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arXiv 1306.3993astro-ph.CO
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用气体称量星系团。I.流体静力质量偏差的计算方法

Weighing Galaxy Clusters with Gas. I. On the Methods of Computing Hydrostatic Mass Bias

Erwin T. Lau, Daisuke Nagai, Kaylea Nelson

摘要:

根据X射线和Sunyeav-Zel'dovich观测对星系团的质量估计,假设星系团内的气体与它们的引力势处于流体静力平衡。然而,由于星系团是动力学活跃的对象,其动力学状态可以偏离平衡配置显着,从流体静力学平衡假设的偏离是在集群宇宙学的系统不确定性的最大来源之一。在文献中,已经有两种方法来计算流体静力学质量偏差的基础上的欧拉方程和修改的Jeans方程,分别,并一直有一些混乱的有效性,这两种方法。“牛仔裤”这个词用词不当,它错误地暗示气体是无碰撞的。为了避免进一步的混淆,我们将这些方法分别称为“求和”和“平均”方法。在这项工作中,我们表明,这两种方法计算的流体静力学质量偏差是等效的,通过证明,在第二种方法中使用的方程可以从欧拉方程的空间平均值。具体来说,我们确定在这两种方法中的个别条款的数学对应关系,并表明这些对应关系是有效的,在百分之几的水平使用流体动力学模拟的星系团形成。此外,我们计算的质量偏差与加速的气体,并表明其贡献是小的维里化区域的内部的星系团,但成为不可忽略的大规模集群的物质被积极吸积的集群郊区。我们讨论的影响,了解流体静力学质量偏差的起源,并利用流体动力学模拟和观测来表征它们。

英文摘要:

Mass estimates of galaxy clusters from X-ray and Sunyeav-Zel'dovich observations assume the intracluster gas is in hydrostatic equilibrium with their gravitational potential. However, since galaxy clusters are dynamically active objects whose dynamical states can deviate significantly from the equilibrium configuration, the departure from the hydrostatic equilibrium assumption is one of the largest sources of systematic uncertainties in cluster cosmology. In the literature there has been two methods for computing the hydrostatic mass bias based on the Euler and the modified Jeans equations, respectively, and there has been some confusion about the validity of these two methods. The word "Jeans" was a misnomer, which incorrectly implies that the gas is collisionless. To avoid further confusion, we instead refer these methods as "summation" and "averaging" methods respectively. In this work, we show that these two methods for computing the hydrostatic mass bias are equivalent by demonstrating that the equation used in the second method can be derived from taking spatial averages of the Euler equation. Specifically, we identify the correspondences of individual terms in these two methods mathematically and show that these correspondences are valid to within a few percent level using hydrodynamical simulations of galaxy cluster formation. In addition, we compute the mass bias associated with the acceleration of gas and show that its contribution is small in the virialized regions in the interior of galaxy cluster, but becomes non-negligible in the cluster outskirts of massive clusters where materials are being actively accreted. We discuss implications for understanding the origin of hydrostatic mass biases and for characterizing them using hydrodynamical simulations as well as observations.

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