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

Abstract 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 z > 3 galaxies in the high-resolution thesan-zoom simulations by assuming 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* = 1010.5 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 JADES 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.