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27-06-2019
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Galaxy clusters caught in the first moment of collision

Submitter: Timothy Shimwell
Description: For the first time, two giant clusters of galaxies that are just about to collide have been found with the help of LOFAR observations (Gu et al. Nature Astronomy 2019)

Large scale structures in the Universe, like galaxies and clusters of galaxies, are thought to grow by collisions and mergers. As the merging process takes a long time, we can just see ‘snapshots’ of the various stages of these collisions on the sky. The challenge has been to find a couple of clusters that are just in the stage of the collision where they first touch each other. In theory, this stage has a relatively short duration and is therefore hard to find but according to simulations it can be identified by a shock wave that is created in between the clusters that travels outwards perpendicular to the merging axis. These observations revealed this type of merger shock for the first time and provide a missing ‘piece of the puzzle’ in our understanding of the formation of structure in the Universe.

The image illustrates the process of a major cluster merger by putting together a set of clusters observed at different merger stages, ordered by the core-to-core distances.  X-ray image (blue) is overlaid with the radio image (red) for each object. The third row from the top shows the object reported in this study. It is rather obvious that this object is seen at the moment of first-touch. The first two rows show objects still far from each other, and the fourth row shows a post-collision object.

This article is available on https://www.nature.com/articles/s41550-019-0798-8
Copyright: Gu et al. Nature Astronomy
 
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