Scientists observe galaxy cluster collisions for the first time in the early universe

Across the vast cosmos, each galaxy stretches some 3,000 to 300,000 light years in diameter. These massive structures tend to be separated from one another by millions of light years. But some are much closer together, grouped to form one of the largest structures in the universe — a galaxy cluster.

Galaxy clusters contain hundreds to thousands of galaxies, bound to one another by the force of gravity.

Incredibly, these huge structures can also collide, smashing into one another over the course of millions of years. Hidden in these collisions are clues to how the universe itself formed and evolved. For the first time, a team of scientists has observed nine massive collisions between galaxy clusters in the early universe. These cataclysmic events, which took place 7 billion years ago, are the most distant to have ever been observed.

The observations are detailed in a study published Monday in the journal Nature Astronomy.

Gabriella Di Gennaro is a researcher at Leiden University in the Netherlands and lead author of the new study. She and her team looked for these collisions by observing a sample of galaxy clusters.

"We didn’t know how many of these clusters were colliding," Di Gennaro tells Inverse. "We looked at ones located far in the universe."

Using the Dutch-European network of linked LOFAR antennas, the researchers gathered unprecedentedly detailed data from galaxy clusters located at greater distances than ever before.

As galaxy clusters merge together, the motion created by the collision accelerates the particles located within the clusters to almost the speed of light. As they come in contact with the magnetic fields of the clusters, the accelerated particles emit radio waves.

Di Gennaro compares it to throwing a stone in a lake, and the ripples that are created from the impact.

"These ripples generate turbulent motions inside the clusters," she says. "It’s all about turbulent motion, shock waves that propagate from the point of impact to the external parts of the lake."

Previous attempts to capture these radio waves were unsuccessful because our instruments were just not powerful enough to catch them from such long distances. The radio waves observed in this study were produced by collisions that took place 7 billion light years away. The universe is currently 13.8 billion years old. So these collisions took place around halfway through its cosmic lifespan.

By observing the galaxy cluster collisions, the scientists were surprised to find that the mergers produced brighter radio emissions than previously expected for a young universe.

"What we see now, and what we see 7 billion years ago, is actually the same," Di Gennaro says. "The only difference we can see is that when the universe was young, there was a higher probability of finding these collisions."

Astronomers study galaxies in order to test out the current theories of cosmology and the model that they have of the universe.

"Galaxies are the final point of evolution," Di Gennaro says.

Observing galaxies can inform scientists of processes such as star formation and understanding the laws of physics that govern the universe.

As the technology behind space telescopes continues to improve, scientists are expecting to observe a lot more of these distant collisions and be able to analyze them in more detail.

Abstract: In the present-day Universe, magnetic fields pervade galaxy clusters¹ and have strengths of a few microgauss, as measured from Faraday rotation². Evidence for cluster magnetic fields is also provided by the observation of megaparsec-scale radio emission, namely radio halos and relics³. These are commonly found in merging systems⁴ and are characterized by a steep radio spectrum S_ν (α < −1, where S_ν∝ν^α and is ν the observing frequency). It is widely believed that magneto-hydrodynamical turbulence and shock waves (re-)accelerate cosmic rays⁵ and produce radio halos and relics. The origin and amplification of magnetic fields in clusters is not well understood. It has been proposed that turbulence drives a small-scale dynamo^{6,7,8,9,10,11} that amplifies seed magnetic fields (which are primordial and/or injected by galactic outflows, such as active galactic nuclei, starbursts or winds¹²). At high redshift, radio halos are expected to be faint, owing to losses from inverse Compton scattering and the dimming effect with distance. Moreover, Faraday rotation measurements are difficult to obtain. If detected, distant radio halos provide an alternative tool to investigate magnetic field amplification. Here, we report Low Frequency Radio Array observations that reveal diffuse radio emission in massive clusters when the Universe was only half of its present age, with a sample occurrence fraction of about 50%. The high radio luminosities indicate that these clusters have similar magnetic field strengths to those in nearby clusters, and suggest that magnetic field amplification is fast during the first phases of cluster formation.