Rare X-shaped radio galaxies formed from supermassive black holes

A team of astrophysicists may have finally solved the mystery of what creates the rarely observed radio-bright X-shaped galaxies.

Sophisticated simulations have shown that these galaxies, featuring four massive jets of matter that are ejected into space, may be the result of a much simpler process than scientists previously thought. Simulations show that the shape can be formed as a result of the interaction of hot gas falling into the galaxy’s supermassive black hole and jets of material ejected from it at a speed close to the speed of light.

If the model is correct, the model could also suggest that X-shaped radio galaxies are more common than currently thought, but short-lived and losing their characteristic X-shaped jets, so astronomers just haven’t been lucky enough to find many galaxies. at this fleeting stage.

Related: Why do galaxies have different shapes?

The simulation shows the development of an X-shaped jet with the infalling gas in red and jets launched by the black hole in blue. (Image credit: Aretaios Lalakos)

The new simulation was conducted by astrophysicists at Northwestern University in Illinois and is the first large-scale simulation that tracks galactic gas traveling a long distance to hit the surface of a titanic object. In the model, the researchers applied simple conditions to a model of a supermassive black hole consuming gas from a thin disk of matter surrounding it, which scientists call an accretion disk. And this simulation showed that simple conditions organically and unexpectedly led to the formation of an X-shaped radio galaxy.

“We found that even with simple, symmetrical initial conditions, you can get a rather confusing result,” Aretaios Lalakos, lead author of the paper and a PhD student in the Department of Physics and Astronomy at Northwestern University, said in a statement. (will open in a new tab).

Simulations have shown that the characteristic X-shape of these galaxies originated when the infalling gas deflected the jets released by the supermassive black hole. The nozzles themselves turned on and off at an early stage of the simulation, as well as randomly “swinging”. This early erratic behavior caused pairs of gaps or cavities in the gas to bulge outward, creating an X-shape.

X-shaped galaxy PKS 2014–55 with bright radio jets extending 2.5 light-years into space. (Image credit: NRAO/AUI/NSF/SARAO/DES)

However, when the jets became strong enough to push through the falling gas, they stabilized, stopped swaying, and merged into a single jet along one axis of the black hole.

This mechanism will replace the popular explanation for the formation of X-shaped radio galaxies – the collision of galaxies. “Two galaxies collide, causing their supermassive black holes to merge, which changes the rotation of the remaining black hole and the direction of the jet,” Lalakos said, describing this scenario. Scientists also suggested that the shape was formed as a result of the interaction between the jet and a large cloud of gas surrounding a supermassive black hole.

“Now, for the first time, we have discovered that X-shaped radio galaxies can actually form in a much simpler way,” Lalakos said.

“X” marks the location of an unexpected discovery

The discovery of this path to the birth of an X-shaped galaxy surprised Lalacos, who started simulations to see how much matter supermassive black holes could consume. It was Sasha Chekovskoy, an astrophysicist from the Northwest, who noticed the significance of the image that appeared as a result of the simulation.

“Hi [Tchekovskoy] said: “Dude, this is very important! It’s an X-shape!” He told me that astronomers had observed this in real life and didn’t know how they formed,” Lalakos said. “We created it in a way that no one had ever imagined before.”

Lalacos suspects that the key to this simulation’s success, and the reason it stumbled upon a previously unseen phenomenon, is its simplicity, and the fact that it didn’t dictate the symmetry of the gas surrounding the supermassive black hole.

“Usually, researchers would put the black hole at the center of the simulation grid and put a large, already formed disk of gas around it, and then they could add surrounding gas from outside the disk,” Lalakos said. “In my simulation, I tried not to assume anything.”

Lalakos explained that in his team’s simulations, the black hole starts without a surrounding disk of gas, which instead forms as the already spinning gas approaches the black hole. The gas from this disk then enters the black hole, creating jets that soon begin to oscillate.

“I made the most basic assumptions, so the whole result came as a surprise,” he said. “This is the first time anyone has seen X-shaped morphology in a simulation from very simple initial conditions.”

Such a simple formation could mean that X-shaped radio galaxies are common but short-lived, potentially explaining why these galaxies account for only 10% of the radio galaxies detected by astronomers.

“They can happen every time a black hole gets new gas and starts eating again,” Lalakos said. “So they can happen often, but we might not be lucky to see them because they only happen as long as the power of the jet is too weak to push the gas away.”

Lalacos will further explore this potential formation model by experimenting with simulation parameters such as the size of the accretion disk and the rotation of the central supermassive black hole.

Sophisticated simulations are critical in research like this because of the difficulties involved in observing black holes, especially new ones, Lalakos said. “For most of the universe, it’s impossible to zoom right in the center and see what’s going on very close to a black hole,” he said.

“And even what we can observe, we are limited by time. If a supermassive black hole has already formed, we can’t watch it evolve because human life is too short,” he added. “Most of the time, we rely on simulations to understand what’s going on near a black hole.”

The team’s results are presented in a paper to be published in the Astrophysical Journal Letters on Monday (August 29).

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