Sept. 21, 2022 — When astronomers use radio telescopes to study the night sky, they typically see elliptical-shaped galaxies, with twin jets blasting from either side of their central supermassive black hole.
But rarely — less than 10% of the time — astronomers might spot an X-shaped radio galaxy, with four jets extending far into space.
A new Northwestern University study sheds new insight into how X-shaped radio galaxies form. The study also found that this galaxy shape might be more common than previously thought.
The study was published August 2022 in the Astrophysical Journal Letters. It marks the first large-scale galaxy accretion simulation that tracks the galactic gas far from its central black hole all the way toward it.
Simple Conditions Lead to Messy Result
Using new simulations, the 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.
Study co-author Alexander Tchekhovskoy, an assistant professor in the Department of Physics and Astronomy at Northwestern, was awarded a large resource allocation on the National Science Foundation-funded Frontera supercomputer at the Texas Advanced Computing Center.
Tchekhovskoy made use of the Longhorn graphics processing unit (GPU)-based subsystem of Frontera, specifically for his team’s GPU-accelerated code H-AMR. It takes advantage of Longhorn’s V100 GPUs and includes advanced features such as adaptive mesh refinement and adaptive time-stepping.
“These amazing machines, with short turnaround and superb tech support, have enabled quick research results and enabled us to efficiently push the science forward,” Tchekhovskoy said.
The surprising science results from the simulations showed that the galaxy’s characteristic X-shape evolved from the interaction between the jets and the gas falling into the black hole.
Early in the simulation, the infalling gas deflected the newly formed jets, which turned on and off, erratically wobbled and inflated pairs of cavities in different directions to resemble an X-shape. Eventually, however, the jets became strong enough to push through the gas. At this point, the jets stabilized, stopped wobbling and propagated along one axis.
“We found that even with simple symmetric initial conditions, you can have quite a messy result,” said Northwestern graduate student Aretaios Lalakos, who led the study. “A popular explanation of X-shaped radio galaxies is that two galaxies collide, causing their supermassive black holes to merge, which changes the spin of the remnant black hole and the direction of the jet. Another idea is that the jet’s shape is altered as it interacts with large-scale gas enveloping 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.”
‘Not Lucky Enough to See Them’
Because the X-shape only emerged early in the simulation — until the jets strengthened and stabilized — Lalakos believes X-shaped radio galaxies might appear more frequently, but last a very short time, in the universe than previously thought.
“They might arise every time the black hole gets new gas and starts eating again,” he said. “So, they might be happening frequently, but we might not be lucky enough to see them because they only happen for as long as the power of the jet is too weak to push the gas away.”
Next, Lalakos plans to continue running simulations to better understand how these X-shapes arise. He looks forward to experimenting with the size of the accretion disks and spins of central black holes. In other simulations, Lalakos included accretion disks that were almost non-existent all the way up to extremely large — none led to the elusive X-shape.
“Supercomputers, such as TACC’s Longhorn, utilize cutting-edge hardware architecture — graphical processing units (GPUs) — which represent the future of supercomputing,” said Tchekhovskoy. He credited the energy efficiency and improved speed compared to central processing units -based systems commonly used by scientists.
“GPUs enable us to carry out simulations previously deemed impossible. The GPU-powered TACC Longhorn supercomputer has enabled us to carry out some of the longest duration simulations of accreting black holes and connect, for the first time, the central supermassive black holes to their galaxies, at the largest scale separation to date,” Tchekhovskoy remarked.
Source: Amanda Morris, Northwestern & Jorge Salazar, TACC