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 HΞ± emitters at
zβ4β6 using deep JWST/NIRCam imaging and wide field slitless
spectroscopy from the EIGER and FRESCO surveys. We identify 20 HΞ± lines
at z=4.2β5.5 that have broad components with line widths from βΌ1200β3700 km sβ1, contributing βΌ30β90 % of the total line flux. We
interpret these broad components as being powered by accretion onto SMBHs with
implied masses βΌ107β8 Mββ. In the UV luminosity range MUVβ = -21 to -18, we measure number densities of β10β5 cMpcβ3.
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 zβΌ5. The SMBH mass function agrees with large cosmological
simulations. In two objects we detect narrow red- and blue-shifted HΞ±
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.Comment: 23 pages, 17 figures. Submitted to ApJ. Main Figs 4, 10, 15 (faint
AGN UV luminosity function) and 16 (SMBH mass function). Fig. 17 summarises
the results. Comments welcom