Kavli Affiliate: Roberto Maiolino
| First 5 Authors: William McClymont, Sandro Tacchella, Xihan Ji, Rahul Kannan, Roberto Maiolino
| Summary:
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 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,
rapidly growing BHs may strongly influence the earliest stages of galaxy
formation.
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