JADES reveals a large population of low-mass black holes at high redshift

James Webb Space Telescope (JWST) has revealed a large population of active galactic nuclei (AGNs) in the distant Universe, which are challenging our understanding of early massive black hole (BH) seeding and growth. We expand the exploration of this population to lower luminosities by stacking ~600 NIRSpec grating spectra from the JWST Advanced Deep Extragalactic Survey (JADES) at 3 < z < 7, in bins of redshift, [Oiii]5007 luminosity and equivalent width, UV luminosity, and stellar mass. In multiple stacks, we detect a broad component of H α without a counterpart in [OIII], implying that it is not due to outflows but traces the broad-line region of a large population of low-luminosity AGNs not detected in individual spectra. The detection, in some stacks, of high [Oiii]4363/H$\gamma$, typical of AGNs, further confirms the detection of a large population of AGNs. We infer that the stacks probe BHs with masses of a few times 10⁶ M_⊙ accreting at rates L/L_Edd ~ 0.02–0.1, i.e. a low-mass and dormant parameter space poorly explored by previous studies on individual targets. We identify populations of BHs that fall within the scatter of the local M_BH-M_* scaling relation, indicating that there is a population of high-z BHs that are not overmassive relative to their host galaxies. Yet, on average, the stacks are still overmassive relative the local relation, with some of them 1–2 dex above it. We infer that the BH mass function at 3 < z < 5 is consistent with models in which BHs evolve through short bursts of super-Eddington accretion.