2018ApJ...852..134T

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_t,gal=7.6±1.4 for the entire region. These overdensities correspond to masses of M_b=(0.76±0.17)×10¹⁵h^–1 M_☉, M_r=(2.15±0.32)×10¹⁵h^–1 M_☉, and M_t=(3.19±0.40)×10¹⁵h^–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 ∼10¹⁵h^–1 M_☉ protocluster and at least one >10¹⁴h^–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 7.4h³ Gpc^–3.