Early Results from GLASS-JWST. I: Confirmation of Lensed $z⩾7$ Lyman-Break Galaxies Behind the Abell 2744 Cluster With NIRISS

Abstract

We present the first search for $z⩾7$, continuum-confirmed sources with NIRISS/WFS spectroscopy over the Abell 2744 Frontier Fields cluster, as part of the GLASS-JWST ERS survey. With $\sim 15$ 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 $z=8.04\pm 0.15$ and $z=7.90\pm 0.13$ 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 ($\sim 2.7-3.8\sigma$) 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 $z⩾7$ sources in the absence of strong emission lines, and gain unprecedented insight into their contributions towards cosmic reionization.

Accepted for publication in ApJL on 1st September 2022, after minor modifications based on the referee report. 10 pages, 3 figures, 1 table

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