Abell 2146 is a pair of colliding galaxy clusters located approximately 2.8 billion light-years away in the constellation of Draco.
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). Image credit: NASA / CXC / University of Nottingham / Russell et al. / NAOJ / Subaru Telescope.
Galaxy clusters contain thousands of galaxies of all ages, shapes and sizes and are among the largest structures in the Universe. Typically, they have a mass of about one million billion times the mass of the Sun.
Collisions between galaxy clusters release enormous amounts of energy unlike anything witnessed since the Big Bang and provide astrophysicists with laboratories that are unavailable here on Earth.
Using NASA’s Chandra X-ray Observatory, University of Nottingham astronomer Helen Russell and colleagues observed Abell 2146 for a total of about 23 days, giving the deepest X-ray image yet obtained of shock fronts in a galaxy cluster merger.
The two shock fronts in Abell 2146 are among the brightest and clearest shock fronts known among galaxy clusters.
Using this powerful data, the researchers studied the gas temperature behind the shock waves in Abell 2146.
They showed that electrons have been mainly heated by compression of gas by the shock, an effect like that seen in the solar wind. The rest of the heating occurred by collisions between particles.
Because the gas is so diffuse this additional heating took place slowly, over about 200 million years.
Chandra makes such sharp images that it can actually measure how much random gas motions are blurring shock front that is expected from theory to be much more narrow.
For Abell 2146, the telescope measured random gas motions of around 650,000 miles per hour.
“In a composite image of Abell 2146, one cluster is moving towards the bottom left and plowing through the other cluster,” the scientists said.
“The hot gas in the former is pushing out a 1.6-million-light-year-long shock wave, like a sonic boom generated by a supersonic jet, as it collides with the hot gas in the other cluster.”
“A second shock wave of similar size is seen behind the collision. Called an ‘upstream shock,’ features like this arise from the complex interplay of stripped gas from the infalling cluster and the surrounding cluster gas.”
“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 travel only about 4 millionths of an inch before colliding with another particle,” they explained.
“Conversely, in galaxy clusters and in the solar wind direct collisions between particles occur too rarely to produce shock waves because the gas is so diffuse, with incredibly low density.”
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
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H.R. Russell et al. The structure of cluster merger shocks: Turbulent width and the electron heating timescale. MNRAS, published online May 17, 2022; doi: 10.1093/mnras/stac1055
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