James Webb Space Telescope Feed Post

UNCOVER: A NIRSpec Identification of a Broad Line AGN at z = 8.50

Paper abstract: Deep observations with JWST have revealed an emerging population of red point-like sources that could provide a link between the postulated supermassive black hole seeds and observed quasars. In this work we present a JWST/NIRSpec spectrum from the JWST Cycle 1 UNCOVER Treasury survey, of a massive accreting black hole at z=8.50, displaying a clear broad-line component as inferred from the H\beta line with FWHM = 3439\pm413 km s^{-1}, typical of the broad line region of an active galactic nucleus (AGN). The AGN nature of this object is further supported by high ionization, as inferred from emission lines, and a point-source morphology. We compute the black hole mass of log_{10}(M_{\rm BH}/M_\odot)=8.17\pm0.42, and a bolometric luminosity of L_{\rm bol}~6.6\times10^{45} erg s^{-1}. These values imply that our object is accreting at ~ 40\% of the Eddington limit. Detailed modeling of the spectral energy distribution in the optical and near-infrared, together with constraints from ALMA, indicate an upper limit on the stellar mass of log_{10}(M_{\rm *}/M_\odot)<8.7, which would lead to an unprecedented ratio of black hole to host mass of at least ~ 30 \%. This is orders of magnitude higher compared to the local QSOs, but is consistent with recent AGN studies at high redshift with JWST. This finding suggests that a non-negligible fraction of supermassive black holes either started out from massive seeds and/or grew at a super-Eddington rate at high redshift. Given the predicted number densities of high-z faint AGN, future NIRSpec observations of larger samples will allow us to further investigate the galaxy-black hole co-evolution in the early Universe.