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Arxiv: Little Red Dots: an abundant population of faint AGN at z~5 revealed by the EIGER and FRESCO JWST surveys Published: 6/8/2023 9:00:00 PM Updated: 2/1/2024 12:58:14 PM

Paper abstract: Characterising the prevalence and properties of faint active galactic nuclei(AGN) in the early Universe is key for understanding the formation ofsupermassive black holes (SMBHs) and determining their role in cosmicreionization. We perform a spectroscopic search for broad H\alpha emitters atz~4-6 using deep JWST/NIRCam imaging and wide field slitlessspectroscopy from the EIGER and FRESCO surveys. We identify 20 H\alpha linesat z=4.2-5.5 that have broad components with line widths from ~1200-3700km s^{-1}, contributing ~30-90 % of the total line flux. We interpretthese broad components as being powered by accretion onto SMBHs with impliedmasses ~10^{7-8} M_{\odot}. In the UV luminosity range M_{\rm UV}=-21to -18, we measure number densities of ~10^{-5} cMpc^{-3}. This isan order of magnitude higher than expected from extrapolating quasar UVluminosity functions. Yet, such AGN are found in only <1 % of star-forminggalaxies at z~5. The SMBH mass function agrees with large cosmologicalsimulations. In two objects we detect narrow red- and blue-shifted H\alphaabsorption indicative, respectively, of dense gas fueling SMBH growth andoutflows. We may be witnessing early AGN feedback that will clear dust-freepathways through which more massive blue quasars are seen. We uncover a strongcorrelation between reddening and the fraction of total galaxy luminosityarising from faint AGN. This implies that early SMBH growth is highly obscuredand that faint AGN are only minor contributors to cosmic reionization.