Galaxy Cluster's Ring Hints at Cosmic Shockwave

A ring-like formation around a distant cluster of galaxies has yielded what may be the first glimpse of a shockwave spawned by either a cosmic collision or interstellar gas.

The ring, made up radio signal-emitting arcs, surrounds a rich collection of galaxies known as Abell 3376.

It appears to be a tell-tale sign of the cluster's growing pains from accreting gas or mergers on a galactic scale, a new study suggests.

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"We think these radio [objects] are tracing out the shockwaves from large-scale structure formation," said radio astronomer Joydeep Bagchi, a professor at Pune, India's Inter-University Centre for Astronomy and Astrophysics, who led the recent study.

Bagchi and his colleagues used the Very Large Array in Socorro, N.M., to scan Abell 3376, where they found a pair of immense radio arcs on the outer fringe of the cluster.

The research appears in the Nov. 3 issue of the journal Science.

Astronomers have known of such objects, previously dubbed "radio relics," since the 1970s, but only recently associated their presence with shockwaves resulting from galaxy cluster mergers, researchers said.

Pinning down their source can help astronomers understand the finer details of the evolution of large-scale structures in the universe, they added.

Bagchi told that computer-model simulations of galaxy-cluster mergers have consistently produced cosmic shockwaves, but the actual phenomenon has not been visible so clearly as seen around Abell 3376.

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The cluster's radio arc signals, he said, are indicative of high-energy particles streaming outward at near the speed of light.

The galaxy cluster's radio arcs may also have stemmed from an accretion shockwave generated by colliding bits of interstellar gas as they fall under the cluster's gravitational spell.

As the interstellar gas falls inward towards the cluster, it collides with the galactic group's own hot, ionized gas to form shockwaves, researchers said.

"Accretion shockwaves are very interesting because they may be the origin of the still-mysterious ultra-high energy cosmic rays," wrote astronomer Torsten Ensslin. of the Max Planck Institute for Astrophysics in Germany, in a Perspectives article related to Bagchi's study in Science.

The accretion model would also call for the presence of magnetic fields on the outer fringe of a galaxy cluster, rather than their typical haunt at its center, setting the stage for the formation of high-energy particles, Bagchi added.

"So there's already all the ingredients of a cosmic particle accelerator," Bagchi said. "That is an exciting possibility for future research."

Bagchi and his colleagues focused on the radio emissions of Abell 3367, but the development of more sensitive telescopes and space-based instruments — such as NASA's planned Gamma-ray Large Area Space Telescope (GLAST) to launch next year — will peer deeper into the source of high-energy particles.

"This is a very exciting result and there are a lot more to come, I believe," Bagchi said. "It's the beginning of [understanding] something that is still hidden beyond our vision."

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