Shining new light on large cosmic structures
Galaxy clusters are some of the largest structures in the universe and are held together by gravity. They contain hundreds to thousands of galaxies, vast amounts of hot gas and dark matter. “Over time, clusters grow by merging with other clusters and by pulling in matter from their surroundings,” explains ClusterWeb project coordinator Reinout van Weeren(opens in new window) from Leiden University(opens in new window) in the Netherlands. “This matter flows along long structures called cosmic web filaments.”
Gas, galaxies and dark matter
These filaments, made of gas, galaxies and dark matter, stretch across the universe and connect different clusters. Radio telescopes have detected faint radio emission from galaxy clusters, produced by highly energetic particles moving close to the speed of light. “A key open question is how these particles get accelerated to such high energies,” says van Weeren. “The aim of this project was to study the origin of this radio emission, and to determine the physical processes that accelerate these particles.” ClusterWeb, supported by the European Research Council(opens in new window), focused on observing galaxy clusters and cosmic web filaments at very low radio frequencies, where this radio emission is typically brighter and thus easier to detect. Van Weeren and his colleagues used observations from the LOFAR(opens in new window) radio telescope, a European array designed to observe the sky at low radio frequencies. “Our study focused on several hundred galaxy clusters,” he explains. “Our aim was to create detailed images and understand how radio emission relates to properties like cluster mass and merger activity.”
High-resolution images of clusters and filaments
While LOFAR provided the project team with much higher sensitivity and resolution than previous telescopes, working with the data was still challenging. “First, the data volume is enormous, requiring advanced computing to process,” notes van Weeren. “Second, at these low frequencies, the Earth’s ionosphere distorts incoming radio waves, which blurs the images. A major part of the project therefore involved developing new techniques to correct for these distortions, allowing us to produce clear, high-resolution images of the clusters and filaments.” The project found strong evidence that shocks and turbulence created during cluster mergers are responsible for accelerating particles to very high energies. The team was also able to measure the properties of magnetic fields within clusters and filaments, which play a key role in shaping the radio emission. “In addition, we observed that jets from supermassive black holes located in cluster member galaxies can inject energetic particles into the cluster’s hot gas and significantly influence its behaviour,” adds van Weeren.
How clusters form and magnetic fields evolve
These results will help scientists to better understand the physical processes – especially particle acceleration – that occur throughout the universe. They also shed light on how the largest cosmic structures, such as clusters and filaments, evolve over time. “On a technical level, the methods we developed to correct distorted radio data can now be applied more broadly, improving image quality for other radio observations and enabling new discoveries,” says van Weeren. The next step is to study more distant clusters and their connections to filaments, to better understand how clusters form and how magnetic fields originate and evolve. This will require deeper observations and combining data from multiple radio frequencies to build a more complete picture of these complex environments.