Astronomers Have Worked Out Why Extragalactic Jets Form Those Huge Plumes

The huge jets of radio galaxy Hercules A. NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)

Extragalactic jets are one of the most spectacular objects in the universe, with the most energetic extending for millions of light-years. Scientists have struggled to recreate these jets in the lab, but a new model could help explain what has been observed.

Some of the jets end up breaking up in huge plumes and according to the new model, it’s due to an instability never considered important before. As reported in Nature Astronomy, the researchers have likened the motion of the jet to water flowing down a circular pipe.

The jets form near the supermassive black holes at the center of galaxies. As they move into intergalactic space, the core of the jet moves further out than the sides, creating a distinctive curved shape. And this shape is responsible for the eventual break-up of the jet.

“These jets have a narrow oval shape which gives them a curved boundary. It is this shape that creates a weak point in the jet,” lead author Dr Kostas Gourgouliatos, from Durham University, said in a statement. “Instability starts at the curved boundary, travels upstream on the jet and then converges at one point – what we refer to as the ‘reconfinement point’.”

Below the reconfinement point, the jet remains narrow and tight, but once material passes this location, it becomes turbulent and forms a plume. Many of these plumes have been observed.

“When the jet disintegrates into a plume it releases heat, making them easier to spot on telescopes," said Gourgouliatos. "The jets and their plumes are so bright that sometimes they outshine their host galaxies and are always more easily spotted than black holes, which are inferred indirectly, in space observations.”

The model shows that these jets are subjected to many sources of instability, which are believed to compete with each other. One of them has to do with the velocity shear, similar to how waves are formed at sea. In the researchers' models, the dominant one turned out to be related to the centrifugal force experienced by material in the jet.

“We did expect instability associated with velocity shear to develop at the jet reconfinement but not as fast,” co-author Professor Serguei Komissarov, from the University of Leeds, added. “Moreover, the observed instability exhibited some rather unexpected features. It turned out that it was related to the centrifugal force acting on the fluid elements traveling along curved streamlines. This centrifugal instability is well studied but nobody expected it to be important for the jet dynamics.”

This is not the final word on extragalactic jet dynamics, but the researchers are already working on improving the current model by adding magnetic fields, which are important in jets and may cause more instabilities along the way.

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