Galaxy clusters are the most massive objects in the universe held together by gravity, containing up to several thousand individual galaxies and huge reservoirs of superheated, X-ray-emitting gas. The mass of this hot gas is typically about five times higher than the total mass of all the galaxies in galaxy clusters. In addition to these visible components, 80% of the mass of galaxy clusters is supplied by dark matter. These cosmic giants are bellwethers not only for the galaxies, stars and black holes within them, but also for the evolution and growth of the universe itself.

It is no surprise then that NASA’s Chandra X-ray Observatory has observed many galaxy clusters over the lifetime of the mission. Chandra’s X-ray vision allows it to see the enormous stockpiles of hot cluster gas, with temperatures as high as 100 million degrees, with exquisite clarity. This blazing gas tells stories about past and present activity within galaxy clusters.

Astronomical Images of Objects Processed Using X-arithmetic Technique (Labeled) Credit: X-ray: NASA/CXC/Univ. of Chicago/H. McCall; Image processing: NASA/CXC/SAO/N. Wolk

Many of these galaxy clusters host supermassive black holes at their centers, which periodically erupt in powerful outbursts. These explosions generate jets that are visible in radio wavelengths, which inflate bubbles full of energetic particles; these bubbles carry energy out into the surrounding gas. Chandra’s images have revealed a wealth of other structures formed during these black hole outbursts, including hooks, rings, arcs, and wings. However, appearances alone don’t tell us what these structures are or how they formed.

To tackle this problem, a team of astronomers developed a novel image-processing technique to analyze X-ray data, allowing them to identify features in the gas of galaxy clusters like never before, classifying them by their nature rather than just their appearance. Prior to this technique, which they call “X-arithmetic,” scientists could only identify the nature of some of the features and in a much less efficient way, via studies of the amounts of X-ray energy dispersed at different wavelengths. The authors applied X-arithmetic to 15 galaxy clusters and galaxy groups (these are similar to galaxy clusters but with fewer member galaxies). By comparing the outcome from the X-arithmetic technique to computer simulations, researchers now have a new tool that will help in understanding the physical processes inside these important titans of the universe.

A new paper looks at how these structures appear in different parts of the X-ray spectrum. By splitting Chandra data into lower-energy and higher-energy X-rays and comparing the strengths of each structure in both, researchers can classify them into three distinct types, which they have colored differently. A pink color is given to sound waves and weak shock fronts, which arise from pressure disturbances traveling at close to the speed of sound, compressing the hot gas into thin layers. The bubbles inflated by jets are colored yellow, and cooling or slower-moving gas is blue. The resulting images, “painted” to reflect the nature of each structure, offer a new way to interpret the complex aftermath of black hole activity using only X-ray imaging data. This method works not only on Chandra (and other X-ray) observations, but also on simulations of galaxy clusters, providing a tool to bridge data and theory.

The images in this new collection show the central regions of five galaxy clusters in the sample: MS 0735+7421, the Perseus Cluster, and M87 in the Virgo Cluster in the top row and Abell 2052 and Cygnus A on the bottom row. All of these objects have been released to the public before by the Chandra X-ray Center, but this is the first time this special technique has been applied. The new treatment highlights important differences between the galaxy clusters and galaxy groups in the study.

The galaxy clusters in the study often have large regions of cooling or slow-moving gas near their centers, and only some show evidence for shock fronts. The galaxy groups, on the other hand, are different. They show multiple shock fronts in their central regions and smaller amounts of cooling and slow-moving gas compared to the sample of galaxy clusters.

This contrast between galaxy clusters and galaxy groups suggests that black hole feedback — that is, the interdependent relationship between outbursts from a black hole and its environment — appears stronger in galaxy groups. This may be because feedback is more violent in the groups than in the clusters, or because a galaxy group has weaker gravity holding the structure together than a galaxy cluster. The same outburst from a black hole, with the same power level, can therefore more easily affect a galaxy group than a galaxy cluster.

There are still many open questions about these black hole outbursts. For example, scientists would like to know how much energy they put into the gas around them and how often they occur. These violent events play a key role in regulating the cooling of the hot gas and controlling the formation of stars in clusters. By revealing the physics underlying the structures they leave behind, the X-arithmetic technique brings us closer to understanding the influence of black holes on the largest scales.

A paper describing this new technique and its results has been published in The Astrophysical Journal and is led by Hannah McCall from the University of Chicago. The other authors are Irina Zhuravleva (University of Chicago), Eugene Churazov (Max Planck Institute for Astrophysics, Germany), Congyao Zhang (University of Chicago), Bill Forman and Christine Jones (Center for Astrophysics | Harvard & Smithsonian), and Yuan Li (University of Massachusetts at Amherst).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.