Universe’s Missing Baryons Found in Intergalactic Medium

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Ordinary (baryonic) matter make up all physical objects in existence, from stars to the cores of black holes. But until now, scientists had only been able to locate about two-thirds of the matter that theorists predict was created by the Big Bang. In new research, a multinational group of astronomers and astrophysicists pinned down the missing third, finding it in hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium (WHIM).

A simulation of the cosmic web, diffuse tendrils of gas that connect galaxies across the Universe. Image credit: Illustris Collaboration.

A simulation of the cosmic web, diffuse tendrils of gas that connect galaxies across the Universe. Image credit: Illustris Collaboration.

“The lost matter exists as filaments of oxygen gas at temperatures of around one million degrees Celsius,” said co-author Professor Michael Shull, a researcher in the Department of Astrophysical Science at the University of Colorado, Boulder.

“This is one of the key pillars of testing the Big Bang theory: figuring out the baryon census of hydrogen and helium and everything else in the periodic table.”

Astrophysicists have a good idea of where to find most of the ordinary matter in the Universe, which is not the same as dark matter: about 10% sits in galaxies, and close to 60% is in the diffuse clouds of gas that lie between galaxies.

In 2012, Professor Shull and co-authors predicted that the missing 30% of baryons were likely in WHIM.

In the new project, they searched for missing atoms in that region between galaxies.

The team pointed a series of space-based telescopes at the quasar 1ES 1553, a massive galaxy with a supermassive black hole at its center that is actively devouring matter and shining brightly from X-rays to radio waves.

First, the researchers used the Cosmic Origins Spectrograph on the NASA/ESA Hubble Space Telescope to get an idea of where they might find the missing baryons.

Next, they homed in on those baryons using ESA’s XMM-Newton satellite.

Artist’s impression of the warm-hot intergalactic medium, a mixture of gas with temperatures ranging from hundreds of thousands of degrees (warm) to millions of degrees (hot) that permeated the Universe in a filamentary cosmic web. After two decades of observations, scientists have detected the hot component of this intergalactic material, closing the gap in the overall budget of normal matter in the cosmos. The discovery was made using observations of 1ES 1553 (shown at the upper left). The researchers observed this quasar for a total of 18 days, split between 2015 and 2017, in the longest X-ray observation ever performed of such a source. Having combed through the data, the team found the signature of oxygen in the hot intergalactic gas between the observatory and the distant quasar, at two different locations along the line of sight (shown in the spectrum in the lower left). The two concentrations of intergalactic gas correspond to redshift z=0.43 (indicated with green arrows) and z=0.35 (indicated with a magenta arrow); the features in the spectrum indicated with blue arrows represent signatures of nitrogen in our Milky Way Galaxy. Image credit: ESA / ATG Medialab / XMM-Newton / F. Nicastro et al / R. Cen.

Artist’s impression of the warm-hot intergalactic medium, a mixture of gas with temperatures ranging from hundreds of thousands of degrees (warm) to millions of degrees (hot) that permeated the Universe in a filamentary cosmic web. After two decades of observations, scientists have detected the hot component of this intergalactic material, closing the gap in the overall budget of normal matter in the cosmos. The discovery was made using observations of 1ES 1553 (shown at the upper left). The researchers observed this quasar for a total of 18 days, split between 2015 and 2017, in the longest X-ray observation ever performed of such a source. Having combed through the data, the team found the signature of oxygen in the hot intergalactic gas between the observatory and the distant quasar, at two different locations along the line of sight (shown in the spectrum in the lower left). The two concentrations of intergalactic gas correspond to redshift z=0.43 (indicated with green arrows) and z=0.35 (indicated with a magenta arrow); the features in the spectrum indicated with blue arrows represent signatures of nitrogen in our Milky Way Galaxy. Image credit: ESA / ATG Medialab / XMM-Newton / F. Nicastro et al / R. Cen.

The scientists observed 1ES 1553, whose light takes more than 4 billion years to reach us, for a total of 18 days, split between 2015 and 2017, in the longest X-ray observation ever performed of such a source.

They found the signatures of a type of highly-ionized oxygen gas lying between the quasar and our Solar System — and at a high enough density to, when extrapolated to the entire Universe, account for the last 30% of ordinary matter.

“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,” said lead author Dr. Fabrizio Nicastro, a researcher at the Istituto Nazionale di Astrofisica in Rome, Italy, and the Harvard-Smithsonian Center for Astrophysics.

“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.”

“We found the missing baryons,” Professor Shull said. “We suspect that galaxies and quasars blew that gas out into deep space over billions of years.”

“We will need to confirm our findings by pointing satellites at more bright quasars.”

A paper on this research was published in the June 20, 2018 edition of the journal Nature.

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F. Nicastro et al. Observations of the missing baryons in the warm-hot intergalactic medium. Nature 558: 406-409; doi: 10.1038/s41586-018-0204-1