X-shaped radio galaxies may arise more easily than previously thought


A still image taken from the 3D simulation of the natural development of an X-shaped jet. The gas (bright red) falls into the black hole, which launches a pair of relativistic jets (light blue). The jets propagate vertically and shock the ambient gas (dark red) The older cavities (dark blue) buoyantly rise at an angle to the vertically propagating jets to form the X-shape. Credit: Aretaios Lalakos/Northwestern University
A still image taken from the 3D simulation of the natural development of an X-shaped jet. The gas (bright red) falls into the black hole, which launches a pair of relativistic jets (light blue). The jets propagate vertically and shock the ambient gas (dark red) The older cavities (dark blue) buoyantly rise at an angle to the vertically propagating jets to form the X-shape. Credit: Aretaios Lalakos/Northwestern University

When astronomers look into the night sky with radio telescopes, they generally observe elliptical galaxies with twin jets erupting from each side of their center supermassive black hole. However, astronomers may notice something unusual and rarely fewer than 10% of the time: An X-shaped radio galaxy with four long-distance jets.


Although astrophysicists have been baffled by these odd X-shaped radio galaxies for two decades, a recent Northwestern University research gives fresh light on how they arise, and it's surprisingly easy. The research also discovered that X-shaped radio galaxies are more prevalent than previously imagined. The findings will be published in The Astrophysical Journal Letters on August 29. It is the first large-scale galaxy accretion simulation that follows galactic gas from far away to the supermassive black hole.

Northwestern astrophysicists used novel simulations to analyze the feeding of a supermassive black hole as well as the organic creation of its jets and accretion disk. When the researchers conducted the simulation, the basic parameters resulted in the development of an X-shaped radio galaxy, which was both organic and surprising.


Surprisingly, the researchers discovered that the galaxy's distinctive X-shape was caused by the interplay of the jets and the gas pouring into the black hole. Early in the simulation, the infalling gas deflected the freshly generated jets, causing them to switch on and off, wobble wildly, and inflate pairs of holes in opposite directions to form an X-shape. However, the jets eventually got powerful enough to push through the gas. The jets had steadied, stopped wobbling, and were propagating along one axis.


According to Aretaios Lalakos of Northwestern University, who conducted the study, "we discovered that even with simple symmetric beginning conditions, you may have quite a complicated conclusion." Two galaxies collide, causing their supermassive black holes to combine, changing the spin of the remaining black hole and the direction of the jet, according to one prevalent interpretation. Another theory is that the jet's form changes as it interacts with large-scale plasma encircling a single supermassive black hole. For the first time, we have discovered that X-shaped radio galaxies may be produced in a considerably easier manner.


Lalakos is a graduate student at the Weinberg College of Arts and Sciences at Northwestern University and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Sasha Tchekhovskoy, an associate professor of physics and astronomy at Northwestern and a major member of CIERA, and Ore Gottlieb, a CIERA postdoctoral scholar, counsel him.


Although radio galaxies emit visible light, they also contain extensive radio emission zones. M87, one of the most enormous galaxies in the cosmos, was made even more renowned in 2019 when the Event Horizon Telescope photographed its center supermassive black hole. X-shaped radio galaxies, first identified in 1992, account for fewer than 10% of all radio galaxies. Lalakos did not anticipate simulating an X-shaped galaxy when he set out to mimic a black hole. Instead, he sought to quantify the quantity of mass consumed by a black hole. He ran the simulation using some simple astronomical circumstances. Lalakos did not realize the significance of the growing X-shape at first, but Tchekhovskoy reacted with zeal.


Lalakos stated, "Dude, this is incredibly crucial!" This is an X-shaped shape! He informed me that astronomers had seen this in real life but didn't know how it happened. We built it in a way that no one had ever imagined.

Other astrophysicists have attempted to generate X-shaped formations artificially in prior simulations in order to explore how they form. However, Lalakos' simulation naturally resulted in the X-shape.


According to Lalakos, in my simulation, I attempted not to make any assumptions. Typically, researchers establish a black hole in the center of a simulation grid, surround it with a big, already-formed gaseous disk, and then add ambient gas outside the disk. The simulation in this study begins without a disk, but one rapidly appears as the whirling gas gets closer to the black hole. This disk then feeds the black hole, causing jets to form. I made the most basic assumptions conceivable, therefore the end result was unexpected. This is the first time that X-shaped morphology has been observed in simulations starting from relatively simple beginning circumstances.


Because the X-shape only appeared early in the simulation until the jets developed and stabilized, Lalakos believes X-shaped radio galaxies may arise more frequently in the cosmos than previously thought, but for a much shorter period of time.


He speculated that they may reappear whenever the black hole acquires more gas and begins devouring again. So they may occur frequently, but we may not be fortunate enough to witness them since they only occur for as long as the jet's force is insufficient to drive the gas away.


Lalakos intends to continue conducting simulations in order to better understand how these X-shapes form. He is excited to experiment with the size of accretion disks and the spins of core black holes. In subsequent simulations, Lalakos incorporated accretion disks ranging from nearly non-existent to exceedingly enormous, none of which resulted in the illusive X-shape.


According to Lalakos, it is difficult to zoom in right at the heart of the universe and see what is happening extremely close to a black hole. Even the things we can notice are limited by time. We cannot watch the evolution of a gigantic black hole if it has already created since human life is too brief. To understand what happens near a black hole, scientists usually depend on simulations.


The study is titled "Bridging the Bondi and Event Horizon Scales: 3D GRMHD simulations reveal X-shaped radio galaxy morphology."

Journal Information: Bridging the Bondi and Event Horizon Scales: 3D GRMHD simulations reveal X-shaped radio galaxy morphology, The Astrophysical Journal Letters (2022). arxiv.org/abs/2202.08281
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