Clusters Have Edges: The Projected Phase SpaceStructure of SDSS redMaPPer Clusters

Abstract

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 $R_{\mathrm{edge}}\approx 2.2R_{\lambda }$, where $R_{\lambda }$ is the cluster radius assigned by redMaPPer. We refer to $R_{\mathrm{edge}}$ 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 $R_{\mathrm{edge}}$ 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.

arXiv comment10 pages, 5 figures, submitted to MNRAS, companion paper to Aung et al. 2020

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