Intercluster filaments in a ΛCDM Universe

The large-scale structure (LSS) in the Universe comprises a complicated filamentary network of matter. We study this network using a high-resolution simulation of structure formation in a Λ Cold Dark Matter cosmology. We investigate the distribution of matter between neighbouring large haloes whose masses are comparable to massive clusters of galaxies. We identify a total of 228 filaments between neighbouring clusters. Roughly half of the filaments are either warped or lie off the cluster–cluster axis. We find that straight filaments, on the average, are shorter than warped ones. Close cluster pairs with separation of 5 h−1 Mpc or less are always connected by a filament. At separations between 15 and 20 h−1 Mpc, about a third of cluster pairs are connected by a filament. On average, more-massive clusters are connected to more filaments than less-massive ones. This finding indicates that the most-massive clusters form at the intersections of the filamentary backbone of LSS. For straight filaments, we compute mass profiles. Radial profiles show a fairly well-defined radius, rs, beyond which the profiles follow an r−2 power law fairly closely. For the majority of filaments, rs lies between 1.5 and 2.0 h−1 Mpc. The enclosed overdensity inside rs varies from a few times up to 25 times the mean density, independent of the length of the filament. Along the filaments' axes, material is not distributed uniformly. Towards the clusters, the density rises, indicating the presence of the cluster infall regions. Filaments have been suggested to cause possible alignments between neighbouring clusters. Looking at the nearest neighbour for each cluster, we find that, up to a separation of about 15 h−1 Mpc, there is a filament present that could account for alignment. In addition, we also find some sheet-like connections between clusters. In roughly a fifth of all cluster–cluster connections where we could not identify a filament or sheet, projection effects lead to filamentary structures in the projected mass distribution.