Early Results from GLASS-JWST. I: Confirmation of Lensed z ≥ 7 Lyman-break Galaxies behind the Abell 2744 Cluster with NIRISS
Santini, Paola; Castellano, Marco; Fontana, Adriano; Pentericci, Laura; Treu, Tommaso; Scarlata, Claudia; Henry, Alaina; Strait, Victoria; Willott, Chris; Glazebrook, Karl; Nanayakkara, Themiya; Brammer, Gabriel; Marchesini, Danilo; Boyett, Kristan; Acebron, Ana; Trenti, Michele; Vulcani, Benedetta; Wang, Xin; Grillo, Claudio; Rosati, Piero; Jones, Tucker; Morishita, Takahiro; Malkan, Matthew; Mason, Charlotte; Vanzella, Eros; Mercurio, Amata; Mascia, Sara; Merlin, Emiliano; Roberts-Borsani, Guido; Bradac, Marusa; Bergamini, Pietro; Calabró, Antonello
United States, Denmark, Slovenia, Italy, Australia, Canada
Abstract
We present the first search for z ≥ 7, continuum-confirmed Lyman break sources with NIRISS/WFS spectroscopy over the Abell 2744 Frontier Fields cluster, as part of the GLASS-JWST-ERS survey. With ~15 hr of preimaging and multiangle 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 ± 0.15 and z = 7.90 ± 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 Lyα in either galaxy, but do observe tentative (~2.7-3.8σ) He II λ1640 Å, O III]λλ1661,1666 Å, and N III]λλ1747,1749 Å line emission in one, suggestive of low-metallicity, star-forming systems with possible nonthermal 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 toward cosmic reionization.