Observations of the cosmic microwave background indicate that baryons account for 5 per cent of the Universe’s total energy content1. In the local Universe, the census of all observed baryons falls short of this estimate by a factor of two2,3. 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 105−107 kelvin, known as the warm–hot intergalactic medium3,4,5,6. There have been previous claims of the detection of warm–hot baryons along the line of sight to distant blazars7,8,9,10 and of hot gas between interacting clusters11,12,13,14. 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 107 kelvin associated with the galaxy cluster Abell 2744. Previous observations of this cluster15 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.
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Work reported here is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. D.E. thanks F. Vazza, S. Paltani and S. Molendi for discussions. We thank H. Ebeling, M. Limousin, B. Clément, H. Atek, D. Harvey, E. Egami, M. Rexroth and P. Natarajan for help with writing the XMM-Newton proposal. M.J., H.I. and R.M. acknowledge support from the UK Science and Technology Facilities Council (grant numbers ST/L00075X/1, ST/H005234/1), the Leverhulme trust (grant number PLP-2011-003) and the Royal Society. J.-P.K. acknowledges support from the ERC advanced grant LIDA and from CNRS. H.Y.S. acknowledges support by a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme, and NSFC of China under grant 11103011. T.E. was supported by the Deutsche Forschungsgemeinschaft through the Transregional Collaborative Research Centre TR 33 ‘The Dark Universe’. E.J. was supported by CNES. J.R. acknowledges support from the ERC starting grant CALENDS.
The authors declare no competing financial interests.
Extended data figures and tables
Shown are XMM-Newton/EPIC surface-brightness profiles (SX); black, obtained by masking the filaments; colours, surface brightness in the sectors NW (northwest, position angle 10°–70°), E (east, 150°–180°) and S (south, 260°–300°). Uncertainties (error bars) are given at the 1σ level.
Extended Data Figure 2 Regions used for the analysis of the thermodynamic properties of the filaments.
The 0.5–2 keV surface brightness level is colour coded (bar at right; units are erg s−1 cm−2 arcmin−2); right ascension and declination are in degrees. Spectra were extracted from the regions indicated as E, S, SW, NW1 and NW2 by the white ellipses. The green circles show the regions labelled as Offset1–4 used to estimate the local background components (see Extended Data Table 1). The dashed cyan sectors show the regions used to extract the radial profiles along the filaments for Extended Data Figs 1, 3 and 5. The grey ellipses show background/foreground structures masked during the analysis (see text).
Galaxy density profiles (Ngal) using spectroscopically confirmed cluster members in sectors encompassing the filaments (same as Extended Data Fig. 1) are compared to the galaxy density of the cluster obtained by masking the filaments (black). Uncertainties (error bars) are given at the 1σ level.
a–f, XMM-Newton/EPIC-pn spectra for the regions shown in Extended Data Fig. 2. The background region (a) refers to Offset1. The fitting procedure was performed jointly on all EPIC instruments; however, only the pn spectra are shown here for clarity. The coloured lines show fitted contributions from the source (red), the NXB (blue), the CXB (green), the Galactic halo (cyan), and the local hot bubble (magenta).
Shown are surface mass density profiles obtained from combined strong and weak lensing. The black curve shows the cluster average, compared to the profiles obtained in the direction of the filaments (same as Extended Data Fig. 1).
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Eckert, D., Jauzac, M., Shan, H. et al. Warm–hot baryons comprise 5–10 per cent of filaments in the cosmic web. Nature 528, 105–107 (2015). https://doi.org/10.1038/nature16058
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