The central black hole and relationships with the host galaxy

More than 40 years ago, astronomers speculated that active galactic nuclei are fundamentally powered by accretion onto supermassive black holes. In this contribution, the basic observations and theoretical considerations that led to this conclusion are reviewed, as is emission-line reverberation mapping, which is now used to measure the masses of black holes in AGNs. Key correlations – the broad-line region radius–luminosity relationship and the relationships between black hole mass and host galaxy properties – are also discussed.