Many galaxies in the universe are found in large groups known as clusters. Just as galaxies can collide with each other, clusters also end up merging. These mergers help test out cosmological models and now, for the first time, researchers have spotted two clusters that are just about to merge.
As reported in Nature Astronomy, clusters 1E 2216.0-0401 and 1E 2215.7-0404 are on the cusp of a collision. Using a mixture of X-ray telescope and radio observatories, the team witnessed merger shocks between two clusters. Clusters are rich in intergalactic gas, which is spread out over millions of light-years and is often very hot.
As clusters crash into each other, the gas from each cluster is compressed and heated to incredible temperatures. Merger shocks have been witnessed before but never like this. These shocks travel perpendicular to the merger axis, outwards along the equatorial plane of the merger.
"These clusters show the first clear evidence for this type of merger shock," lead author Liyi Gu, from the RIKEN National Science Institute in Japan and the SRON Netherlands Institute for Space Research, said in a statement. "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. Therefore, the observed shock has a huge impact on the evolution of galaxy clusters and large-scale structures."
Cluster collisions can take many billions of years to complete, so the picture of how these events unfold was crafted by looking at “snapshots” of many different merging clusters. And that’s also why finding the beginning of a merger has been so difficult. It’s been a needle in a haystack situation. But the future is looking brighter.
"More merger clusters like this one will be found by eROSITA, an X-ray all-sky survey mission that will be launched this year. Two other upcoming X-ray missions, XRISM and Athena, will help us understand the role of these colossal merger shocks in the structure formation history," co-author and SRON researcher Hiroki Akamatsu added.
Clusters of galaxies are the largest gravitationally bound objects in the universe. They're important in terms of how galaxies change and evolve, and how matter is distributed within the universe. These observations provide another missing piece of the puzzle that is our knowledge of the cosmos.