Discovery of a Broad-Line Active Galactic Nucleus at z = 9.98

The James Webb Space Telescope (JWST) has uncovered a previously unexplored population of faint active galactic nuclei (AGNs) in the early Universe. We present MoM-BL-z10 at z=9.9837 plus or minus0.0009, identified through a systematic search for broad-line AGNs at z at least6 using public JWST/NIRSpec PRISM spectra from the DAWN JWST Archive. The PRISM spectrum exhibits a broad component in H β at 5.35 microns. A narrow+broad Gaussian fit yields a broad H β detection at S/N = 3.87, with FWHM = 3480+2040-950 km s-1 and L H β, broad=1.25+0.33-0.31 times10 42 erg s-1. The detection of [Ne III] lambda3870 and the high [Ne III]/[O II] and [O III]/H β ratios further support the AGN interpretation. Its rest-frame UV and optical continuum slopes, compact NIRCam morphology, and a tentative MIRI F770W detection suggest similarities to Little Red Dots (LRDs). The broad H β detection enables the first virial SMBH mass estimate at z about10. We derive M BH=3.6+5.8-1.7 times10 7 M and λ Edd=0.11+0.10-0.07 using the standard single-epoch virial calibration and quasar bolometric correction. We also consider an alternative scenario adopting the LRD-specific bolometric correction and assuming λ Edd=1, which yields M BH=2.1+0.6-0.5 times10 6 M. Under either scenario, the inferred SMBH mass at a cosmic age of only 470 Myr highlights the challenge of rapid BH growth in the early Universe, suggesting super-Eddington accretion from a stellar-mass seed or a more massive seed. As the highest-redshift broad-line AGN identified to date, MoM-BL-z10 provides a new observational benchmark for understanding the early growth of supermassive black holes.