When astronomers use radio telescopes to look at the night sky, they often see elliptical galaxies, with twin jets exploding from either side of their central supermassive black holes. But every now and then – less than 10% of the time – astronomers can spot something special and rare: the X-shaped radio galaxywith four jets reaching far into space.
Although these mysterious X-shaped radio galaxies have puzzled astrophysicists for two decades, a new Northwestern University study offers new insight into how they form – and it’s amazingly simple. The study also found that X-shaped radio galaxies may be more common than previously thought.
The research has been published in the Astrophysical Journal Letters. It marks the first large-scale galactic accretion simulation to track galactic gas far away from the supermassive black hole towards it.
Simple conditions lead to messy results
Using new simulations, Northwestern astrophysicists implemented simple conditions to model the feeding of a supermassive black hole and the organic formation of jets and the accretion disk of a supermassive black hole. it. When the researchers ran the simulations, surprisingly simple conditions led to the formation of an X-shaped radio galaxy.
A still image captured from a 3D simulation of the natural evolution of the X-shaped jet. The gas (magenta) falls into the black hole, ejecting a pair of relativistic jets (light blue). ). The sparks travel vertically and vibrate the surrounding gas (dark red) Older chambers (dark blue) rise at an angle to the vertically propagated rays to form an X shape. Image credit: Northwestern University
Surprisingly, the researchers discovered that the galaxy’s signature X-shaped shape is the result of an interaction between jet rays and gas falling into the black hole. In the early stages of the simulation, the gas giant deflected newly formed jets, which turned on and off, pairs of chambers oscillating and erratic inflating in different directions to resemble an X. Eventually, the jets become strong enough to push the gas through. At this point, the jet stabilizes, stops oscillating and propagates along an axis.
We found that even with simple symmetric initial conditions, you can get a rather messy result,” says Northwestern’s. Aretaios Lalakos, who led the study. “A common explanation for X-shaped radio galaxies is that two galaxies collide, causing their supermassive black holes to merge, which changes the residual black hole’s rotation and orientation. of jet rays. Another idea is that the jet’s shape is altered when it interacts with the large-scale gas surrounding an isolated supermassive black hole. Now, we have revealed for the first time that X-shaped radio galaxies can, in fact, be formed in a much simpler way. ”
Lalakos is a graduate student at Northwestern’s Weinberg College of Arts and Sciences and a member of Center for Interdisciplinary Discovery and Research in Astrophysics (CIERA). He was consulted by co-author of the paper Sasha Tchekhovskoyan assistant professor of physics and astronomy at Northwestern and principal member of CIERA, and Ore Gottlieb, a CIERA postdoctoral fellow.
A random X shape
Although radio galaxies emit visible light, they also include large regions of radio emission. Perhaps the most famous radio galaxy is M87, one of the most massive galaxies in the universe, which became more widely known in 2019 when the Event Horizon Telescope photographed the central supermassive black hole. its. First formed in 1992, X-shaped radio galaxies make up less than 10% of all radio galaxies.
When Lalakos started modeling a black hole, he didn’t expect to simulate an X-shaped galaxy. Instead, he aimed to measure the mass a black hole eats up. He entered simple astronomical conditions into the simulation and let it run. At first Lalakos did not realize the importance of the emerging X, but Tchekhovskoy responded with enthusiasm.
“He said, ‘Dude, this is important! This is an X shape! ‘ We created it in a way that nobody could have speculated before.” – Aretaios Lalakos, PhD student, CIERA
“He said, ‘Dude, this is important! This is an X shape! ‘” Lalakos said. “He told me that astronomers had observed these in real life and had no idea how they formed. We created it in a way that no one could have speculated before.”
In previous simulations, other astrophysicists have attempted to artificially create X-shaped structures to study how they form. But Lalakos’ simulation essentially resulted in an X-shape.
“In my simulation, I tried to assume nothing,” Lalakos said. “Normally, researchers place a black hole in the middle of a simulated grid and place a large, already formed disk of gas around it, then they can add gas around the outside of the disk. In this study, the simulation started without the disk, but soon a disk formed as the gas rotated closer to the black hole. This disk then feeds the black hole and creates jet jets. I made the simplest possible assumptions, so the whole result was a surprise. This is the first time anyone has seen an X-shaped morphology in a simulation from very simple initial conditions.”
‘Not lucky enough to see them’
Because X-shapes appear only early in the simulation – until the jets are strong and stable – Lalakos believes X-shaped radio galaxies may appear more frequently, but exist in a very short time, in the universe than previously thought.
“They can arise every time the black hole receives new gas and starts eating again,” he said. “So they can happen often, but we might not be lucky enough to see them because they only happen when the jet’s power is too weak to push the gas away.”
Next, Lalakos plans to continue running simulations to better understand how these X-shapes form. He wanted to experiment with the size of the accretion disks and the rotation of the central black holes. In other simulations, Lalakos consists of accretion disks that are almost nonexistent up to the megalithic – none of which results in an elusive X-shape.
“For most of the universe, it’s not possible to zoom in right at the center and see what’s happening so close to a black hole,” Lalakos said. “And even the things we can observe, we are limited by time. If a supermassive black hole had already formed, we would not be able to observe its evolution because the lifespan of humans is too short. We often rely on simulations to understand what happens near black holes.”
Source: WOMENorthwestern university
