Why some black hole jets blast out their galaxy – and others fail


Why do some supermassive black holes blast immensely powerful jets that punch through their galaxy and can be seen millions of light-years away, while others seem to fizzle and fall apart?

A computer simulation may have the answer – the stability of a black hole’s spinning, coiled magnetic field. A pair of astrophysicists – University of California, Berkeley’s Alexander Tchekhovskoy and Princeton University’s Omer Bromberg – published modelling showing this in the Monthly Notices of the Royal Astronomical Society.

Around one in 10 galaxies have jets of hot, ionised gas blasting from the supermassive black hole lying in its core, which spins like a cinnamon doughnut. 

Some of these galaxies’ jets are broad and strong, while others are narrow and weak. So what dictates the power of a set of jets?

It’s a mystery known as the “Fanaroff-Riley morphological dichotomy of jets”, first pointed out by astrophysicists Bernie Fanaroff of South Africa and Julia Riley of the UK in 1974.

Previous simulations easily demonstrated how a thick, powerful jet could punch out of its galaxy. But no one had sufficiently explained how they could fizzle out, and not come anywhere near the edge of the galaxy. Some thought stars in the jets’ path toppled them off course, Tchekhovskoy says. 

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The black hole at the centre of the galaxy M87 produced a weak jet that could not break out of the galaxy, as seen in this radio image from 1989. As in the new computer simulation, stalled jets dump hot gas into giant bubble-like structures that heat up the galaxy. These stalled jets may be part of the black hole feedback mechanism that periodically halts the inflow of gas that feeds the black hole.

So Tchekhovskoy and Bromberg incorporated the magnetic field that barrels out of a black hole and pushes out the jets into their modelling.

They saw the power of a jet, and its success in escaping the galaxy, is heavily dependent on the magnetic field. Without a stable field, the jet is narrow and prone to twisting and snapping – like a twig.

But if with the power of a beefy magnetic field driving it, a jet is thick, like a tree trunk.

Gas density of the galaxy also played a big role. If it drops off quickly, jets can travel farther. But if there’s thick gas extending further, it impedes the jets’ range.

The simulation agrees well with observations. An example of a weak jet is in the galaxy M87, one of the closest such jets to Earth at a distance of about 50 million light-years, and has a central black hole equal to about six billion suns.

Tchekhovskoy says the unstable jets contribute to “black hole feedback”, when the material around the black hole tends to slow its intake of gas and thus its growth.

It’s not forever, though – supermassive black holes go through bingeing cycles, interrupted by unstable jets.

Next, the pair wants to improve their simulation and incorporate the smaller effects of gravity, buoyancy and the thermal pressure of the gas between stars and galaxies.

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