title: Warm-hot baryons comprise 5-10 per cent of filaments in the cosmic web.
abstract: Observations of the cosmic microwave background indicate that baryons account for 5 per cent of the Universe's total energy content. In the local Universe, the census of all observed baryons falls short of this estimate by a factor of two. Cosmological simulations indicate that the missing baryons have not condensed into virialized haloes, but reside throughout the filaments of the cosmic web (where matter density is larger than average) as a low-density plasma at temperatures of 10(5)-10(7) kelvin, known as the warm-hot intergalactic medium. There have been previous claims of the detection of warm-hot baryons along the line of sight to distant blazars and of hot gas between interacting clusters. These observations were, however, unable to trace the large-scale filamentary structure, or to estimate the total amount of warm-hot baryons in a representative volume of the Universe. Here we report X-ray observations of filamentary structures of gas at 10(7) kelvin associated with the galaxy cluster Abell 2744. Previous observations of this cluster were unable to resolve and remove coincidental X-ray point sources. After subtracting these, we find hot gas structures that are coherent over scales of 8 megaparsecs. The filaments coincide with over-densities of galaxies and dark matter, with 5-10 per cent of their mass in baryonic gas. This gas has been heated up by the cluster's gravitational pull and is now feeding its core. Our findings strengthen evidence for a picture of the Universe in which a large fraction of the missing baryons reside in the filaments of the cosmic web.
authors.affiliation: Department of Astronomy, University of Geneva, Chemin d'Ecogia 16, 1290 Versoix, Switzerland.|INAF - IASF Milano, Via E. Bassini 15, 20133 Milan, Italy.
authors.forename: Dominique
authors.initials: D
authors.lastname: Eckert
authors.affiliation: Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.|Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, University of KwaZulu-Natal, Durban 4041, South Africa.
authors.forename: Mathilde
authors.initials: M
authors.lastname: Jauzac
authors.affiliation: Laboratoire d'Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix, Switzerland.
authors.forename: HuanYuan
authors.initials: H
authors.lastname: Shan
authors.affiliation: Laboratoire d'Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix, Switzerland.|Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille) UMR 7326, 13388 Marseille, France.
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Kneib
authors.affiliation: Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Erben
authors.affiliation: Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Holger
authors.initials: H
authors.lastname: Israel
authors.affiliation: Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille) UMR 7326, 13388 Marseille, France.
authors.forename: Eric
authors.initials: E
authors.lastname: Jullo
authors.affiliation: Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Klein
authors.affiliation: Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Richard
authors.initials: R
authors.lastname: Massey
authors.affiliation: CRAL, Observatoire de Lyon, Université Lyon 1, 9 Avenue Ch. André, F-69561 Saint Genis Laval Cedex, France.
authors.forename: Johan
authors.initials: J
authors.lastname: Richard
authors.affiliation: Department of Astronomy, University of Geneva, Chemin d'Ecogia 16, 1290 Versoix, Switzerland.
authors.forename: Céline
authors.initials: C
authors.lastname: Tchernin