Galaxy clusters are the largest structures held together by gravity in the universe.
Each cluster contains hundreds or even thousands of galaxies, along with hot gas and dark matter.
When two of these enormous structures crash into each other, the impact sends gigantic shock waves rolling through the clusters.
These shock waves release an amount of energy that has not been seen since the time of the Big Bang.
As the shock waves move through space, they sweep up electrons and give them extra energy. These energized electrons spiral around magnetic fields and produce radio waves.
This creates what astronomers call a “radio relic”—a huge, glowing arc of radio emission. Some radio relics stretch more than 6 million light years, which is like lining up 60 or 70 Milky Way galaxies in a row.
Although radio relics have been known for years, they have also presented scientists with several stubborn mysteries. One major puzzle is that the magnetic fields inside radio relics appear stronger than current theories can explain.
Another issue is that astronomers get different answers when they measure the strength of the shock waves using radio signals compared to using X-ray observations.
Most troubling of all, X-ray data suggest that many of the shock waves are too weak to energize electrons to the levels needed to create radio relics. These contradictions made it difficult to understand how radio relics actually form.
A research team at the Leibniz Institute for Astrophysics Potsdam (AIP) has now solved these long-standing problems using a new, multi-scale approach. Dr. Joseph Whittingham, the lead author of the study, explains that they examined the problem from the largest scales down to the smallest.
They first studied how shock waves form in full cosmological simulations, which model the entire evolution of the universe.
They then zoomed in to study these shocks in simplified, high-resolution setups. Finally, they followed the behavior of the energized electrons and the resulting radio emission from the ground up.
Their method linked the physics of massive galaxy clusters with the behavior of electrons orbiting magnetic fields—a scale difference of a trillion times.
The researchers discovered something crucial. When shock waves reach the outer edges of a galaxy cluster, they collide with other shocks created by cold gas falling into the cluster. This collision squeezes the gas and forms a dense layer that moves outward and crashes into more clumps of gas.
This entire process creates turbulence that twists, stretches, and compresses magnetic fields until they reach the strong levels observed in radio relics. This solves the first major mystery.
They also found that when shock waves move through clumpy gas, some parts of the shock become much stronger. These strong sections produce the bright radio emission we see. Meanwhile, X-ray observations mostly measure the average shock strength, which remains much weaker. This explains why radio and X-ray measurements do not match, solving the second puzzle.
Finally, the team showed that radio relics are mostly produced by the strongest sections of the shock wave. This means the weaker average values seen in X-ray data are not a problem after all. With these discoveries, the long-standing contradictions surrounding radio relics have been resolved.
Whittingham says the team now hopes to expand this work and explore the remaining mysteries of these spectacular cosmic structures.
Source: KSR.
