Colliding Giant Galaxy Clusters Create Shockwaves 1.6 Million Light-Years Across

Abell 2146 is a giant merger of two galaxy clusters and the bow shock and upstream shock seen from their encounter is enormous
Abell 2146 is a giant merger of two galactic clusters. The shockwaves created by their meeting are 1.6 million light years across. Image credit: X-ray: NASA/CXC/Univ. of Waterloo/H. Russell et al.; Optical: NASA/STScI

The universe is littered with examples of galaxies colliding, but Abell 2146 is something much bigger, being formed from two giant galaxy clusters merging, each containing hundreds of galaxies. Unsurprisingly, the shockwaves from this impact are enormous, and the first detailed study of cluster merger shockwaves has revealed the scale.

Two shock fronts exist within Abell 2146, each of which is around 1.6 million light years across. One is the bow shock,  which parallels the bow wave created by a moving boat, and the other is the upstream shock trailing behind the moving cluster.

In Monthly Notices of the Royal Astronomical Society a team led by Dr Helen Russell of the University of Nottingham describe and analyze data from the Chandra X-Ray and Subaru optical telescopes on these two shockwaves.

Abell 2146 is around 2.8 billion light-years away, so even at its immense scale, it isn’t easy to see. Nevertheless, the two powerful telescopes reveal the shocks with surprising clarity.

Abell 2146’s shockwaves are what is known as collisionless. This might seem like an internally contradictory statement, but collisionless shocks occur when the density of particles in colliding media are so low they almost never interact directly. The gasses between galaxies within a cluster are so diffuse that when the clusters meet, unless individual galaxies collide, a particle would need to travel 30,000-50,000 light-years before encountering one from the other cluster.

With such an enormous amount of space, the total number of particles is large and a few will run into each other, but such collisions are insufficient to create a shockwave themselves. However, many of the particles in each intergalactic medium are charged. Shockwaves are the consequence of the encounters between the particles of one cluster and the magnetic field of the other.

Extraordinarily, given the lack of direct impact between particles, the paper notes that “the shocks…dissipate most of the merger’s ∼ 1064 erg of kinetic energy,” causing the clusters to merge rather than pass through each other and keep going.

A marked combination of X-ray and optical images of the galactic cluster merger Abell 2146

Abell 2146 with its major features labelled including the two shock waves and brightest galaxies. Image credit: X-ray: NASA/CXC/Univ. of Nottingham/H. Russell et al.; Optical: NAOJ/Subaru

The interstellar medium, and the bubble that defines the solar system, also produce a collisionless shock wave as they meet, even though the interstellar medium is considerably denser than the gas between galaxies. Consequently, by providing us with a rare opportunity to witness collisionless shockwaves from the outside, Abell 2146 could help astronomers model the shock our solar system is creating. 

“Our results for Abell 2146 are expected to be valid for collisionless shocks with similar parameters in other environments and support the existing picture from the solar wind and supernova remnants,” the paper notes.

“I first detected these shock fronts in an earlier, short Chandra observation when I was a Ph.D. student. It was a thrilling discovery and a fantastic journey to this deep, legacy observation revealing the detailed shock structure,” Russell said in a statement

The paper notes Abell 2146 is one of only three cluster mergers known with shock fronts bright enough to study.

Close-up image of Abell 2146 with shockwaves labelled

Composite image of Abell 2146. Cluster #2 is moving towards the bottom left and plowing through cluster #1. The hot gas in the former is pushing out a shock wave as it collides with the hot gas in the other cluster. Image credit: Chandra/University of Nottingham

Even the width of Abell 2146’s two shock layers is extraordinary – more than 50,000 light-years for the bow shock, approximately equal to the radius of the Milky Way.

Understanding collisionless shockwaves has practical implications, as they can produce radiation that could affect the safety of spaceprobes and crewed missions.

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