Colossal collisions linked to solar system science

June 07, 2022

(News from Nanowerk) A new study shows a deep connection between some of the largest, most energetic events in the universe and much smaller, fainter events powered by our own Sun.

The results come from a long observation with NASA’s Chandra X-ray Observatory of Abell 2146, a pair of colliding galaxy clusters located about 2.8 billion light-years from Earth. The new study (Royal Astronomical Society Monthly Notices, “The structure of cluster fusion shocks: turbulent width and electron heating time scale”) was led by Helen Russell of the School of Physics and Astronomy at the University of Nottingham.

In this composite image of Abell 2146, X-ray data from Chandra (purple) shows hot gases and optical data from the Subaru Telescope shows galaxies (red and white). One group (labeled #2) moves down left in the direction shown and crosses the other group (#1). The hot gas in the first produces a shock wave, like a sonic boom generated by a supersonic jet, as it collides with the hot gas in the other cluster. (Photo: University of Nottingham)

Galaxy clusters contain hundreds of galaxies and huge amounts of hot gas and dark matter and are among the largest structures in the universe. Collisions between galaxy clusters release enormous amounts of energy not seen since the big bang and provide scientists with physics laboratories not available here on Earth.

The shock wave is about 1.6 million light-years away and is most easily seen in a version of the X-ray image that has been processed to accentuate sharp features. Also labeled are the central core of hot gas in cluster #2 and the gas tail it left behind. A second shock wave of similar size is visible behind the collision. Called “upstream shock,” features like this result from the complex interaction of gas extracted from the incoming cluster and gas from the surrounding cluster. The brightest and most massive galaxy in each cluster is also labeled.

Shock waves like those generated by a supersonic jet are collisional shocks, involving direct collisions between particles. In Earth’s atmosphere near sea level, gas particles typically only travel about 4 millionths of an inch before colliding with another particle.

Conversely, in galaxy clusters and in the solar wind – streams of particles carried by the Sun – direct collisions between particles occur too rarely to produce shock waves because the gas is very diffuse, with an incredibly low density. For example, in galaxy clusters, particles typically have to travel about 30,000 to 50,000 light-years before colliding. Instead, shocks in these cosmic environments are “collisionless”, generated by interactions between charged particles and magnetic fields.

Chandra observed Abell 2146 for a total of about 23 days, providing the deepest X-ray image of shock fronts in a galaxy cluster ever. The two shock fronts of Abell 2146 are among the brightest and clearest shock fronts known among galaxy clusters.

Helen commented: “I first detected these shock fronts in a previous short observation of Chandra when I was a PhD student. It was an exciting discovery and a fantastic journey to this deep and legacy observation revealing the detailed structure of the shock.

Using this powerful data, Russell and his team studied the temperature of the gas behind the shock waves in Abell 2146. They showed that the electrons were mainly heated by the compression of the gas by the shock, an effect similar to that observed in the solar wind. The rest of the heating occurred through particle collisions. Because the gas is so diffuse, this additional warming happened slowly, over about 200 million years.

Chandra creates images so sharp that he can actually measure how much random gas motion blurs the shock front which, according to theory, should be much narrower. For this cluster, they measure random gas movements of about 650,000 miles per hour.

Collisionless shock waves are important in several other areas of research. For example, radiation produced by solar wind shocks can negatively impact the operation of spacecraft, as well as the safety of humans in space.

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