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 $z=9.9837\pm 0.0009$, identified through a systematic search for broad-line AGNs at $z\ge 6$ using public JWST/NIRSpec PRISM spectra from the DAWN JWST Archive. The PRISM spectrum exhibits a broad component in H$\beta$ at 5.35 microns. A narrow+broad Gaussian fit yields a broad H$\beta$ detection at S/N = 3.87, with FWHM = $3480_{-950}^{+2040}$ km s${}^{-1}$ and $L_{\mathrm{H}\beta ,\mathrm{b}\mathrm{r}\mathrm{o}\mathrm{a}\mathrm{d}}=1.25_{-0.31}^{+0.33}\times {10}^{42}$ erg s${}^{-1}$. The detection of [Ne III] $\lambda 3870$ and the high [Ne III]/[O II] and [O III]/H$\beta$ 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$\beta$ detection enables the first virial SMBH mass estimate at $z\sim 10$. We derive $M_{\mathrm{B}\mathrm{H}}=3.6_{-1.7}^{+5.8}\times {10}^7{M}_{\odot }$ and ${\lambda }_{\mathrm{E}\mathrm{d}\mathrm{d}}=0.11_{-0.07}^{+0.10}$ 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 ${\lambda }_{\mathrm{E}\mathrm{d}\mathrm{d}}=1$, which yields $M_{\mathrm{B}\mathrm{H}}=2.1_{-0.5}^{+0.6}\times {10}^6{M}_{\odot }$. 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.