James Webb Space Telescope Feed Post

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

JWST/NIRCam 1."5 stamps and the RGB color image comprised of the F277W, F356W and F444W bands. The MSA slitlet layout is highlighted in white. An unambiguous point-like morphology of ID: 20466 can be observed in all filters. On each panel we show total magnitudes, with 1s uncertainty, as presented in the UNCOVER photometric catalog of Weaver et al. (2023). Middle: 2D MSA PRISM spectrum produced by msaexp. We optimally scaled the trace to highlight all the significant line detections. Bottom: A collapsed 1D spectrum of our source. We show the data in black, while the uncertainty on the spectrum is in red. Assuming the best-fit msaexp zspec = 8.502 ± 0.003 we show the positions of all the prominent emission with significant (> 3alpha) detections as solid vertical lines. Emission for which we only obtain an upper limit are shown with dashed lines. 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 Hß line with FWHM = 3439±413 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 log10(MBH/M?)=8.17±0.42, and a bolometric luminosity of Lbol~6.6×1045 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 log10(M*/M?)<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.