A simulation of the powerful jets generated by supermassive black holes at the centers of the largest galaxies¹ explains why some burst forth² as bright beacons³ visible across the universe, while others fall apart and never pierce the halo of the galaxy⁴. About 10 percent of all galaxies with active nuclei⁵ - all presumed to have supermassive black holes within the central bulge⁶ - are observed to have jets of gas spurting⁷ in opposite directions from the core. The hot ionized gas is propelled by the twisting magnetic fields of the rotating black hole, which can be as large as several billion suns.

 

A 40-year-old puzzle was why some jets are hefty and punch out of the galaxy into intergalactic space, while others are narrow and often fizzle out before reaching the edge of the galaxy. The answer could shed light on how galaxies and their central black holes evolve, since aborted⁸ jets are thought to roil⁹ the galaxy and slow star formation, while also slowing the infall of gas that has been feeding the voracious¹⁰ black hole. The model could also help astronomers¹¹ understand other types of jets, such as those produced by individual stars, which we see as gamma-ray bursts or pulsars.

 

"Whereas it was rather easy to reproduce the stable jets in simulations, it turned out to be an extreme challenge to explain what causes the jets to fall apart," said University of California, Berkeley theoretical astrophysicist Alexander Tchekhovskoy, a NASA Einstein postdoctoral fellow, who led the project. "To explain why some jets are unstable¹², researchers had to resort to explanations such as red giant stars in the jets' path loading the jets with too much gas and making them heavy and unstable so that the jets fall apart."

 

By taking into account the magnetic fields that generate these jets, Tchekhovskoy and colleague Omer Bromberg, a former Lyman Spitzer Jr. postdoctoral fellow at Princeton University, discovered that magnetic instabilities in the jet determine their fate. If the jet is not powerful enough to penetrate¹³ the surrounding gas, the jet becomes narrow or collimated, a shape prone¹⁴ to kinking and breaking. When this happens, the hot ionized gas funneled¹⁵ through the magnetic field spews into the galaxy, inflating¹⁶ a hot bubble of gas that generally heats up the galaxy. 

 

Powerful jets, however, are broader and able to punch through the surrounding gas into the intergalactic medium. The determining factors are the power of the jet and how quickly the gas density¹⁷ drops off with distance, typically dependent on the mass and radius¹⁸ of the galaxy core.

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