Sun, FengwuFengwuSunEgami, EiichiEiichiEgamiPirzkal, NorNorPirzkalRieke, MarciaMarciaRiekeBoyer, MarthaMarthaBoyerCORRENTI, MatteoMatteoCORRENTIGennaro, MarioMarioGennaroGirard, JulienJulienGirardGreene, Thomas P.Thomas P.GreeneKelly, DougDougKellyKoekemoer, Anton M.Anton M.KoekemoerLeisenring, JarronJarronLeisenringMisselt, KarlKarlMisseltNikolov, NikolayNikolayNikolovRoellig, Thomas L.Thomas L.RoelligStansberry, JohnJohnStansberryWilliams, Christina C.Christina C.WilliamsWillmer, Christopher N.A.Christopher N.A.Willmer2025-03-072025-03-0720222041-8205http://hdl.handle.net/20.500.12386/36472We report the serendipitous discovery of an [O iii] λ λ4959/5007 and Hα line emitter in the Epoch of Reionization (EoR) with the James Webb Space Telescope (JWST) commissioning data taken in the NIRCam wide-field slitless spectroscopy (WFSS) mode. Located 1/455″ away from the flux calibrator P330-E, this galaxy exhibits bright [O iii] λ λ4959/5007 and Hα lines detected at 3.7σ, 9.9σ, and 5.7σ, respectively, with a spectroscopic redshift of z = 6.112 ± 0.001. The total Hβ+[O iii] equivalent width is 664 ± 98 Å (454 ± 78 Å from the [O iii] λ5007 line). This provides direct spectroscopic evidence for the presence of strong rest-frame optical lines (Hβ+[O iii] and Hα) in EoR galaxies as inferred previously from the analyses of the Spitzer/IRAC spectral energy distributions (SEDs). Two spatial and velocity components are identified in this source, possibly indicating that this system is undergoing a major merger, which might have triggered the ongoing starburst with strong nebular emission lines over a timescale of 1/42 Myr, as our SED modeling suggests. The tentative detection of He ii λ4686 line (1.9σ), if real, may indicate the existence of very young and metal-poor star-forming regions with a hard UV radiation field. Finally, this discovery demonstrates the power and readiness of the JWST/NIRCam WFSS mode, and marks the beginning of a new era for extragalactic astronomy, in which EoR galaxies can be routinely discovered via blind slitless spectroscopy through the detection of rest-frame optical emission lines.STAMPAenArticle10.3847/2041-8213/ac89382-s2.0-85137691834https://api.elsevier.com/content/abstract/scopus_id/85137691834https://iopscience.iop.org/article/10.3847/2041-8213/ac8938FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation