First Peek with JWST/NIRCam Wide-field Slitless Spectroscopy: Serendipitous Discovery of a Strong [O iii]/Hα Emitter at z = 6.11

We report the serendipitous discovery of an [O iii ] λ λ 4959/5007 and H α 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 ∼55″ away from the flux calibrator P330-E, this galaxy exhibits bright [O iii ] λ λ 4959/5007 and H α lines detected at 3.7 σ , 9.9 σ , and 5.7 σ , respectively, with a spectroscopic redshift of z = 6.112 ± 0.001. The total H β +[O iii ] equivalent width is 664 ± 98 Å (454 ± 78 Å from the [O iii ] λ 5007 line). This provides direct spectroscopic evidence for the presence of strong rest-frame optical lines (H β +[O iii ] and H α ) 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 ∼2 Myr, as our SED modeling suggests. The tentative detection of He ii λ 4686 line (1.9 σ ), 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.