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For the first time, astronomers have found two giant clusters of galaxies that are just about to collide.

Clusters of galaxies are thought to mainly grow through collisions and mergers, but two clusters this close to colliding had never been observed—until now, providing a missing piece in the understanding of the formation of structure in the Universe.

Clusters of galaxies are the largest known objects bound by gravity, and consist of hundreds of galaxies (that each contain hundreds of billions of stars) surrounded by a halo of hot gas. Ever since the Big Bang, these objects have been growing, mainly by colliding and merging with each other. Due to their large size—with diameters of a few million light years—these collisions can take about a billion years to complete. After the dust has settled, the two colliding clusters will have merged into one bigger cluster.

Because the merging process takes much longer than a human lifetime, we only see snapshots of the various stages of these collisions. Astronomers have recorded galaxy clusters mid-collision and post-collision, and have theorized about pre-collision, but this stage has a relatively short duration and is therefore hard to find.

An international team of astronomers have announced the discovery of two clusters at the verge of colliding. They’re only 300-600 million years away from impact – cosmically close when you consider it will take a billion years to merge. This will enable astronomers to test their computer simulations, which show that in the first moments a shock wave is created between the cluster and travels out perpendicular to the merging axis.
“X-ray and radio images of these clusters show the first clear evidence for this type of merger shock”, says RIKEN-researcher and first author Liyi Gu. “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.”

This animation illustrates the process of a major merger by putting together a set of clusters observed at different merger stages, ordered by the core-to-core distances. The X-ray image (blue) is overlaid with the radio image (red) for each object. This particular study showcases the cluster pair 300-600 million years before impact.
ICRAR-Curtin’s Dr Huib Intema, from the team who found the cluster, says they are planning to collect more snapshots to ultimately build up a model describing 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. Australian-based radio telescopes like the Murchison Widefield Array (MWA) and the future Square Kilometre Array (SKA) are particularly sensitive to these sign posts. 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.”

The team studied the colliding pair during an observation campaign carried out with three X-ray satellites (ESA’s XMM-Newton satellite, the NASA’s Chandra satellite, and JAXA’s Suzaku satellite) and two radio telescopes (the Low-Frequency Array [LOFAR], a European project led by the Netherlands, and the Giant Metrewave Radio Telescope [GMRT] operated by National Centre for Radio Astrophysics of India).

Read the paper here.