Cosmic filaments in galaxy cluster outskirts: quantifying finding filaments in redshift space

TL;DR

This study investigates how peculiar velocities, causing 'Fingers of God' distortions in redshift space, affect the detection of cosmic filaments around galaxy clusters, finding that statistical correction methods are insufficient for reliable filament identification.

Contribution

The paper evaluates the effectiveness of statistical FoG correction techniques in filament detection around galaxy clusters using simulated data, highlighting their limitations.

Findings

FoG corrections do not reliably recover filament networks within 5 R200.

Complex galaxy motions overwhelm filament signals in redshift space.

Future surveys may need to rely on 2D galaxy positions for filament detection.

Abstract

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). 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. 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. Using zoom-in resimulations of 324 massive galaxy clusters and their outskirts from The ThreeHundred project, we test methods typically applied to large-scale spectroscopic data sets. 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. We find that within 5 R200 (~15 Mpc/h) statistically correcting for FoG elongations of virialized regions does not achieve reliable filament networks compared to reference filament networks based on true positions. 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 we probe. 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.