An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

8.2 Extended radio sources

In section 7.5 we studied the spectrum of the synchrotron emission, i.e. how the flux density of radiation depends on the frequency or wavelength of the radio emission. Using radio telescopes such as the VLA (Figure 22) radio astronomers can also spatially resolve the celestial radio emission, that is to say they can study how the flux density of radiation at one particular frequency depends on position in the sky. Hence pictures can be made with radio emission, just as we make pictures showing how optical emission depends on position in the sky (e.g. Figure 1). Figure 23 shows two examples of radio pictures. Note: Do not worry about the information in the caption until you undertake Activity 5. Radio emission pictures are often shown graphically as contour maps. You probably noticed Peterson's Figure 1.4 as you completed the previous reading activity: this figure is an example of a radio intensity map. The interpretation of these figures is analogous to reading altitude contours on an everyday map: each contour line represents a particular value, so the highest value in the picture will be inside the largest number of contour lines.

Figure 22
Figure 22 (a) and (b) The ‘Very Large Array’ (VLA) is used to make radio images of the sky. It consists of many individual radio dishes, the signals from which are combined. These individual dishes can be moved, and the spatial resolution of the radio images depends on their separation
Figure 23
Figure 23 Images made by using a false-colour scale to encode the intensity of radio emission. (a) A classic double-lobed radio source (3C 35), showing the core, extended lobes, and hotspots. (b) Cygnus A showing the radio jet

Activity 5: Pictures via radio: extended radio sources

Read the remainder of Section 1.3.1 of Peterson [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .


Surface brightness refers to the amount of radiation per unit solid angle. An extended source may have a large total brightness, without having a particularly high surface brightness. In contrast, the small nucleus of a Seyfert galaxy emits about the same amount of radiation as the total emitted from the entire host galaxy. The nucleus has high surface brightness, the surrounding galaxy has low surface brightness.

Keywords: Fanaroff–Riley (FR) classes I and II, core, lobe, jet

Question 8

Figure 24 shows the radio intensity maps of the active galaxies 3C 465 and 3C 105. (a) Is 3C 465 an FR I or an FR II type radio galaxy? (b) Is 3C 105 an FR I or an FR II type radio galaxy?

Figure 24
Figure 24 Radio intensity maps of two active galaxies taken with the Very Large Array (VLA). (a) 3C 465 at wavelength 6 cm. (b) 3C 105 at 3.6 cm

The ‘3C’ in the designations of the two active galaxies shown in Figure 24 stands for the ‘third Cambridge’ catalogue. This was an early survey, made when the instruments were of poor resolution and relatively insensitive, consequently these are among the strongest radio sources in the sky.


  • (a) 3C 465 is a FR I galaxy. It is clearly brightest in the centre, with the surface brightness decreasing towards the edges of the lobes. Figure 25 shows an annotated version of the intensity maps in Figure 24.

  • (b) 3C 105 shows the regions of enhanced emission at the edge of the radio lobes characteristic of the FR II class.

Figure 25
Figure 25 (a) The central regions of this radio galaxy (3C 465) are brightest: they are surrounded by the most contour lines. Hence it can be classified as a member of the FR I class. (b) This radio galaxy (3C 105) shows regions of bright emission at the extreme edges of the lobes, i.e. it exhibits ‘limb-brightening’. Hence it can be classified as a member of the FR II class


Question: What frequency radio emission was observed to produce the map in Figure 24a?


The observation was made at a wavelength of 6 cm. We can work out the corresponding frequency using the equation relating speed, frequency, and wavelength for electromagnetic radiation, = c /λ. Inserting values (using cgs units), we find,


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