About 2.8 billion light-years from Earth, two clusters of galaxies are engaged in a cosmic battle. These huge structures last clashed about a billion years ago, but now they appear to be returning for a second round.
The clusters of galaxies involved in this clash are part of a larger system, a “merged” galaxy cluster called PSZ2 G181. In a study published in April in The Astrophysical Journal, scientists analyzed new observations of PSZ2 G181, which indicate that its constituent clusters are ready to collide a second time.
According to the European Space Agency (ESA), large collisions between clusters of galaxies are generally considered rare, but repeated collisions are certainly more unusual. Furthermore, the total mass of PSZ2 G181 is uncharacteristically small compared to other merged galaxy clusters that have formed as a result of collisions, adding another level of unusualness to the results.
Astronomers know that the individual clusters of galaxies that make up PSZ2 G181 have already collided with each other thanks to previous radio observations, NASA said in a statement.
This work has revealed bracket-shaped regions of diffuse radio emission on the outskirts of the system. These structures were most likely formed as a result of the initial collision billions of years ago.
The new study, led by Andra Stroe, a postdoctoral researcher at the Harvard and Smithsonian Centers for Astrophysics, further supports the idea that a previous collision occurred. The researchers analyzed new observations of PSZ2 G181 from two X-ray observatories – NASA’s Chandra and the European Space Agency’s XMM-Newton. In their study, they found a bridge of cold gas connecting the two clusters, which was probably formed as a result of gas liquefaction during the first collision, according to their report.
Over the past billion years, the two clusters have moved apart and are now about 11 million light-years apart. According to NASA, this is the largest distance between such structures that astronomers have ever seen.
But Stroh and her colleagues found evidence that these clusters of galaxies are now on a collision course again. The team found three more shock fronts aligned with the previous collision axis – a potential early sign of a second, oncoming collision.
The discovery of this rare event provides new insight into the dynamics of merging galaxy clusters, especially those involving low-mass systems that are poorly understood. As the researchers note, finding radio emission from low-mass clusters such as PSZ2 G181 is challenging, but “with the advent of new generation radio telescopes and research, we may be discovering the ‘tip of the iceberg,'” they state in their report.
