From nuclear star clusters to intermediate mass black holes

 Nuclear star clusters, that fragment into metal-poor stars in situ at the centers of forming galaxies, provide ideal environments for the formation of massive black holes. Stellar collisions, black hole ejections,  and gas accretion/expulsion  all contribute to the dynamical formation of intermediate-mass black holes via repeated mergers of stellar black-hole seeds. Million solar mass black holes can form within 100 Myr in the heaviest and most compact nuclear  star clusters. These  are the missing link for seeding supermassive black holes. There are potential gravitational-wave signatures of the massive black-hole formation process.