Abell 2744 may be a supercluster aligned along the sightline

Abstract

To explain the unusual richness and compactness of the Abell 2744, we propose a hypothesis that it may be a rich supercluster aligned along the sightline, and present a supporting evidence obtained numerically from the MultiDark Planck 2 simulations with a linear box size of $1\,h^{-1}$Gpc. Applying the friends-of-friends (FoF) algorithm with a linkage length of $0.33$ to a sample of the cluster-size halos from the simulations, we identify the superclusters and investigate how many superclusters have filamentary branches that would appear to be similar to the Abell 2744 if the filamentary axis is aligned with the sightline. Generating randomly a unit vector as a sightline at the position of the core member of each supercluster and projecting the positions of the members onto the plane perpendicular to the direction of the sightline, we measure two dimensional distances ($R_{2d}$) of the member halos from the core for each supercluster. Defining a Abell 2744-like spuercluster as the one having a filamentary branch composed of eight or more members with $R_{2d}\le 1\,$Mpc and masses comparable to those of the observed Abell 2744 substructures, we find one Abell 2744-like supercluster at $z=0.3$ and two at $z=0$. Repeating the same analysis but with the data from the Big MultiDark Planck simulations performed on a larger box of linear size of $2.5\,h^{-1}$Mpc, we find that the number of the Abell 2744-like superclusters at $z=0$ increases up to eighteen, among which three are found more massive than $5\times 10^{15}\,M_{\odot}$.