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Published October 2014 | Submitted + Published
Journal Article Open

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 δ_g = 12 ± 2. This overdensity has a total mass of M ~ 8.1 × 10^(14) M_⊙ in a volume of 13 × 15 × 17 Mpc^3. Simulations indicate that such an overdensity at z ~ 2.9 is a proto-cluster, which will collapse in a cluster of total mass M_(z = 0) ~ 2.5 × 10^(15) 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 × 10^(19) 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.

Additional Information

© 2014 ESO. Article published by EDP Sciences. Received 14 March 2014. Accepted 23 July 2014. Published online 07 October 2014. Based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791. We thank the referee for the useful comments that improved the paper. We thank Nico Cappelluti for his help with X-ray data and Mauro Roncarelli for helpful discussion. We thank ESO staff for their continuous support of the VUDS survey, particularly the Paranal staff conducting the observations and Marina Rejkuba and the ESO user support group in Garching. This work is supported by funding from the European Research Council Advanced Grant ERC-2010-AdG-268107-EARLY and by INAF Grants PRIN 2010, PRIN 2012, and PICS 2013. A.C., O.C., M.T., and V.S. acknowledge the grants MIUR PRIN 2010–2011 and ASI n.I/023/12/0 "Attività relative alla fase B2/C per la missione Euclid". D.M. gratefully acknowledges LAM hospitality during the initial phases of the project. This work is based on data products made available at the CESAM data centre, Laboratoire d'Astrophysique de Marseille. This work partly uses observations obtained with MegaPrime/MegaCam, a joint project of the CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of the NRC and CNRS.

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Published - aa23811-14.pdf

Submitted - 1403.3691v2.pdf


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