Extragalactic jets, and why they collapse

Weak points in the structure of extragalactic jets may be what causes them to collapse into enormous plumes, according to researchers at the University of Leeds, UK.

First observed in 1918 zooming out from the massive galaxy Messier 87, extragalactic jets are powerful streams of particles blasted from feeding supermassive black holes, usually at the hearts of active galaxies such as quasars and radio galaxies. They are remarkably energetic and stretch out for millions of light-years.

Scientists still don’t know much about these powerful phenomena. Even their composition is uncertain. Two popular models suggest they are made up of either positron-electron plasma or a mix of electrons, positrons, and atomic nuclei. It is well-known, however, that although they mostly remain stable, sometimes these monstrous streamers disintegrate into huge plume-like structures.

The study, published in Nature Astronomy, discovered that this disintegration is likely caused by unexpected weak points, causing instabilities similar to those that can develop in water flowing through a curved pipe.

According to lead author Kostas Gourgouliatos, the weak points are created by their narrow oval shape, which gives them a curved boundary.

“Instability starts at the curved boundary, travels upstream on the jet and then converges at one point,” explains Gourgouliatos. “Below this point the jet stays tidy and tight but everything above will be destroyed and creates a large cosmic plume.”

The research was conducted mathematically, using 3D computer simulations to study how instabilities and turbulence develop. But the collapse can also be observed.

“When the jet disintegrates into a plume it releases heat, making them easier to spot on telescopes,” Gourgouliatos says. “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.”

The formation and evolution of these highly complex phenomena are still open areas of research. It is thought that as a black hole devours gas and dust from its accretion disc, particles can be accelerated to immense speeds and form two narrow but highly energetic beams, like giant party poppers in space.

“The observed instability exhibited some rather unexpected features,” adds co-author Serguei Komissarov. The stability seems to be related to the centrifugal force acting on the fluid elements that zoom out along curved streamlines. According to Komissarov, nobody expected such centrifugal instability to be important in jet dynamics.