First Peek with JWST/NIRCam Wide-Field Slitless Spectroscopy: Serendipitous Discovery of a Strong [O III]/H$\hat{I}\pm$ Emitter at $z=6.11$

Abstract

We report the serendipitous discovery of an [O III] $\hat{I}»\hat{I}»$4959/5007 and H$\hat{I}\pm$ line emitter in the Epoch of Reionization (EoR) with the JWST commissioning data taken in the NIRCam wide field slitless spectroscopy (WFSS) mode. Located $\sim$55" away from the flux calibrator P330-E, this galaxy exhibits bright [O III] $\hat{I}»\hat{I}»$4959/5007 and H$\hat{I}\pm$ lines detected at 3.7, 9.9 and 5.7$\ddot{I}ƒ$, respectively, with a spectroscopic redshift of $z=6.112\pm 0.001$. The total H${\hat{I}}^2$+[O III] equivalent width is 664$\pm$98 à (454$\pm$78 à from the [O III] $\hat{I}»$5007 line). This provides direct spectroscopic evidence for the presence of strong rest-frame optical lines (H${\hat{I}}^2$+[O III] and H$\hat{I}\pm$) in EoR galaxies as inferred previously from the analyses of Spitzer/IRAC spectral energy distributions. 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 $\sim$2 Myr as our SED modeling suggests. The tentative detection of He II $\hat{I}»$4686 line ($1.9\ddot{I}ƒ$), 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.

10 pages, 4 figures, published in the ApJL