Overmassive black holes in the early Universe can be explained by gas-rich, dark matter-dominated galaxies

ABSTRACT James Webb Space Telescope (JWST) has revealed the apparent evolution of the black hole (BH) –stellar mass (MBH–M_) relation in the early Universe, while remaining consistent with the BH–dynamical mass (MBH–Mdyn) relation. We predict BH masses for z3 galaxies in the high-resolution thesan-zoom simulations by assuming that the MBH–Mdyn relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed MBH distribution, including overmassive BHs relative to the local MBH–M_ relation. We find that MBH/M_ declines with M_, evolving from 0. 1 at M_ =10⁶\ M_ to 0. 01 at M_ =10^10. 5\ M_. This trend reflects the dark matter (fDM) and gas fractions (fgas), which decrease with M_ but show little redshift evolution down to z=3, resulting in small M_ /Mdyn ratios and thus overmassive BHs in low-mass galaxies. We use prospector-derived stellar masses and star formation rates to infer fgas across 48 022 galaxies in the JWST Advanced Deep Extragalactic Survey at 3 z 9, finding excellent agreement with our simulation. Our results demonstrate that overmassive BHs would naturally result from a fundamental MBH–Mdyn relation and be typical of the gas-rich, dark matter-dominated nature of low-mass, high-redshift galaxies. Such overmassive BHs may strongly influence early galaxy formation, and we caution that our approach does not include the self-consistent BH–galaxy co-evolution required for a complete understanding.