by a parameter β, we find that the eccentricity distribution is a monotonically increasing function of e for the case of highly radially anisotropic velocity dispersions (β> 0.6), while showing a hump-like shape for the cases from radial through tangential velocity anisotropy (β< 0.6). We also find that when the velocity anisotropy agrees with that observed for the Milky Way halo stars (β= 0.5-0.7), a nearly linear eccentricity distribution of N(e) αe results at e < 0.7, largely independent of the potential adopted. Our theoretical eccentricity distribution would be a vital tool of examining how far out in the halo the dynamical equilibrium has been achieved, through comparison with kinematics of halo stars sampled at greater distances. Given that large surveys of the SEGUE and Gaia projects would be in progress, we discuss how our results would serve as a new guide in exploring the formation and evolution of the Milky Way halo.
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