The GLASS James Webb Space Telescope Early Release Science Program. I. Survey Design and Release Plans

TL;DR

GLASS-JWST-ERS presents a deep, multi-instrument JWST ERS program targeting Abell 2744 and two parallel blank fields to address how the universe was reionized and how baryons cycle through galaxies. By pairing high-resolution NIRSpec/NIRISS spectroscopy with NIRCam imaging, the project enables measurements of Lyα velocity offsets, metallicity gradients, and dust attenuation across a wide redshift range, while exploiting lensing magnification to push to the faintest galaxies. Extensive simulations using MIRAGE, Grizli, Lenstronomy, and Pandeia forecast sensitivities, line-flux completeness, and photometric redshift accuracy, informing observing strategies and analysis pipelines. The data will be public in two stages, with Stage I delivering core products and Stage II refining analyses through cross-instrument comparisons, ultimately advancing our understanding of reionization, galaxy evolution, and the accuracy of high-z inferences from JWST data.

Abstract

The GLASS James Webb Space Telescope Early Release Science (hereafter GLASS-JWST-ERS) Program will obtain and make publicly available the deepest extragalactic data of the ERS campaign. It is primarily designed to address two key science questions, namely, "what sources ionized the universe and when?" and "how do baryons cycle through galaxies?", while also enabling a broad variety of first look scientific investigations. In primary mode, it will obtain NIRISS and NIRSpec spectroscopy of galaxies lensed by the foreground Hubble Frontier Field cluster, Abell 2744. In parallel, it will use NIRCam to observe two fields that are offset from the cluster center, where lensing magnification is negligible, and which can thus be effectively considered blank fields. In order to prepare the community for access to this unprecedented data, we describe the scientific rationale, the survey design (including target selection and observational setups), and present pre-commissioning estimates of the expected sensitivity. In addition, we describe the planned public releases of high-level data products, for use by the wider astronomical community.

Figures (11)

• Figure 1: Layout of the GLASS-JWST-ERS observing program. The position of the NIRISS (red solid line) and NIRSpec (yellow solid line) primary fields are shown, along with the NIRCam parallel pointings (dashed lines, color corresponding to the relevant primary instrument) and the extant HFF central (green solid line) and parallel (green dashed line). The background image has been obtained with the Magellan Telescope and will be released as part of the high level data products.
• Figure 2: NIRSpec's power to accurately measure Ly$\alpha$ velocity offsets with respect to systemic. G140H with $R\sim2700$ has a velocity resolution of $\sim100$ km/s, enabling the characterization of the probability of line transmission through the reionizing intergalactic medium (IGM).
• Figure 3: NIRSpec losses for a slit positioned on the UV continuum as a function of projected offset with Ly$\alpha$, in the idealized case of point-like emission for both. For the $\sim1-3$kpc scale offsets typically observed at $z\sim2$ the loss is $\gtrsim$60%--99% for a point source.
• Figure 4: A simulated $\sim26.5$ AB mag $z=8.38$ galaxy laporte17 showing Ly$\alpha$, C4, He II. [O3] and C3] as observed with our NIRISS program and simulated with MIRAGE and Grizli (using the full set of exposures from both GR150C and GR150R dispersers). The input SED is taken from the ASTRODEEP catalogs and artificial emission lines are added for illustration: Ly$\alpha$ flux is assumed to have an integrated flux of $1\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$, while all other rest-frame UV lines have $4\times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ with simple Gaussian profiles . From left to right, the panels show results using the F115W, F150W and F200W filters, respectively. No contamination by foreground galaxies or intra cluster light, and no lensing magnification, have been included in this simulation. Simulations including the effects of lensing magnification and foreground contamination are presented in Section \ref{['sec:NIRISS']}, and Figures \ref{['fig:niriss_sims']} and \ref{['fig:lya_complete']}.
• Figure 5: A $z=1.34$ galaxy magnified by A2744, illustrates the power of combining NIRISS and NIRSpec data. From left: F160W image, and GLASS H$\alpha$, [O3]$\lambda5007$, and gas-phase metallicity maps Wang:2016umWang:2019cfWang:2020bp. Two NIRSpec slits ($0\farcs2\times0\farcs46$) centered on the F160W and [O3] peaks are shown as red/blue boxes. A forecast of the gas-phase metallicity, nebular dust attenuation, and SFR estimates derived from the proposed full-galaxy NIRISS (green), and partial-galaxy NIRSpec data is shown at right. Inferences change dramatically with aperture: metallicity varies as much as 0.4 dex (2.5$\times$) between the two slits. Our sample will showcase the variations within sources and help build statistical recipes to account for aperture effects.