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 $z=9.9837\pm0.0009$, identified through a systematic search for broad-line AGNs at $z\geq6$ 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 $L_{{\rm H}β,{\rm broad}}=1.25^{+0.33}_{-0.31}\times10^{42}$ 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 $z\sim10$. We derive $M_{\rm BH}=3.6^{+5.8}_{-1.7}\times10^7 M_\odot$ and $λ_{\rm Edd}=0.11^{+0.10}_{-0.07}$ 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 $λ_{\rm Edd}=1$, which yields $M_{\rm BH}=2.1^{+0.6}_{-0.5}\times10^6 M_\odot$. 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.