X-shaped radio galaxies could form more easily than expected, study says


New research has found that X-shaped radio galaxies could form more easily than expected and be more common than previously believed, according to their authors published in the journal ‘Astrophysical Journal Letters’.

When astronomers use radio telescopes to look at the night sky, they typically see elliptical-shaped galaxies with twin jets exiting on either side of their central supermassive black holebut occasionally (less than 10% of the time) they can see something of an X-shaped radio galaxy, with four jets stretching across space.

Although these mysterious X-shaped radio galaxies have stumped astrophysicists for two decades, a new study from Northwestern University sheds new light on how they form — and it’s surprisingly straightforward — and also reveals that radio galaxies in X-shaped could be more common than previously thought.

This is the first simulation of large-scale galaxy accretion that follows galactic gas away from the supermassive black hole all the way to it.highlight the researchers.

Using new simulations, 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, They discovered that the characteristic X-shape of the galaxy was the result of the interaction between the jets and the gas that fell into the black hole..

Early in the simulation, gas entering the black hole deflected newly formed jets, which flickered on and off, wobbled erratically, and inflated pairs of cavities in different directions to resemble an X-shape. jets became strong enough to pierce through the gas. At that point, the jets stabilized, stopped wobbling, and spread along one axis.

We find that, even with simple symmetric initial conditions, a rather messy result can be obtained explains Aretaios Lalakos of Northwestern, who led the study. A popular explanation for X-shaped radio galaxies is that two galaxies collide, causing their supermassive black holes to merge, which changes the remnant black hole’s spin and jet direction.”

“Another idea is that the shape of the jet is altered by interacting with the large-scale gas that surrounds an isolated supermassive black hole,” he continues. “Now, we have revealed, for the first time, that X-shaped radio galaxies can in fact, to train 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) and is advised by the paper’s co-author Sasha Tchekhovskoy, an adjunct professor of physics and astronomy at Northwestern and CIERA key member, and by Ore Gottlieb, CIERA postdoctoral fellow.

Although radio galaxies emit visible light, they also encompass large regions of radio emission. Perhaps the most famous radio galaxy is M87, one of the most massive galaxies in the universe, made even more popular in 2019 when the Event Horizon telescope imaged its central supermassive black hole.. First recognized in 1992, X-shaped radio galaxies make up less than 10% of all radio galaxies.

When Lalakos set out to model a black hole, he did not expect to simulate an X-shaped galaxy, but instead intended to measure the amount of mass consumed by a black hole. To do this he introduced simple astronomical conditions into the simulation and let it run. As he recalls, he did not initially recognize the importance of the emerging X shape, but Tchekhovskoy did and reacted enthusiastically.

“He told me, ‘Dude, this is very important! This is an X shape,'” recalls Lalakos. “And he told me that astronomers had observed this in real life and didn’t know how it formed. way no one had ever speculated before.

In previous simulations, other astrophysicists have tried to artificially create X-shaped structures to study how they arise. But the Lalakos simulation led organically to the X shape.

“In my simulation, I tried not to assume anything,” says Lalakos. Typically, researchers put a black hole in the center of a simulation grid. and they put a big already-formed gaseous disk around it, and then they can add ambient gas outside the disk.”

As he explains, in his study “the simulation starts with no disk, but one soon 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, for what the result was a surprise. This is the first time anyone has seen an X-shaped morphology in simulations from very simple initial conditions.”Add.

Since the X-shape only emerged early in the simulation, until the jets strengthened and stabilized, Lalakos believes that X-shaped radio galaxies could appear more frequently, but last very briefly, in the universe than previously thought. thought.

“They could arise every time the black hole gets new gas and starts eating again,” he says. So they might be happening frequently, but we might not be lucky enough to see them because they only happen while the power of the jet is too weak to push the gas.”

Now Lalakos plans to continue running simulations to better understand how these X-shapes arise and hopes to experiment with the size of accretion disks and the spins of central black holes. In other simulations, Lalakos included almost nonexistent to extremely large accretion disks, but none gave rise to the elusive X-shape.

“For most of the universe, it’s impossible to get close to the center and see what’s going on very close to a black hole,” Lalakos said. “And even the things we can observe, we’re limited by time. If the supermassive black hole is already formed, we cannot observe its evolution because human life is too short. In most cases, we rely on simulations to understand what happens near a black hole,” he concludes.

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