X-shaped radio galaxies might form more simply than expected

Astronomers frequently observe elliptical-shaped galaxies with twin jets erupting from either side of their central supermassive black hole when using radio telescopes to observe the night sky. However, astronomers may occasionally—less than 10% of the time—discover something unique and uncommon: a radio galaxy in the shape of an X with four long-distance jets.

Despite the fact that astrophysicists have been baffled by these unusual X-shaped radio galaxies for the past two decades, a recent study from Northwestern University offers new information about their shockingly straightforward formation process. Additionally, the study discovered that X-shaped radio galaxies might be more widespread than previously believed.

The Astrophysical Journal Letters will publish the paper on August 29. The galactic gas far from the supermassive black hole is followed all the way toward it, making it the first large-scale galaxy accretion simulation to do so.

The Northwestern astrophysicists used fresh simulations to incorporate straightforward circumstances to simulate the feeding of a supermassive black hole and the organic creation of its jets and accretion disk. When the simulation was run, the straightforward circumstances organically and unpredictably resulted in the construction of an X-shaped radio galaxy.

Unexpectedly, the team discovered that the galaxy's distinctive X-shape came about as a result of the interplay between the jets and the gas accreting into the black hole. The freshly created jets were deflected by the infalling gas early in the simulation; these jets then erratically wobbled, turned on and off, and inflated pairs of cavities in opposite directions to approximate an X-shape. However, the jets eventually developed enough force to cut through the gas. The jets then steadied, stopped swaying, and began to spread along a single axis.

Aretaios Lalakos of Northwestern University, who conducted the investigation, said, "We found that even with simple symmetric initial circumstances, you can have quite a complex conclusion." "According to a widely accepted theory, two galaxies collide, causing their supermassive black holes to combine. This results in a change in the leftover black hole's spin and the jet's direction. Another hypothesis is that the jet interacts with the vast gas surrounding an isolated supermassive black hole, changing its shape. Now, we have demonstrated that X-shaped radio galaxies can actually form in a much more straightforward manner for the first time."

The Center for Interdisciplinary Exploration and Research in Astrophysics is where Lalakos, a PhD student in Northwestern's Weinberg College of Arts and Sciences, participates (CIERA). He is co-advised by Ore Gottlieb, a CIERA postdoctoral scholar, and Sasha Tchekhovskoy, an assistant professor of physics and astronomy at Northwestern University and an important member of CIERA.

Radio galaxies have substantial areas of radio emission in addition to visible light emission. One of the universe's most massive galaxies, M87, which gained additional notoriety in 2019 when the Event Horizon Telescope captured an image of its center supermassive blackhole, is possibly the most well-known radio galaxy. Less than 10% of all radio galaxies are X-shaped radio galaxies, a term that was first used in 1992.

Lalakos did not intend to simulate an X-shaped galaxy when he started out to simulate a black hole. Instead, he sought to quantify the mass that a black hole consumes. He ran the simulation after entering some basic astronomical parameters. Lalakos was originally unaware of the significance of the developing X-shape, but Tchekhovskoy responded with enthusiasm.

"Dude, this is really essential, he exclaimed. This shape is an X." stated Lalakos. "He informed me that astronomers have seen this in the actual world but were unsure of their origin. It was made by us in a way that had never even been considered before."

Other astrophysicists have sought to intentionally produce X-shaped structures to understand how they develop in earlier simulations. But the X-shape resulted naturally from Lalakos' simulation.

Lalakos stated, "In my simulation, I sought to make no assumptions. "Typically, scientists center a black hole in the center of a simulation grid, surround it with a sizable gaseous disk that has already formed, and then they may add ambient gas outside the disk. In this study, a disk does not initially appear in the simulation, but one does as the rotating gas approaches the black hole. The black hole is then fed by this disk, which also produces jets. The entire result was unexpected because I only made the most basic assumptions. This is the first time X-shaped morphogenesis has been observed in simulations starting from very basic initial conditions."

Lalakos speculates that X-shaped radio galaxies may be more common than previously assumed in the universe, despite they only exist for relatively brief periods of time. This is because the X-shape only appeared early in the simulation, before the jets strengthened and stabilized.

He speculated that they might appear each time the black hole acquires more gas and resumes its feeding. Since they only occur while the jet's force is insufficient to push the gas away, they may occur frequently but we may not be fortunate enough to witness them.

Lalakos will then keep conducting simulations to learn more about how these X-shapes form. He is eager to conduct experiments to determine the spin and accretion disk sizes of central black holes. Other simulations by Lalakos tried accretion disks with sizes ranging from nearly nonexistent to extremely huge, but none of them produced the illusive X-shape.

It is impossible to zoom in to the very heart of the cosmos and observe what is happening in close proximity to a black hole, according to Lalakos. "Even the things we can notice are limited by the passage of time. We cannot see the evolution of a supermassive black hole if it has already created because the lifespan of an individual is too short. Most of the time, scientists rely on simulations to comprehend what transpires close to a black hole."

Northwestern University