Supermassive Black Hole Growth in Massive Galaxies at Cosmic Dawn

Among the emerging excess of massive, bright galaxies at Cosmic Dawn $z \gtrsim 9$ seen by the James Webb Space Telescope, several exhibit spectral features associated with active galactic nuclei (AGN). These AGN candidates suggest that supermassive black holes (SMBHs) grow rapidly in the early Universe. In a series of numerical experiments, we investigate how SMBHs grow within and influence the most massive galaxies at Cosmic Dawn using cosmological hydrodynamic zoom-in simulations run with the adaptive mesh refinement code \textsc{ramses}. Our suite of simulations explore how super-Eddington accretion, seed mass, and the strength of feedback influence SMBH-galaxy co-evolution in the most massive galaxies ($M_\star \gtrsim 10^8 M_\odot$) of the early Universe ($z \sim 15 - 9$). The environment which our numerical experiments reside in is an overdensity that collapses into a $\sim 10^{11} M_\odot$ halo by $z \sim 9$. Within this type of environment we find that SMBH growth is sensitive to stellar feedback which generates a turbulent-multiphase interstellar medium (ISM) that stochastically starves the SMBH. In the absence of AGN feedback, we find that the SMBH is starved $\sim 50\%$ of the time after the onset of star formation in the galaxy. SMBH growth can become self-regulated by AGN feedback if the SMBH becomes massive enough, either by accretion or seeding, for its feedback to dominate the surrounding nuclear region. We find no evidence of galaxy-scale, AGN-driven quenching in the star formation rate (SFR) across all simulations in our suite.