Using spectroscopic observations taken for the Visible Multi-Object Spectrograph ( VIMOS ) Ultra-Deep Survey ( VUDS ) we report here on the discovery of PCl J1001+0220 , a massive proto-cluster of galaxies located at z _ { spec } \sim 4.57 in the COSMOS field .
With nine spectroscopic members , the proto-cluster was initially detected as a \sim 12 \sigma spectroscopic overdensity of typical star-forming galaxies in the blind spectroscopic survey of the early universe ( 2 < z \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ < $ } } 6 ) performed by VUDS .
It was further mapped using a new technique developed which statistically combines spectroscopic and photometric redshifts , the latter derived from a recent compilation of incredibly deep multi-band imaging performed on the COSMOS field .
Through various methods , the descendant mass of PCl J1001+0220 is estimated to be \log ( \mathcal { M } _ { h } / \mathcal { M } _ { \odot } ) _ { z = 0 } \sim 14.5 - 15 with a large amount of mass apparently already in place at z \sim 4.57 .
An exhaustive comparison was made between the properties of various spectroscopic and photometric member samples and matched samples of galaxies inhabiting less dense environments at the same redshifts .
Tentative evidence is found for a fractional excess of older galaxies more massive in their stellar content amongst the member samples relative to the coeval field , an observation which suggests the pervasive early onset of vigorous star formation for proto-cluster galaxies .
No evidence is found for the differences in the star formation rates ( SFRs ) of member and coeval field galaxies either through estimating by means of the rest-frame ultraviolet or through separately stacking extremely deep Very Large Array 3 GHz imaging for both samples .
Additionally , no evidence for pervasive strong active galactic nuclei ( AGN ) activity is observed in either environment .
Analysis of Hubble Space Telescope images of both sets of galaxies as well as their immediate surroundings provides weak evidence for an elevated incidence of galaxy-galaxy interaction within the bounds of the proto-cluster .
The stacked and individual spectral properties of the two samples are compared , with a definite suppression of Ly \alpha seen in the average member galaxy relative to the coeval field ( f _ { esc, Ly \alpha } = 1.8 ^ { +0.3 } _ { -1.7 } % and 4.0 ^ { +1.0 } _ { -0.8 } % , respectively ) .
This observation along with other lines of evidence leads us to infer the possible presence of a large , cool , diffuse medium within the proto-cluster environment evocative of a nascent intracluster medium forming in the early universe .