Radio galaxies are recognizable by their enormous jets and lobes of radiating plasma, driven outwards at nearly the speed of light by supermassive black holes harbored in galaxy cores. Pretty amazing right? But I wouldn’t want to get anywhere near one.
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| Cygnus A, the classic view of a radio galaxy, showing the tiny central host galaxy and the enormous jets and lobes which punch through the gas environment at nearly the speed of light. Credit: NRAO/AUI |
Sadly, I think the name ‘radio galaxy’ is far too boring to capture their essence. Even worse is the technical term for the black hole engine that powers radio galaxies: active galactic nucleus or AGN. Much better nicknames have been suggested by astrophysicists over the years ranging from ‘monster’ to the ‘beast below’. Some have even suggested the acronym DRAGN, standing for Double Radiosource Associated with Galactic Nucleus. This has the fun property that we can talk about the jets being the fiery breath of the DRAGN. In the press, a recently popular term has been the ‘death ray jets‘.
Naming conventions aside, there is a lot we still don’t know about radio galaxies, including how the jets actually form in the first place — the white elephant in the field!
To get a feel for our current model of radio galaxies, check out this artist’s simulation of a black hole, an accretion disk of in-falling matter, and twin plasma jets speeding outwards. You’ll notice the thin twisting lines which seem to confine the jets. These represent strong magnetic field lines, arising from the flow of charged particles. Astrophysicists think that magnetic fields play an important role in creating the jets, but the details are unclear.
Keep in mind this video is just a model. Even our most powerful telescopes cannot resolve the very central black hole and accretion disk, where we think the jets actually form. So astrophysicists have to extrapolate based on images and spectra at larger distances.
You might think, wouldn’t it be great if we could send an astronaut to a radio galaxy and find out what’s really happening down in the core? Well, distance impracticalities aside (the closest radio galaxy is about 10 million light years away), there are a lot of good reasons why radio galaxies are the coolest places you would never want to actually visit. I worked hard to limit the list to just five…
1. Radio galaxies are absolutely massive
They are the biggest structures in the universe by far, but even that statement cannot do justice to their incomprehensible sizes. I wanted to place a model of our solar system next to the nearby radio galaxy, Hercules A, for size comparison but the solar system is too small! The length of just one of these jets is about a billion times larger than the diameter of our solar system.
So that you can actually visualize things, let’s compare Hercules A to the size of our Milky Way galaxy. It turns out the Milky Way is about the same size (diameter ~100,000 light-years) as that spiral galaxy I have circled in white in the image below . That’s our entire galaxy (never mind our solar system) being completely dwarfed by these enormous jets. Our intrepid astronaut would be lost in the sheer scale of things.
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| Hercules A radio galaxy. Radio light is shown in pink highlighting the massive double jets. Visible light shows the nearby galaxies. White circle surrounds a Milky Way-sized spiral galaxy. Image adapted from a Hubble Heritage Release. Credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA) |
2. Radio galaxies emit synchrotron radiation!
As well as being the biggest objects around, radio galaxies and quasars (believed to be a radio galaxy viewed down the length of the jet) are also some of the brightest objects in the universe. They allow us to peer out in space, and back in time, to when the first galaxies were beginning to take shape.
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| Model of a supermassive black hole with accretion disk and helix magnetic fields directing jets. Credit: B. Saxton, NRAO/AUI/NSF |
How do radio galaxies emit this enormous amount of radio emission? The process is called synchrotron radiation, which sounds like something out of a science fiction novel. Synchrotron radiation is named after a type of circular accelerator where particles, often electrons, are guided by strong magnets and whiz around at speeds very, very close to the speed of light. Because the particles are traveling on a curved path and are constantly changing direction, they emit light to conserve energy.
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| The former Ryle Telescope. Credit: Public domain |
The jets in radio galaxies are essentially the largest particle accelerators we know of. Charged particles like electrons speed down the length of the jet and spiral along entangled magnetic field lines. As they spiral, the particles emit loads of synchrotron radiation in the radio band, which is why radio telescopes are the classic way of studying these beasts.
Fun to study from a distance, these jets of relativistic particles emitting copious amounts of radiation in radio, optical, and X-ray are not exactly astronaut friendly.
3. Radio galaxies are bullies
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| Perseus cluster showing giant dark cavities and ripples in the X-ray emitting cluster gas due to radio galaxy activity. Credit: NASA/CXC/SAO/E.Bulbul, et al. |
Radio galaxies hollow out enormous cavities, shoving gas aside and sending rippling waves out to large distances from the core. By measuring the size of these cavities, astrophysicists can estimate how much work is done by radio galaxies on their environment, and extrapolate how powerful the driving jets must be. This can be upwards of 1039 Watts! It’s hard to fathom just how powerful these jets actually are and how much of an impact they have on their environment. Radio galaxies could be called the bullies of the celestial playground.
4. Extreme cluster weather can disrupt even the most powerful radio galaxy jets
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| A pair of co-orbiting radio galaxies. The jets are shown in pink from the radio waves they emit. The blue environment is X-ray emitting hot gas in the cluster. Credit: Public domain via NASA |
5. Sometimes radio galaxies eat other galaxies
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| Credit: CSIRO |
This is Centaurus A, our closest radio galaxy at only 10 million light years away from us. The giant pink plumes are the plasma lobes typical of a powerful radio galaxy. The central galaxy in the middle harbors a supermassive black hole monster that doesn’t like to play nice.
Scientists think this may be what happened to cause the strange parallelogram shape around the galaxy core. A simulation in which Centaurus A feeds on a nearby spiral galaxy is able to recreate the same parallelogram illusion. As the spiral galaxy falls in, the arms are warped and stretched and glow brightly from the released energy. Astrophysicists think that large meals like this one are what feed the black hole and trigger jet formation.
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| Credit: NASA/JP-Caltech/J.Keene (SSC/Caltech) |
So there you have it. Five reasons why I’d recommend not booking that next holiday to your local radio galaxy. There are, of course, many more good reasons (i.e. radio galaxies sometimes open fire on their neighbors), but I’ll leave it there for now.
Radio galaxies are extreme, epic, extraordinary, and lots of grandiose adjectives. Words cannot do them justice. So these pictures (and countless others) will have to suffice. After all, you wouldn’t want to actually pay them a visit.
1 Full disclosure, these cosmic beasts were the subject of my PhD thesis so I kinda like them a lot
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By Tamela Maciel, also known as “pendulum”