Tag Archive | Radio Galaxy Zoo

How do black holes form jets?

Credit: ESA, NASA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)

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.

RGZ Team Spotlight: Stas Shabala

To introduce you to the Radio Galaxy Zoo team, we’re doing a series of blog posts written by each team member — in no particular order. Meet Stas Shabala, our team Project Manager from the University of Tasmania, Australia:

Stas Shabala

I grew up in Tasmania, a gorgeous part of the world which also happens to be the place Grote Reber, the world’s first radio astronomer, called home for 50 years. After finishing university, I made a pilgrimage that these days is more or less standard for young Australians – I moved to the UK. I ended up staying for six years, and it was during my time in Oxford that I became involved with Galaxy Zoo. Normal galaxies are interesting but – given our history- a Tasmanian’s true heart will always be with radio astronomy. That’s why I have such a soft spot for Radio Galaxy Zoo.

Recently, I’ve been trying to figure out why radio galaxies come in so many different shapes, sizes and luminosities. Data from Radio Galaxy Zoo will go a long way to answering these questions. I’ve also had lots of fun using active black holes as beacons to accurately measure positions on Earth. It’s just like navigation by stars, but much more precise because stars move around in the sky a fair bit, whereas black holes don’t. The neat thing is, these measurements make it possible to study all sorts of geophysical processes here on Earth. It’s such a cool concept- using black holes to measure the movement of tectonic plates!