Radio galaxy Centaurus A
With the invention of radio astronomy, scientists had a new way to study the night sky. One amazing finding was the discovery of radio galaxies – objects that were millions and even billions of light-years away pouring out energy in the radio spectrum. These were first identified in the 1970s, but it wasn’t until recently that astronomers finally figured out what was going on.
Radio galaxies are actually just one aspect of a phenomenon known as active galaxies. These are just like regular galaxies, but the supermassive black holes at their hearts are actively feeding on material. If too much material piles up, it forms a flattened accretion disk around the black hole, and becomes so hot that it gives off a tremendous amount of radiation. At the same time, a torus (or donut) of gas and dust forms around the accretion disk that blocks our view when we see the galaxy from the side.
Radio galaxies are active galaxies seen from the side. All the radiation that we would normally see blasting out of the center of the galaxy is blocked by the torus of gas and dust; instead, we can only see the radio waves generated by the synchrotron process. These radio waves are generated when charged particles moving close to the speed of light move through a powerful magnetic field.
When we see the galaxy at an angle, we see above this torus of gas and dust, and can see the radiation generated around the black hole. Then the object is observed as a quasar. And when the active galaxy is seen face on, one of the relativistic jets streaming out of the galaxy is pointed directly at the Earth; that’s a blazar.
We have written many articles about galaxies for Universe Today. Here’s an article about a recent image of a radio galaxy.
If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.
Comments on this entry are closed.