In the early 1960s, radio astronomers from England completed the 3rd Cambridge Catalog of radio sources. As a result, most of the radio sources in the sky are designated with a label 3C...* They found that many of these radio sources were coincident with distant galaxies. Now, with the much better angular resolution of the VLA and VLBA radio telescope arrays, we can see that these radio sources often consist of point sources at the center of the galaxy and extended sources that extend thousands or even millions of light years beyond the galaxy into intergalactic space.

*An important astronomical source can have several different names that arise from the different surveys by which it is observed. For example, the giant elliptical galaxy M87 in the Virgo Cluster is also called: NCG 4486, from the New General Catalog; Virgo A, because it is the brightest radio source in the constellation Virgo; and 3C 274, from the 3rd Cambridge Catalog.

The image on the left is Cygnus A, one of the brightest radio sources in the sky and a classical example of a double-lobed radio galaxy. The radio emission comes primarily from two giant lobes in intergalactic space, each displaced more than 250 thousand light years from the central galaxy. The tiny spot in the center of the image lies at the center of the galaxy. You can see faint jets of radio emission extending from this point to both lobes. They are narrow beams of relativistic electrons (i.e., electrons moving at nearly the speed of light). At the bright radio lobes, the beams are stopped by collisions with intergalactic gas, as illustrated here: Jet Near Light Speed, or by this movie (source). Powerful turbulent flows generate electrical currents and magnetic fields, which cause the electrons to spiral and emit polarized synchrotron radiation.

The jets in radio galaxies might remind you of the jets seen in star-forming regions (see Lesson 9). But they are very different. The jets in star-forming regions are expanding with velocities of a few hundred km/s, while the jets from radio galaxies are expending at nearly the speed of light (300,000 km/s). The jets in star-forming regions extend for a few light years, beyond a dark interstellar cloud, while those in radio galaxies can extend for hundreds of thousands or even millions of light years into intergalactic space. Despite these differences, jets in star-forming regions and those in radio galaxies may have something in common. Evidently both are produced when gas swirls through an accretion disk into a central object.

On the right is a HST optical image of the central region of the giant elliptical galaxy M87 (Virgo A). The scale is much smaller here: the jet extending in the 2 o'clock direction from the point-like nucleus of the galaxy has a length of about 2 kpc. You can see the same core-jet structure in radio and X-ray images of M87, and time-lapse images show that the bright knots in the jet are moving at a substantial fraction of the speed of light.

Radio astronomers at Manchester University have coined the name, DRAGNs, meaning Double Radio Sources Associated with Galactic Nuclei. I think it's a great name, because these DRAGNs are certainly breathing fire! You should look at Atlas of DRAGNs from Manchester University, where you can find an excellent summary of radio galaxies and many images. You can also find some more good Images of Radio Galaxies and Quasars by Alan Bridle of the National Radio Astronomy Observatory.

As you will soon see in more detail, we believe that these DRAGNs contain supermassive black holes at their centers. Collisions with other galaxies cause gas to flow in toward the black hole. Close to the black hole, the gas forms an accretion disk, and jets of particles flow out from the poles of the disk into intergalactic space. Check this wonderful Movie of Cen A (source, with a few more great movies), which zooms into the center of Cen A and then shows an illustration of the central disk and jets.

Recently, the Hubble Space Telescope has observed a few radio galaxies and has found very complex optical structures within them. See Hubble Observes Radio Galaxies. The optical counterparts of most radio galaxies appear to be either elliptical galaxies (e.g., M87) or colliding galaxies (e.g., Cygnus A, Centaurus A).

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Last modified November 10, 2000
Copyright by Richard McCray