Is there a big "X" in the universe? It's a unique radio galaxy formed by a supermassive black hole!

Jets of galaxies are extending out into space, and their formation may be far simpler than previously thought.

A complex model shows that these X-shaped ejections of celestial material into outer space may be formed by some simple mechanisms, and are much simpler than scientists previously thought. The model shows that this special shape is formed by the interaction between hot gas falling into the supermassive black hole in the galaxy and the light-speed jet near the black hole.

If this hypothesis is correct, then the model suggests that X-shaped radio galaxies are far more common than scientists thought. However, because X-shaped jets exist for a very short time, astronomers have not been lucky enough to discover more of them so far.

Figure: A simulation showing the development of an X-shaped jet. The red gaseous material is the gaseous material that is constantly falling into the black hole, while the blue material is the jet that is ejected from the black hole.

The new model, proposed by astrophysicists at Northwestern University in Illinois, is the first large model to track the long-distance infall of galactic gaseous matter onto the surface of a large celestial body. In this model, the researchers set up a simple condition for the supermassive black hole, which is that it absorbs a thin layer of surrounding matter, which scientists call an accretion disk. After adding this condition, the model unexpectedly formed an X-shaped radio galaxy structure.

"We found that even simple initial symmetry conditions can lead to chaotic structures," said Aretaeos Larakos, a graduate student in Northwestern's Department of Physics and Astronomy and an author of the study.

The model shows that X-shaped galaxies appear when infalling gaseous matter deflects ejected matter from supermassive black holes. Early in the model run, these jets appear to be created and destroyed on their own, and in irregular waves. These early irregular jets cause gas to expand, flow in different directions, and form X-shaped galaxies.

Image: The X-shaped galaxy PKS2014-55 forms a jet that extends into space 2.5 light-years away.

However, when the jets become powerful enough to push out the infalling gas, they stop fluctuating and become stable in a single dimension around the black hole.

This principle could replace the previously popular view that X-shaped radio galaxies are caused by galaxy collisions. Larakos described the scenario: "Two galaxies collided to form a supermassive black hole and changed the motion of the remaining black hole and the direction of its jets." Scientists have also proposed another hypothesis that the X-shape is caused by the interaction of the jets around the black hole with large gaseous nebulae.

"Now, our results show for the first time that X-shaped radio galaxies can form in a simpler way," Larakes said.

'X' shaped galaxy is a surprising discovery

Larax originally wanted to use the model to understand how much matter a supermassive black hole would consume, but he was surprised to discover the formation process of an X-shaped galaxy. When Sasha Chekovsky, an astrophysicist at Northwestern University, saw the shape presented by the model, she immediately realized the significance of this discovery.

"He told me, 'Silly, this is huge! It's an X-shape,'" Lalakos recalled. "He said astronomers had been observing galaxies of this shape their whole lives and had never been able to figure out how it formed. And we created it in a way that no one had before."

Larakos speculates that the key to the model's success and the huge attention it has received is its simplicity, and the fact that it does not set the gas around the black hole to be symmetric.

"Typically, researchers put the black hole in the middle of their model and have a large, built-in gaseous accretion disk around it," Lalakos said. "Afterwards, they might add gas around the disk. But in my model, I didn't have these complications."

Lalakos explained that in his team's model, there is no gas disk around the black hole in the early stages, but the gas disk appears when the rotating gas approaches the black hole. The gas around the gas disk falls into the black hole, forming a jet, which begins to fluctuate.

"I simplified my assumptions as much as possible, and the results I obtained were like a surprise. With such simple initial conditions, we saw the X pattern in the model for the first time," said Lalakos.

Such a simple structure may also indicate that X-shaped radio galaxies are very common in the universe, but they do not exist for long, which may also explain why astronomers have only observed about 10% of radio galaxies.

"X-shaped galaxies may appear when a black hole acquires new gas and re-absorbs it, so they may appear quite frequently, but we are just not lucky enough to see them," Lalakos said. "This shape only appears when the jets are energetic enough to push out gas."

Larakos will further study other possible shapes by modifying the model's parameters, such as the size of the accretion disk or the rotation speed of the supermassive black hole.

Such complex models are invaluable in research because it is difficult to observe real black holes, especially newly formed ones. "In most cosmic conditions, it is impossible to zoom in on the space around a black hole and see what is happening around it," Lalakos said.

"And even if we could observe it, we don't have enough time to observe it. If a supermassive black hole has already formed, we won't be able to observe its evolution because human lifespan is too short. In most cases, we rely on models to understand the phenomena around black holes," Lalakos added.

BY: Robert Lea

FY: Autumn

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