Warm-hot baryons comprise 5-10 per cent of filaments in the cosmic web

TL;DR

This study provides direct X-ray evidence that 5-10% of baryonic matter in cosmic filaments at 10^7 K is associated with galaxy clusters, helping to locate the Universe's missing baryons.

Contribution

First direct detection of warm-hot baryons in cosmic filaments associated with a galaxy cluster using X-ray observations.

Findings

Filaments contain 5-10% of their mass in baryonic gas.

Gas structures are coherent over 8 Mpc scales.

Baryons in filaments are heated by gravitational processes.

Abstract

Observations of the cosmic microwave background indicate that baryons account for 5% 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 might not have 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 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 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 reveal hot gas structures that are coherent over scales of 8 mergaparsecs. 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. 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.