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The properties of the black hole population we observe are given by the combination of their birth rate , merger rate, and growth rate on each black hole. The mass and the frequency of the seeds, as well as the dynamical evolution of black hole pairs ultimately dictate the distribution of MBHs in galaxies, that is, how often a MBH sits in a galaxy center. We study the connection between galaxy mergers, black hole mergers and black hole growth using high-resolution simulations. We estimate the lifetime over which black holes light-up as quasars during galaxy mergers, and how the luminosity of the (two) active galactic nuclei depends on the merger properties. In the first movie below we simulate the merger of two spiral galaxies, with one galaxy twice as massive as the other one. Each galaxy hosts a massive black hole, which partakes into the dynamical 'dance' occurring during the collision.

Strong instabilities appear in the merging galaxies, causing burst of star formation, and leading to the morphological transformation of the two colliding galaxies into a brand new one. Over time, the two cores where the massive black holes are embedded get closer and closer. Eventually they merge into a single nuclear disk: the core of the newly formed galaxy. A few million years thereafter the massive black holes form a bound pair. The final act of the dynamical ballet of the two black holes occurs in the center of this merger remnant. The second movie shows a second simulation where we have zoomed-in into the core of the remnant of a galaxy merger. Here two massive black holes orbiting around each other in a death spiral towards coalescence. During the merger phase both black holes are enveloped by dense gas, that becomes fuel for the black holes to become active and shine as quasars.