The astronomers analysed a filament connecting four clusters of galaxies: A3532 and A3530 on one side and A3528-N and A3528-S on the other side. The clusters are part of the Shapley Supercluster, a large collection of more than 8000 galaxies located about 650 million light-years away from Earth in the constellation of Centaurus.
The analysis reveals that the filament consists mainly of free electrons and protons with a temperature of more than 10 million degrees Celsius. The density is about 10 particles per cubic meter. That is 30 to 40 times the average density of the universe. In total, the filament accounts for 10 times the mass of the Milky Way.
Combining multiple telescope observations
Hot gas in filaments was observed before, but it's the first time that its properties were determined with an accurate spectroscopic analysis, without significant contamination from black holes and galaxies. To pull off the contamination, the research team used an elegant combination of methods. First, with data from optical telescopes, they determined the orientation of the filament in the sky. Then, with the Japanese Suzaku X-ray space telescope, they obtained a spectrum of the whole region. After that, they used data from the European XMM-Newton telescope to model the contaminating black holes and cancel them out. Finally, they could isolate a spectrum of the filament, which they used to determine its density and temperature.
The problem of the “missing” normal matter
Cosmological observations support the standard model that describes the origins and evolution of the Universe, which presupposes that various forms of matter and energy must exist in certain quantities. Two of these—dark matter and dark energy—are yet to be discovered by particle physicists but are crucial to the continued viability of the model. The former is believed to explain the relatively fast rotational velocities of galaxies and the latter the fact that the Universe is expanding at an ever-faster rate. Baryonic matter, which simply means “ordinary” matter such as electrons and protons, constitutes a mere 5 percent of the total. “It’s not only that the physicists of today don’t understand 95 percent of what makes up our Universe, we’re not even in a position yet to say where half of the remaining 5 percent actually is,” explains Dr. Florian Pacaud from the Argelander Institute for Astronomy at the University of Bonn.
Large-scale cosmological simulations have shown that what is missing is the warm, ionized matter known as WHIM. This is found in huge cosmic filaments, elongated strands of gas that link galaxy clusters to one another. Previous studies, several of them spearheaded by the University of Bonn, had only succeeded in proving the existence of a few isolated examples of the densest filaments, because these are brighter and easier to spot. However, these observations had been at odds with the simulations.
"We did not expect that our new method isolated the signal of the missing baryons so effectively," says research lead Konstantinos Migkas, who began his work at the University of Bonn and is now conducting research at Leiden Observatory and the Space Research Organisation Netherlands (SRON) on an Oort Postdoctoral Fellowship. The results of the analysis show that the filament is four times less dense than those discovered previously, making it typical of those predicted by numerical simulations. "We now show that the properties of cosmic filaments agree with the simulations after all. So, it seems the cosmological simulations were right all along. That is a great reward."
According to the team, the research could lead the way for future studies that search similar locations in the universe for filaments and their properties. It allows the researchers to better understand how the largest structures in the universe connect to each other.
