Active Galactic Nuclei
Written by Fraser Cain
Artist illustration of a quasar with jets.
Astronomers now think that there's a supermassive black hole at the heart of every large galaxy in the Universe. Most of the time, these black holes are quiet and invisible. But during the times when material is falling onto these supermassive black holes, they blaze with radiation, putting out more light than the rest of the galaxy combined. These bright centers are called active galactic nuclei.
The enormous amount of energy isn't coming from the black hole itself; nothing, not even light itself can escape the clutches of a black hole. Instead, the radiation is coming from a vast disk of gas and dust surrounding the black hole. This is the material that's backed up around the black hole, waiting go be consumed. The incredible force of gravity in this region compresses the material until it reaches millions of degrees kelvin. This generates bright radiation, up into the X-ray and gamma ray spectrum.
The interaction between the supermassive black hole's rotating magnetic field and the accretion disk creates powerful magnetic jets that fire material above and below the black hole at relativistic speeds. These jets can extend for hundreds of thousands of light-years.
The discovery of active galactic nuclei has allowed astronomers to group together several classes of objects. They realized that an active galactic nucleus is surrounded by a torus of gas and dust that can obscure our view to the core of the galaxy.
When we see an active galactic nucleus edge on, the torus blocks the radiation pouring out, and we only see radio waves – this is known as a radio galaxy or Seyfert galaxy. When the galaxy is at an angle, the active galactic nucleus is visible and bright; this is a quasar. And when the active galactic nucleus is seen face on, the jet is pointed directly towards us, and we see a blazar.
Our own Milky Way has probably been host to an active galactic nucleus in the past, but it's in a quiet phase right now. But it might become active again in a few million (or billion) years.
We have written many articles about galaxies for Universe Today. Here's an article about a spacecraft searching for active galaxies.
Want more resources on galaxies? Here's a link to the Messier catalog's section on Galaxies. And here's NASA's World Book on galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.
Filed under: Astronomy
