OVERDENSITIES OF GALAXIES AT z ∼ 3.7 IN CHANDRA DEEP FIELD-SOUTH

We report the discovery of possible overdensities of galaxies at z similar to 3.7 in Chandra Deep Field-South (CDF-S). These overdensities are identified from a photometric redshift-selected sample and a BV z-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 7 sigma significance level. The other overdensity is less significant. It is identified around six z(spec) similar or equal to 3.6 galaxies at 3 sigma significance level. The line-of-sight velocity dispersions of these overdensities are found to be sigma(upsilon) similar or equal to 500-800 km s(-1), comparable to the velocity dispersions of clusters of galaxies today. Through spectral energy distribution fitting, we find similar to 15 massive galaxies with M greater than or similar to 10(11)M(circle dot) around the z similar or equal to 3.7 overdensity. The mass of the z similar or equal to 3.7 overdensity is found to be a few x 10(14)M(circle dot). 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 similar to 3.7.