Intermediate-mass black holes from Population III remnants in the first galactic nuclei

We report the formation of intermediate-mass black holes (IMBHs) in suites of numerical N-body simulations of Population III remnant black holes (BHs) embedded in gas-rich protogalaxies at redshifts z & 10. We model the eects of gas drag on the BHs’ orbits, and allow BHs to grow via gas accretion, including a mode of hyper-Eddington accretion in which photon trapping and rapid gas inow suppress any negative radiative feedback. Most initial BH congurations lead to the formation of one (but never more than one) IMBH in the center of the protogalaxy, reaching a mass of 10 3 5 M through hyper-Eddington growth. Our results suggest a viable pathway to forming the earliest massive BHs in the centers of early galaxies. We also nd that the nuclear IMBH typically captures a stellar-mass BH companion, making these systems observable in gravitational waves as extreme mass-ratio inspirals (EMRIs) with eLISA.