Accidental discovery shows that rare X-shaped galaxies can form more easily than thought

Lthe galaxies are not just spirals. There are elliptical galaxies as well as rare X-shaped or winged galaxies.

This week, new 3D modeling offers insight into how these X-shaped astronomical objects formed, but almost didn’t.

Header image, from study: Volumetric rendering of an X-shaped galaxy simulated in this study. (Aretaios Lalakos et al./Astrophysical Journal Letters)

When astronomers observe the universe through radio telescopes, they can see the two relativistic jets of radiation escaping from either side of supermassive black holes in elliptical galaxies.

But occasionally, less than 10% of the time, astronomers can spot the rare X-shaped radio galaxies with not two, but four beams reaching into space.

Below is an example with the giant radio galaxy in X, PKS 2014-55, whose formation was described in a previous study presented by your guru here.


These mysterious objects have puzzled astronomers for decades.

A new study from Northwestern University in the US shows that training them could be surprisingly simple. The study is the first to use a simulation of large-scale galaxy accretion to trace the motion of gas stretching away from the supermassive black hole.

Northwestern astrophysicists have found that all it takes are simple conditions to “seed” their supermassive black hole to form the four jets seen in X-shaped galaxies.

The researchers showed that the characteristic X-shape originates from the interaction between the black hole’s jets and the gas falling into it. At the start of the simulation, the newly formed jets are deflected by gas falling into the black hole, causing the radiation to turn on and off.

But the jets eventually overcome the throttle and propel themselves along a single axis.

As with the header image, the three-dimensional volume rendering of the density illustrates the natural evolution of the X-shaped jet morphology. The inflow of gas forms an accretion flow (pink), within which an accretion disk (yellow) forms, which feeds the black hole, which emits a pair of relativistic jets (light blue), which propagate vertically and shock the surrounding gas (dark red ). Ancient voids (dark blue), inflated by previously misaligned jets, float up at an angle to the vertically propagating jets, forming the X-shaped morphology of the jets. (Aretaios Lalakos/ Northwestern University)

According to Aretaios Lalakos, study director and graduate student at Northwestern University:

We found that even with simple symmetric initial conditions, the result can be quite messy. A popular explanation for X-shaped radio galaxies is that two galaxies collide, causing their supermassive black holes to merge, changing the spin of the remaining black hole and the direction of the jet.

Another idea is that the jet’s shape changes when it interacts with large amounts of gas covering an isolated supermassive black hole. Now, for the first time, we have revealed that X-shaped radio galaxies can actually form in a much simpler way.

Lalakos did not intend to simulate an X-shaped galaxy. His goal was to determine the amount of radio galaxies forming. His goal was to measure the amount of mass pulled into a black hole using simple data. While Lalakos did not initially see the importance of the X-shape created by his simulation, his supervisor and co-author of the paper, assistant professor Sasha Chekhovskoy, reacted to this discovery with great enthusiasm:

It is very important! It’s an X shape! He told me that astronomers have observed this in real life and don’t know how they form. We created it in a way that no one had ever imagined.

Where previous simulations failed, Lalakos’ model organically created the iconic X shape.

According to Lalakos:

In my simulation I tried not to assume anything. Usually, researchers place a black hole in the center of a simulation grid and place a large gaseous disk that has already formed around it, then they can add ambient gas to the outside of the disk. In this study, the simulation begins without a disc, but a disc quickly forms as the rotating gas approaches the black hole. This disk then feeds the black hole and creates jets. I made the simplest possible assumptions, so the result was a surprise. This is the first time anyone has seen an X-shaped morphology in simulations from very simple initial conditions.

Because the X shape only appeared early in the simulation, Lalakos believes that these rare galaxies may actually be more common in the universe, but only survive for a short time.

Still according to Lalakos:

They can appear every time the black hole gets new gas and starts eating again. It may therefore be that they are frequent, but that we have no chance of seeing them, because they only survive as long as the force of the jet is too weak to repel the gas.

Lalakos plans to perform additional simulations to better understand the formation of the X-shaped galaxies. In other simulations, very small accretion discs and extremely large accretion discs did not lead to the elusive shape of the X.

In most of the universe, it is impossible to zoom into the center and see what is happening very close to a black hole. In most cases, we rely on simulations to understand what is happening.

The study published in Astrophysical Journal Letters: Bridging the Bondi and Event Horizon Scales: 3D GRMHD Simulations Reveal X-shaped Radio Galaxy Morphology and presented on Northwestern University’s website: X-shaped radio galaxies may form more simply than expected.

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