Supermassive black holes

The existence of black holes (BHs) is a natural prediction of Einstein's theory of general relativity (GR). The gravitational field of black holes is strong enough to prevent light from escaping and to distort the space and the flow of time around them. In 1963 extremely luminous distant objects were discovered and called quasars. Their discovery was quickly associated to the presence of very massive black holes. Quasars belong to a class of galaxies known as active galactic nuclei (AGN). What makes these galaxies active is the emission of staggering amounts of energy from their cores across the whole electromagnetic spectrum, from radio to gamma rays. Moreover, the luminosities of AGN fluctuate on very short timescales, within days or even minutes. These fluctuations and the finite velocity of light set an upper limit to the size of the emitting region. For these reasons we know that nuclei of some active galaxies responsible for the X-ray emission have a size ≲ 0.001 pc, i.e. are at least 1 billion times smaller than the host galaxy. Astronomers had thus to explain how a luminosity of a hundred of times that of an entire galaxy could be emitted from a volume billions of times smaller. The answer is the release of gravitational energy from matter falling toward a central black hole. Black holes in AGN would need to be enormous (∼106–109 M⊙) in order to produce the luminosities of quasars. To distinguish these objects from stellar mass black holes left behind supernova explosions, the term supermassive black hole (SMBH) was coined. In this Chapter we give an introduction to the astrophysics of supermassive black holes and the basics of gas accretion onto these compact objects.