We have known for some twenty years that every galaxy has a supermassive black hole at its center, with a mass of between 1 million and a few billion solar masses. The best-known black hole is that at the center of the Milky Way, with a mass of 4 million solar masses. When these black holes accrete matter, they become active nuclei (or AGN, for Active Galactic Nuclei): the energy recovered by the gravitational force is of the order of 20% of the mass energy mc2, far greater than the efficiency of nuclear fusion energy, which sustains stars. This accretion then gives rise to the quasar phenomenon, where the galaxy's nucleus, in an infinitesimal volume, radiates 1,000 times more than the entire host galaxy.
This phenomenal energy would be enough to destroy the galaxy itself, if it were more intimately coupled with matter. But this is not the case: the energy escapes through regions of least resistance. Nevertheless, the impact on the gas and star formation of galaxies is not negligible, and this feedback could be at the origin of the proportional relationship between black hole mass and galaxy bulge mass. One of the spectacular phenomena of AGNs is the ejection of matter perpendicular to the accretion disks, in the form of radio jets. Electrons escape at relativistic speed, making these jets appear superluminal. When two spiral galaxies interact through tidal forces, dynamic friction brings them together, eventually merging them into a single elliptical galaxy. The black holes at the center then form a binary, which, as it closes in, emits gravitational waves. Detection and timing of these waves by a network of pulsars will enable us to quantify the number of mergers in the future. The detection by ground-based interferometers (Laser Interferometer Gravitational wave Observatory or LIGO, Virgo) of the merging of stellar-mass black holes, announced in February 2016, holds great promise for the exploration of this new window on the Universe.