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Backward Shockwaves From Deep Space Are Puzzling Scientists

Photo credit: Andriy Onufriyenko - Getty Images
Photo credit: Andriy Onufriyenko - Getty Images
  • A faraway galaxy cluster boasts an unusual backward shockwave phenomenon.

  • Galaxy clusters contain up to thousands of galaxies and tons of plasma in motion.

  • The backward shockwave may be from overlapping collisions among subclusters.

Scientists from the University of Western Australia and Italy’s Università di Bologna have been studying an extremely faraway galaxy cluster, Abell 3266, where they’ve identified three phenomena that are unheard of elsewhere—at least so far. And all three emit radio waves that have allowed us to observe them across a distance so far it almost defies units of measure altogether.

Abell 3266 is a galaxy cluster that’s 809 million light years away from Earth in an area known as the Horologium-Reticulum Supercluster. (As you zoom out further and further into space, you see that we, in the Milky Way, are part of a supercluster as well. Some supercluster-like structures we believe to exist are so large they potentially unmake our existing theory of the universe itself!)

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In a piece for the Conversation, a website that publishes content from academics, the scientists explain that the term “galaxy cluster” is a little bit misleading. Yes, a cluster may contain “hundreds, or even thousands” of galaxies, they say, but the overwhelming majority of mass in these clusters is dark matter, with “hot plasma ‘soup’” making up the rest. The Milky Way is made up of an estimated 85 percent dark matter, for example. Galaxies, themselves, make up a measly “few percent.”

On Earth, we primarily understand plasma as the makeup of stars, including our sun. But plasma physics is a huge field, since plasma itself is just another state of matter. The more we can examine it in different contexts, the more we can understand—knowledge that may also eventually help people on Earth who work on projects like nuclear fusion energy.

Abell 3266 is “a particularly dynamic” cluster, named for prolific astronomer and public scientist George O. Abell, who surveyed both the northern and southern skies for galaxy clusters during his lifetime. (Abell 3266 is in the southern sky.) While observing the cluster with the Australian Square Kilometre Array Pathfinder and Australia Telescope Compact Array radio telescopes, the researchers observed three different unusual phenomena: radio relics, radio haloes, and fossil radio sources.

All three of these phenomena are created by energy slamming into abundant plasma. With radio relics, this reaction sends out shockwaves like the sonic booms we experience on Earth. They themselves aren’t unusual in galaxy clusters, but the one these researchers found is backward. The brightness in this area of the cluster suggests that a shockwave should travel from north to south, but instead it goes from south to north. The researchers dubbed this a “wrong-way” radio relic.

Photo credit: Risely et. al
Photo credit: Risely et. al

The researchers say other scientists have begun to observe other backward radio relics in similar work, showing that this is an uncommon, but not nonexistent, phenomenon. And they theorize that the wrong-way radio relics result from not just energy striking plasma, but multiple subclusters striking each other simultaneously. Think about biting down on a crunchy potato chip versus biting down on a crunchy potato chip that was folded over during manufacturing.

In their paper, published August 1 in Monthly Notices of the Royal Astronomical Society, the researchers present all three radio anomalies with the follow-up note that they all require a lot more study; this paper is kind of an announcement, rather than a conclusion. “[F]urther work is required to fully unpack the history of Abell 3266 and its constituent radio galaxies, and the answers to a number of questions remain elusive,” they conclude.

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