How do black holes form jets?
This post was written by Radio Galaxy Zoo team member Stas Shabala, an astronomer at the University of Tasmania.
The supermassive black holes at the hearts of galaxies are supposed to be simple. For someone looking at a black hole from afar, physicists tell us all black holes can be described by just three parameters: their mass, electric charge, and spin. For really big black holes, such as the ones astronomers deal with, things are even simpler: there is no charge (that’s because they are so big that there would always be enough neighbouring positive and negative charges to more or less cancel out). So if you know how heavy a black hole is, and how fast it spins, in theory at least you have enough information to predict a black hole’s behaviour.
Of course the black holes in Radio Galaxy Zoo often have at least one other, quite spectacular, feature – bright jets of radio plasma shooting through their host galaxy and into intergalactic space. Where do these jets come from?, I hear you ask. This is a very good question, and one to which astronomers are yet to find a wholly convincing answer. We have some pretty good hunches though.
The fact that black holes can spin might be quite important. Matter accreted by a black hole will rotate faster and faster as it falls in. Stuff closer to the equator will also rotate faster than stuff at the poles, and that causes the accreting material to flatten out into a pancake, which astronomers call the accretion disk.
The accreting matter near the black hole event horizon (a fancy term for the point of no return – any closer to the black hole, and not even light is fast enough to escape the gravitational pull) is subject to friction, which heats it up so much that individual atoms dissociate into plasma. These plasma (i.e. positively and negatively charged) particles are moving, so they are in fact driving an electrical current. When this current interacts with the rotating magnetic field of the black hole and the accretion disk, the charged particles are flung out at close to the speed of light along the axis of black hole rotation. We can see these fast-moving particles as jets in the radio part of the electromagnetic spectrum. A useful analogy is a car alternator, where electrical currents and magnetic fields are also combined to generate energy.
This artist’s conception is on a *really* different scale than the image at the top of this post. Compared to those (real) jets, this is zoomed in 100,000 times or so. Jets are big.
There are many things we don’t know. For example, we don’t know for sure where most of the jet energy comes from. It could be from the accreted matter, or the spin of the black hole, or a combination of both. We are also not sure exactly what sort of charged particles these jets are made up of. Understanding black hole jets is one of the great unsolved mysteries in astronomy. By studying a huge number of these jets at different points in their lifetimes, Radio Galaxy Zoo — with your help — will help us solve this puzzle.
13 responses to “How do black holes form jets?”
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Galaxy Zoo 
Very interesting!
If a black hole has significant angular momentum, isn’t its event horizon a bit more complicated than you’ve described (what’s the ergosphere)? Also, ‘spin’ when used to discuss (massive) black holes means rotating, not spin in the quantum mechanical sense, doesn’t it?
“black holes can be described by just three parameters: their mass, electric charge, and spin” but “…interacts with the rotating magnetic field of the black hole” – isn’t this contradictory? If a supermassive black hole has essentially zero charge, it cannot have a magnetic field, even if it’s rotating, can it?
Black holes happened also because a huge star. Like humans, stars haves lives and when the huge star, a supernova explosion happens in space leaving a black hole
@Mars: this is correct. In our own Galaxy, there are plenty of black holes with masses comparable to the Sun (but a bit bigger). These are remnants of massive stars that have collapsed under gravity. The black holes we are studying with Radio Galaxy Zoo are much bigger. These monsters are typically a million to a billion times more massive than the Sun, and live at the centres of galaxies.
Every Interesting
Good questions Jean.
You are right, by “spin” in this article I really meant angular momentum (i.e. this is different from the quantum mechanical definition). Rotating black holes with angular momentum have a region called the ergosphere, from which energy can be extracted using a so-called Penrose process. I quite like the linen sheet analogy in this Wikipedia article (http://en.wikipedia.org/wiki/Penrose_process). During this process some of the black hole’s rotation energy is used to launch the jets, and the black hole spins down. This is what I was referring to when I said that the spin of the black hole can help launch the jets.
Regarding magnetic field, the black hole itself has zero charge. However, immediately outside it (the ergosphere again) this is not the case.
Good questions Jean.
You are right, by “spin” in this article I really meant angular momentum (i.e. this is different from the quantum mechanical definition). Rotating black holes with angular momentum have a region called the ergosphere, from which energy can be extracted using a so-called Penrose process. I quite like the linen sheet analogy in this Wikipedia article (http://en.wikipedia.org/wiki/Penrose_process). During this process some of the black hole’s rotation energy is used to launch the jets, and the black hole spins down. This is what I was referring to when I said that the spin of the black hole can help launch the jets.
Regarding magnetic field, the black hole itself has zero charge. However, immediately outside it (the ergosphere again) this is not the case.
What is the reason for believing that a black hole is electrically neutral? Positive and negative charge carriers have different masses, so surely they will behave rather differently in the maelstrom of the accretion disk due to their different momentum? Also, won’t they respond differently to radiation pressure due to this different mass? I imagine this would result in charge imbalance, and a predominance of one polarity of charged particle in the jets?