Characterising superclusters with the galaxy cluster distribution

Abstract

Superclusters are the largest observed matter density structures in the Universe. Recently, we presented the first supercluster catalogue constructed with a well-defined selection function based on the X-ray flux-limited cluster survey, REFLEX II. To construct the sample we proposed a concept to find large objects with a minimum overdensity such that it can be expected that most of their mass will collapse in the future. The main goal is to provide support for our concept here by using simulation that we can, on the basis of our observational sample of X-ray clusters, construct a supercluster sample defined by a certain minimum overdensity. On this sample we also test how superclusters trace the underlying dark matter distribution. Our results confirm that an overdensity in the number of clusters is tightly correlated with an overdensity of the dark matter distribution. This enables us to define superclusters within which most of the mass will collapse in the future. We also obtain first-order mass estimates of superclusters on the basis of the properties of the member clusters. We also show that in this context the ratio of the cluster number density and dark matter mass density is consistent with the theoretically expected cluster bias. Our previous work provided evidence that superclusters are a special environment in which the density structures of the dark matter grow differently from those in the field, as characterised by the X-ray luminosity function. Here we confirm for the first time that this originates from a top-heavy mass function at high statistical significance that is provided by a Kolmogorov-Smirnov test. We also find in close agreement with observations that the superclusters only occupy a small volume of a few per cent, but contain more than half of the clusters in the present-day Universe.

Sloan Great Wall as a complex of superclusters with collapsing cores

“…The mass-to-light ratios of the SGW superclusters, M/L ≈ 300 h M /L , are close to the values of the mass-to-light ratios for other rich superclusters (Gavazzi et al 2004;Schirmer et al 2011;Einasto et al 2015). Moreover, the components in rich SGW superclusters have masses that are comparable to the masses of simulated superclusters of average richness (Chon et al 2014;Araya-Melo et al 2009). The masses of rich SGW superclusters are of the same order as the high end of simulated supercluster masses.…”

“…The main uncertainty of supercluster masses comes from missing groups and clusters, and from the mass estimate of faint groups. The bias between the masses of superclusters determined using group masses and the total mass of the supercluster have been studied from simulations (Chon et al 2014). They showed that the bias between masses depends on the richness of groups; it is lower when low-mass groups are used to construct the supercluster catalogues.…”

On the definition of superclusters

“…For the details of the construction of the superclusters in simulations, we refer the readers to Chon et al (2014). The value of r Δ was taken at the radius where the density ratio reaches the threshold value for collapse, and r v is defined as the radius where the required infall velocity is reached.…”

“…To evaluate the total mass of Shapley with the REFLEX II clusters, we also considered the total cluster mass as a function of a radius for the same volume. We converted the total cluster mass to the total supercluster mass by adopting a scaling relation found with cosmological N-body simulations in Chon et al (2014). We find that the required density ratio for collapse is satisfied out to about 12.4 h −1 Mpc.…”

“…There have been attempts to explore this definition in simulations (Dünner et al 2006). We have been exploring a similar approach observationally in our construction of an X-ray supercluster catalogue (Chon et al 2013(Chon et al , 2014. We selected the superclusters from the X-ray galaxy cluster distribution by means of a friends-of-friends (FoF) algorithm in such a way that we expect the superclusters to have their major parts gravitationally bound and to collapse in the future.…”