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 ($M_\mathrm{BH}$–$M_\mathrm{\ast }$) relation in the early Universe, while remaining consistent with the BH–dynamical mass ($M_\mathrm{BH}$–$M_\mathrm{dyn}$) relation. We predict BH masses for $z>3$ galaxies in the high-resolution thesan-zoom simulations by assuming that the $M_\mathrm{BH}$–$M_\mathrm{dyn}$ relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed $M_\mathrm{BH}$ distribution, including overmassive BHs relative to the local $M_\mathrm{BH}$–$M_\mathrm{\ast }$ relation. We find that $M_\mathrm{BH}/M_\mathrm{\ast }$ declines with $M_\mathrm{\ast }$, evolving from $\sim$0.1 at $M_\mathrm{\ast }=10^6\ \mathrm{M_\odot }$ to $\sim$0.01 at $M_\mathrm{\ast }=10^{10.5}\ \mathrm{M_\odot }$. This trend reflects the dark matter ($f_\mathrm{DM}$) and gas fractions ($f_\mathrm{gas}$), which decrease with $M_\mathrm{\ast }$ but show little redshift evolution down to $z=3$, resulting in small $M_\mathrm{\ast }/M_\mathrm{dyn}$ ratios and thus overmassive BHs in low-mass galaxies. We use prospector-derived stellar masses and star formation rates to infer $f_\mathrm{gas}$ 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 $M_\mathrm{BH}$–$M_\mathrm{dyn}$ 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.