Discovery of a rich proto-cluster at z = 2.9 and associated diffuse cold gas in the VIMOS Ultra-Deep Survey (VUDS)

High-density environments are crucial places for studying the link between hierarchical structure formation and stellar mass growth in galaxies. In this work, we characterise a massive proto-cluster at z = 2.895 that we found in the COSMOS field using the spectroscopic sample of the VIMOS Ultra-Deep Survey (VUDS). This is one of the rare structures at z ~ 3 not identified around an active galactic nucleus (AGN) or a radio galaxy, thus it represents an ideal laboratory for investigating the formation of galaxies in dense environments. The structure comprises 12 galaxies with secure spectroscopic redshift in an area of ~ 7' × 8', in a total z range of Δz = 0.016. The measured galaxy number overdensity is δg = 12 ± 2. This overdensity has a total mass of M ~ 8.1 × 1014 M⊙ in a volume of 13 × 15 × 17 Mpc3. Simulations indicate that such an overdensity at z ~ 2.9 is a proto-cluster, which will collapse in a cluster of total mass Mz = 0 ~ 2.5 × 1015 M⊙ at z = 0, i.e. a massive cluster in the local Universe. We analysed the properties of the galaxies within the overdensity, and we compared them with acontrol sample at the same redshift but outside the overdensity. We could not find any statistically significant difference between the properties (stellar mass, star formation rate, specific star formation rate, NUV-r and r - K colours) of the galaxies inside and outside the overdensity, but this result might be due to the lack of statistics or possibly to the specific galaxy population sampled by VUDS, which could be less affected by environment than the other populations not probed by the survey. The stacked spectrum of galaxies in the background of the overdensity shows a significant absorption feature at the wavelength of Lyα redshifted at z = 2.895 (λ = 4736 Å), with a rest frame equivalent width (EW) of 4 ± 1.4 Å. Stacking only background galaxies without intervening sources at z ~ 2.9 along their line of sight, we find that this absorption feature has a rest frame EW of 10.8 ± 3.7 Å, with a detection S/N of ~4. We verify that this measurement is not likely to be due to noise fluctuations. These EW values imply a high column density (N(HI) ~ 3-20 × 1019 cm-2), consistent with a scenario where such absorption is due to intervening cold streams of gas that are falling into the halo potential wells of the proto-cluster galaxies. Nevertheless, we cannot rule out the hypothesis that this absorption line is related to the diffuse gas within the overdensity.