The VIMOS Ultra-Deep Survey: Emerging from the dark, a massive proto-cluster at z ∼ 4.57

Using spectroscopic observations taken for the Visible Multi-Object Spectrograph (VIMOS) Ultra-Deep Survey (VUDS) we report here on the discovery of PC1 J1001+0220, a massive proto-cluster of galaxies located at z(spec) similar to 4.57 in the COSMOS field. With nine spectroscopic members, the proto-cluster was initially detected as a similar to 12 sigma-spectroscopic overdensity of typical star-forming galaxies in the blind spectroscopic survey of the early universe (2 < z less than or similar to 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 PC1 J1001+0220 is estimated to be log(M-h/M-circle dot)(z=0) similar to 14,5-15 with a large amount of mass apparently already in place at z similar to 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 Lya seen in the average member galaxy relative to the coeval field (f(esc, Ly alpha) = 1.8-(+0.3)(1.7)% and 4.0(-0.8)(+1.0)%, 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.