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

¹ Department of Physics, University of California, 1 Shields Ave, Davis, CA 95616, USA
e-mail: bclemaux@ucdavis.edu
² Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
³ INAF – Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁴ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretañ a 1111, Playa Ancha, Valparaíso, Chile
⁵ INAF – IASF, via Bassini 15, 20133 Milano, Italy
⁶ University of Bologna, Department of Physics and Astronomy (DIFA), viale Berti Pichat 6/2, 40127 Bologna, Italy
⁷ Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia
⁸ Max-Planck-Institut für extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
⁹ Sorbonne Université, UPMC Université Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis Boulevard Arago, 75014 Paris, France
¹⁰ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² European Space Agency, ESA/ESTEC, Keplerlaan 1, 22001 AZ Noordwijk, The Netherlands
¹³ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Ave, Cambridge CB30HE, UK
¹⁴ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB30HA, UK
¹⁵ Department of Astronomy, University of Geneva, ch. d’Écogia 16, 1290 Versoix, Switzerland
¹⁶ Observatoire de Genève, 51 ch. des Maillettes, 1290 Versoix, Switzerland

Received: 25 March 2017
Accepted: 3 January 2018

Abstract

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 ~ 4.57 in the COSMOS field. With nine spectroscopic members, the proto-cluster was initially detected as a ~12σ spectroscopic 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 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 (M_h/M_⊙)_z=0 ~ 14.5–15 with a large amount of mass apparently already in place at z ~ 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 Lyα seen in the average member galaxy relative to the coeval field (f_esc, _Lyα = 1.8_−1.7^+0.3% 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.