Black holes in the shadows: The missing high-ionization lines in the earliest JWST active galactic nuclei

Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift, broad-line active galactic nuclei (AGNs), whose properties challenge standard models of black-hole growth and AGN emission. We analyzed a spectroscopic sample of 34 Type 1 AGNs from the JWST Advanced Deep Survey (JADES) survey, spanning redshifts 1.7 < z < 9, to constrain the physical nature of the accretion flows powering these sources with broad-line diagnostics statistically for the first time. At z > 5, we find a marked suppression of high-ionization emission lines (HeII, CIV, NV) relative to prominent broad Hα and narrow [OIII] features. This contrast places strong constraints on the shape of the ionizing spectral energy distribution (SED) and on the physical conditions in the broad-line region (BLR). By comparing the observations to photoionization models based on SEDs of black holes accreting at sub-Eddington ratios, we show that standard AGN continua struggle to reproduce the observed broad-line ratios and equivalent widths (EWs) across a wide ionization parameter range. These results suggest the need for modified SEDs – either intrinsically softened due to super-Eddington accretion or radiative inefficiencies in the innermost disk, or externally filtered by intervening optically thick gas that absorbs or scatters the highest energy photons before they reach the BLR.