A radio galaxy is a galaxy that emits large amounts of radio energy. They are powered by the supermassive black hole in their center, and thus can have active and dormant periods. During the active periods lobes of ionized material streams from the region near the black hole, and this energized material emits radio waves. During a dormant period this material doesn’t emit energy, but it’s still there in the region of the galaxy. As a result, which two galaxies collide this material can be re-energized by colliding with material from the other galaxy, radio energy is again emitted, creating a kind of “radio phoenix.”

The radio phoenix period of colliding galaxies doesn’t last long on the cosmic scale, so it can be difficult to observe in action. But new observations of a galaxy cluster known as Abell 1033 has found a region that is just beginning to re-energize. By observing the cluster at visible, x-ray, and radio wavelengths, the team found that lobes emitted earlier by a galaxy’s black hole has now undergone compression due to shock waves from collision with another galaxy. The compressed regions are the very regions where radio waves are being emitted. You can see this effect in the composite image. Blue represents the distribution of the lobe material, pink shows x-ray emissions indicating hot material, and cyan indicates the region of radio emissions.

This work gives us insight on the dynamics of galaxy collisions. It’s also a great demonstration of how combining observations from different regions of the electromagnetic spectrum allows us to determine far more than any single region could.

Paper: F. de Gasperin, et al. Abell 1033: birth of a radio phoenix. MNRAS 448 (3): 2197-2209 (2015)

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A radio galaxy is a galaxy that emits large amounts of radio waves. They were first discovered in the 1950s, but it wasn’t until the 1960s when a technique known as aperture synthesis was developed that we could resolve the distribution of radio emissions within a radio galaxy. It then became clear that many radio galaxies had a double-lobed structure emanating from a galactic core. It was suggested that these Double Radio Sources Associated with Galactic Nuclei be known a DRAGNs, though the term has never really caught on.

We now know that these lobed radio sources are powered by a 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 cause some of the surrounding material to stream off at high speed as jets. The jets stream away from the galactic nucleus at tremendous speed, and can collide with the intergalactic medium. This forms lobes of radio bright material, as can be seen in the image above.

A 1965 radio map of our galaxy, showing the double lobed structure. Credit: M. Ryle et al

We see this in our own galaxy, which is one of the ways we know the Milky Way contains a supermassive black hole (although we also have more direct evidence). With our own galaxy we can see these lobes from past activity, but at present the region around the Milky Way black hole is fairly quiet. So it seems that supermassive black holes go through active and quiet periods. It is a pattern we see in other galaxies as well.

When they were first discovered, radio galaxies and DRAGNs were seen as a distinct phenomena, different from other objects such as quasars an blazars. We now know that they are all driven by a similar process, and that how they appear to us depends largely on our view of the galaxy. We still use various terminology for radio galaxies, quasars, and the like, but we now understand that the lines between different categories are fuzzy.

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