For the first time, astronomers have found two giant clusters of galaxies that are just about to collide. This observation is one of the missing pieces of the puzzle in our understanding of the formation of structure in the Universe, because large-scale structures—such as galaxies and clusters of galaxies—are thought to grow by collisions and mergers. The discovery is published today in Nature Astronomy.
Clusters of galaxies are the largest known bound objects and consist of hundreds of galaxies that each contain hundreds of billions of stars. Ever since the Big Bang, these objects have been growing by colliding and merging with each other. Due to their large size, with diameters of a few million light years, these collisions can take 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. The challenge is to find colliding clusters that are just at the stage of first touching each other. In theory, this stage has a relatively short duration and is therefore hard to find.
Now an international team of astronomers announces the discovery of two clusters at the verge of colliding. This enables astronomers to test their computer simulations, which predict that in the first moments a shock wave is created in between the clusters and travels out perpendicular to the merging axis. “These clusters show the first clear evidence for this type of merger shock”, says first author Liyi Gu from the Japanese RIKEN laboratory and SRON Netherlands Institute for Space Research. “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.”
Astronomers are planning to collect more ‘snapshots’ to ultimately build up a continuous model describing the evolution of cluster mergers. SRON-researcher Hiroki Akamatsu: “More merger clusters like this one will be found by eROSITA, an X-ray all-sky survey mission that will be launched in July. Two other upcoming X-ray observatories, XRISM and Athena, will observe these objects with state-of-art X-ray spectrometers which helps us understand the role of these colossal merger shocks in the structure formation history.”
Liyi Gu and his colleagues 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 (LOFAR, a European project led by ASTRON, and the Giant Metrewave Radio Telescope operated by National Centre for Radio Astrophysics of India).
The figure above shows the process of merging by putting together a set of different merger stages, ordered by the core-to-core distances. X-ray images (blue) are overlaid with radio images (red). The third row from the top shows the 'first kiss'.