1 Meet your first active galactic nuclei
Figure 1 compares two nearby spiral galaxies of similar distance and type. NGC 5548, on the left, has a brighter nucleus than that of NGC 3277, on the right. This extra emission from the central regions of NGC 5548 is not generated by stars. Instead this light is thought to be ultimately powered by material falling in the gravitational field of a supermassive black hole at the centre of NGC 5548. Similar non-stellar emission is also seen at the centre of many other galaxies. These bright central regions are generally known as active galactic nuclei or AGN. In this course you will gain an overview of these enormously energetic objects and how they are observed and studied.
The first AGN was discovered in 1908 when E. A. Fath took a spectrum of M77, also known as NGC 1068. This nearby spiral galaxy is shown in Figure 2. Carl Seyfert was the first to realise that there were other similar galaxies, when he discovered that NGC 1275, NGC 3516, NGC 4015, NGC 4151 and NGC 7469 all had spectra similar to that of NGC 1068. Consequently these objects, and others like them, are called Seyfert galaxies.