Early Results from GLASS-JWST. VI. Extreme Rest-optical Equivalent Widths Detected in NIRISS Wide Field Slitless Spectroscopy

Wide Field Slitless Spectroscopy (WFSS) provides a powerful tool for detecting strong line emission in star-forming galaxies (SFGs) without the need for target preselection. As part of the GLASS-JWST Early Release Science (ERS) program, we leverage the near-infrared wavelength capabilities of NIRISS (1-2.2 μm) to observe rest-optical emission lines out to z ~ 3.4, to a depth and with a spatial resolution higher than ever before (Hα to z < 2.4; [O III]+Hβ to z < 3.4). In this Letter we constrain the rest-frame [O III]λ5007 equivalent width (EW) distribution for a sample of 76 1 < z < 3.4 SFGs in the A2744 Hubble Frontier Field and determine an abundance fraction of extreme emission line galaxies with EW > 750Å in our sample to be 12%. We determine a strong correlation between the measured Hβ and [O III]λ5007 EWs, supporting that the high [O III]λ5007 EW objects require massive stars in young stellar populations to generate the high-energy photons needed to doubly ionize oxygen. We extracted spectra for objects up to 2 mag fainter in the near-infrared than previous WFSS studies with the Hubble Space Telescope. Thus, this work clearly highlights the potential of JWST/NIRISS to provide high-quality WFSS data sets in crowded cluster environments.