Early Results from GLASS-JWST. I: Confirmation of Lensed $z\geqslant7$ Lyman-Break Galaxies Behind the Abell 2744 Cluster With NIRISS (2207.11387v2)

Abstract: 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.

• The paper confirms z≥7 galaxies via continuum break detection using JWST/NIRISS, providing a robust alternative to Lyα emission methods.
• It employs extensive pre-imaging and multi-filter grism exposures to accurately measure redshifts of lensed galaxies GLASSz8-1 (z=8.04) and GLASSz8-2 (z=7.90).
• The methodology reduces biases from IGM effects on Lyα, enhancing our understanding of early galaxy formation and cosmic reionization.

Spectroscopic Confirmation of High-Redshift Galaxies Using JWST/NIRISS

The paper "Early Results from GLASS-JWST. I: Confirmation of Lensed $z \geqslant 7$ Lyman-Break Galaxies Behind the Abell 2744 Cluster With NIRISS" by Roberts-Borsani et al. presents significant observations of early-universe galaxies using the James Webb Space Telescope (JWST). This paper is part of the GLASS-JWST Early Release Science (ERS) program, focusing on the Abell 2744 galaxy cluster, a well-studied region due to its gravitational lensing properties, which can amplify distant and faint objects.

Observational Methodology

The research employs the Near-Infrared Imager and Slitless Spectrograph (NIRISS) aboard the JWST for slitless spectroscopy, a technique valuable for observing multiple objects simultaneously without the need for pre-defined slits. The authors utilized extensive pre-imaging and grism exposures in multiple filters (F115W, F150W, and F200W) over approximately 15 hours, allowing them to handle complex observations involving data reduction, contamination modeling, and spectral extraction.

Key Results

The paper achieved notable results by confirming the redshifts of two lensed galaxies, designated GLASSz8-1 and GLASSz8-2, at redshifts 8.04 and 7.90 respectively, using the presence of Lyman-breaks, which are significant drops in brightness occurring due to hydrogen absorption at specific wavelengths. The galaxies exhibited sharp breaks indicative of the Lyman-alpha transition, with blue UV slopes (-1.69 and -1.33) and magnitudes ($\sim$ -20.37 and -19.68 respectively).

Additionally, both galaxies showed no significant Ly$\alpha$ emission, confirming their detection primarily through continuum break rather than emission lines. This methodological approach mitigates the bias introduced by Ly$\alpha$ transmission being affected by the intergalactic medium, thus enabling a broader and more accurate view of the high-redshift universe.

Implications and Future Scope

The implications of this research are twofold—empirically enhancing our understanding of high-redshift galaxy properties and methodologically setting a precedent for future spectroscopic studies using JWST. By confirming galaxies via continuum breaks, the paper opens avenues for unbiased sampling of distant galaxies during the reionization epoch.

From a theoretical standpoint, the paper contributes to the understanding of cosmic reionization. The ability to confirm galaxies without relying on Ly$\alpha$ serves as a methodological advantage, offering a clearer view into galaxy formation and evolution phenomena of the universe at epochs where direct information is sparse.

Future developments with JWST are likely to expand upon these findings by deepening the understanding of the early universe through higher resolution and more sensitive spectroscopic capabilities. Ensuing studies will benefit from the refinements in data processing and modeling techniques as demonstrated by Roberts-Borsani et al., further unraveling the characteristics and influence of early galaxies on cosmic reionization.

In summary, this research exemplifies the potential of JWST/NIRISS in astrophysical explorations of the early universe, offering a robust framework for observing and analyzing galaxies from a period pivotal in cosmic history. As further data from JWST emerges, we can anticipate notable advances in our comprehension of the universe's infancy.