Mapping and characterisation of cosmic filaments in galaxy cluster outskirts: strategies and forecasts for observations from simulations

Upcoming wide-field surveys are well-suited to studying the growth of galaxy clusters by tracing galaxy and gas accretion along cosmic filaments. We use hydrodynamic simulations of volumes surrounding 324 clusters from The ThreeHundred project to develop a framework for identifying and characterising these filamentary structures, and associating galaxies with them. We define 3-dimensional reference filament networks reaching 5R200 based on the underlying gas distribution and quantify their recovery using mock galaxy samples mimicking observations such as those of the WEAVE Wide-Field Cluster Survey. Since massive galaxies trace filaments, they are best recovered by mass-weighting galaxies or imposing a bright limit (e.g. > L ∗ ) on their selection. We measure the transverse gas density profile of filaments, derive a characteristic filament radius of ' 0.7–1 h −1Mpc, and use this to assign galaxies to filaments. For different filament extraction methods we find that at R > R200, ∼ 15–20% of galaxies with M∗ > 3 × 109M are in filaments, increasing to ∼ 60% for galaxies more massive than the Milky-Way. The fraction of galaxies in filaments is independent of cluster mass and dynamical state, and is a function of cluster-centric distance, increasing from ∼ 13% at 5R200 to ∼ 21% at 1.5R200. As a bridge to the design of observational studies, we measure the purity and completeness of different filament galaxy selection strategies. Encouragingly, the overall 3-dimensional filament networks and ∼ 67% of the galaxies associated with them are recovered from 2-dimensional galaxy positions.