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.
| Characteristic | Active galaxies | ||||
|---|---|---|---|---|---|
| Normal | Seyfert | Quasar | Radio galaxy | Blazar | |
| Narrow emission lines | weak | yes | yes | yes | no |
| Broad emission lines | no | some cases | yes | some cases | some cases |
| X-rays | weak | some cases | some cases | some cases | yes |
| UV excess | no | some cases | yes | some cases | yes |
| Far-infrared excess | no | yes | yes | yes | no |
| Strong radio emission | no | no | some cases | yes | some cases |
| Jets and lobes | no | no | some cases | yes | no |
| Variability | no | yes | yes | yes | yes |
Activity: the AGN zoo
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
View document [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]