Astronomers have discovered a pair galaxy clusters on the verge of collision.

Galaxy clusters are the largest cosmic structures organized by gravity. These three-dimensional webs of galaxies, numbering in the thousands, contain billions and billions of stars.

Just as galaxies collide to form larger galaxies, clusters can collide and merge. But the latest study marks the first time scientists have spotted a pair of clusters just prior to collision.

The study of large-scale cosmic mergers and the resulting shocks can help scientists better understand the structural evolution of the universe.

Collisions between galaxy clusters take billions of years. Previously, astronomers have previously documented mid-collision and post-collision galaxy-cluster mergers. Pre-collision formations last between 300 million and 600 million years, a short amount of cosmic time, making them harder to find.

"X-ray and radio images of these clusters show the first clear evidence for this type of merger shock," Liyi Gu, a cosmologist at RIKEN, a large scientific research institute in Japan, said in a news release.

"The shock created a hot belt region of 100-million-degree gas between the clusters, which is expected to extend up to, or even go beyond the boundary of the giant clusters," Gu said. "Therefore, the observed shock has a huge impact on the evolution of galaxy clusters and large-scale structures."

Researchers described their discovery this week in the journal Nature Astronomy.

Scientists plan to continue imaging the galaxy-cluster merger and use their observations to build a comprehensive model of the evolution of cluster mergers.

"Radio observations at very low frequencies provide a unique view on these clusters, in this case pinpointing the locations of old radio jets that light up again due to the merger," said Huib Intema, scientist at International Center for Radio Astronomy Research in Australia. "Mapping out the whole radio sky will inevitably lead to the discovery of even more of these systems. This will help us to complete our understanding of the role of merger shocks in the formation of the largest structures in the universe."

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