The baryon content of the Cosmic Web

• Published in: Nature (London) 2015-12, Vol.528 (7580), p.105-107
• Main Authors: Eckert, Dominique, Jauzac, Mathilde, Shan, HuanYuan, Kneib, Jean-Paul, Erben, Thomas, Israel, Holger, Jullo, Eric, Klein, Matthias, Massey, Richard, Richard, Johan, Tchernin, Céline
• Format: Article
• Language: English
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature16058

Big-Bang nucleosynthesis indicates that baryons account for 5% of the Universe’s total energy content[ 1 ]. In the local Universe, the census of all observed baryons falls short of this estimate by a factor of two[ 2 , 3 ]. Cosmological simulations indicate that the missing baryons have not yet condensed into virialised halos, but reside throughout the filaments of the cosmic web: a low-density plasma at temperature 10 5 –10 7 K known as the warm-hot intergalactic medium (WHIM)[ 3 , 4 , 5 , 6 ]. There have been previous claims of the detection of warm baryons along the line of sight to distant blazars[ 7 , 8 , 9 , 10 ] and hot gas between interacting clusters[ 11 , 12 , 13 , 14 ]. These observations were however unable to trace the large-scale filamentary structure, or to estimate the total amount of warm baryons in a representative volume of the Universe. Here we report X-ray observations of filamentary structures of ten-million-degree gas associated with the galaxy cluster Abell 2744. Previous observations of this cluster[ 15 ] were unable to resolve and remove coincidental X-ray point sources. After subtracting these, we reveal hot gas structures that are coherent over 8 Mpc scales. The filaments coincide with over-densities of galaxies and dark matter, with 5-10% of their mass in baryonic gas. This gas has been heated up by the cluster's gravitational pull and is now feeding its core.