Little Red Dots: An Abundant Population of Faint Active Galactic Nuclei at z 5 Revealed by the EIGER and FRESCO JWST Surveys

Characterizing the prevalence and properties of faint active galactic nuclei (AGNs) 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 H alpha emitters at z approximate to 4-6 using deep JWST/NIRCam imaging and wide field slitless spectroscopy from the EIGER and FRESCO surveys. We identify 20 H alpha lines at z = 4.2-5.5 that have broad components with line widths from similar to 1200-3700 km s(-1), contributing similar to 30%-90% of the total line flux. We interpret these broad components as being powered by accretion onto SMBHs with implied masses similar to 10(7-8)M(circle dot). In the UV luminosity range M-UV,M-AGN+host = -21 to -18, we measure number densities of approximate to 10(-5) cMpc(-3). This is an order of magnitude higher than expected from extrapolating quasar UV luminosity functions (LFs). Yet, such AGN are found in only <1% of star-forming galaxies at z similar to 5. The number density discrepancy is much lower when compared to the broad H alpha LF. The SMBH mass function agrees with large cosmological simulations. In two objects, we detect complex H alpha profiles that we tentatively interpret as caused by absorption signatures from 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.