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

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.

💡 Research Summary

The authors address the puzzling observation from JWST that black holes (BHs) in low‑mass, high‑redshift (z > 3) galaxies appear “over‑massive” when compared to the local BH–stellar‑mass (M_BH–M_) relation, while remaining consistent with the local BH–dynamical‑mass (M_BH–M_dyn) relation. They propose that the M_BH–M_dyn relation is the fundamental scaling law and that the apparent evolution of M_BH–M_ is a secondary effect driven by the changing baryonic composition of early galaxies.

To test this hypothesis they use the THESAN‑ZOOM suite of high‑resolution cosmological zoom‑in simulations. These runs employ the AREPO‑RT moving‑mesh code with a state‑of‑the‑art multi‑phase interstellar‑medium (ISM) model, radiation‑hydrodynamics, and realistic star‑formation and feedback prescriptions. Importantly, the simulations contain no live black‑hole particles; therefore the authors must infer BH masses a posteriori.

For each central galaxy they compute a dynamical mass M_dyn by summing the stellar, gas, and dark‑matter mass within the stellar half‑mass radius and multiplying by two, mimicking the observational estimator. Assuming the local Kormendy & Ho (2013) M_BH–M_bulge relation (with M_bulge ≈ M_dyn for gas‑poor early‑type systems) they assign a BH mass to every galaxy, adding a 0.29 dex intrinsic scatter via Monte‑Carlo sampling.

The resulting M_BH–M_* distribution shows a clear mass‑dependent offset: at M_* ≈ 10^6 M_⊙ the median M_BH/M_* ≈ 0.1, while at M_* ≈ 10^10.5 M_⊙ the ratio drops to ≈0.01. This reproduces the JWST AGN data, including the “over‑massive” low‑mass points, without invoking any special BH growth physics. The authors attribute the trend to two correlated quantities: the dark‑matter fraction f_DM and the gas fraction f_gas. Both decrease with increasing stellar mass but exhibit little evolution from z ≈ 12 down to z ≈ 3. Consequently, low‑mass galaxies have small stellar‑to‑dynamical mass ratios (M_/M_dyn), so a fixed M_BH–M_dyn relation yields relatively large M_BH/M_ values.

To validate the simulated gas fractions they analyse 48,022 galaxies from the JWST Advanced Deep Extragalactic Survey (JADES) using Prospector SED fitting. Gas masses are estimated via the Tacconi et al. (2018) scaling with specific star‑formation rate, stellar mass, and redshift. The inferred f_gas–M_* relation matches the simulation remarkably well, and also aligns with independent high‑z measurements (e.g., de Graaff et al. 2024), confirming that the simulated galaxies have realistic baryonic compositions.

The study acknowledges several limitations. By construction it omits BH accretion, feedback, and any dynamical impact of BHs on their hosts, which could modify both M_dyn and the observed scaling relations. The neglect of the dark‑matter contribution to M_dyn (estimated at 10–40 % for early‑type systems) may cause a modest over‑estimate of BH masses (0.05–0.2 dex). Nonetheless, the core result—that a universal M_BH–M_dyn relation combined with the gas‑rich, dark‑matter‑dominated nature of low‑mass high‑z galaxies naturally yields over‑massive BHs—holds robustly.

In conclusion, the paper provides a compelling semi‑empirical framework that reconciles JWST observations with a fundamental BH–dynamical‑mass scaling. It suggests that the apparent evolution of the BH–stellar‑mass relation is largely a reflection of the evolving baryonic fractions in early galaxies rather than exotic BH seeding or super‑Eddington growth. Future work incorporating live BH particles and feedback will be essential to assess how these over‑massive BHs influence early galaxy formation, star‑formation quenching, and the emergence of the first quasars.