Galaxy overdensities at intermediate to high redshift

We searched and studied galaxy overdensities at 0.6 < z < 4.5 in the areas of two GOODS fields. These overdensities are identified by running top-hat filters on the two dimensional spatial distribution of two galaxy samples - a K-band limited, photometric redshift sample at 0.6 < z < 4.5, and BVz-color selected sample targeting overdensities at z ~ 3.7 and z ~ 4.0. Here photometric redshifts are derived from multi-wavelength data ranging from U-band through 8 micron band of the Spitzer. We find 52 overdensities with significances of 3.5-7 ${\sigma}$. The derived masses found to be a > $10^{13}\;M_{\odot}$ with the comoving number density of a few ${\times}10^{-6}\;Mpc^{-3}$ at z ~ 3. In order to understand the high number density of massive overdensities at high redshift, we carried out an analysis of galaxy overdensities using the mock galaxy catalog based on Millennium simulation selected in the same way as the analysis of the observational data. In the simulation, we find 650 galaxy overdensities with a 3.5 ${\sigma}$ detection threshold over $2^{{\circ}2}$ sky field. The number density of the very massive overdensities (M > $10^{14}\;M_{\odot}$) in simulation shows a similar trend with the observation. We further discuss implications of our results.