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

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 δ_(b.gal) = 4.8 ± 1.8 and δ_(r.gal) = 9.5 ± 2.0 for the blue and red peaks, respectively, and δ_(r.gal) = 7.6 ± 1.4 for the entire region. These overdensities correspond to masses of M_b = (0.76 ± 0.17) x 10^(15)h^(-1) M_⊙ M_r = (2.15 ± 0.32) x 10^(15)h^(-1) M_⊙, and M_t = (3.19 ± 0.40) x 10^(15)h^(-1),M_⊙ for the red, blue, and total peaks, respectively. We use the Small MultiDark Planck (SMDPL) simulation to identify comparably massive z ~ 3 protoclusters, and uncover the underlying structure and ultimate fate of the SSA22 protocluster. For this analysis, we construct mock redshift histograms for each simulated z ~ 3 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 ~ 10^(15)h^(-1) M_⊙ protocluster and at least one 〉 10^(14)h^(-1) M_⊙ 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 7.4h^3 Gpc^(-3).

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