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Characterising the prevalence and properties of faint active galactic nuclei (AGN) in the early Universe is key for understanding the formation of supermassive black holes (SMBHs) and determining their role in cosmic reionization. We perform a spectroscopic search for broad Hα emitters at z4-6 using deep JWST/NIRCam imaging and wide field slitless spectroscopy from the EIGER and FRESCO surveys. We identify 20 Hα lines at z=4. 2-5. 5 that have broad components with line widths from 1200-3700 km s^-1, contributing 30-90 % of the total line flux. We interpret these broad components as being powered by accretion onto SMBHs with implied masses 10^7-8 M_. In the UV luminosity range M ₔₕ=-21 to -18, we measure number densities of 10^-5 cMpc^-3. This is an order of magnitude higher than expected from extrapolating quasar UV luminosity functions. Yet, such AGN are found in only <1 % of star-forming galaxies at z5. The SMBH mass function agrees with large cosmological simulations. In two objects we detect narrow red- and blue-shifted Hα absorption indicative, respectively, of dense gas fueling SMBH growth and outflows. We may be witnessing early AGN feedback that will clear dust-free pathways through which more massive blue quasars are seen. We uncover a strong correlation between reddening and the fraction of total galaxy luminosity arising from faint AGN. This implies that early SMBH growth is highly obscured and that faint AGN are only minor contributors to cosmic reionization.
Matthee et al. (Thu,) studied this question.