Astronomers using the European Space Agency’s XMM-Newton space observatory have found evidence of hot, diffuse gas permeating the cosmos, closing a gap in the overall total of ‘normal’ matter in the Universe.
While dark matter and dark energy make up about 25% and 70% of our cosmos, the ordinary matter that makes up everything we see amounts to only 5%, and astronomers have been using the XMM-Newton space observatory to try and track down this matter.
The amount of ordinary matter, known as baryons, can be estimated from observations of the Cosmic Microwave Background, which, according to ESA, is the most ancient light in the history of the Universe, dating back to only 380,000 years after the Big Bang.
What methods allow astronomers to follow the evolution of matter?
Observing very distant galaxies allows astronomers to follow the evolution of this matter throughout the Universe’s first couple of billion years. After that, however, more than half of it seems to have gone missing.
Fabrizio Nicastro, lead author of the paper, said: “The missing baryons represent one of the biggest mysteries in modern astrophysics.
“We know this matter must be out there, we see it in the early Universe, but then we can no longer get hold of it. Where did it go?”
Counting the population of stars in galaxies across the Universe, plus the interstellar gas that permeates galaxies, only gets as far as a mere 10% of all ordinary matter. Adding up the hot, diffuse gas in the haloes that encompass galaxies and the even hotter gas that fills galaxy clusters raises the total to less than 20%.
Astronomers suspect that the ‘missing’ baryons must be in the ubiquitous filaments of this cosmic web, where matter is, however, less dense and therefore more challenging to observe.
Through different techniques, astronomers were able to locate a large amount of this material, bringing the total amount to 60%.
Using XMM-Newton space observatory
Nicastro and other astronomers have been looking for the remaining baryons for almost two decades, ever since the X-ray observatories such as ESA’s XMM-Newton space observatory and NASA’s Chandra became available to the scientific community.
For this project, Fabrizio and his collaborators used XMM-Newton to look at a quasar – a massive galaxy with a supermassive black hole at its centre that is actively consuming matter and shining brightly from X-rays to radio waves.
They observed this quasar, whose light takes more than four billion years to reach us, for a total of 18 days, in the longest X-ray observation ever performed of such a source.
Nicastro said: “After combing through the data, we succeeded at finding the signature of oxygen in the hot intergalactic gas between us and the distant quasar, at two different locations along the line of sight.
“This is happening because there are huge reservoirs of material – including oxygen – lying there, and just in the amount we were expecting, so we finally can close the gap in the baryon budget of the Universe.”
Fabrizio and his colleagues are planning to study more quasars with XMM-Newton and Chandra in the coming years. However, to fully explore the distribution and properties of this intergalactic medium, more sensitive instruments will be needed, like ESA’s Athena, the Advanced Telescope for High-Energy Astrophysics, scheduled for launch in 2028.