Multiblackhole

Multiblackhole is a term used in astrophysics to describe a gravitationally bound system containing three or more black holes, ranging from stellar-mass remnants to supermassive black holes found in galactic nuclei. The most common configurations are hierarchical triples, with an inner binary orbited by a third black hole, and potential further companions forming quadruple or higher-order systems. Such systems are expected to arise during hierarchical galaxy mergers and in dense star clusters where dynamical interactions are frequent. In galactic nuclei, multiple SMBHs can coexist briefly or persist for longer if dynamical friction and gas dynamics keep them bound.

Their dynamics are governed by gravitational three-body interactions, which can drive rapid changes in orbital elements, induce Kozai-Lidov oscillations, hasten mergers via gravitational radiation, or eject one component. If any BHs accrete gas, the system may host multiple active galactic nuclei.

Observationally, multiblackhole systems are challenging to detect directly, especially at sub-parsec separations. Confirmations rely on high-resolution imaging that reveals multiple compact nuclei, spectroscopy showing multiple velocity components, or correlations in periodic variability that hint at a binary or triple. Gravitational waves from mergers of BHs provide a direct probe of their history and population.

Notable examples include systems identified as dual SMBHs in merging galaxies, such as NGC 6240, with ongoing efforts to find and confirm higher-multiplicity configurations. The study of multiblackholes informs models of black hole growth, galaxy evolution, and the gravitational wave background.

See also: binary black hole, supermassive black hole, active galactic nucleus, gravitational waves.