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–Ma) 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 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∗ = 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 48022 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.