A new study shows a deep connection between some of the biggest, most energetic events in the universe and much smaller, weaker events driven 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 was led by Helen Russell from the School of Physics and Astronomy at the 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 supply scientists with physics laboratories that are not available here on Earth.
In this composite image of Abell 2146 *, X-ray data from Chandra (purple) show hot gases, and optical data from the Subaru telescope show galaxies (red and white). One group (marked # 2) moves down to the left in the direction shown and crosses the other group (# 1). The hot gas in the first cluster produces a shock wave, like a sound boom generated by a supersonic jet, as it collides with the hot gas in the second cluster.
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 highlight sharp features. Also tagged is the central core of hot gas in cluster # 2 and the gas tail it left behind. Another shock wave of similar size is visible behind the collision. Called “upstream shock”, functions like this are due to the complex interaction between 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 collision shocks that involve direct collisions between particles. In Earth’s atmosphere near the sea surface, gas particles typically travel only about 4 millionths of an inch before colliding with another particle.
Conversely, in galaxy clusters and in solar wind – currents of particles blown by the Sun – direct collisions between particles occur too rarely to produce shock waves because the gas is so diffuse, with incredibly low density. For example, particles in galaxy clusters typically have to travel about 30,000 to 50,000 light-years before colliding. Instead, shocks in these cosmic environments are “collision-free”, generated by interactions between charged particles and magnetic fields.
Chandra observed Abell 2146 in a total of about 23 days, giving the deepest X-ray image of shock fronts in a galaxy cluster ever. The two shock fronts on Abell 2146 are among the brightest and clearest shock fronts known among galaxy clusters.
Helen commented: “I first discovered these shock fronts in a previous brief observation of Chandra when I was a PhD student. It was an exciting discovery and an amazing journey to this deep and hereditary observation that revealed the detailed structure of the shock.
Using these powerful data, Russell and his team studied the temperature of the gas behind the shock waves of 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 sun wind. The rest of the heating happened through particle collisions. Because the gas is so diffuse, this further warming happened slowly over about 200 million years.
Chandra creates images so sharp that he can actually measure how much random gas movement obscures the shock front, which according to the theory should be much narrower. For this cluster, they measure random gas motions at about 650,000 miles per hour.
Collision-free shock waves are important in several other research areas. For example, radiation produced by solar gusts can adversely affect the operation of spacecraft as well as human safety in space.
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