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.