An article published in the journal “Astronomy & Astrophysics” reports the discovery of a filamentary structure of hot gas connecting two galaxy clusters about 50 million light-years long. A team of researchers used data collected by the eROSITA instrument, the ASKAP radio telescope, and the DECam instrument to study a system of three galaxy clusters called Abell 3391/95 and found what is the longest filament ever discovered. Its estimated mass confirms that the so-called missing baryon matter could be in those filaments.

For years, astronomers have been trying to reconstruct what is a sort of cosmic web formed by gas filaments that connect different galaxy clusters. The gas is mostly ionized hydrogen that is very difficult to detect due to its very low emissions. Instruments like space telescopes and new ground-based instruments help a lot, but it’s still a complex task. An article published in the journal “Nature” in June 2018 reported the discovery of that type of baryon matter, but finding all the intergalactic gas filaments is a huge task.

This new research focused on Abell 3391/95, a system made up of three different galaxy clusters that are likely gravitationally binding to merge into a single cluster, or supercluster. At a distance of about 700 million light-years from Earth, this system is relatively close for a triple galaxy cluster.

Abell 3391/95 was mainly studied using observations conducted with eROSITA, one of the instruments of the Spektr-RG space telescope, launched on July 13, 2019. Other data was collected by the ASKAP (Australian Square Kilometer Array Pathfinder) radio telescope during the EMU (Evolutionary Map of the Universe) survey, and the DECam (Dark Energy Camera) instrument.

The top image (Courtesy Reiprich et al., Astronomy & Astrophysics. All rights reserved) shows an optical frequency image of the Abell 3391/95 system captured with the DECam. Superimposed are an image captured with eROSITA and the radio outlines in yellow from observations with the ASKAP radio telescope.

The bottom image (Courtesy Reiprich et al., Astronomy & Astrophysics. All rights reserved) shows a comparison between an image of Abell 3391/95 captured by eROSITA (on the right) and a simulation (on the left).

Thanks in particular to the very precise observations of the eROSITA instrument, it was possible to discover a gas emission between the two main clusters that make up Abell 3391/95. This gas bridge joins a series of galaxies for a length of about 50 million light-years. According to the researchers, this filament could be even longer and only part of it is visible in the available images.

Thomas Reiprich of the University of Bonn, the first author of this study, explained that he and his colleagues compared their observations with the results of a computer simulation of the evolution of the universe called Magneticum. The images obtained by eROSITA are very similar to those generated by the computer and this suggests that the so-called standard model is correct.

Cosmological models will continue to be discussed, especially in relation to dark matter and dark energy problems. However, in this case there’s direct evidence that the missing baryon matter is indeed hidden in the intergalactic filaments which adds to the evidence already uncovered in recent years. It’s important to be able to identify the baryon matter to understand if the calculations on its amount are correct and to have more reliable data in the tests of models concerning the parts of the universe that we can’t yet see.