With the discovery of active galaxies and quasars, astrophysicists were challenged. It was necessary, to understand the different types of active galaxies, to imagine a source of energy capable of supplying the power of a thousand galaxies, being located in a space barely larger than the solar system.
Two ideas were advanced, but quickly rejected: the collisions between galaxies, which would not have produced enough energy, and the supernova chain bursts, which should have occurred in unimaginable quantities.
Finally, it was the British Donald Lynden-Bell who proposed in 1968 the idea to the base of modern explanation. According to him, the energy source of active galaxies must be a supermassive black hole, a gigantic version of black holes resulting from the death of a massive star. Instead of having just a few solar masses, supermassive black holes contain between a million and a billion solar masses, and have a size that can often reach the distance between Earth and the Sun.
Above: Artistic representation of a supermassive black hole absorbing matter from a nearby star. Below: images of a supposed supermassive black hole devouring a star in the galaxy RXJ 1242-11. Left in x-rays; Right in visible light. Credit: NASA / CXC / M.Weiss; X-ray: NASA / CXC / MPE / S.Komossa et al .; Optical: ESO / MPE / S.Komossa
As there are no stars of such large mass, supermassive black holes can not directly be the fruit of a stellar collapse. In fact, its mechanism of formation is still not very clear. It could be a massive star that collapses and gives rise to a black hole that grows slowly feeding on other stars, or a huge cloud of gas that collapses directly under its own gravity.
Whatever the origin of the supermassive black holes, their existence is, in any case, entirely possible. General relativity is clear on this point: if you can concentrate one billion solar masses in a region the size of the solar system, the situation is mathematically similar to that of a black hole of star origin, space and time will be extremely deformed , and even light will no longer be able to escape.
The preceding script is all the more realistic, as the required density of matter is low. Indeed, the critical parameter for the formation of a black hole is not the density, that is, the amount of matter in a given volume, but rather the compactibility, the ratio of the mass to the size of the object. Thus, a black hole of star origin needs enormous densities, while a supermassive black hole can be formed in a low density region. In fact, an average density lower than that of water is sufficient, which is not difficult to achieve by regrouping enough stars in a limited space.
Obviously, by definition, the luminosity of the nucleus of an active galaxy or a quasar does not come from the supermassive black hole itself. The cause is the action of the latter on the surrounding matter. In effect, in the unified model of active galaxies, the black hole is surrounded by an accretion disk, whose size is of the order of a hundred times the Earth-Sun distance.
Viscous phenomena have as a consequence the progressive fall of the disc matter in the black hole. In this process, the gas transforms its gravitational energy into thermal energy, which causes a strong increase in the temperature of the disk. For this reason, the latter is going to start producing a very powerful continuous thermal radiation, particularly in the ultraviolet and X-rays. The process is similar in the beginning and the collapse of a molecular cloud that heats up contracting.
This way of producing energy is much more effective than the nuclear reactions that feed the stars, which is the reason why active nuclei can shine like several galaxies.
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