Supermassive black holes from runaway mergers and accretion in nuclear star clusters

Rapid formation of supermassive black holes occurs in dense nuclear star clusters that are initially gas-dominated. Stellar-mass black hole remnants of the most massive cluster sink into the core, where a massive runaway black hole forms as a consequence of combined effects of repeated mergers and Eddington-limited gas accretion. The associated gravitational-wave signals of high-redshift extreme mass-ratio inspirals are a unique signature of the nuclear star cluster scenario.