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Overdensities of galaxies at z ~ 3.7 in CDF-S

We report the discovery of possible overdensities of galaxies at z ~ 3.7 in the Chandra Deep Field South (CDF-S). These overdensities are identified from the photometric redshift selected sample, and the BVz-selected sample. One over-density is identified in the proximity of 2 AGNs and LBGs at z=3.66 and z=3.70 at 7-sigma significance level. The other over-density is less significant. It is identified around six z_{spec} ~ 3.6 galaxies at 3-sigma significance level. The line of sight velocity dispersions of these overdensities are found to be sigma_{v} ~ 500-800 km/sec, comparable to the velocity dispersions of clusters of galaxies today. Through the spectral energy distribution (SED) fitting, we find ~15 massive galaxies with M > 10^{11}M_sun around the z ~ 3.7 overdensity. The mass of the z ~ 3.7 overdensity is found to be a few times 10^{14}M_sun. Our result suggests that high redshift over-dense regions can be found in a supposedly blank field, and that the emergence of massive structures can be traced back to redshift as high as z ~ 3.7.