The outskirts of galaxy clusters are, thus, areas of increasing interest for both cosmology and astrophysics. Consequently, cosmic filaments are fundamental in transporting both dark matter and baryonic matter into clusters (Cautun et al. More specifically, X-ray observations of the hot intracluster medium have located the bulk of gas to just beyond their virial radius and, importantly, within the filaments that connect clusters to the cosmic web (see Walker et al. Today, the majority of mass relative to the volume occupied in the Universe lies in the small regions of clusters and groups (Cautun et al. de Lapparent, Geller & Huchra 1986 Colless et al. ![]() 2014) have been able to describe the formation and evolution of large-scale structures that largely match the observed Universe from galaxy surveys on comparable scales (e.g. Springel & Hernquist 2005 Klypin, Trujillo-Gomez & Primack 2011 Vogelsberger et al. In a series of successes, ever improving cosmological simulations (e.g. This general view of structure formation is strengthened by comparing results from numerical simulations that have implemented ΛCDM cosmological models (Bond & Szalay 1983 Doroshkevich & Khlopov 1984) to observations. In the Zel’dovich approximation, anisotropic collapse has a well-defined sequence, whereby regions first compress to form walls, then filaments, before finally collapsing along each direction to form clusters (Lin, Mestel & Shu 1965 Arnold, Shandarin & Zeldovich 1982 Shandarin & Klypin 1984 Shandarin & Zeldovich 1989 Cautun, van de Weygaert & Jones 2012 Hidding, Shandarin & van de Weygaert 2013). Comparatively empty voids expand accordingly, growing to dominate the overall volume in the Universe. This is the environment in which galaxy clusters form, grow and continue to grow as ultimate manifestations of hierarchical structure formation through a series of mergers and accretion from the cosmic web. The contrast of the Universe increases with time as rich overdensities grow in mass and density at the intersection of filaments while contracting in physical size. The structure develops from the anisotropic gravitational collapse of initial density perturbations (Zel’dovich 1970 Bond, Kofman & Pogosyan 1996) building the backbone of the cosmic web. In standard cosmology, this highly anisotropic distribution of matter on large scales is the natural consequence of a hierarchical assembly under the effect of gravity. The present day Universe is pervaded by a network of filaments that connect groups and clusters brimming with galaxies. Methods: data analysis, methods: observational, galaxies: clusters: general, galaxies: distances and redshifts, cosmology: observations, large-scale structure of Universe 1 INTRODUCTION ![]() While information from spectroscopic redshifts is still important to isolate the cluster regions, and thereby reduce background and foreground interlopers, we expect future spectroscopic surveys of galaxy cluster outskirts to rely on 2D positions of galaxies to extract cosmic filaments. This is due to the complex flowing motions of galaxies towards filaments in addition to the cluster infall, which overwhelm the signal of the filaments relative to the volume that we probe. We find that within 5 R 200 (∼15 h −1 Mpc) statistically correcting for FoG elongations of virialized regions does not achieve reliable filament networks compared to reference filament networks based on true positions. This paper describes our investigation of whether a statistical compression of the FoG of cluster centres and galaxy groups can lead to correct filament extractions in the cluster outskirts. Using zoom-in resimulations of 324 massive galaxy clusters and their outskirts from the three hundred project, we test methods typically applied to large-scale spectroscopic data sets. Upcoming targeted wide-field spectroscopic surveys of galaxy clusters and their infall regions, such as the WEAVE Wide-Field Cluster Survey, motivate our investigation of the impact of FoG on finding filaments connected to clusters. This presents a significant challenge for finding filaments in large observational data sets as these artificial elongations can be wrongly identified as cosmic web filaments by extraction algorithms. ![]() Inferring line-of-sight distances from redshifts in and around galaxy clusters is complicated by peculiar velocities, a phenomenon known as the ‘Fingers of God’ (FoG).
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