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 ($M_{\mathrm{BH}}$-$M_\rm{\ast}$) 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 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 $M_{\mathrm{BH}}$-$M_{\ast }$ relation. We find that $M_{\mathrm{BH}}/M_{\ast }$ declines with $M_{\ast }$, evolving from $\sim$0.1 at $M_{\ast }=10^6{\mathrm{M}}_{\odot }$ to $\sim$0.01 at $M_{\ast }=10^{10.5}{\mathrm{M}}_{\odot }$. This trend reflects the dark matter ($f_{\mathrm{DM}}$) and gas fractions ($f_{\mathrm{gas}}$), which decrease with $M_{\ast }$ but show little redshift evolution down to $z=3$, resulting in small $M_{\ast }/M_{\mathrm{dyn}}$ ratios and thus overmassive BHs in low-mass galaxies. We use $\text{Prospector}$-derived stellar masses and star-formation rates to infer $f_{\mathrm{gas}}$ 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 $M_{\mathrm{BH}}$-$M_{\mathrm{dyn}}$ 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.

6 pages, 2 figures, accepted in MNRAS