Study Reveals The Mystery Behind Giant Radio Relics In The Universe

At the largest scales of the universe, huge clusters of galaxies crash into one another in extremely slow but powerful collisions. These events leave behind giant, faint arcs, long ribbons of radio waves stretching across millions of light-years. These ghostly structures, called radio relics, are created when massive shock waves speed up electrons to nearly the speed of light, reported Space.com.

Astronomers have found many such relics, but their strange behaviour has been very hard to understand.

Scientists have discovered that the magnetic fields within radio relics are much stronger than expected. Measurements using radio and X-ray light reveal different strengths of the shock waves. Further complicating the puzzlement is that X-ray data suggests that these shocks are not strong enough to accelerate electrons, raising the question of how radio relics form.

A recent study by scientists at the Leibnitz Institute for Astrophysics Potsdam (AIP) in Germany offers a solution to these long-standing questions. The team used high-resolution simulations to understand how radio relics are form and how they change over time. Their model recreated the mysterious features seen in actual observations, further shedding light on the nature of these massive arc-like structures.

Joseph Whittingham, the lead author of the study and a postdoctoral researcher at AIP, explains that studying the problem at different scales was key to their success. In their paper, he and his team write that they used a large set of cosmological simulations that show how galaxy clusters evolve and collide over billions of years.

In this data, they focused on an extremely powerful collision between two clusters, one about 2.5 times more massive than the other. This collision generated massive, arched shock waves extending nearly 7 million light-years.

Based on these initial findings, the team further developed extremely high-resolution simulations called "shock-tubes." Through these, they were able to understand the precise physics of a single shock wave traveling through the uneven and turbulent outer regions of a galaxy cluster. From this, they developed models that show how electrons are accelerated at the shock front and how the radio waves emitted would appear in a telescope.

This multi-scale approach enables them to understand the microscopic physics that current large cosmological simulations don't fully capture.

The simulations revealed that as the shock wave moves outward from the cluster, it collides with shocks created by cold gas coming from the cosmic web. This collision transforms the plasma into a dense layer, which in turn collides with smaller clumps of gas. This creates a turbulent environment, strengthening the magnetic fields to the point where they reach values comparable to those recorded in actual observations.

Study co-author and AIP scientist Christoph Frommer explains that this entire process creates turbulence, which twists and compresses the magnetic fields, leading to the strengths astronomers have observed. This solves a previously major mystery.

The study also explains why radio and X-ray measurements differ. When the shock wave collides with dense gas clumps, some parts of the shock become more powerful and accelerate electrons more efficiently. These bright and dense parts are the primary contributors to radio signals.

However, X-ray instruments measure an average of the entire width of the shock, including the weaker parts. This explains the discrepancy between the two.

Simulations also show that the strongest and most confined parts of the shock wave produce almost all of the radio emission. Therefore, the low average shock strength measured by X-rays does not conflict with the mechanism of radio emission.

These multi-scale simulations together reproduced the same combination of magnetic, radio, and X-ray properties seen in real radio emission. This has resolved many long-standing questions about these mysterious structures.

Whittingham says this achievement inspires him to continue his research and understand the remaining mysteries of radio waves.