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

2023/08/22 by Vasily Kokorev, Seiji Fujimoto, Kokorev, Vasily +60 · 33 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena #Astronomy and Astrophysical Research

paper · doi:10.48550/arxiv.2308.11610

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 Hβ 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(M_{\rm BH}/M_\odot)=8.17±0.42, and a bolometric luminosity of L_{\rm bol}∼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_{\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.

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