Growth of high redshift supermassive black holes from heavy seeds in the BRAHMA cosmological simulations: Implications of overmassive black holes

JWST has recently revealed a large population of accreting black holes (BHs) in the early Universe. Even after accounting for possible systematic biases, the high-z M_*-M ₁₇ relation derived from these objects by Pacucci et al. (2023 P23 relation) is above the local scaling relation by >3. To understand the implications of potentially overmassive high-z BH populations, we study the BH growth at z4-7 using the 18~Mpc³ BRAHMA suite of cosmological simulations with systematic variations of heavy seed models that emulate direct collapse black hole (DCBH) formation. In our least restrictive seed model, we place 10⁵~M_ seeds in halos with sufficient dense and metal-poor gas. To model conditions for direct collapse, we impose additional criteria based on a minimum Lyman Werner flux (LW flux =10~J₂₁), maximum gas spin, and an environmental richness criterion. The high-z BH growth in our simulations is merger dominated, with a relatively small contribution from gas accretion. For the most restrictive simulation that includes all the above seeding criteria for DCBH formation, the high-z M_*-M ₁₇ relation falls significantly below the P23 relation (by factor of 10 at z4). Only by excluding the spin and environment based criteria, and by assuming 750~Myr delay times between host galaxy mergers and subsequent BH mergers, are we able to reproduce the P23 relation. Overall, our results suggest that if high-z BHs are indeed systematically overmassive, assembling them would require more efficient heavy seeding channels, higher initial seed masses, additional contributions from lighter seeds to BH mergers, and / or more efficient modes for BH accretion.