星系团合并的模拟模拟对Viewing Angle的约束
Simulated Analogs of Merging Galaxy Clusters Constrain the Viewing Angle
摘要：
在解释双峰合并星系团的观测结果时，一个关键的不确定性是子星系团分离矢量和天空平面之间的未知角度。我们提出了一种新的方法来约束这个关键参数。我们在宇宙学n体模拟中发现了观测到的系统的类似物，并量化了它们与观测到的投影分离和子集群之间的相对径向速度相匹配的可能性，作为视角的函数。我们推导出许多观察到的双峰合并，包括子弹团（1 E 0657-558）和埃尔戈多（ACT-CL J 0102 -4915）的视角上的限制。我们还提出了更通用的约束作为投影分离和相对径向速度的函数，这可以用来评估更多的集群，因为它们的信息变得可用。仅从这两个观测量的约束是弱的（通常$\gtrsim 70-75^\circ$在68%置信度和$\gtrsim 55-60^\circ$在95%置信度），但是比经典的定时参数包含更多的宇宙学背景，边缘化子结构的许多实现、特殊速度等。与在NFW晕上实现定时参数的MCMAC代码相比，我们的约束通常预测更接近天空平面的子集群分离向量。这是因为在实际的合并中，子星系团的速度向量并不完全平行于分离向量（即合并并不是完全迎面的）。因此，非零相对径向速度的观测并不排除天空平面中的分离矢量，就像MCMAC采用的迎面定时参数一样。
英文摘要：
A key uncertainty in interpreting observations of bimodal merging galaxy clusters is the unknown angle between the subcluster separation vector and the plane of the sky. We present a new method for constraining this key parameter. We find analogs of observed systems in cosmological n-body simulations and quantify their likelihood of matching the observed projected separation and relative radial velocities between subclusters, as a function of viewing angle. We derive constraints on the viewing angle of many observed bimodal mergers including the Bullet Cluster (1E 0657-558) and El Gordo (ACT-CL J0102-4915). We also present more generic constraints as a function of projected separation and relative radial velocity, which can be used to assess additional clusters as information about them becomes available. The constraints from these two observables alone are weak (typically $\gtrsim 70-75^\circ$ at 68\% confidence and $\gtrsim 55-60^\circ$ at 95\% confidence) but incorporate much more cosmological context than the classical timing argument, marginalizing over many realizations of substructure, peculiar velocities, and so on. Compared to the MCMAC code, which implements the timing argument on NFW halos, our constraints generally predict subcluster separation vectors closer to the plane of the sky. This is because in realistic mergers the subcluster velocity vectors are not entirely parallel to the separation vector (i.e, the mergers are not perfectly head-on). As a result, observation of a nonzero relative radial velocity does not exclude a separation vector in the plane of the sky, as it does in the head-on timing argument employed by MCMAC.
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