James Webb Space Telescope Feed Post
Little Red Dots: an abundant population of faint AGN at z~5 revealed by the EIGER and FRESCO JWST surveys
Zoom-in stamps of all BL Halpha emitters. For each object we show the F200W (EIGER) or F182M+F210M (FRESCO) image that has a PSF FWHM~ 0.06" and the F356W or F444W image (PSF FWHM ~ 0.12") that contains the Ha line emission. The colour scaling follows a power-law with exponent y = 0.6 to highlight low surface brightness emission. 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 Ha emitters at z˜4-6 using deep JWST/NIRCam imaging and wide field slitless spectroscopy from the EIGER and FRESCO surveys. We identify 20 Halpha 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 ~107-8 M. In the UV luminosity range MUV = -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 z~5. The SMBH mass function agrees with large cosmological simulations. In two objects we detect narrow red- and blue-shifted Ha 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. Read more at the article.