Composition of images of Centaurus A in the optical range (ESO/WFI) and X-rays (NASA/CXC/CfA). Centaurus A is a massive galaxy (similar to those analysed in this study) which is in the process of merging with a neighbouring spiral. At its centre is a supermassive black hole which is accreting matter, which makes it an active galactic nucleus (AGN). This produces X-ray and radio emission as well as the two jets of matter emitted from its centre. Credits: ESO/WFI (Optical), MpIfR/ESO/Apex/ A.Weiss et al. (submillimetre); NASA/CXC/CfA/R. Kraft et al. (X-rays).