Untangling the knot of galaxy clusters from NASA


Astronomers have recorded an impressive permanent collision of at least three clusters of galaxies. Data from NASA’s Chandra X-ray Observatory, the European Space Agency’s (ESA) XMM-Newton and three radio telescopes are helping astronomers figure out what’s going on in this confusing scene. Collisions and mergers like these are the primary way that galaxy clusters can grow into the giant cosmic structures seen today. They also act as the largest particle accelerators in the universe.

The massive galaxy cluster that resulted from this collision is Abell 2256, located 780 million light-years from Earth. This composite image of Abell 2256 combines blue X-rays from Chandra and XMM with radio data collected by the Giant Metrowave Radio Telescope (GMRT), the Low Frequency Array (LOFAR), and the Karl G. Jansky Very Large Array (VLA). all in red, plus optical and infrared data from Pan-STARR in white and pale yellow.

Astronomers studying this object are trying to figure out what led to this unusual-looking structure. Each telescope tells a different part of the story. Galaxy clusters are some of the largest objects in the universe, containing hundreds or even thousands of individual galaxies. In addition, they contain huge reservoirs of superheated gas with temperatures of several million degrees Fahrenheit. Only X-ray telescopes like Chandra and XMM can see this hot gas. The labeled version of the figure shows gas from two galaxy clusters, with the third mixed too tightly to separate from the others.

The radio emission in this system arises from an even more complex set of sources. First, these are the galaxies themselves, in which the radio signal is generated by particles jetting out from the supermassive black holes at their centers. These jets either shoot out into space in straight, narrow lines (labeled “C” and “I” in the annotated image, using the astronomers’ naming system), or slow down as the jets interact with the gas they collide with to create complex shapes and filaments (” A”, “B” and “F”). The F source contains three sources, all produced by the black hole in the galaxy that aligns with the leftmost source of the trio.

The radio waves also come from huge filamentary structures (labeled “relics”) located mostly north of the radio-emitting galaxies, which likely formed when the collision created shock waves and accelerated particles in the gas more than two million light-years away. A paper analyzing this structure was published earlier this year by Kamlesh Rajpurohit of the University of Bologna in Italy in the March 2022 issue of The Astrophysical Journal and available online . This is Article I of an ongoing series examining various aspects of this galaxy cluster system.

Finally, there is a “halo” of radio emission located near the collision center. Because this halo overlaps with the X-ray emission and is fainter than the filamentary structure and galaxies, a different radio image was created to highlight the faint radio emission. Paper II, led by Rajpurohit, was recently published in the journal Astronomy and Astrophysics and available online , presents a model in which halo emission can be caused by re-acceleration of particles due to rapid changes in gas temperature and density as collisions and mergers proceed. This model, however, is unable to explain all the features of radio data, emphasizing the need for a more theoretical study of this and similar objects.

Paper III by Rajpurohit and colleagues will examine the radio-producing galaxies in Abell 2256. This cluster contains an unusually large number of such galaxies, possibly because collisions and mergers cause the growth of supermassive black holes and subsequent eruptions. More details about the LOFAR image of Abell 2256 will be reported in a future article by Eric Osinga.

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