Keywords

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 ${\delta }_{b,\mathrm{gal}}=4.8\pm 1.8$ and ${\delta }_{r,\mathrm{gal}}=9.5\pm 2.0$ for the blue and red peaks, respectively, and ${\delta }_{t,\mathrm{gal}}=7.6\pm 1.4$ for the entire region. These overdensities correspond to masses of ${M}_{b}=(0.76\pm 0.17)\times {10}^{15}{h}^{-1}\ {M}_{\odot }$, ${M}_{r}=(2.15\pm 0.32)\times {10}^{15}{h}^{-1}\,{M}_{\odot }$, and ${M}_{t}=(3.19\pm 0.40)\times {10}^{15}{h}^{-1}\,{M}_{\odot }$ for the red, blue, and total peaks, respectively. We use the Small MultiDark Planck (SMDPL) simulation to identify comparably massive $z\sim 3$ protoclusters, and uncover the underlying structure and ultimate fate of the SSA22 protocluster. For this analysis, we construct mock redshift histograms for each simulated $z\sim 3$ 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 $\sim {10}^{15}{h}^{-1}\,{M}_{\odot }$ protocluster and at least one $\gt {10}^{14}{h}^{-1}\,{M}_{\odot }$ 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 $7.4{h}^{3}\,{{\rm{Gpc}}}^{-3}$.

We present the results of a densely sampled spectroscopic survey of the SSA22 protocluster at z ≈ 3.09. Our sample with Keck/LRIS spectroscopy includes 106 Lyα emitters (LAEs) and 40 Lyman break galaxies (LBGs) at z = 3.05–3.12. These galaxies are contained within the 9' × 9' region in which the protocluster was discovered, which also hosts the maximum galaxy overdensity in the SSA22 region. The redshift histogram of our spectroscopic sample reveals two distinct peaks, at z = 3.069 (blue; 43 galaxies) and z = 3.095 (red; 103 galaxies). Furthermore, objects in the blue and red peaks are segregated on the sky, with galaxies in the blue peak concentrating toward the western half of the field. These results suggest that the blue and red redshift peaks represent two distinct structures in physical space. Although the double-peaked redshift histogram is traced in the same manner by LBGs and LAEs, and brighter and fainter galaxies, we find that 9 out of 10 X-ray AGNs in SSA22, and all 7 spectroscopically confirmed giant Lyα "blobs," reside in the red peak. We combine our data set with sparsely sampled spectroscopy from the literature over a significantly wider area, finding preliminary evidence that the double-peaked structure in redshift space extends beyond the region of our dense spectroscopic sampling. In order to fully characterize the three-dimensional structure, dynamics, and evolution of large-scale structure in the SSA22 overdensity, we require the measurement of large samples of LAE and LBG redshifts over a significantly wider area, as well as detailed comparisons with cosmological simulations of massive cluster formation.

We present the results of the extensive narrowband survey of Lyα emission-line objects at z = 3.1 in the 1.38 deg² area surrounding the high-density region of star-forming galaxies at z = 3.09 in the SSA22 field, as well as in the 1.04 deg² area of the three separated general blank fields. In total, of 2161 Lyα emitters, there are 1394 in the SSA22 fields and 767 in the general fields detected at the narrowband AB magnitude limit of 25.73, which corresponds to the line flux of ≈1.8 × 10⁻¹⁷ erg s⁻¹ cm⁻² or the luminosity of ≈1.5 × 10⁴² erg s⁻¹ at z = 3.1, above the observed equivalent-width threshold, ≈190 Å. The average surface number density of the emitters at z = 3.1 in the general fields above the thresholds is 0.20 ± 0.01 arcmin⁻². The SSA22 high-density region at z = 3.09, whose peak local density is six times that of the average, is found to be the most prominent outstanding structure in the whole surveyed area and is firmly identified as a robust "protocluster." We also compared the overdensity of the 100 arcmin² and 700 arcmin² areas which contain the protocluster with the expected fluctuation of the dark matter as well as those of the model galaxies in cosmological simulations. We found that the peak height values of the overdensity are 8–10 and 3–4 times the expected standard deviations for the counts of Lyα emitters at z = 3.1 in the corresponding volume, respectively. We conclude that the structure at z = 3.09 in the SSA22 field is a very significant and rare density peak up to the scale of ≈60 Mpc.