Astronomers are pretty used to the idea of collisions between large objects and structures. They’ve spotted colliding neutron stars. Four billion years from now, the Andromeda Galaxy will collide with the Milky Way Galaxy. Now they’ve found evidence of one of the largest possible collisions: three galaxy clusters that are currently fusing into one big cluster.
Galaxy clusters are groups of galaxies that are bound together by gravitational forces and are typically composed of hundreds to thousands of individual galaxies, dark matter, hot gas, and cosmic dust. These structures are the largest known gravitationally bound objects in the universe.
A big enough galaxy cluster is referred to as a “supercluster” that might include several mini-clusters. The Milky Way, for instance, is a member of the Local Group, a small, isolated cluster of galaxies that also includes the Andromeda galaxy and several smaller galaxies. The Local Group is part of a larger structure known as the Virgo Supercluster, which is a cluster of galaxy clusters that includes several thousand galaxies. The center of the Virgo Supercluster is located in the constellation Virgo and is approximately 60 million light-years away from the Milky Way.
Of course, the idea of “clusters within a cluster” sort of goes out the window when galaxy clusters collide. A NASA statement on the newly discovered collision between galaxy clusters referred to it as a “jumbled scene” due to the amount of chaos caused by the collision. The collision is forming a new, larger cluster called Abell 2256 and resides about 780 million light-years from Earth.
Researchers are using data from NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton, the Giant Metrewave Radio Telescope (GMRT), the Low Frequency Array (LOFAR), and the Karl G. Jansky Very Large Array (VLA) to study the collision between clusters. They created this composite image using data from Chandra’s X-ray instruments and radio data from GMRT, LOFAR, and VLA.
Each telescope gives them a unique view of Abell 2256. Chandra and XMM-Newton provide valuable views of the hot gas in the colliding clusters with their X-ray instruments. Even with their sensitive instruments, one of the three clusters is hard to see.
However, the radio instruments on GMRT, LOFA, and VLA can pick up additional important details by detecting radio waves produced by the jets shooting away from supermassive black holes as they feed on material that may have been stirred up by the collisions. Radio waves are also coming from huge filamentary structures that might have been created when collisions between objects in the clusters generated shock waves that accelerated material in the clusters. The radio telescopes also picked up a fainter “halo” of radio waves near the center of Abell 2256 that overlaps with the more powerful X-ray radiation.
The research team behind the study of Abell 2256 already published three papers on their work. Kamlesh Rajpurohit of the University of Bologna in Italy was the lead author of a paper published in the Astrophysical Journal in March 2022, describing the overall structure of the colliding clusters. A second paper led by Kamlesh Rajpurohit took a closer look at the faint halo of radio waves near the center of the collision. Rajpurohit will also lead a third research paper that takes a closer look at the galaxies that are emitting radio waves.