An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

8 Basic properties and historical perspective

8.1 Continuum spectra

Activity 4: General properties of quasars and power-law emission

Timing: 0 hours 30 minutes

Read Section 1.3 of Peterson, up to and including the first two paragraphs of Section 1.3.1 here [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .


Morphology is a technical way of referring to shape. Thus the ‘radio morphology’ referred to in the opening section of Peterson 1.3.1 is simply the shape(s) of the spatially resolved radio emission.

Keywords: spectral energy distribution (SED), power law, power-law index,

Question 7

Give six common general properties of quasars.


The first paragraph of Peterson section 1.3 gives:

  • star-like objects identified with radio sources

  • time-variable continuum flux

  • large UV flux

  • broad emission lines

  • large redshifts

  • large X-ray flux

as typical common characteristics of quasars.


Question: A quasar has a power-law spectrum obeying , how would this spectrum be described in terms of F and ?


. This comes from the relationship arising from on Peterson page 8: , so – 2 = 1, therefore = 1 + 2 = 3.

Example 3

Figure 20 shows a schematic spectral energy distribution. In which part of the electromagnetic spectrum is most energy emitted per unit interval on the logarithmic frequency axis? Fully justify your answer, including an explanation of why log10is the most convenient choice of variable on the horizontal axis of a graph for this purpose.

Figure 20
Figure 20 A schematic energy distribution of a hypothetical quasar


The graph in Figure 20 shows log10( F) versus log10. The flux emitted between any two frequencies 1 and 2 is given by

Looking at this final expression for F12, we see it is of the form , where x = log10, i.e. F12 is the area under the curve of F between the two specified values on the horizontal (or log10) axis. Generally F will have values varying by many orders of magnitude over the full electromagnetic spectrum, so usually the logarithm of F is a more useful quantity to plot on the graph than F itself. The part of the electromagnetic spectrum where the most energy is emitted per unit interval on the logarithmic frequency axis will be where F reaches a maximum: this occurs at log10 ~ 12.5, i.e. at wavelength

i.e. in the far infrared. It is for this reason that spectral energy distributions are generally plotted using this particular choice of axes: the eye easily picks out the location of the dominant contribution to the area under the curve. These axes allow immediate identification of the part of the spectrum which radiates most energy. As Figure 21 shows, different choices of axes can make it much more difficult to do accomplish this. If the horizontal axis was rather than log10, the entire low-energy part of the spectrum would be compressed into a tiny space on the extreme left-hand edge of the graph.

Figure 21
Figure 21 The same spectral energy distribution as in Figure 20, but with two different choices of axes

The two different choices of axes in Figure 21 are (a) The vertical axis shows logF. (b) The horizontal axis shows .


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