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Introduction to active galaxies
Introduction to active galaxies

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3.6 A 'non-active' class - the starburst galaxies

We end this section by drawing a distinction between the classes of active galaxy that are described in the previous subsections and the starburst galaxies mentioned earlier. As you have seen, starburst galaxies are essentially ordinary galaxies in which a massive burst of star formation has taken place. Their spectra show emission lines from their many HII regions and infrared emission from dust but, in the main, they do not show unusual activity in their nuclei. In the past they were regarded as active galaxies but modern practice is to place them in a class of their own.

Although it is clear that there are starburst galaxies that are not active galaxies, it does appear that some active galaxies are undergoing a burst of star formation. It is not clear at present whether there is a link between these two types of phenomenon where they are seen in the same galaxy but, as you will see later, it is possible that both types of phenomenon - rapid star formation and activity in the galactic nucleus - may be triggered by galactic collisions and mergers.

Question 6

Take a few minutes to jot down as many differences that you can think of between normal galaxies and each of the four types of active galaxy. Are there any characteristics which all active galaxies have in common?

Answer

There are several things you may have thought of. Table 1 summarises many of the characteristics and includes some pieces of new information as well. What all active galaxies have in common is a powerful, compact nucleus which appears to be the source of their energy.

Table 1: Features of active galaxies compared to those of normal galaxies
CharacteristicActive galaxies
NormalSeyfertQuasarRadio galaxyBlazar
Narrow emission linesweakyesyesyesno
Broad emission linesnosome casesyessome casessome cases
X-raysweaksome casessome casessome casesyes
UV excessnosome casesyessome casesyes
Far-infrared excessnoyesyesyesno
Strong radio emissionnonosome casesyessome cases
Jets and lobesnonosome casesyesno
Variabilitynoyesyesyesyes

Activity: the AGN zoo

Timing: 1 hour

Open the AGN zoo document linked below and use the online NED database of galaxies to complete the activity as instructed in the document.

Click below 6 pages 0.15 MB

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