Radio Galaxies

Radio galaxies are AGNs associated with a large nonthermal emission of radio waves. Powerful radio galaxies are elliptical and we often observe jet structure from a compact nucleus. It is likely that strong radio frequency emission from an AGN is always associated with jets, but the jets may not always be visible, depending on the details of the way we view the galaxy. Weaker radio sources are found associated with smaller jets in some spiral galaxies (for example, Seyfert galaxies). Although a radio galaxy can be a spectacular sight at radio-frequency wavelengths, even powerful radio galaxies may appear at first glance to be rather normal elliptical galaxies when observed at optical wavelengths. Often abnormalities at optical wavelengths become obvious only if the very core of the galaxy is resolved.

A Question of Geometry
We believe that the RF emission in radio galaxies is produced by high-speed jets from the core of the galaxy. The difference between core-halo and lobed radio galaxies is then probably mostly geometry and strength. If strong jets are perpendicular to our line-of-sight we see a lobed radio galaxy. If the jets are aligned more along our line-of-sight (or are weak) we see instead a core-halo radio source.

Lobes and Core-Halos

There are two broad classes of radio galaxies. Core-halo radio galaxies exhibit radio emission from a region concentrated around the nucleus of the galaxy. In this case the region of radio emission is comparable in size to the optically visible galaxy. Lobed radio galaxies display great lobes of radio emission extending in some cases for millions of light years beyond the optical part of the galaxy (we saw one example of this earlier in the radio galaxy 3C219). Some radio galaxies have a single lobe, but more often the lobes are double, arrayed on both sides of the optical galaxy as we saw for 3C219. As noted in the box, it is likely that the difference between these two classes of radio galaxies is associated basically with the orientation of the jets relative to the observer.

The Radio Galaxy NGC 4261

The following figure shows a giant elliptical galaxy in the Virgo Cluster of galaxies named NGC 4261. This galaxy is also a strong double-lobed radio source with gigantic radio frequency jets emerging from its nucleus (see the left side of the following figure).

The optical image is the white area in the center of the above left image; the superposed radio lobes are shown in false color. The right side of the above figure shows a Hubble Space Telescope image of the central region of NGC 4261 where an enormous black hole is suspected to exist. This black hole is thought to be the source of the radio frequency jets in NGC 4261. We shall have more to say about it in the next module.

Jets in M87

The giant elliptical galaxy M87 in the Virgo Cluster is another example of a nearby radio galaxy. This galaxy is associated with the radio source Virgo A. It is a core-halo type galaxy, with the region of radio emission comparable in size with the optically-visible galaxy. Some radio galaxies, like M87, exhibit optical jets in addition to those observed at radio frequencies. The optical jet of M87 is quite spectacular in the adjacent image, which is a composite constructed from Hubble Space Telescope near-IR and near-UV images to enhance the jet relative to the rest of the galaxy. The optical jet of M87 is about 2 kpc long. It can only be seen well in short exposures since it is dwarfed by the light of the full galaxy. The M87 jet is seen more clearly in radio frequency observations where there is little interference from the rest of the galaxy. Here is an animation giving a more extensive introduction to the radio jet of M87. The M87 jet exhibits significant polarization, indicating that it is produced by synchrotron radiation from electrons moving at a significant fraction of the velocity of light.