Gravitational lensing detection of an extremely dense environment around a galaxy cluster


Galaxy clusters form at the highest-density nodes of the cosmic web1,2. The clustering of dark matter halos hosting these galaxy clusters is enhanced relative to the general mass distribution, with the matter density beyond the virial region being strongly correlated to the halo mass (halo bias)3. Halo properties other than mass can further enhance the halo clustering (secondary bias)4,5,6,7. Observational campaigns have ascertained the halo bias8,9,10, but efforts to detect this secondary bias for massive halos have been inconclusive11,12,13. Here, we report the analysis of the environment bias in a sample of massive clusters, selected through the Sunyaev–Zel’dovich effect by the Planck mission14,15, focusing on the detection of the environment dark matter correlated to a single cluster, PSZ2 G099.86+58.45. The gravitational lensing signal of the outskirts is very large and can be traced up to 30 megaparsecs with a high signal-to-noise ratio (about 3.4), implying environment matter density in notable excess of the cosmological mean. Our finding reveals this system to be extremely rare in the current paradigm of structure formation and, implies that enhancing mechanisms around high-mass halos can be very effective. Future lensing surveys will probe the surroundings of single haloes, enabling the study of their formation and evolution of structure.

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Fig. 1: Visible light and total mass.
Fig. 2: Lensing profile.
Fig. 3: Differential surface density of correlated matter around PSZ2 G099.86+58.45.
Fig. 4: Environment bias of PSZ2LenS.


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We thank J. A. R. Martín for coordinating the spectroscopic campaign and L. D’Avino for suggestions on the rendering of Fig. 1. S.E. and M.S. acknowledge financial support from contracts ASI-INAF I/009/10/0, NARO15 ASI-INAF I/037/12/0, ASI 2015-046-R.0 and ASI-INAF n.2017-14-H.0. C.G. acknowledges support from the Italian Ministry for Education, University, and Research (MIUR) through the SIR individual grant SIMCODE, project number RBSI14P4IH, and the Italian Ministry of Foreign affairs and International Cooperation, Directorate General for Country Promotion for Country Promotion. L.I. acknowledges support from the Spanish research project AYA 2014-58381-P. L.M. acknowledges support from the grants ASI n.I/023/12/0 and PRIN MIUR 2015. A.F., A.S. and R.B. acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under AYA 2014-60438-P, ESP2013-48362-C2-1-P and the 2011 Severo Ochoa Program MINECO SEV-2011-0187 projects. This article includes observations made with the Gran Telescopio Canarias (GTC) operated by Instituto de Astrofísica de Canarias (IAC) with telescope time awarded by the CCI International Time Programme at the Canary Islands observatories (programme ITP13-8). The simulations were run on the Marconi supercomputer at Cineca thanks to the projects IsC10_MOKAlen3 and IsC49_ClBra01.

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All authors contributed to the interpretation and presentation of the results. M.S.: lead author; project concept, planning, and design; writing; lensing, statistical, and cosmological analyses. C.G.: numerical simulations. L.I.: X-ray analysis. F.M. and A.V.: cosmological analysis. S.E. and L.M.: planning and interpretation. G.C.: cluster sample selection. A.F., A.S. and R.B.: galaxy kinematics.

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Correspondence to Mauro Sereno.

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Supplementary Figures 1–5, Supplementary Tables 1–2

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Sereno, M., Giocoli, C., Izzo, L. et al. Gravitational lensing detection of an extremely dense environment around a galaxy cluster. Nat Astron 2, 744–750 (2018).

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