High redshift formation and evolution of central massive objects I: model description

Galactic nuclei host central massive objects either in the form of supermassive black holes or nuclear stellar clusters. Recent investigations have shown that both components co-exist in at least a few galaxies. In this paper we explore the possibility of a connection between nuclear star clusters and black holes that establishes at the moment of their formation. We here model the evolution of high redshift discs, hosted in dark matter halos with virial temperatures 10^4 K, whose gas has been polluted with metals just above the critical metallicity for fragmentation. A nuclear cluster forms as a result of a central starburst from gas inflowing from the unstable disc. The nuclear stellar cluster provides a suitable environment for the formation of a black hole seed, ensuing from runaway collisions among the most massive stars. Typical masses for the nuclear stellar clusters at the time of black hole formation (z~10) are inthe range 10^4-10^6 solar masses and have half mass radii < 0.5 pc. The black holes forming in these dense, high redshift clusters can have masses in the range ~300-2000 solar masses.