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 characterizing these filamentary structures and associating galaxies with them. We define three-dimensional reference filament networks reaching 5R(200) based on the underlying gas distribution and quantify their recovery using mock galaxy samples mimicking observations such as those of the WEAVEWide-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 similar or equal to 0.7(-1) h(-1) Mpc, and use this to assign galaxies to filaments. For different filament extractionmethods, we find that at R > R-200,similar to 15-20 per cent of galaxies with M-* > 3 x 10(9)M(circle dot) are in filaments, increasing to similar to 60 per cent 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 similar to 13 per cent at 5R200 to similar to 21 per cent at 1.5R(200). As a bridge to the design of observational studies, we measure the purity and completeness of different filament galaxy selection strategies. Encouragingly, the overall three-dimensional filament networks and similar to 67 per cent of the galaxies associated with them are recovered from two-dimensional galaxy positions.