Enormous Radio Ridge Connecting Two Galaxy Clusters Through Cosmic Web Discovered

A radio ridge that connects two huge, merging galaxy clusters has been discovered by scientists. This is the first time a magnetic field has been observed in the filaments in intergalactic space that connect clusters, researchers say.

Galaxy clusters are enormous structures potentially containing thousands of galaxies. They are some of the biggest structures in the universe that are bound by galaxy. They grow by merging with smaller structures in a process that produces a huge amount of turbulence in the gas surrounding both galaxies. In the intergalactic space between two merging galaxies are filaments.

The Abell catalogue is a collection of around 4,000 galaxy clusters and two of these, Abell 0399 and Abell 0401 are currently in the process of merging.

"The matter distribution of the Universe is not uniform, but forms the so-called 'cosmic web,'—a structure of filaments and galaxy clusters surrounding large voids," Federica Govoni, from Italy's National Institute for Astrophysics (INAF), told Newsweek.

Govoni is lead author of a study published in Science that provides the first observation of a magnetic field within the filaments connecting two galaxy clusters—what the researchers call a "radio ridge."

"Galaxy clusters ... form at the intersections of the cosmic web filaments and grow by accreting substructures in a merging process," Govoni said. "Observations made so far with radio telescopes show a 'halo' of radio emission in the central areas of some clusters that confirms the existence of a magnetic field and relativistic electrons. Up to now, however, a magnetic field has never been observed in the filaments connecting clusters."

Researchers used the Low-Frequency Array telescope network to observe the two galaxy clusters. Previously the team had found radio halos at the middle of both Abell 0399 and Abell 0401, and recent data from the Planck satellite suggested that the two systems are connected by a tenuous filament.

"The presence of this filament stimulated our curiosity and led us to investigate whether the magnetic field could extend beyond the central regions of clusters, permeating the filament of matter that connects them," Govoni said.

Abell galaxy clusters
The two colliding galaxy clusters in the Abell catalogue connected by a "radio ridge." : DSS and Pan-STARRS1 optical), XMM-Newton X-rays, PLANCK satellite (yparameter, F. Govoni, M. Murgia, INAF

In the space between Abell 0399 and Abell 0401, they found a strip of radio emissions extending for millions of light years. The presence of the emissions suggest that within the filament, there is a magnetic field and relativistic electrons—the structure is larger than any other models can account for.

Electrons could only travel at relativistic speeds required for distances far shorter than the length of the filament. This, they say, means some unknown mechanism that re-accelerates the electrons is happening. "Our simulations show that a volume-filling distribution of weak shocks may re-accelerate a pre-existing population of relativistic particles, producing emission at radio wavelengths that 'illuminates' the magnetic ridge." Govoni said.

The team says their findings suggest current theories about particle acceleration in intergalactic space need to be revisited.

Leah Morabito, from the Department of Physics at the University of Oxford, U.K., who was not involved in the study, said the discovery was exciting as it provides an insight into how the cosmic web really works. She told Newsweek we know filaments between galaxies exist—but how gas in them is moved around is poorly understood. "This study is the first time astronomers have seen direct emission along the length of a filament—in this case, it goes from one galaxy cluster to another.

"The implications are that we now have direct evidence that magnetic fields in the cosmic web exist, and that they run from one [galaxy cluster] to another. The radio emission is a bit brighter than expected from theory, which is great for trying to detect more filaments around massive galaxy clusters in the future."

Julie Hlavacek-Larrondo, from the Université de Montréal, Canada, who was also not involved in the research, said: "The detection of the radio bridge is a direct testimony to the dynamic nature of galaxy clusters and the dynamic nature of the Universe on the largest scales. Its discovery implies that part of the energy related to galaxy cluster mergers goes into the acceleration of non-thermal particles on much faster time scales than we initially thought.

"This discovery implies that a large population of preexisting relativistic electrons is likely embedded within the cosmic web on scales much larger than initially thought and that the Universe contains a vast network of magnetic fields spread out across the cosmos," she told Newsweek.