Supercritical growth pathway to overmassive black holes at cosmic dawn:
  coevolution with massive quasar hosts
  Observations of the most luminous quasars at high redshifts ($z > 6$) have
revealed that the largest supermassive black holes (SMBHs) at those epochs tend
to be substantially overmassive relative to their host galaxies compared to the
local relations, suggesting they experienced rapid early growth phases. We
propose an assembly model for the SMBHs that end up in rare massive
$\sim10^{12}~M_{\odot}$ host halos at $z \sim 6-7$, applying a kinetic feedback
prescription for BHs accreting above the Eddington rate, provided by radiation
hydrodynamic simulations for the long-term evolution of the accretion-flow
structure. The large inflow rates into these halos during their assembly enable
the formation of $>10^9~M_{\odot}$ SMBHs by $z \sim 6$, even starting from
stellar-mass seeds at $z \sim 30$, and even in the presence of outflows that
reduce the BH feeding rate, especially at early times. This mechanism also
naturally yields a high BH-to-galaxy mass ratio of $> 0.01$ before the SMBH
mass reaches $M_{\rm BH} > 10^9~M_{\odot}$ by $z \sim 6$. These fast-growing
SMBH progenitors are bright enough to be detected by upcoming observations with
the James Webb Space Telescope over a wide range of redshift ($7 < z < 15$),
regardless of how they were seeded.
