The Redshift Evolution of the $M_\bullet-M_\star$ Relation for JWST's Supermassive Black Holes at $z > 4$

Abstract: JWST has detected many overmassive galactic systems at $z > 4$, where the mass of the black hole, $M_\bullet$, is $10-100$ times larger than expected from local relations, given the host's stellar mass, $M_\star$. This Letter presents a model to describe these overmassive systems in the high-$z$ Universe. We suggest that the black hole mass is the main driver of high-$z$ star formation quenching. SMBHs globally impact their high-$z$ galaxies because their hosts are physically small, and the black holes have duty cycles close to unity at $z > 4$. In this regime, we assume that black hole mass growth is regulated by the quasar's output, while stellar mass growth is quenched by it and uncorrelated to the global properties of the host halo. We find that the ratio $M_\bullet/M_\star$ controls the average star formation efficiency: if $M_\bullet/M_\star > 8\times 10^{18} (n \Lambda/f_{edd})[(\Omega_b M_h)/(\Omega_m M_\star) - 1]$, then the galaxy is unable to form stars efficiently. Once this ratio exceeds the threshold, a runaway process brings the originally overmassive system towards the local $M_\bullet - M_\star$ relation. Furthermore, the $M_\bullet - M_\star$ relation evolves with redshift as $\propto (1+z)^{5/2}$. At $z \sim 5$, we find an overmassive factor of $\sim 55$, in excellent agreement with current JWST data and the high-$z$ relation inferred from those. Extending the black hole horizon farther in redshift and lower in mass will test this model and improve our understanding of the early co-evolution of black holes and galaxies.