Early Results from GLASS-JWST. I: Confirmation of Lensed z ≥ 7 Lyman-break Galaxies behind the Abell 2744 Cluster with NIRISS
Guido Roberts-Borsani, Takahiro Morishita, Tommaso Treu, Gabriel Brammer, Victoria Strait, Xin Wang, Maruša Bradač, Ana Acebrón, P. Bergamini, Kristan Boyett, Antonello Calabrò, M. Castellano, A. Fontana, Karl Glazebrook, C. Grillo, Alaina Henry, Tucker Jones, Matthew A. Malkan, Danilo Marchesini, Sara Mascia, Charlotte Mason, A. Mercurio, E. Merlin, Themiya Nanayakkara, L. Pentericci, Piero Rosati, P. Santini, Claudia Scarlata, Michele Trenti, E. Vanzella, Benedetta Vulcani, Chris J. Willott
Abstract
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 Ly α in either galaxy, but do observe tentative (∼2.7–3.8 σ ) He ii λ 1640 Å, O iii ] λλ 1661,1666 Å, and N iii ] λλ 1747,1749 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.