Colossal collisions linked to solar system science

Newswise – A new study shows a deep connection between some of the largest, most energetic events in the universe and much smaller, fainter ones 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 about 2.8 billion light-years from Earth. The new study was led by Helen Russell of the University of Nottingham’s School of Physics and Astronomy.

Clusters of galaxies contain hundreds of galaxies and enormous amounts of hot gas and dark matter and are among the largest structures in the universe. Collisions between galaxy clusters release massive amounts of energy not seen since the Big Bang and provide scientists with physics labs unavailable here on Earth.

In this composite image from Abell 2146, Chandra X-ray data (purple) shows hot gas and Subaru Telescope optical data shows galaxies (red and white). One cluster (labeled #2) moves to the bottom left in the indicated direction and plows through the other cluster (#1). The hot gas in the former propels a shock wave, like a sonic boom generated by a supersonic jet, as it collides with the hot gas in the other cluster.

The shock wave is about 1.6 million light-years long and is most easily seen in a version of the X-ray that has been edited to emphasize sharp features. Also labeled are the central core of hot gas in cluster #2, and the tail of the gas it left behind. A second shock wave of similar size can be seen behind the collision. This is called an “upstream shock” and features like this arise from the complex interplay of stripped gas from the infalling cluster and the surrounding cluster gas. The brightest and most massive galaxy in each cluster is also labeled.

Shock waves such as those generated by a supersonic jet are collision shocks, where direct collisions between particles occur. In Earth’s atmosphere near sea level, gas particles usually travel only about 4 millionths of an inch before colliding with another particle.

Conversely, in galaxy clusters and in the solar wind — streams of particles blown away from the sun — direct collisions between particles are too rare to produce shock waves because the gas is so diffuse, with an incredibly low density. For example, in galaxy clusters, particles usually have to travel about 30,000 to 50,000 light-years before colliding. Instead, the 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 yet obtained of shock fronts in a galaxy cluster. The two shock fronts in Abell 2146 are among the brightest and clearest shock fronts known among galaxy clusters.

Helen noted: ‘I first discovered these shock fronts in a previous short Chandra observation when I was a PhD student. It was an exciting discovery and a fantastic journey into this deep, legacy observation that revealed the detailed shock structure.”

Using this powerful data, Russell and her team studied the gas temperature behind the shock waves in Abell 2146. They showed that electrons are heated primarily by compression of gas by the shock, an effect similar to that seen in the solar wind. The rest of the heating took place through particle collisions. Because the gas is so diffuse, this additional heating took place slowly, over about 200 million years.

Chandra creates such sharp images that it can actually measure how many random gas movements blur the shock front which the theory says will be much narrower. For this cluster, they measure random gas movements at about 650,000 miles per hour.

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

A paper describing these results was accepted by: The Monthly Notices of the Royal Astronomical Society and appears online† The authors are Helen Russell (University of Nottingham, United Kingdom), Paul Nulsen (Center for Astrophysics Harvard | Smithsonian, or CfA), Damiano Caprioli (University of Chicago), Urmila Chadayammuri (CfA), Andy Fabian (Cambridge University, United Kingdom ), Matthew Kunz (Princeton University), Brian McNamara (University of Waterloo, Canada), Jeremy Sanders (Max Planck Institute for Extraterrestrial Physics, Germany), Annabelle Richard-Laferriere (Cambridge University, United Kingdom), Maya Beleznay (Massachusetts Institute of Technology), Becky Canning (University of Portsmouth, United Kingdom), Julie Hlavacek-Larrondo (University of Montreal, Canada) and Lindsay King (University of Texas at Dallas).

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

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