Astronomers have observed a dramatic, ongoing collision involving at least three galaxy clusters. Astronomers are using data from NASA’s Chandra X-ray Observatory, the European Space Agency’s XMM-Newton and three radio telescopes to understand this chaotic spectacle.

These collisions and mergers are the main mechanisms that allow galaxy clusters to grow into the massive cosmic structures we observe today. They serve as the largest particle accelerators in existence.

Abell 2256, a massive cluster of galaxies formed as a result of this collision, is located 780 million light-years from Earth. This composite image of Abell 2256 combines radio data from the Giant Metrewave Radio Telescope (GMRT), the Low-Frequency Array (LOFAR), and the Karl G. Jansky Very Large Array (VLA) in red with X-rays from Chandra and XMM in blue, as well as optical and infrared data from Pan-STARRs in white and light yellow.

Astronomers investigating this object are trying to decipher the origin of this peculiar-looking structure. The story is told differently through every telescope. Some of the largest objects in the universe are galaxy clusters, which are made up of hundreds or even thousands of individual galaxies.

They also have huge reserves of gas that are extremely hot, with temperatures of several million degrees Fahrenheit. X-ray telescopes such as Chandra and XMM can only see this heated gas. A labeled version of the figure shows gas from two galaxy clusters, with the third one too closely mixed to separate from the other.

The radio emissions from this system come from a more complex combination of sources. The first are the galaxies, whose supermassive black holes at their centers produce the radio signal by ejecting particles into space-time in jets.

These jets are either slowed down as they interact with the gas they collide into, forming intricate shapes and filaments (“A”, “B” and “F”), or they fly into space in straight, narrow lines (which named “C” and “I” in the annotated image, using the astronomer’s naming method).

Three sources are present in Source F, all of which were produced as a result of a black hole in a galaxy aligned with the center source of this trio. Radio waves also come from huge filamentary structures (labeled “relic”), mostly located north of the radio-emitting galaxies, likely generated when the collision created shock waves and accelerated particles in the gas over two million light-years.

Finally, a “halo” of radio emissions is located near the center of the collision. Because this halo overlaps the X-rays and is fainter than the filamentary structure and the galaxies, another radio image was produced to emphasize the faint radio emission.

In another paper, astronomers present a model that the halo emission may be caused by the re-acceleration of particles from rapid changes in the temperature and density of the gas as the clusters collide and merge. However, this model cannot explain all features of the radio data, highlighting the need for a more theoretical study of this and similar objects.

Rajpurohit and colleagues Paper III will investigate the radio-emitting galaxies in Abell 2256. These galaxies are unusually abundant in this cluster, possibly due to the merger and collision that fueled the expansion of supermassive black holes and their resulting outbursts. Erik Osinga will publish more information about the LOFAR image of Abell 2256 in a future article.

Magazine reference:

  1. K. Rajpurohit et al. Deep low-frequency radio observations of A2256. I. The filamentous radio remnant. The Astrophysical Journal. DOI: 10.3847/1538-4357/ac4708
  2. K. Rajpurohit et al. Deep low-frequency radio observations of Abell 2256 II. The ultra-steep spectrum radiohalo⋆. Astronomy and astrophysics. DOI: 10.1051/0004-6361/202244925