Cosmic rays are the highest energy particles in the Universe, produced by the most extreme astrophysical events, including gamma ray bursts and the accretion of matter onto super massive black holes. Cosmic rays can reach energies millions of times greater than those seen in the LHC, packing the energy of a hard-thrown baseball into a single proton. In a recent Science paper, researchers identified a radio relic from colliding galaxy clusters that should be able to produce cosmic rays in abundance. The finding confirms theoretical models that indicate cluster-merging shocks can push cosmic rays to energies up to 1019 eV.

Massive clusters of galaxies collide regularly and form the large scale structure of the cosmic web which we observe in the Universe. Their collisions release massive amounts of energy (1064 erg on timescales of 1-2 billion years), but not in the way many might expect. None of the stars collide; instead the diffuse gas in each galaxy collides, creating a shock front like that created by a jet fighter going supersonic.

The gas carries with it the magnetic fields of the intracluster space. These field lines become compressed and aligned parallel with the shock front, which packs in a lot of energy into the structure. Even as the gas cools and its emissions shift to radio frequencies (hence the "radio relic" term), theory suggests that these field lines can still accelerate particles to extremely high energies.

The challenge was confirming this via observations. High energy cosmic rays are thought to originate from several sources that are common during cluster collisions. These include gamma ray bursts triggered by the formation of a black hole from a collapsing star, and active galactic nuclei, which are super massive black holes at the center of energetic galaxies. Adding to the challenge, it's not possible to specifically assign observed high energy cosmic rays to a shock front because magnetic fields throughout the Universe scramble the path that the particles travel.

Still, it’s possible to determine whether a shock front has the properties necessary to act as particle accelerators. The researchers observed a radio relic that extended over nine million light years in an arc with a radius of curvature of five million light years. They were careful to rule out other interpretations of the observation, including a gravitational lens, a separate radio jet source, or a supernova remnant.

The half-arc shape is consistent with a head-on merger of clusters of roughly equal mass. Modeling suggests the relic is traveling at 1,000 km per second and has a a magnetic field between 5 and 7 micro Gauss, which is about 100,000 times smaller than the Earth's north-pointing magnetic field, but on the order of the magnetic fields within the Milky Way.

This radio is a perfect example of what a particle-accelerating shock front would look like, and provides an observational confirmation of the theory that merging galaxies could be a major source of high energy cosmic rays. The maximum energy to which protons can be accelerated in these shocks is only limited be the lifetime of the shock itself, and they can last for over a billion years. Further, the arching radio signature left is the sky fits previous models incredibly well and fills in details of the growth of the cosmic web we see in the Universe.

Science, 2010. DOI: 10.1126/science.1194293 (About DOIs).

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