Accidental discovery shows rare X-shaped galaxies might form more simply than expected

Galaxies don’t just come in spirals. There are elliptical galaxies as well as the rare X-shaped, or winged, galaxies.

Now, new 3D modelling gives us insight into the formation of these X-shaped astronomical objects, but it almost didn’t happen.

When astronomers gaze into the universe using radio telescopes, they can see the twin jets of radiation that blast out of either side of the supermassive black holes in elliptical galaxies.

But occasionally – less than 10% of the time – astronomers might spot the rare X-shaped radio galaxies with not just two, but four jets of radiation extending into space.

These mysterious objects have puzzled astronomers for decades.

A new study coming out of Northwestern University in the US shows that their formation may be surprisingly straightforward. The study is the first to use large-scale galaxy accretion simulation to track the movement of gas stretching far from the supermassive black hole. The findings are published in the Astrophysical Journal Letters.

Northwestern astrophysicists found they needed only simple conditions to “feed” their supermassive black hole to form the four jets seen in X-shaped galaxies.

The researchers showed that the characteristic X-shape results from the interaction between the black hole’s jets and the gas falling into the black hole. As the simulation begins, the newly formed jets are deflected by infalling gas, effectively turning the radiation blasts on and off.

Three-dimensional volume rendering of density illustrates the natural development of X-shaped jet morphology. Gas infall forms an accretion flow (bright red) deep within which we witness the formation of an accretion disk (yellow) that feeds the black hole, that launches a pair of relativistic jets (light blue), which propagate vertically and shock the ambient gas (dark red). The older cavities (dark blue), which were inflated by previous misaligned jet activity, buoyantly rise at an angle to the vertically-propagating jets and form the X-shaped jet morphology. Credit: Aretaios Lalakos/Northwestern University.

Eventually, however, the jets overcome the gas to push through along a single axis.

“We found that even with simple symmetric initial conditions, you can have quite a messy result,” says study leader Aretaios Lalakos and a graduate student at Northwestern.

“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 amounts of 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.”

Read more: Scientists believe they have resolved a fundamental signature of gravity around a black hole

Lalakos did not intend to simulate an X-shaped galaxy. His aim was to measure the amount of mass drawn into a black hole using simple inputs. While Lalakos did not initially see the significance of the X-shape that his simulation created, his supervisor, and co-author of the paper, Assistant Professor Sasha Tchekhovskoy, responded to the discovery very enthusiastically.

“He said, ‘Dude, this is very important! This is an X-shape!’” Lalakos recalls. “He told me that astronomers have observed this in real life and didn’t know how they formed. We created it in a way that no one had even speculated before.”

Where previous simulations had failed, Lalakos’s model organically created the iconic X-shape.

“In my simulation, I tried to assume nothing,” Lalakos explains. “Usually, researchers put a black hole in the middle of a simulation grid and place a large, already-formed gaseous disk around it, and then they may add ambient gas outside the disk. In this study, the simulation starts without a disk, but soon one forms as the rotating gas gets closer to the black hole. This disk then feeds the black hole and creates jets. I made the simplest assumptions possible, so the whole outcome was a surprise. This is the first time anyone has seen X-shaped morphology in simulations from very simple initial conditions.”

Because the X-shape only emerged early in the simulation, Lalakos believes these rare galaxies might actually be more common in the universe, but only survive a short time.

“They might arise every time the black hole gets new gas and starts eating again,” Lalakos says. “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.”

Lalakos plans to run more simulations to better understand the formation of X-shaped galaxies. In other simulations, very small accretion disks and extremely large accretion disks did not lead to the elusive X-shape.

“For most of the universe, it’s impossible to zoom in right at the centre and see what’s happening very near a black hole,” Lalakos adds. “In most cases, we rely on simulations to understand what happens.”

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