2009ApJ...691L..33K

We report the discovery of possible overdensities of galaxies at z ∼ 3.7 in Chandra Deep Field-South (CDF-S). These overdensities are identified from a photometric redshift-selected sample and a BVz-selected sample. One overdensity is identified in the proximity of two active galactic nuclei and Lyman break galaxies at z = 3.66 and z = 3.70 at 7σ significance level. The other overdensity is less significant. It is identified around six z_spec≃ 3.6 galaxies at 3σ significance level. The line-of-sight velocity dispersions of these overdensities are found to be σ_v ≃ 500-800 km/s, comparable to the velocity dispersions of clusters of galaxies today. Through spectral energy distribution fitting, we find ∼15 massive galaxies with M ≳ 10¹¹ M_☉ around the z ≃ 3.7 overdensity. The mass of the z ≃ 3.7 overdensity is found to be a few x10¹⁴ M_☉. Our result suggests that high-redshift overdense regions can be found in a supposedly blank field, and that the emergence of massive structures can be traced back to redshifts as high as z ∼ 3.7.