A new study reveals a deep link between some of the universe’s largest, most energetic events and much smaller, weaker ones powered by our own Sun. The findings are the result of 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. Helen Russell from the University of Nottingham’s School of Physics and Astronomy led the new study.
Galaxy clusters, which contain hundreds of galaxies as well as massive amounts of hot gas and dark matter, are among the largest structures in the universe. Collisions between galaxy clusters release enormous amounts of energy unlike anything witnessed since the big bang and provide scientists with physics laboratories that are unavailable here on Earth.
In this composite image of Abell 2146,* Chandra X-ray data (purple) shows hot gas, and Subaru Telescope optical data shows galaxies (red and white). One cluster (labeled #2) is moving towards the bottom left in the direction shown and plowing through the other cluster (#1). The hot gas in the former is pushing out a shock wave, like a sonic boom generated by a supersonic jet, as it collides with the hot gas in the other cluster.
A study reveals a deep link between some of the universe’s largest, most energetic events and much smaller, weaker ones powered by our own Sun. Collisions between galaxy clusters release enormous amounts of energy unlike anything witnessed since the big bang and provide scientists with physics laboratories that are unavailable here on Earth.
Richard Zeebe
The shock wave is approximately 1.6 million light-years long and is best seen in an X-ray image that has been processed to emphasize sharp features. The central core of hot gas in cluster #2, as well as the tail of gas it has left behind, are also labeled. Behind the collision, a second shock wave of comparable size can be seen. This type of feature, known as a “upstream shock,” results from the complex interplay of stripped gas from the infalling cluster and the surrounding cluster gas. Each cluster’s brightest and most massive galaxy is also labeled.
Collisional shocks, such as those produced by a supersonic jet, involve direct collisions between particles. Gas particles in the Earth’s atmosphere near sea level typically travel only about 4 millionths of an inch before colliding with another particle.
Direct collisions between particles occur too rarely in galaxy clusters and the solar wind (streams of particles blown away from the Sun) to produce shock waves because the gas is so diffuse and has such a low density. In galaxy clusters, for example, particles must typically travel 30,000 to 50,000 light-years before colliding. The shocks in these cosmic environments, on the other hand, are “collisionless,” caused by interactions between charged particles and magnetic fields.
Computing the positions of the planets over a 5 billion-year period as they interact gravitationally is a difficult task. But physicist Richard Zeebe had a once-in-a-lifetime opportunity: The University of Hawaii, Manoa, where he works, was putting a new Cray supercomputer through its paces. “No one will ever get six weeks on this machine for free again,” Zeebe says. That is the amount of computer time he spent running 1600 simulations of the solar system’s future. Each simulation was distinct from the others because Mercury began in a slightly different position.
Chandra observed Abell 2146 for approximately 23 days, yielding the most detailed X-ray image of shock fronts in a galaxy cluster yet obtained. Abell 2146’s two shock fronts are among the brightest and clearest shock fronts known in galaxy clusters.
‘I first detected these shock fronts in an earlier, brief Chandra observation when I was a PhD student,’ Helen said. It was an exciting discovery and an incredible journey to this deep, legacy observation that revealed the detailed shock structure.’
Russell and her colleagues used this powerful data to investigate the gas temperature behind the shock waves in Abell 2146. They demonstrated that electrons were primarily heated by the shock’s compression of gas, an effect similar to that seen in the solar wind. The remainder of the heating was caused by particle collisions. Because the gas is so diffuse, the additional heating occurred gradually over about 200 million years.
Chandra produces such sharp images that it can actually measure how much random gas motions blur a shock front that should be much narrower according to theory. They measured random gas motions of around 650,000 miles per hour for this cluster.
Collisionless shock waves are useful in a variety of other fields of study. For example, radiation produced by solar wind shocks can have a negative impact on spacecraft operation as well as human safety in space.
