Abstract
Utilizing spectroscopic observations taken for the VIMOS Ultra-Deep Survey
(VUDS) we report here on the discovery of Cl J1001+0220, a massive
proto-cluster located at $z_spec\sim4.57$ in the COSMOS field. The
proto-cluster was initially detected as a $\sim12\sigma$ overdensity of typical
star-forming galaxies in the blind spectroscopic survey of the early universe
($2<z<6$) 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 deep multi-band imaging.
Through various methods, the descendant halo mass of Cl J1001+0220 is estimated
to be $łog(M_h/M_ødot)_z=0\sim14.5-15$ with a large amount of mass
apparently already in place at $z\sim4.57$. Tentative evidence is found for a
fractional excess of older and more massive galaxies within the proto-cluster,
an observation which suggests the pervasive early onset of vigorous star
formation. No evidence is found for the differences in the star formation rates
of member and a matched sample of coeval field galaxies either through
rest-frame ultraviolet methods or through stacking extremely deep Very Large
Array 3 GHz imaging. Additionally, no evidence for pervasive strong active
galactic nuclei (AGN) activity is observed. Analysis of Hubble Space Telescope
images provides weak evidence for an elevated incidence of galaxy-galaxy
interaction within the proto-cluster. The 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.
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