Tour: Painting Galaxy Clusters by Numbers (and Physics)
(Credit: NASA/CXC/A. Hobart)
[Runtime: 03:54]

With closed-captions (at YouTube)

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.

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.

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. Galaxy groups 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. The researchers were able to classify features in the galaxy clusters and groups into three distinct types, which they have colored differently, by splitting Chandra data into lower-energy and higher-energy X-rays and comparing the strengths of each structure in both. The physical features included bubbles, sound waves and cooler or slower-moving gas.

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.

Quick Look: Painting Galaxy Clusters by Numbers (and Physics)
(Credit: NASA/CXC/A. Hobart)
[Runtime: 00:45]

With narration (video above with voiceover)

Astronomers and the public can examine galaxy clusters and groups like never before.

A new technique identifies X-ray features in these cosmic giants and applies color.

The researchers developed this technique and applied it to galaxy clusters and groups.

Using X-ray data from NASA’s Chandra, this gives a fresh take on these objects.