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

JWST has revealed a large population of accreting black holes (BHs) in the early Universe. Recent work has shown that even after accounting for possible systematic biases, the high-z$M_*{\!-\!}M_{\rm \rm bh}$ relation can be above the local scaling relation by $\gt 3\sigma$. To understand the implications of these overmassive high-z BHs, we study the BH growth at $z\sim 4{\!-\!}7$ using the $[18~\mathrm{Mpc}]^3$BRAHMA 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 $\sim 10^5~{\rm M}_{\odot }$ seeds in haloes 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 $=10~J_{21}$), 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. The simulation that includes all the above seeding criteria fails to reproduce an overmassive high-z$M_*{\!-\!}M_{\rm bh}$ relation consistent with observations (by factor of $\sim 10$ at $z\sim 4$). However, more optimistic models that exclude the spin and environment based criteria are able to reproduce the observed relations if we assume $\lesssim 750~\mathrm{Myr}$ delay times between host galaxy mergers and subsequent BH mergers. Overall, our results suggest that current JWST observations may be explained with heavy seeding channels if their formation is more efficient than currently assumed DCBH conditions. Alternatively, we may need higher initial seed masses, additional contributions from lighter seeds to BH mergers, and / or more efficient modes for BH accretion.