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Newton-telescope finds missing intergalactic material

Astronomers from, among others, SRON and Leiden Observatory have discovered long-sought intergalactic gas with ESA’s space telescope XMM-Newton. This gas is one of the pieces of the puzzle to map the total amount of ‘normal’ matter in the universe. The research will be published in Nature on 21 June.

Seeing is believing

The universe consist of 25 and 70 percent of mysterious dark matter and dark energy respectively. Only 5 percent consists of ‘normal’ matter: things that we can see, such as stars, planets and people. But, even that 5 percent is difficult to detect. Tthe total amount of normal matter is called ‘baryons’ by astronomers. They estimate these baryons based on the cosmic background radiation, the oldest light in the universe that originates from the period of only 380,000 years after the Big Bang.


Through observation on distant galaxies, astronomers can follow the evolution of this matter during the first few billion years of the universe. After that, almost half seems to be traceless. First author Fabrizio Nicastro (Istituto Nazionale di Astrofisica) calls these missing baryons one of the greatest mysteries in modern astrophysics. ‘We know that matter must be there, but we have no control over it. Where did it go?’

Incomplete sum

If you add up all the stars and galaxies in the universe, including the interstellar gas, you come to about 10 percent of all normal matter. If you add the hot diffuse gas in the halos around galaxies, plus the even hotter gas in clusters of galaxies, you’ll stick to less than 20 percent. That is no surprise in itself, because stars, galaxies and clusters are formed in the most rare places of the universe: the densest nodes of the cosmic web, the threadlike large-scale structure.


Astronomers therefore think that the ‘missing’ baryons are hiding in the filaments of the cosmic web, where matter is more rare and therefore more difficult to perceive. Up to now, only 60 percent of this intergalactic matter has been localised.

Fingerprint of oxygen

In 2015 and 2017, Nicastro and his colleagues spent a total of 18 days with ESA’s x-ray telescope looking at a quasar at 4 billion light-years away. Quasars are large galaxies with a super heavy black hole in the centre and shine brightly on X-ray and radio wavelengths. In the data, the researchers found the fingerprint of oxygen in the hot intergalactic gas at two locations along the line of sight between us and the distant quasar. Second author Jelle Kaastra (Space research institute SRON): ‘There are enormous stocks of matter there, including oxygen, in the quantities we expected. It seems that we can finally solve the mystery of the missing baryons.’

New search

The result is the beginning of a new search. The astronomers are now going to investigate new quasars with both XMM-Newton and NASA’s Chandra observatory. Co-author Nastasha Wijers (PhD student at Leiden Observatory): ‘We now want to look at other sources in the universe to confirm that our results are universal. We also want to further investigate the long-sought matter.’ Co-author Joop Schaye (Leiden Observatory) adds: ‘This is an exciting first step. We are also looking forward to the launch of Athene in 2031, which allows us to study the warm intergalactic medium in great detail through the much greater sensitivity.

Artistic impression of the warm-hot intergalactic medium, a mix of gas with temperatures ranging from hundreds of thousands to millions of degrees, in the filaments of the cosmic web. Credit: Illustrations and composition: ESA / ATG medialab; data: ESA / XMM-Newton / F. Nicastro et al. 2018; cosmological simulation: R. Cen
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