A European team of astronomers led by the Netherlands have uncovered a large filament of hot gas connecting four galaxy clusters. The filament contains so-called warm-hot intergalactic medium. The hot gas spans over a distance of 23 million light years, is more than ten million degrees hot, and may be part of what astronomers call the ‘missing normal matter’. The team used two X-ray space telescopes: the Japanese Suzaku and the European XMM-Newton. Publication on June 19th in Astronomy and Astrophysics.
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 8,000 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 1.2 x 10^13 solar masses of hot gas, or 10 times the mass of the Milky Way.
Elegant combination of methods
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 missing normal matter problem
Observations suggest that the Universe contains much more substance than what we can currently see. For instance, galaxies are rotating faster than they should based on the total mass of their stars. Astronomers therefore suspect that they also contain so-called dark matter. And they cite dark energy to explain the increasing expansion rate of the Universe. But even while accounting for dark matter and dark energy, a large chunk of the normal matter is also missing.
The standard cosmological model predicts that about 30 to 40 percent of normal matter is difficult or impossible to detect with telescopes. Large-scale cosmological simulations show that this matter may be hiding in huge cosmic gas filaments connecting clusters of galaxies.
Early observations did not agree with simulations
Earlier, astronomers tried to ‘stack’ the observations of collections of filaments to overcome contaminations by X-ray bright black holes. However, those stacked observations did not align with the predictions of the standard cosmological model.
‘We did not expect that our new method isolated the signal of the warm-hot intergalactic medium so effectively,’ says research leader Konstantinos Migkas, (Leiden University / SRON). ‘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 Migkas and his 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.
Publication
Detection of pure warm-hot intergalactic medium emission from a 7.2 Mpc long filament in the Shapley supercluster using X-ray spectroscopy. Door: K. Migkas, F. Pacaud, T. Tuominen & N. Aghanim, Astronomy & Astrophysics