Growth of high redshift supermassive black holes from heavy seeds in the BRAHMA cosmological simulations: Implications of overmassive black holes
Abstract
JWST has recently revealed a large population of accreting black holes (BHs) in the early Universe. Even after accounting for possible systematic biases, the highz $M_*M_{\rm \rm bh}$ relation derived from these objects by Pacucci et al. (2023 P23 relation) is above the local scaling relation by $>3\sigma$. To understand the implications of potentially overmassive highz BH populations, we study the BH growth at $z\sim47$ using the $[18~\mathrm{Mpc}]^3$ 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 $\sim10^5~M_{\odot}$ seeds in halos with sufficient dense and metalpoor 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 highz 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 highz $M_*M_{\rm bh}$ relation falls significantly below the P23 relation (by factor of $\sim10$ at $z\sim4$). Only by excluding the spin and environment based criteria, and by assuming $\lesssim750~\mathrm{Myr}$ delay times between host galaxy mergers and subsequent BH mergers, are we able to reproduce the P23 relation. Overall, our results suggest that if highz 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.
 Publication:

arXiv eprints
 Pub Date:
 June 2024
 DOI:
 10.48550/arXiv.2406.14658
 arXiv:
 arXiv:2406.14658
 Bibcode:
 2024arXiv240614658B
 Keywords:

 Astrophysics  Astrophysics of Galaxies
 EPrint:
 21 pages, 15 figures