New observations support the idea that hot, diffuse threads of gas called cosmic filaments connect clusters of galaxies across the cosmos. That is the conclusion of Konstantinos Migkas at Leiden University and colleagues who say that their study strengthens the idea that much of the normal matter in the universe resides in these structures.
About 5% of the universe’s mass–energy content appears to be baryonic matter – the familiar nuclei and particles that make up atoms and molecules. The rest is believed to be dark energy and dark matter, which are both hypothetical entities. Although they know what baryonic matter is, astronomers have a poor understanding of where much of it is distributed in the universe.
Combining the Standard Model of cosmology with the rigid constraints enforced by observations of cosmic microwave background radiation tells us that structures including stars, black holes, and gas clouds account for around 60% of baryonic matter in the universe. This leaves 40% of baryonic matter unaccounted for.
Previously, cosmologists have argued that this discrepancy could point to a fundamental error in the Standard Model. Recently, however, a growing body of evidence suggests that this matter could be found in vast yet elusive structures, hidden deep within intergalactic space.
On a WHIM
“Large-scale structure simulations of the universe tell us this material should reside within long strings of gas called ‘cosmic filaments’, which connect clusters of galaxies,” Migkas explains. “These missing baryons should be found in the so-called ‘warm-hot intergalactic medium’ (WHIM).”
Despite being extremely sparse, models also predict that the WHIM should be extremely hot – primarily heated by shock waves produced as matter collapses into the large-scale cosmic web, as well as by phenomena including active galactic nuclei and mergers between galaxy clusters. As a result, these cosmic filaments should be emitting a faint yet detectable X-ray signal.
On top of this, the Standard Model places tight theoretical constraints on several physical properties of the WHIM – including its density, temperature, and composition. If X-rays are indeed being emitted by cosmic filaments, these properties should be encoded in their energies, intensities, and frequency spectra – providing astronomers with a clear target in their search for the elusive structures.
These X-ray signals have so far evaded detection because they are extremely faint compared to powerful X-ray signals such as those coming from supermassive black holes
To overcome this, researchers combined data from two of the world’s most advanced X-ray observatories. One is the Suzaku satellite, which was jointly operated by JAXA and NASA and was very good at detecting very faint signals. The other is the ESA’s XMM-Newton, which is very good at observing powerful X-ray signals.
Eliminating black holes
“Combining the two instruments, we carefully and appropriately eliminated the contaminating signal of the black holes throughout our filament,” Migkas explains. “This enabled us to isolate the signal of WHIM and measure its density and temperature for the very first time with such accuracy.”
For an observational target, Migkas’ team searched for cosmic filaments in the Shapley supercluster. This vast structure around 650 million light-years from the Milky Way contains one of the highest concentrations of galaxies in the known universe.
With the combined abilities of Suzaku and XMM-Newton, the researchers detected an X-ray signal indicating the presence of a filament – consistent with predictions of the Standard Model. As they expected, this intergalactic material was extremely hot and sparse: boasting temperatures close to 10 million Kelvin, while containing just around 10 electrons per cubic metre.
“We also found that on average, the filament is around 40 times denser than the average density of the universe – which is pretty empty in general – and around 1000 times less dense than the cores of the four-galaxy cluster it connects,” Migkas describes. Despite having gone undetected so far, this filament also carries a total mass around 10 times that of the Milky Way – making it a vast reservoir of previously hidden matter.
“For the very first time, our work confirms the validity of the predictions of the Standard Model of cosmology regarding the properties of a big part of the missing baryons,” Migkas concludes.
The research is described in Astronomy and Astrophysics.
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