First Peek with JWST/NIRCam Wide-field Slitless Spectroscopy: Serendipitous Discovery of a Strong [O III]/Hα Emitter at z = 6.11
Koekemoer, Anton M.; Pirzkal, Nor; Gennaro, Mario; Nikolov, Nikolay; Leisenring, Jarron; Egami, Eiichi; Rieke, Marcia; Sun, Fengwu; Williams, Christina C.; Willmer, Christopher N. A.; Misselt, Karl; Girard, Julien; Correnti, Matteo; Greene, Thomas P.; Stansberry, John; Roellig, Thomas L.; Boyer, Martha; Kelly, Doug; Members of the JWST/NIRCam Commissioning Team
United States
Abstract
We 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 ~55″ 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 ~2 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.