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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

TL;DR

The paper investigates why high-redshift BHs appear overmassive relative to local $M_ ext{BH}$–$M_ ext{*}$ relations by testing whether a fundamental $M_ ext{BH}$–$M_ ext{dyn}$ relation can reproduce JWST observations using THESAN-zoom simulations that lack live BHs. By inferring BH masses from dynami­cal masses with the local $M_ ext{BH}$–$M_ ext{bulge}$ relation (assuming $M_ ext{bulge} oughly M_ ext{dyn}$) and incorporating intrinsic scatter, the authors show overmassive BHs arise naturally in gas-rich, dark matter-dominated, low-mass galaxies, with $M_ ext{BH}/M_ ext{*}$ rising toward lower $M_ ext{*}$ and declining from $ ext{≈0.1}$ at $M_ ext{*} ext{ around }10^6 M_\odot$ to $ ext{≈0.01}$ at $M_ ext{*} ext{ around }10^{10.5} M_\odot$. The results are supported by comparisons to JADES gas fractions derived from Prospector SED fits and Tacconi scaling, and by cross-checks with TNG50, though the approach omits self-consistent BH feedback. Overall, the work suggests that the presence of overmassive BHs at high redshift is a natural consequence of fundamental dynamical-mass coupling in the early, gas-rich, DM-dominated regime, with significant implications for early galaxy formation and the need for future live-BH–galaxy co-evolution modeling.

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.

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

TL;DR

The paper investigates why high-redshift BHs appear overmassive relative to local – relations by testing whether a fundamental – relation can reproduce JWST observations using THESAN-zoom simulations that lack live BHs. By inferring BH masses from dynami­cal masses with the local – relation (assuming ) and incorporating intrinsic scatter, the authors show overmassive BHs arise naturally in gas-rich, dark matter-dominated, low-mass galaxies, with rising toward lower and declining from at to at . The results are supported by comparisons to JADES gas fractions derived from Prospector SED fits and Tacconi scaling, and by cross-checks with TNG50, though the approach omits self-consistent BH feedback. Overall, the work suggests that the presence of overmassive BHs at high redshift is a natural consequence of fundamental dynamical-mass coupling in the early, gas-rich, DM-dominated regime, with significant implications for early galaxy formation and the need for future live-BH–galaxy co-evolution modeling.

Abstract

JWST has revealed the apparent evolution of the black hole (BH)-stellar mass (-) relation in the early Universe, while remaining consistent with the BH-dynamical mass (-) relation. We predict BH masses for galaxies in the high-resolution THESAN-ZOOM simulations by assuming the - relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed distribution, including overmassive BHs relative to the local - relation. We find that declines with , evolving from 0.1 at to 0.01 at . This trend reflects the dark matter () and gas fractions (), which decrease with but show little redshift evolution down to , resulting in small ratios and thus overmassive BHs in low-mass galaxies. We use -derived stellar masses and star-formation rates to infer across 48,022 galaxies in JADES at , finding excellent agreement with our simulation. Our results demonstrate that overmassive BHs would naturally result from a fundamental - 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.

Paper Structure

This paper contains 9 sections, 1 equation, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methods
  3. The thesan-zoom project
  4. Inferring black hole masses in thesan-zoom
  5. Observations and SED fitting
  6. Overmassive black holes
  7. The black hole--stellar mass relation
  8. Baryonic and dark mass content
  9. Discussion and Conclusions

Figures (2)

  • Figure 1: The masses of BHs compared to their host galaxies, where $M_\mathrm{BH}$ is calculated from $M_\mathrm{dyn}$ assuming a universal $M_\mathrm{BH}$–$M_\mathrm{dyn}$ relation (see text for details). Red contours show the log-scaled distribution of thesan-zoom galaxies. The solid blue line shows the median trend, whereby the $M_\mathrm{BH}/M_\mathrm{\ast}$ ratio evolves from $\sim$0.1 at $M_\mathrm{\ast}=10^6~\mathrm{M_\odot}$ to $\sim$0.01 at $M_\mathrm{\ast}=10^{10.5}~\mathrm{M_\odot}$. Blue errorbars show the 16th--84th percentile scatter. Golden circles show $M_\mathrm{BH}$ for JWST-observed AGN at $z\approx4-11$Ubler:2023aaUbler:2024abHarikane:2023aaKokorev:2023aaCarnall:2023aaMaiolino:2024aaMaiolino:2024acFurtak:2024aaJuodzbalis:2024abNatarajan:2024aa, with dashed golden lines connecting candidate merging BHs. Grey triangles show QSOs at $z\approx5-7$Ding:2022aaStone:2024aaYue:2024aa. Dashed red lines follow constant $M_\mathrm{BH}/M_\mathrm{\ast}$ of 1, 0.1, and 0.01. We show best fit relations of $M_\mathrm{BH}$–$M_\mathrm{bulge}$ for local early type galaxies Kormendy:2013aa and $M_\mathrm{BH}$–$M_\mathrm{\ast}$ for local disk galaxies Reines:2015aa. Even without including any BH physics, our approach well reproduces JWST-observed BH and host galaxy masses, including overmassive BHs at low $M_\mathrm{\ast}$. We plot results following the same method for low-mass and bulge-dominated galaxies in TNG50, which show a similar trend at $z=3$ and evolve to the local elliptical relation at $z=0$.
  • Figure 2: Baryonic and dark matter mass content of galaxies as a function of stellar mass. Contours show the log-scaled distribution of thesan-zoom galaxies and the lines show median trends. Left:$f_\mathrm{DM}$ decreases with $M_\mathrm{\ast}$. thesan-zoom galaxies have higher $f_\mathrm{DM}$ than the high-redshift observational results of de-Graaff:2024aa, this may be due to their overestimated gas masses. Our $f_\mathrm{DM}$ values are consistent with the $z\approx1$ sample of Sharma:2021aa, except for the highest mass bin at $M_\mathrm{\ast}\gtrsim10^{10.5}$. There is a weak trend of increasing $f_\mathrm{DM}$ with redshift. TNG50 at $z=0$ shows somewhat lower $f_\mathrm{DM}$ at higher $M_\mathrm{\ast}$, whereas $f_\mathrm{DM}$ is significantly lower for higher masses at $z=3$. Right:$f_\mathrm{gas}$ decreases with $M_\mathrm{\ast}$, showing agreement with JADES galaxies. We also show $f_\mathrm{gas}$ from de-Graaff:2024aa, which are higher than both JADES and the low-redshift xGASS survey Catinella:2018aa. TNG50 shows lower $f_\mathrm{gas}$ for all $M_\mathrm{\ast}$ at $z=0$ and lower $f_\mathrm{gas}$ for higher $M_\mathrm{\ast}$ ($M_\mathrm{\ast}\gtrsim10^{9.5}$) at $z=3$. The median lines show that thesan-zoom$f_\mathrm{gas}$ values tend to be higher at lower redshifts, contrary to expectations. We attribute this to a population of galaxies with $f_\mathrm{gas}\approx0$ at higher redshift, which is caused by galaxies ejecting their ISMs during the burst-quench cycle. The median lines, excluding this population, show no redshift evolution and lie on the plotted $3<z<6$ line, in agreement with the lack of evolution in JADES. The combination of decreasing $f_\mathrm{DM}$ and $f_\mathrm{gas}$ with $M_\mathrm{\ast}$ leads to increasing $M_\mathrm{\ast}/M_\mathrm{dyn}$ with $M_\mathrm{\ast}$.