Untangling a Knot of Galaxy Clusters

Multiwavelength image of Abell 2256
Abell 2256
Credit: X-ray: Chandra: NASA/CXC/Univ. of Bologna/K. Rajpurohit et al.; XMM-Newton: ESA/XMM-Newton/Univ. of Bologna/K. Rajpurohit et al. Radio: LOFAR: LOFAR/ASTRON; GMRT: NCRA/TIFR/GMRT; VLA: NSF/NRAO/VLA; Optical/IR: Pan-STARRS

Astronomers have captured a spectacular, ongoing collision between at least three galaxy clusters. Data from NASA’s Chandra X-ray Observatory, ESA’s (European Space Agency’s) XMM-Newton, and a trio of radio telescopes is helping astronomers sort out what is happening in this jumbled scene. Collisions and mergers like this are the main way that galaxy clusters can grow into the gigantic cosmic edifices seen today. These also act as the largest particle accelerators in the universe.

The giant galaxy cluster forming from this collision is Abell 2256, located 780 million light-years from Earth. This composite image of Abell 2256 combines X-rays from Chandra and XMM in blue with radio data collected by the Giant Metrewave 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-STARRs in white and pale yellow.

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

The main image of this release with labels showing different features.
Abell 2256, Labeled (Credit: X-ray: Chandra: NASA/CXC/Univ. of Bologna/K. Rajpurohit et al.; XMM-Newton: ESA/XMM-Newton/Univ. of Bologna/K. Rajpurohit et al.

The radio emission in this system arises from an even more complex set of sources. The first are the galaxies themselves, in which the radio signal is generated by particles blasting away in jets from supermassive black holes at their centers. These jets are either shooting into space in straight and narrow lines (those labeled “C” and “I” in the annotated image, using the astronomer’s naming system) or slowed down as the jets interact with gas they are running into, creating complex shapes and filaments (“A”, “B,” and “F”). Source F contains three sources, all created by a black hole in a galaxy aligning with the left-most source of this trio.

Radio waves are also coming from huge filamentary structures (labeled "relic"), mostly located to the north of the radio-emitting galaxies, likely generated when the collision created shock waves and accelerated particles in the gas across over two million light-years. A paper analyzing this structure was published earlier this year by Kamlesh Rajpurohit from the University of Bologna in Italy in the March 2022 issue of The Astrophysical Journal, and is available online. This is Paper I in an ongoing series studying different aspects of this colliding galaxy cluster system.

Finally, there is a "halo" of radio emission located near the center of the collision. Because this halo overlaps with the X-ray emission and is dimmer than the filamentary structure and the galaxies, another radio image has been produced to emphasize the faint radio emission. Paper II led by Rajpurohit, recently published in the journal Astronomy and Astrophysics and available online, presents a model that the halo emission may be caused by the reacceleration of particles by rapid changes in the temperature and density of the gas as the collision and merging of the clusters proceed. This model, however, is unable to explain all the features of the radio data, highlighting the need for more theoretical study of this and similar objects.

Halo of Radio Emission
Halo of Radio Emission (Credit: LOFAR/ASTRON)

Paper III by Rajpurohit and collaborators will study the galaxies producing radio waves in Abell 2256. This cluster contains an unusually large number of such galaxies, possibly because the collision and merger are triggering the growth of supermassive black holes and consequent eruptions. More details about the LOFAR image of Abell 2256 will be reported in an upcoming paper by Erik Osinga.

The full list of co-authors for papers I and II include researchers from the University of Bologna, Italy (Franco Vazza, Annalisa Bonafede, Andrea Botteon, Christopher J. Riseley, Paola Domínguez-Fernández, Chiara Stuardi, and Daniele Dallacasa); Leiden Observatory, Leiden University, the Netherlands (Erik Osinga, Reinout J. van Weeren, Timothy Shimwell, Huub Röttgering, and George Miley); Thüringer Landessternwarte, Tautenburg, Germany (Matthias Hoeft and Alexander Drabent); INAF-Istituto di Radio Astronomia, Bologna, Italy (Gianfranco Brunetti and Rossella Cassano); Hamburger Sternwarte, Germany (Denis Wittor, Marcus Brüggen, and Francesco de Gasperin); Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Italy (Marisa Brienza); Center for Astrophysics, Harvard | Smithsonian (William Forman); Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley (Sangeeta Rajpurohit); Physical Research Laboratory, Ahmedabad, India (Arvind Singh Rajpurohit); Universität Würzburg, Würzburg, Germany (Etienne Bonnassieux), and INAF–IASF Milano, Italy (Mariachiara Rossetti).

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