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

William McClymont, Sandro Tacchella, Xihan Ji, Rahul Kannan, Roberto Maiolino, Charlotte Simmonds, Aaron Smith, Ewald Puchwein, Enrico Garaldi, Mark Vogelsberger, Francesco D'Eugenio, Laura Keating, Xuejian Shen, Bartolomeo Trefoloni, Oliver Zier

First listed 2025-06-16 | Last updated 2026-02-06

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 $M_\mathrm{BH}$-$M_\mathrm{\ast}$ 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 $\texttt{Prospector}$-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.

Short digest

Using THESAN-ZOOM zoom-ins, the authors assign black-hole masses by assuming the M_BH–M_dyn relation is fundamental and then compare to JWST-selected AGN and JADES host properties at z>3. This simple prescription reproduces the observed distribution of “overmassive” black holes relative to local M_BH–M_* relations, with M_BH/M_* declining from ~0.1 at M_*≈10^6 Msun to ~0.01 at M_*≈10^10.5 Msun. The trend naturally follows the high dark-matter and gas fractions of low-mass galaxies—f_DM and f_gas decrease with M_* and show little evolution to z≈3—yielding small M_*/M_dyn and thus large apparent BH offsets; Prospector-inferred f_gas for 48,022 JADES galaxies (3<z<9) agrees with the simulations. A caveat is that BH growth and feedback are not modeled live, so full BH–galaxy co-evolution is not captured.

Key figures to inspect

• Figure 1 (M_BH vs M_* assuming M_BH–M_dyn): Inspect the median track where M_BH/M_* falls from ~0.1 to ~0.01, the constant-ratio lines, and the overlay of JWST broad-line AGN and z>6 QSOs; note how the simulated distribution reproduces the observed overmassive regime and how the TNG50 overlay trends toward the local e…
• Figure 2 (left; dark-matter fraction vs M_*): Check how f_DM decreases with stellar mass with only weak redshift dependence; compare to de Graaff et al. points and TNG50 trends to see how high f_DM at low M_* drives small M_*/M_dyn and thus large apparent M_BH/M_*.
• Figure 2 (right; gas fraction vs M_*): Verify the declining f_gas with mass and the close agreement with JADES (Prospector-derived) medians; note the higher de Graaff values, the xGASS low-z reference, and the near-zero-f_gas outliers attributed to burst–quench ISM ejection; observe the minimal redshift evolution once…