Even in images taken with the most modern equipment on a large telescope, it can be difficult to pick out the galaxies now known as 'active' from the other more normal galaxies. But if your telescope were equipped to examine the spectra of the galaxies, then the active galaxies would stand out. Normal galaxies contain stars that are generally similar to those in our own Galaxy; and spiral galaxies have additional similarities to the Milky Way in their gas and dust content. Active galaxies show extra emission of radiation, and this is most apparent from the spectra.
Active galaxies come in a variety of types, including Seyfert galaxies, quasars, radio galaxies and blazars. These types were discovered separately and at first seemed quite different, but they all have some form of spectral peculiarity. There is also evidence in each case that a very large amount of energy is being released in a region that is tiny compared with the size of the galaxy, and so they are classified together. It is usually found that the tiny source region can be traced to the nucleus of the galaxy, so the origin of the excess radiation is attributed to the active galactic nucleus or AGN. An active galaxy may be regarded as a normal galaxy plus an AGN with its attendant effects.
Active galaxies seem to be quite rare in the nearby Universe. Whether every galaxy goes through an active phase in its lifetime, or whether active galaxies are a separate class of object is not clear. We have been aware of these objects only since the 1940s, and the galaxies have been around for at least 1010 years. So the fact that we observe a small percentage of galaxies in an active phase could mean that every galaxy becomes active for the same small percentage of its lifetime, but it could also mean that a small proportion of galaxies become active for a longer time. At present we cannot tell which of these scenarios may be correct. A further complication is that some nearby galaxies, including our own, show evidence of a low level of activity in their nuclei, but we shall concentrate in this course on the prominent and powerful active galaxies.
The engine that powers the AGN, the tiny nucleus of the active galaxy, is a great mystery. It has to produce 1011 or more times the power of our own Sun, but it has to do this in a region little larger than the Solar System. To explain this remarkable phenomenon, a remarkable explanation is required. This has proved to be within the imaginative powers of astronomers, who have proposed that the engine consists of an accreting supermassive black hole, around which gravitational energy is converted into electromagnetic radiation.
In Section 2 you will learn how spectroscopy can be used to distinguish different kinds of galaxy and to measure their properties. Section 3 then introduces the four main classes of active galaxies and describes how they can be recognized. Section 4 examines the evidence for the existence of black holes at the centres of active galaxies, and in Section 5 you will study a simple model that attempts to explain the key characteristics of active galaxies in an illuminating way. Finally, in Section 6, we consider some of the outstanding questions about the origin and evolution of active galaxies.
We begin by looking at the spectra of galaxies.