Most galaxies, including our galaxy the Milky Way, contain a gigantic black hole in their centre. Black holes are the densest objects in the Universe and their gravitational field is so extreme, that even light cannot escape the black hole, hence they are not visible. These black holes in the centres of galaxies are called 'supermassive' black holes and they have a mass that is between a million and ten billion times the mass of our Sun! In some galaxies, lots of matter spirals onto the supermassive black hole and forms an accretion disc. The matter heats up so that it emits all kinds of light including X-ray light. Sometimes strong winds or jets of material and light are emitted and we call these systems “Active Galactic Nuclei”.
Active Galactic Nuclei are very bright, especially in X-ray light and we can see them with telescopes even if they are a long way away. Studying these galaxies allows us to understand how black holes can become supermassive. We observe many different types of Active Galactic Nuclei, some of which have very strong radio emission. The radio light comes from the highly collimated jets of gas that originate in the centre of the galaxy and can extend up to millions of light years from black hole!