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Deep observations with JWST have revealed an emerging population of redpoint-like sources that could provide a link between the postulatedsupermassive black hole seeds and observed quasars. In this work we present aJWST/NIRSpec spectrum from the JWST Cycle 1 UNCOVER Treasury survey, of amassive accreting black hole at $z=8.50$, displaying a clear broad-linecomponent as inferred from the H$\beta$ line with FWHM = $3439\pm413$ kms$^{-1}$, typical of the broad line region of an active galactic nucleus (AGN).The AGN nature of this object is further supported by high ionization, asinferred from emission lines, and a point-source morphology. We compute theblack hole mass of log$_{10}(M_{\rm BH}/M_\odot)=8.17\pm0.42$, and a bolometricluminosity of $L_{\rm bol}\sim6.6\times10^{45}$ erg s$^{-1}$. These valuesimply that our object is accreting at $\sim 40\%$ of the Eddington limit.Detailed modeling of the spectral energy distribution in the optical andnear-infrared, together with constraints from ALMA, indicate an upper limit onthe stellar mass of log$_{10}(M_{\rm *}/M_\odot)<8.7$, which would lead to anunprecedented ratio of black hole to host mass of at least $\sim 30 \%$. Thisis orders of magnitude higher compared to the local QSOs, but is consistentwith recent AGN studies at high redshift with JWST. This finding suggests thata non-negligible fraction of supermassive black holes either started out frommassive seeds and/or grew at a super-Eddington rate at high redshift. Given thepredicted number densities of high-$z$ faint AGN, future NIRSpec observationsof larger samples will allow us to further investigate the galaxy-black holeco-evolution in the early Universe.

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