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X-shaped radio galaxies might form more simply than expected

Simple simulation accidentally leads to X-shaped galaxy for first time

When astronomers use radio telescopes to gaze into the night sky, they typically see elliptical-shaped galaxies, with twin jets blasting from either side of their central supermassive black hole. But every once in a while — less than 10% of the time — astronomers might spot something special and rare: An X-shaped radio galaxy, with four jets extending far into space.

Although these mysterious X-shaped radio galaxies have confounded astrophysicists for two decades, a new Northwestern University study sheds new insight into how they form — and its surprisingly simple. The study also found that X-shaped radio galaxies might be more common than previously thought.

The study was published today (Aug. 29) in the Astrophysical Journal Letters. It marks the first large-scale galaxy accretion simulation that tracks the galactic gas far from the supermassive black hole all the way toward it. 

Simple conditions lead to messy result

Using new simulations, the Northwestern astrophysicists implemented simple conditions to model the feeding of a supermassive black hole and the organic formation of its jets and accretion disk. When the researchers ran the simulation, the simple conditions organically and unexpectedly led to the formation of an X-shaped radio galaxy.

Surprisingly, the researchers found that the galaxy’s characteristic X-shape resulted from the interaction between the jets and the gas falling into the black hole. Early in the simulation, the infalling gas deflected the newly formed jets, which turned on and off, erratically wobbled and inflated pairs of cavities in different directions to resemble an X-shape. Eventually, however, the jets became strong enough to push through the gas. At this point, the jets stabilized, stopped wobbling and propagated along one axis.

“We found that even with simple symmetric initial conditions, you can have quite a messy result,” said Northwestern’s Aretaios Lalakos, who led the study. “A popular explanation of X-shaped radio galaxies is that two galaxies collide, causing their supermassive black holes to merge, which changes the spin of the remnant black hole and the direction of the jet. Another idea is that the jet’s shape is altered as it interacts with large-scale gas enveloping an isolated supermassive black hole. Now, we have revealed, for the first time, that X-shaped radio galaxies can, in fact, be formed in a much simpler way.”

Lalakos is a graduate student in Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He is co-advised by paper coauthor Sasha Tchekhovskoy, an assistant professor of physics and astronomy at Northwestern and key member of CIERA, and Ore Gottlieb, a CIERA Postdoctoral Fellow.

Continue to the full Northwestern News story.

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Top Image and Video Credit: Aretaios Lalakos