Understanding Large-scale Structure in the SSA22 Protocluster Region Using Cosmological Simulations∗

Abstract

Abstract We investigate the nature and evolution of large-scale structure within the SSA22 protocluster region at z = 3.09 using cosmological simulations. A redshift histogram constructed from current spectroscopic observations of the SSA22 protocluster reveals two separate peaks at z = 3.065 (blue) and z = 3.095 (red). Based on these data, we report updated overdensity and mass calculations for the SSA22 protocluster. We find and for the blue and red peaks, respectively, and for the entire region. These overdensities correspond to masses of , , and for the red, blue, and total peaks, respectively. We use the Small MultiDark Planck (SMDPL) simulation to identify comparably massive protoclusters, and uncover the underlying structure and ultimate fate of the SSA22 protocluster. For this analysis, we construct mock redshift histograms for each simulated protocluster, quantitatively comparing them with the observed SSA22 data. We find that the observed double-peaked structure in the SSA22 redshift histogram corresponds not to a single coalescing cluster, but rather the proximity of a protocluster and at least one cluster progenitor. Such associations in the SMDPL simulation are easily understood within the framework of hierarchical clustering of dark matter halos. We finally find that the opportunity to observe such a phenomenon is incredibly rare, with an occurrence rate of .