In the currently favored scenario for the formation of cosmic structures in the Universe, present-day galaxies have been built up, via a series of mergers, from small building blocks that formed at early cosmic times. Galaxies experience multiple mergers during their lifetime. A single large galaxy, now containing a massive black hole, can be traced back to the stage when it was split up in hundreds of components with masses a million times smaller than today's galaxies.
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. In the movie to the right 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. The white "flickering" lights in the centers of the merging galaxies show the black holes going off when they accrete gas and become active.
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 movie to the left shows a second simulation where we have zoomed-in into the core of the remnant of a galaxy merger. Here two massive black holes orbit 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.
The mergers of black holes that exist in binary systems are predicted to be the strongest signals that low-frequency gravitational waves telescope can detect. Gravitational waves are ondulatory perturbations of the spacetime generated by rapidly accelerating bodies, and merging massive black holes are the strongest source of gravitational radiation in the Universe.
To understand how black holes are born and grow up we need to collect as much information as possible, using techniques at the edge of what technology ca do. To learn about the massive black hole population we can look for signatures of their mergers through emission of gravitational radiation. In the currently favored cosmological model, galaxies form in a hierarchical fashion, starting from small systems at early times, and then growing via mergers. The formation history of galaxies is often described as a merger tree: the 'trunk', a galaxy that we observe today, can be traced back through its merger history to the high-redshift smaller branches (left). If black holes formed at early times, they are bound to follow the merger hierarchy experienced by their host galaxies. Black hole mergers are therefore expected to be common in the Universe.