Arc-like shock waves travel outwards during a galaxy cluster merger, converting an enormous amount of energy into heat. The left image shows the gas density, the right image the dissipated energy with clearly visible shock fronts. Credit: AIP/J. Whittingham
Galaxy clusters are the largest gravitationally bound structures in the universe, with each containing hundreds or even thousands of galaxies. When two of these giants collide, they send powerful shock waves through one another, releasing energy on a scale not seen since the Big Bang.
The shock waves sweep over electrons, energizing them and causing them to emit radio waves as they spiral around magnetic field lines. The result is a “radio relic”: a vast arc of radio emission, which can stretch for more than 6 million light years across, or approximately 60–70 Milky Way galaxies lined up end to end.
In recent years, however, the puzzles surrounding radio relics have been mounting. First, when observers measure the magnetic field strength in a relic, they find the value to be unexplainably high. Just as curiously, the strength of the underlying shock wave appears to change depending on whether it is observed using radio or X-ray wavelengths.
Finally, and perhaps most concerning of all, X-ray data imply that many of the shock waves powering radio relics are actually too weak to properly energize electrons. This puts these results in contradiction with the very existence of radio relics.
Researchers at AIP, however, have finally been able to crack these problems using an innovative multi-scale approach. “Key to our success was tackling the issue using a range of scales,” explains Dr. Joseph Whittingham, postdoctoral researcher at AIP and leading author of the study posted to the arXiv preprint server.
“We first traced how shock waves form in cosmological simulations, before replicating what we saw in a more idealized setup, with significantly better resolution.” In their final step, the authors mapped the evolution of the energized electrons and the resultant radio emission from first principles. Hence, their modeling connects physics on the size of galaxy clusters with processes that happen on scales as small as an electron’s orbit—scales a trillion-fold apart.
Arc-like shock waves travel outwards during a galaxy cluster merger, converting an enormous amount of energy into heat (top left). When these shocks collide with others at the cluster’s edge, they compress the surrounding material, forming a dense sheet of gas (shown here in cross-section in the zoomed-in panel, top right). By replicating this feature in even higher resolution simulations (bottom right), evolving the magnetic fields (bottom center), and modeling the resulting radio emission (bottom left), researchers have been able to gain key insights into the origin of radio relics. Credit: AIP/J. Whittingham
The researchers found that when shock waves reach the edge of a galaxy cluster, they collide with other shocks produced by cold, infalling gas. This process compresses the surrounding material, forming a dense sheet of gas that moves outwards, where it runs into further gas clumps. “The whole mechanism generates turbulence, twisting and compressing the magnetic field up to the observed strengths, thereby solving the first puzzle,” reports co-author Prof. Christoph Pfrommer.
Additionally, when the shock wave passes through gas clumps, a portion of the shock-front becomes stronger, boosting the radio emission. By contrast, the X-ray emission continues to reflect the average, overall weaker shock strength, thereby explaining why the data from the two radiation types typically disagree with one another, thus solving the second riddle.
Finally, because the overwhelming majority of a radio relic is formed by just the strongest parts of the shock-front, the lower average values inferred from X-ray data are not an issue for the theory of electron energization at shocks after all. “This success motivates us to build on our study to answer the remaining unresolved mysteries surrounding radio relics,” says Whittingham.
More information:
Joseph Whittingham et al, Zooming-in on cluster radio relics—I. How density fluctuations explain the Mach number discrepancy, microgauss magnetic fields, and spectral index variations, arXiv (2024). DOI: 10.48550/arxiv.2411.11947
Journal information:
arXiv
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Leibniz Institute for Astrophysics Potsdam
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Shocked galaxy clusters: Unraveling the mysteries of radio relics (2025, November 14)
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