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

JWST has revealed a large population of active galactic nuclei (AGN) 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, [O iii]5007 luminosity and equivalent width, UV luminosity and stellar mass. In various stacks, we detect a broad component of Hα without a counterpart in [O iii], implying that it is not due to outflows but traces the Broad Line Region of a large population of low-luminosity AGN not detected in individual spectra. The detection, in some stacks, of high [O iii]4363/Hγ, typical of AGN, further confirms the detection of a large population of AGN. We infer that the stacks probe BHs with masses of a few times 106M⊙ accreting at rates L/LEdd ∼ 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 MBH − 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.