We study the distribution of line-of-sight velocities of galaxies in the vicinity of SDSS redMaPPer galaxy clusters. Based on their velocities, galaxies can be split into two categories: galaxies that are dynamically associated with the cluster, and random line-of-sight projections. Both the fraction of galaxies associated with the galaxy clusters, and the velocity dispersion of the same, exhibit a sharp feature as a function of radius. The feature occurs at a radial scale $R_{\rm edge} \approx 2.2R_{\rm{\lambda}}$, where $R_{\rm{\lambda}}$ is the cluster radius assigned by redMaPPer. We refer to $R_{\rm edge}$ as the "edge radius." These results are naturally explained by a model that further splits the galaxies dynamically associated with a galaxy cluster into a component of galaxies orbiting the halo and an infalling galaxy component. The edge radius $R_{\rm edge}$ constitutes a true "cluster edge", in the sense that no orbiting structures exist past this radius. A companion paper (Aung et al. 2020) tests whether the "halo edge" hypothesis holds when investigating the full three-dimensional phase space distribution of dark matter substructures in numerical simulations, and demonstrates that this radius coincides with a suitably defined splashback radius.

中文翻译：

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簇有边缘：SDSS redMapPer簇的投影相空间结构

我们研究了 SDSS redMapPer 星系团附近星系的视线速度分布。根据它们的速度，星系可以分为两类：与星团动态关联的星系和随机视线投影的星系。与星系团相关的星系比例以及它们的速度色散都表现出作为半径函数的尖锐特征。该特征发生在径向尺度 $R_{\rm edge} \approx 2.2R_{\rm{\lambda}}$，其中 $R_{\rm{\lambda}}$ 是由 redMapPer 分配的集群半径。我们将 $R_{\rm edge}$ 称为“边缘半径”。这些结果自然可以通过一个模型来解释，该模型将与星系团动态相关的星系进一步分裂成一个围绕光晕运行的星系成分和一个下落星系成分。边缘半径 $R_{\rm edge}$ 构成了真正的“集群边缘”，因为在这个半径之外不存在任何轨道结构。一篇配套论文（Aung 等人，2020 年）测试了在数值模拟中研究暗物质子结构的完整三维相空间分布时“光环边缘”假设是否成立，并证明该半径与适当定义的飞溅半径一致。