Introduction to active galaxies
Introduction to active galaxies

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Introduction to active galaxies

3.4 Radio galaxies

Radio galaxies were discovered accidentally by wartime radar engineers in the 1940s, although it took another decade for them to be properly studied by the new science of radio astronomy. Radio galaxies dominate the sky at radio wavelengths. They show enormous regions of radio emission outside the visible extent of the host galaxy - usually these radio lobes occur in pairs.

The first radio galaxy to be discovered, and still the brightest, is called Cygnus A (Figure 20). Radio maps show the two characteristic lobes on either side of a compact nucleus. A narrow jet is apparent to the right of the nucleus and appears to be feeding energy out to the lobe. There is a hint of a similar jet on the left. Jets are a common feature of radio galaxies, especially at radio wavelengths. They trace the path by which material is being ejected from the AGN into the lobes.

Figure 20
NASA/IPAC Extragalactic database (NED); Bob Fosbury/European Southern Observatory ©
NASA/IPAC Extragalactic database (NED); Bob Fosbury/European Southern Observatory
Figure 20: (a) The Cygnus A radio galaxy consists of two bright 'lobes' on either side of a compact nucleus. The lobe on the right is connected to the nucleus by a narrow jet. The white box shows the extent of (b), the host galaxy of Cygnus A. It is believed to be a giant elliptical galaxy with morphological peculiarities. The galaxy is at a distance of about 240 Mpc. This optical image combines observations made in the blue, visual (V) and near-infrared bands

Cygnus A is an example of the more powerful class of radio galaxy with a single narrow jet. The second jet is faint, or even absent, in many powerful radio galaxies; we will consider the reasons for this shortly. Note the relatively inconspicuous nucleus and the bright edge to the lobes, as if the jet is driving material ahead of it into the intergalactic medium.

The jets of weaker radio galaxies spread out more and always come in pairs. These galaxies have bright nuclei, but the lobes are fainter and lack sharp edges. You can see an example in Figure 21. This is M84, a relatively nearby radio galaxy in the Virgo cluster of galaxies.

Figure 21
Copyright © NRAO/AUI 1998/1999 ©
Copyright © NRAO/AUI 1998/1999
Figure 21: The radio galaxy M84. The radio emission is shown in red while the optical image of the galaxy is indicated in blue. The distance to M84 is about 18 Mpc. The inset shows an expanded view of the inner regions of the jets and the bright nucleus

Each radio galaxy has a point-like radio nucleus coincident with the nucleus of the host galaxy. It is this feature that is reminiscent of other classes of active galaxies and which is believed to be the seat of the activity. The nucleus shows many of the properties of other AGNs, including emission lines, a broadband spectrum which is far wider than that of a normal galaxy, and variability.

The optical spectrum of the nucleus of a radio galaxy looks very much like that of any other AGN. Like Seyferts, radio galaxies can be classified into two types depending on whether broad lines are present (broad-line radio galaxies) or only narrow lines (narrow-line radio galaxies). Figure 22 shows an example of a spectrum of a broad-line radio galaxy.

Figure 22
© 1976 The American Astronomical Society ©
© 1976 The American Astronomical Society
Figure 22: The optical spectrum of the nucleus of the radio galaxy 3C 445 (adjusted to zero redshift)

Figure 23 shows maps of radio, optical and X-ray wavelengths of Centaurus A, which is the nearest radio galaxy to the Milky Way. The optical image (Figure 23b) shows that it is an elliptical galaxy with a dust lane bisecting it.

Figure 23
From Peculiar Galaxies, Chapter 5 in Foundations of Astronomy by Michael A. Seeds, Wadsworth Publishing Co. Inc., 1988 (2nd edn.); © 1987-2002, Anglo-Australian Observatory, photograph by David Malin; NASA/SAO/R. Kraft et al. ©
From Peculiar Galaxies, Chapter 5 in Foundations of Astronomy by Michael A. Seeds, Wadsworth Publishing Co. Inc., 1988 (2nd edn.); © 1987-2002, Anglo-Australian Observatory, photograph by David Malin; NASA/SAO/R. Kraft et al.
Figure 23: The Centaurus A radio galaxy. (a) A radio map shows the lobes that extend over more than 9 degrees of the sky. (b) An image at visible wavelengths shows that the host galaxy is an elliptical galaxy with a dust lane bisecting it (the inner radio lobes are shown superimposed on this image). (c) An X-ray image from the Chandra X-ray Observatory clearly shows the jet and the point-like nucleus in the inner parts of the galaxy
  • Given that Centaurus A is an elliptical galaxy, does anything strike you as incongruous about Figure 23b?

  • Elliptical galaxies are supposed to have negligible amounts of dust, so the thick dust lane seems very strange indeed!

The galaxy is obviously not a normal elliptical and this is a clue to the nature of radio galaxies. In fact, it is now thought that Centaurus A was formed by the collision of a spiral galaxy with a massive elliptical, the dust lane being the remains of the spiral's disc. We will come back to this interesting topic later in the course.

M87 (also known as Virgo A) is such a well-known radio galaxy that it must be mentioned at this point. In the optical region it, too, appears as a giant elliptical galaxy at the centre of the nearby Virgo cluster of galaxies. It seems that most radio galaxies are ellipticals. The single bright jet in the galaxy (Figure 24) is reminiscent of the jet in the quasar 3C 273 shown in Figure 18.

Figure 24
NASA and J. Biretta (STScI/JHU)l; NASA, NRAO and J Biretta (STScI) ©
NASA and J. Biretta (STScI/JHU)l; NASA, NRAO and J Biretta (STScI)
Figure 24: (a) Optical and (b) radio images of the giant elliptical galaxy M87 clearly show the presence of a 'one-sided' jet that extends from the active nucleus. Note that (a) and (b) are at the same scale
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