A Selection Aware View of Black Hole–Galaxy Coevolution at High Redshift

The co-evolution of supermassive black holes and their host galaxies during the first gigayear of cosmic history remains a central open question in galaxy formation, particularly in the context of black hole feedback. The large population of broad-line AGN discovered by JWST at z>4 now enables direct empirical constraints on this connection. Using the JADES survey-level compilation, we develop a forward-modeling Bayesian framework that explicitly accounts for broad Halpha detectability and associated selection effects. We find that the M_BH–M_* relation at z=4–6 is broadly consistent with the local scaling in both slope and normalization, indicating that the average black hole–galaxy coupling was already established at early times. However, the intrinsic scatter is substantially larger than in the nearby Universe, revealing a wide diversity of growth pathways likely driven by stochastic accretion, mergers, and feedback processes in young galaxies. This increased dispersion disfavors finely tuned seeding or self-regulated feedback models, and instead points to a more chaotic interplay between black hole growth and host-galaxy evolution at cosmic dawn.