XMM-Newton discovered part of the missing matter in the Universe

The existence of the tenuous hot gas, which is believed to be located within the connecting threads of the enormous cosmic web, was predicted by theoreticians about 10 years ago. However, its very low density hampered many attempts to detect it. Now astronomers discovered the hottest part of the missing matter made of atoms.

Most of the matter/energy in the Universe is of unknown nature – and astronomers call it dark. 72% of the Universe is a mysterious dark energy, causing an accelerated expansion of the Universe. Some 23% of the total amount of matter/energy is constituted by the so called dark matter, which is made of heavy particles still waiting to be discovered by particle physicists. Only 4.6% percent of the Universe is made of normal matter as we know it, consisting of protons and neutrons – called baryons – which together with electrons are the building blocks of atoms. Small as this percentage might be, still a big part of this “ordinary” baryonic matter is also missing. All the stars, galaxies, and gas that astronomers observe in the Universe account for less than a half of all the baryons that should be around.

All the matter in the Universe, including the galaxies observed with optical telescopes, is distributed in a web-like structure. Dense nodes of this cosmic web are clusters of galaxies, the biggest objects in the Universe. For about the past ten years, astronomers suspected that the missing baryonic matter is in the form of hot gas with very low densities which permeates the filamentary structure of the cosmic web. Due to its high temperature this gas is expected to emit primarily in the far-ultraviolet and X-ray band. However, the very low density of the gas makes its observation difficult.

Astronomers using the European XMM-Newton X-ray satellite observed a pair of clusters of galaxies – Abell 222 and Abell 223. The images and the spectra of this system revealed a bridge of hot gas connecting the clusters. “The hot gas that we see in this bridge or filament is probably the hottest and densest part of the diffuse gas in the cosmic web, which is believed to constitute about half of the baryonic matter in the Universe” says Norbert Werner from SRON Netherlands Institute for Space Research, the leader of the team reporting the discovery.

“The discovery of the warmest of the missing baryons is important as various models, while all predicting the missing baryons in some form of warm gas, tend to disagree about the extremes.” adds Alexis Finoguenov, member of the team from the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany. “The discovery was made possible by a very fortunate geometry, where we see the filament along the line of sight, looking into it, instead of looking at it from the side.

This means that the entire emission from the filament is concentrated in a small region of the sky, making the observation of this low density gas possible for the first time” explains Jelle Kaastra, team member and senior scientist at SRON. “Prior to the sensitivity level achieved with deep XMM-Newton observations, we could only see the clusters, the dense knots of the web. Now we are starting to see the connecting wires of the immense cosmic 'spider' web” adds Aurora Simionescu, team member from MPE. “We saw the filament years ago as a bridge between the clusters in the distribution of the galaxies, and the gravitational weak lensing data also indicated the presence of a massive structure.

The discovery of the hot gas associated with this structure will help us to better understand the evolution of the cosmic web” says Jörg Dietrich, team member from the European Southern Observatory, who investigated this pair of clusters of galaxies for many years. “This is only the beginning. To understand the distribution of the matter within the cosmic web, we have to see more systems like this one. And ultimately launch a dedicated space observatory to observe the cosmic web with a much higher sensitivity than possible with the current satellites. Our result allows to set up reliable requirements for those new missions.” concludes Norbert Werner.

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