For the second time ever, astronomers observed a giant bridge of radio emissions between two galaxy clusters that are readying themselves for collision. This bridge is among the largest structures observed in the universe, and it brings scientists one step closer to a fundamental goal: observing direct emissions from the cosmic web, the vast network of dark matter and hydrogen gas believed to form the backbone of the universe.
The scientists discovered the bridge between galaxy clusters Abell 1758N and Abell 1758S using the Low-Frequency Array (LOFAR), a radio telescope network based in the Netherlands. LOFAR was designed in part to look for these radio emissions, which have longer wavelengths and lower frequencies than light in the visible spectrum. This discovery was published on August 22 as an accepted manuscript in the journal Monthly Notices of the Royal Astronomical Society: Letters.
According to Andrea Botteon, a postdoctoral researcher at the Leiden Observatory in the Netherlands and the first author of the manuscript, discovering radio bridges with LOFAR is "a first step to what astrophysicists would like to detect for the very first time, which is the cosmic web," he said.
“You can imagine bridges as small filaments, and the cosmic web is formed by a net of filaments much larger than bridges that connect all the clusters in the universe.”
Intergalactic cartographers have been able to probe the cosmic web before, discovering other filaments in the forms of walls made of galaxies. The South Pole Wall, for example, stretches 1.4 billion light years, and its discovery was announced in July. The largest known structure—the Hercules-Corona Borealis Great Wall—is 10 billion light years wide.
A particle moving at the speed of light would take 6.5 million years to traverse the radio bridge discovered between the A1758 clusters, which are each a few million light years wide.
This latest observation comes at the heels of the first discovery of a radio bridge by an overlapping group of researchers, which was published last year in the journal Science. The A1758 clusters in the new paper, however, are much earlier in the process of colliding and merging than the first two galaxy clusters, Abell 0399 and Abell 0401. As a result, they are much farther apart from one another.
Botteon said that magnetic fields, combined with relativistic particles (those moving close to the speed of light), created the bridge of radio-emitting plasma.
LOFAR enabled the discoveries of these bridges by accessing a very low-frequency observational window that astronomers had not been able to probe as recently as a few years ago, Botteon said. Now that two bridges have been observed using LOFAR, Botteon and his collaborators want to know how common these are between galaxy clusters.
Last year, he observed a third pair of galaxy clusters that were not linked by a radio bridge—the two cluster pairs that did have bridges were both more massive and dynamic than this third pair. Still, it’s unclear whether the two bridges are exceptions or rules.
“I think that these two radio bridges that have been reported are in very exceptional pairs of galaxy clusters that are basically the most massive clusters we can observe with LOFAR,” he said. “What we are doing now is observing more galaxy clusters in every phase of collision and searching for diffuse radio emissions.”