Automated detection of filaments in the large-scale structure of the Universe

Abstract

We present a new method to identify large-scale filaments and apply it to a cosmological simulation. Using positions of haloes above a given mass as node tracers, we look for filaments between them using the positions and masses of all the remaining dark matter (DM) haloes. In order to detect a filament, the first step consists in the construction of a backbone linking two nodes, which is given by a skeleton-like path connecting the highest local DM density traced by non-node haloes. The filament quality is defined by a density and gap parameters characterizing its skeleton, and filament members are selected by their binding energy in the plane perpendicular to the filament. This membership condition is associated to characteristic orbital times; however if one assumes a fixed orbital time-scale for all the filaments, the resulting filament properties show only marginal changes, indicating that the use of dynamical information is not critical for the method. We test the method in the simulation using massive haloes (M > 1014 h-1M⊙) as filament nodes. The main properties of the resulting high-quality filaments (which corresponds to ≃33 per cent of the detected filaments) are (i) their lengths cover a wide range of values of up to 150 h-1 Mpc, but are mostly concentrated below 50 h-1 Mpc; (ii) their distribution of thickness peaks at d= 3.0 h-1 Mpc and increases slightly with the filament length; (iii) their nodes are connected on average to 1.87 ± 0.18 filaments for ≃ 1014.1 M⊙ nodes; this number increases with the node mass to ≃ 2.49 ± 0.28 filaments for ≃ 1014.9M⊙ nodes; (iv) on average, the central density along the filaments starts at almost a hundred times the average density in the regions surrounding the nodes and then drops to about a few times the mean density at larger distances, where it remains roughly constant over 20-80 per cent of the filament length (this result may depend on the filament length); (v) there is a strong relation between length, quality and how straight a filament is, where shorter filaments are those characterized by higher qualities and more straight-line-like geometries. © 2010 The Authors. Journal compilation © 2010 RAS.