Early Results from GLASS-JWST. I: Confirmation of Lensed \(z\geqslant7\) Lyman-Break Galaxies Behind the Abell 2744 Cluster With NIRISS

We present the first search for \(z\geqslant7\), continuum-confirmed sources with NIRISS/WFS spectroscopy over the Abell 2744 Frontier Fields cluster, as part of the GLASS-JWST ERS survey. With \(\sim15\) hrs of pre-imaging and multi-angle grism exposures in the F115W, F150W, and F200W filters, we describe the general data handling (i.e., reduction, cleaning, modeling, and extraction processes) and analysis for the GLASS-JWST survey. We showcase the power of JWST to peer deep into reionization, when most intergalactic hydrogen is neutral, by confirming two galaxies at \(z=8.04\pm0.15\) and \(z=7.90\pm0.13\) by means of their Lyman breaks. Fainter continuum spectra are observed in both the F150W and F200W bands, indicative of blue (\(-1.69\) and \(-1.33\)) UV slopes and moderately-bright absolute magnitudes (\(-20.37\) and \(-19.68\) mag). We do not detect strong Ly\(\alpha\) in either galaxy, but do observe tentative (\(\sim2.7-3.8\sigma\)) HeII\(\lambda\)1640 A, OIII]\(\lambda\lambda\)1661,1666 A, and NIII]\(\lambda\lambda\)1747,1749 A line emission in one, suggestive of low metallicity, star-forming systems with possible non-thermal contributions. These novel observations provide a first look at the extraordinary potential of JWST/NIRISS for confirming representative samples of bright \(z\geqslant7\) sources in the absence of strong emission lines, and gain unprecedented insight into their contributions towards cosmic reionization.