2013MNRAS.436.3331B

Using data from the Sloan Digital Sky Survey we assess the current dynamical state of the Corona Borealis supercluster (CSC), a highly dense and compact supercluster at z ~ 0.07. The Fundamental Plane relation is used to determine redshift-independent distances to six clusters in the densest region of the supercluster, with mean accuracy in the relative distance estimates of 4 percent. Peculiar velocities determined from these distance estimates indicate that the clusters have broken from the Hubble flow, suggesting that the CSC likely contains two regions that have reached turnaround and are currently undergoing gravitational collapse. These results provide the strongest observational evidence to date that the CSC is a bound system similar to the much more extensive Shapley supercluster, which is the most extensive confirmed bound supercluster yet identified in the Universe. When compared with simulations of the CSC our results require substantially more mass than is contained within the clusters, possibly indicating a significant intercluster dark matter component. In order to facilitate comparison with studies for which spectroscopic data are not available, an alternative analysis of the dynamics is made using the Kormendy relation as a distance indicator. The results are generally consistent with those of the Fundamental Plane and suggest similar global dynamics, but we find that the relatively sparse sampling of the clusters makes the Kormendy relation less reliable overall and more susceptible to small systematic differences between the cluster samples.