2025MNRAS.542L..78R

Motivated by JWST observations of dense, clumpy, and clustered high-redshift star formation, we simulate the hierarchical assembly of nine $M_{\mathrm{cl}}={10^6}\:\mathrm{M_\odot }$ star clusters using the BIFROST N-body code. Our low-metallicity models ($0.01Z_\odot$) with post-Newtonian equations of motion for black holes include evolving populations of single, binary, and triple stars. Massive stars grow by stellar collisions and collapse into intermediate-mass black holes (IMBHs) up to $M_\mathrm{\bullet }\sim {6200}\:\mathrm{M_\odot }$, stellar multiplicity boosting the IMBH masses by a factor of 2-3. The IMBHs tidally disrupt (TDE) $\sim 50$ stars in 10 Myr with peak TDE rates up to $\Gamma \sim 5\times 10^{-5}$ $\rm{yr}^{-1}$ per cluster. These IMBHs are natural seeds for supermassive black holes (SMBHs) and the hierarchical assembly frequently leads to $>2$ SMBH seeds per cluster and their rapid mergers ($t< 10$ Myr). We propose that a gravitational wave (GW)-driven merger of IMBHs with ${1000}\:\mathrm{M_\odot } \lesssim M_\bullet \lesssim {10\,000}\:\mathrm{M_\odot }$ with comparable masses is a characteristic GW fingerprint of SMBH seed formation at redshifts $z>10$, and IMBH formation in similar environments at lower redshifts. Massive star clusters provide a unique environment for the early Universe GW studies for the next-generation GW observatories including the Einstein Telescope and the Laser Interferometer Space Antenna.