This is an image of a black hole. Actually, the black hole is in the center and can’t be directly seen. What we see are two large jets streaming away from the black hole. When matter accretes around a black hole, some of it is captured, but some of it is pushed away into long jets. In the case of this image, the jets stream out for about 1.5 million light years. It’s known as Hercules A, and it’s one of the brightest radio galaxies.

The 3C 348 galaxy in the visible range.

The purple lobes in this image show radio emissions, not visible light. It was taken using the Very Large Array (VLA). Since the jets are made of plasma, they are bright at radio wavelengths, but not very bright at optical wavelengths. The supermassive black hole driving Hercules A is at the center of a rather bland elliptical galaxy known as 3C 348. The image above combines an image of the galaxy in the visible spectrum with the radio image of the lobes.

Since the VLA is an array of radio antennas, it can produce detailed radio images. We can see, for example how the jets stream out in a narrow beam at nearly the speed of light, eventually slowing and interacting to create wide turbulent lobes. By studying images such as this we can better understand how high energy plasma interacts in space.

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A radio galaxy is a galaxy that emits large amounts of radio waves.  They are powered by the galaxy’s supermassive black hole.  When a supermassive black hole eats nearby gas, dust and stars, it is known as an active galactic nucleus (AGN).  These active black holes have superheated material swirling around them in a circular disk, known as an accretion disk.  The material in the accretion disk is swirling around the black hole in a tight circle, which means all the charged particles in the disk are being accelerated.  When you accelerate charges, they give off radio waves along their direction of motion, known as synchrotron radiation.  If you look at an AGN from a bit of an angle, you would see intense visible light from the superheated accretion disk, and depending on your angle you might (or might not) also see intense radio waves.  If that’s the case, then it is a radio galaxy.

Radio image of Centaurus A.
Credit: ALMA (ESO/NAOJ/NRAO); ESO/Y. Beletsky

One of the more famous radio galaxies is Centaurus A, which is one of the closest radio galaxies.  Since its distance is only 10-15 million light years, we can actually get good observations.  We know, for example, that its supermassive black hole is about 50 million solar masses, and that it is likely active due to a collision with a smaller galaxy.

Radio galaxies are also related to quasars.  The term quasar comes from “quasi-stellar radio source”, as they were bright sources of radio energy that appeared point-like and thus star-like.  We now know that quasars are also powered by active galactic nuclei, and radio loud quasars are distant radio galaxies.

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