Overdensities of galaxies at z ~ 3.7 in CDF-S

TL;DR

This study reports the discovery of galaxy overdensities at redshift z ~ 3.7 in the CDF-S, indicating early formation of massive structures comparable to present-day galaxy clusters.

Contribution

First identification of galaxy overdensities at z ~ 3.7 in a blank field, linking high-redshift structures to present-day clusters.

Findings

Overdensities detected with 7-sigma and 3-sigma significance.

Velocity dispersions of 500-800 km/sec suggest cluster-like structures.

Presence of ~15 massive galaxies with M > 10^{11} M_sun.

Abstract

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.