Kavli Affiliate: Roberto Maiolino
| Summary:
JWST has revealed the apparent evolution of the black hole (BH)-stellar mass ($M_mathrmBH$-$M_rmast$) relation in the early Universe, while remaining consistent with the BH-dynamical mass ($M_mathrmBH$-$M_mathrmdyn$) relation. We predict BH masses for $z>3$ galaxies in the high-resolution THESAN-ZOOM simulations by assuming the $M_mathrmBH$-$M_mathrmdyn$ relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed $M_mathrmBH$ distribution, including overmassive BHs relative to the local $M_mathrmBH$-$M_mathrmast$ relation. We find that $M_mathrmBH/M_mathrmast$ declines with $M_mathrmast$, evolving from $sim$0.1 at $M_mathrmast=10^6,mathrmM_odot$ to $sim$0.01 at $M_mathrmast=10^10.5,mathrmM_odot$. This trend reflects the dark matter ($f_mathrmDM$) and gas fractions ($f_mathrmgas$), which decrease with $M_mathrmast$ but show little redshift evolution down to $z=3$, resulting in small $M_mathrmast/M_mathrmdyn$ ratios and thus overmassive BHs in low-mass galaxies. We use $textttProspector$-derived stellar masses and star-formation rates to infer $f_mathrmgas$ 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 $M_mathrmBH$-$M_mathrmdyn$ 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.
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