星系团中的三轴性：质量与势的重建
Triaxiality in galaxy clusters: Mass versus Potential reconstructions
摘要：
解释星系团的三轴形状将在即将到来的宇宙学调查中变得重要。我们表明，相比气体密度分布，集群引力势可以更好地表征一个简单的3D模型，是更强大的波动。在气体密度分布的扰动可以有一个实质性的影响，推导出的热力学性质，而集群的潜力是顺利的，以及近似的球体模型。我们使用了一个大型宇宙流体动力学模拟的85个星系团的统计样本来研究星系团的形状作为半径的函数。特别是，我们检查的形状等密度和等势壳和分析它是如何影响的组件的选择（气体与潜在的），子结构去除（气体密度）和计算域的定义（内部与壳）。我们发现，气体等密度线的方向和轴比是退化的子结构的存在和不稳定的波动。我们观察到，作为派生的集群形状取决于用于去除子结构的方法，从例如X射线发射率分布提取的热力学性质遭受这种额外的，往往被低估的偏见。与此相反，形状重建的潜力是在很大程度上不受这些因素和收敛到简单的几何模型，为放松和动态活跃的集群。星系团的势可以用简单的几何模型来更好地表示，并且对动态活跃和松弛的星系团都用低水平的系统学来重建，这一观察表明，通过用它们的势而不是它们的质量来表征星系团，动态活跃和松弛的星系团可以在宇宙学研究中结合起来，改善统计和降低散射。
英文摘要：
Accounting for the triaxial shapes of galaxy clusters will become important in the context of upcoming cosmological surveys. We show that, compared to the gas density distribution, the cluster gravitational potential can be better characterised by a simple 3D model and is more robust against fluctuations. Perturbations in the gas density distribution can have a substantial influence on the derived thermodynamic properties, while cluster potentials are smooth and well-approximated by a spheroidal model. We use a statistical sample of 85 galaxy clusters from a large cosmological hydrodynamical simulation to investigate cluster shapes as a function of radius. In particular, we examine the shape of isodensity and isopotential shells and analyze how it is affected by the choice of component (gas vs. potential), substructure removal (for the gas density) and the definition of the computation domain (interior vs. shells). We find that the orientation and axis ratios of gas isodensity contours are degenerate with the presence of substructures and unstable against fluctuations. We observe that, as the derived cluster shape depends on the method used for removing the substructures, thermodynamic properties extracted from e.g. X-ray emissivity profiles suffer from this additional, often underestimated bias. In contrast, the shape reconstruction of the potential is largely unaffected by these factors and converges towards simple geometric models for both relaxed and dynamically active clusters. The observation that cluster potentials are better represented by simple geometrical models and reconstructed with a low level of systematics for both dynamically active and relaxed clusters suggests that by characterising galaxy clusters by their potential rather than by their mass, dynamically active and relaxed clusters could be combined in cosmological studies, improving statistics and lowering scatter.
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