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

Sketch of the three phases that describe our sample of faint AGN, illustrated by their apparent morphology and Halpha line profile. In the top row, we show false-color JWST/NIRCam images of representative objects for each phase. We sketch the relative importance of star formation and AGN activity to the rest-frame UV-optical light in the middle row, where the red star shows the AGN, the black spiral the star-forming component and clouds illustrate dust content. In the bottom row we show the observed Ha line profiles for the representative sources. We also list typical BH masses, FWHM of the broad components and the relative broad-to-total Ha luminosity. 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.