Galaxy Clusters' Radio Relics Provide Insights into Cosmic Mysteries

Galaxy clusters, the largest gravitationally bound structures in the universe, contain hundreds or even thousands of galaxies. According to Phys.org, when two of these massive clusters collide, they generate powerful shock waves, releasing an immense amount of energy similar to that of the Big Bang.

These shock waves energize electrons, causing them to emit radio waves as they spiral around magnetic field lines, creating what are known as 'radio relics.' These radio relics can stretch over six million light years, equivalent to about six thousand seventy Milky Way galaxies lined up end to end.

However, puzzling issues have arisen regarding the properties of these radio relics. Observations show that the magnetic field strength within these relics is unexpectedly high. Additionally, discrepancies have been noted in the strength of shock waves when measured with radio versus X-ray wavelengths.

Most concerning is that X-ray data suggest many shock waves are too weak to adequately energize electrons, challenging the very existence of radio relics. A team of researchers at AIP has made significant strides in addressing these mysteries through a multi-scale approach.

Dr. Joseph Whittingham, the leading author, explains that the team began by tracking the formation of shock waves in cosmological simulations. They then replicated their findings with higher resolution, ultimately mapping the evolution of energized electrons and the resulting radio emissions.

This process connected physics on the scale of galaxy clusters with phenomena occurring on the scale of electron orbits. The researchers discovered that when shock waves reach the edges of a galaxy cluster, they collide with shocks from cold, infalling gas.

This interaction compresses surrounding material, forming a dense sheet of gas that expands outward and collides with further gas clumps. Prof. Christoph Pfrommer, a co-author of the study, noted that this mechanism generates turbulence, twisting and compressing the magnetic field to observed strengths, thereby resolving the first puzzle.

Furthermore, when shock waves pass through gas clumps, parts of the shock front become stronger, increasing radio emissions. This explains the discrepancies between X-ray and radio data, as X-ray emissions reflect a lower average shock strength.

Ultimately, the majority of radio relics are formed by the strongest parts of the shock front, making the lower average values from X-ray data non-problematic for theories of electron energization at shocks.

Whittingham expressed motivation to build on this study to further explore unresolved questions surrounding radio relics. More details can be found in their study available on the arXiv preprint server under the title 'Zooming-in on cluster radio relics.'