For the first time, astronomers have spotted the "first kiss" of two giant galaxy clusters that are destined to marry and merge together.
Clusters of galaxies are the largest known bound objects in the universe, 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 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.
The challenge is to find colliding clusters that are just at the stage of first touching each other.
This stage has a relatively short duration and is therefore hard to find. It is like finding a raindrop that just touches the water surface in a photograph of a pond during a rain shower.
Now an international team of astronomers claims to have discovered two clusters on the verge of colliding for the first time.
They spotted the colliding pair using three X-ray satellites and two radio telescopes spread out across the world.
The discovery has enabled them to test their computer simulations, helping to shed light on the formation of structure in the universe.
The simulations show 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 RIKEN national science institute in Japan 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.
The findings were published today in the journal Nature Astronomy .
