How X-shaped radio galaxies are formed

Scientists have put forward a new simple theory of how radio galaxies are formed whose regions of radiation look like X. It turned out that the reason for this could be due to the pressure of the gas falling on the black hole.

Х-подібні радіогалактики
Gas jets escaping from a black hole. Source: Aretaios Lalakos/Northwestern University

New model

Radio galaxies are star systems that emit a much larger amount of energy in the corresponding range than the rest of their kind. The signal sources are the plasma flows surrounding them. They are emitted by supermassive black holes at the center, which accelerate gas from their poles. As a result, most radio galaxies look like jets that stream in opposite directions.

But among them there are those having four jets that resemble the letter “X”. These X-shaped radio galaxies make up about 10 percent of the total number of such systems. Their origin has been a mystery to scientists for several decades. And now specialists proposed a new model of the formation of these systems, depicted in a video.

According to the model, free gas (red in the video) begins to fall onto the supermassive black hole and forms an accretion disk (yellow) around it. At the same time, jets (blue) begin to shoot from the poles, breaking through the gas and causing its ionization.

However, at first they are rather weak and therefore strongly shift to the side. At the same time, the gas remains heated and emits radio waves. Later, the jets become stronger and the gas falling on the black hole can no longer stop them. This is how X-shaped radio galaxies arise.

X-shaped radio galaxies: other theories of origin

The proposed theory of how X-shaped radio galaxies arise is valuable in that it does not require any special conditions. Scientists have found out that in order for it to be realized, it is not even necessary that the gaseous environment be heterogeneous. This means that many such objects can be formed in this way.

Scientists have already proposed several other scenarios for the formation of X-shaped radio galaxies. For example, scientists have assumed that they can be formed when two star systems collide. Then such a cruciform pattern is generated due to the interaction of two black holes, each of which forms its own pair of jets.

Another possible scenario involved a change in the shape of the jet upon its encounter with a large gas cloud. But this does not explain at all why the four jets look more or less symmetric. Furthermore, it doesn’t explain why X-shaped radio galaxies are so widespread.

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