For the first time, the huge shock wave created by the earliest phases of a collision between some of the Universe's most massive objects has been observed and photographed.
The discovery was made in the Abell 98 galaxy cluster, which is a massive structure made up of three smaller sub-clusters of galaxies situated more than 1.2 billion light-years away from the Solar System.
There, a massive filament of gas holds the massive shock along the merger axis that has been expected to be the initial 'contact' between two sub-clusters of galaxies as they begin to join hypothetically.
"With this discovery, we caught two sub-clusters of a galaxy cluster in a crucial early epoch of the merging process, with a strong shock between them, providing a missing link to the formation of the most massive structures in our Universe," said University of Kentucky physicist and astronomer Arnab Sarkar.
The Universe is continuously interacting with itself and organising itself. Clusters, superclusters, megaclusters, and filaments dance around each other, forming larger and larger structures. Galaxies aren't isolated entities drifting through space; gravity is everywhere, and the constant push-pull and interplay results in clusters, superclusters, megaclusters, and filaments, dancing around each other and forming larger and larger structures.
Of course, these interactions don't happen on anything close to human timeframes; but, astronomers may recreate how these collisions happen by seeing clusters at various stages of merging.
As you might expect, the gravitational environment within clusters of galaxies is rather powerful, with sub-clusters combining to form bigger structures within the entire cluster.
Astronomers discovered in 2014 that two sub-clusters inside Abell 98 – A98N and A98S – seemed to be merging, as demonstrated by brightness and temperature signals in A98N compatible with a merger shock.
Sarkar and his colleagues used the Earth-orbiting Chandra X-ray Observatory to investigate the area between the two sub-clusters, which they reported at the 240th meeting of the American Astronomical Society. They discovered what they call "definitive evidence" of a shock edge south of A98N there.
They claim that this is a significant problem. Although inter- and intra-cluster mergers must be very common (because to the fact that the Universe is replete with galaxy clusters), capturing one in the early stages is extremely unusual. We witness a lot in the latter phases, such as the shock waves caused by these severe contacts, but very few when the clusters come together.
That might be because they're more difficult to find, but Sarkar and his colleagues' discovery could help future searches by revealing what to look for. And, of course, it closes some important gaps in our knowledge of how cluster mergers occur and evolve. This implies we'll be able to make more accurate forecasts regarding galaxy cluster evolution.
"This result is important because different computer simulations seem to be telling us different things about what we should observe early on in a galaxy cluster merger," said Sarkar. "Here, we have a picture of what this process actually looks like, and that can be used to inform our theoretical models."
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