JWST's GLIMPSE: an overview of the deepest probe of early galaxy formation and cosmic reionization

TL;DR

GLIMPSE addresses how the faintest galaxies contributed to cosmic dawn and reionization by combining ultra-deep JWST/NIRCam imaging with strong gravitational lensing from Abell S1063. The program uses seven broadband and two medium-band filters to reach $5\sigma$ depths of $\sim$30.8–30.9 mag over a $\sim$4.4–4.5 arcmin$^2$ source-plane area, enabling a robust, lensing-boosted census of $z>6$ galaxies ($\sim540$ candidates, including $z\sim16$). A two-halo analytic strong-lensing model calibrated with abundant spectroscopic redshifts yields accurate magnification maps, while advanced foreground subtraction and PSF-homogenized photometry deliver a clean, deep catalog of $>6\times10^4$ sources. Early science results constrain the faint-end UV luminosity function, reveal a modest $[\mathrm{O}\mathrm{III}]+\mathrm{H}\beta$ LF slope at $z\sim7$–9, and enable spatially resolved studies of star formation down to $\sim10$–$100$ pc scales in the first galaxies, all of which refine models of galaxy formation and reionization and demonstrate the power of lensing-enabled JWST surveys.

Abstract

We present an overview of the JWST GLIMPSE program, highlighting its survey design, primary science goals, gravitational lensing models, and first results. GLIMPSE provides ultra-deep JWST/NIRCam imaging across seven broadband filters (F090W, F115W, F200W, F277W, F356W, F444W) and two medium-band filters (F410M, F480M), with exposure times ranging from 20 to 40 hours per filter. This yields a 5$σ$ limiting magnitude of 30.9 AB (measured in a 0.2 arcsec diameter aperture). The field is supported by extensive ancillary data, including deep HST imaging from the Hubble Frontier Fields program, VLT/MUSE spectroscopy, and deep JWST/NIRSpec medium-resolution multi-object spectroscopy. Exploiting the strong gravitational lensing of the galaxy cluster Abell S1063, GLIMPSE probes intrinsic depths beyond 33 AB magnitudes and covers an effective source-plane area of approximately 4.4 arcmin$^2$ at $z \sim 6$. The program's central aim is to constrain the abundance of the faintest galaxies from $z \sim 6$ up to the highest redshifts, providing crucial benchmarks for galaxy formation models, which have so far been tested primarily on relatively bright systems. We present an initial sample of $\sim 540$ galaxy candidates identified at $6 < z < 16$, with intrinsic UV magnitudes spanning $M_{\mathrm UV}$ = $-$20 to $-$12. This enables unprecedented constraints on the extreme faint end of the UV luminosity function at these epochs. In addition, GLIMPSE opens new windows for spatially resolved studies of star clusters in early galaxies and the detection and characterization of faint high-$z$ active galactic nuclei. This paper accompanies the first public data release, which includes reduced JWST and HST mosaics, photometric catalogs, and gravitational lensing models.

Figures (14)

• Figure 1: We compare the depth and survey area of various HST and JWST programs with that of GLIMPSE. Wide-field surveys like COSMOS-Web span hundreds of arcmin$^2$ but are relatively shallow, with magnitude limits around 28--29 AB. In contrast, our GLIMPSE survey takes the opposite approach: it probes extremely deep, reaching intrinsic magnitudes fainter than 33 AB, but over a much smaller area in the source (physical) plane. At $z \sim 6$, the total effective survey area of GLIMPSE is 4.5 arcmin$^2$.
• Figure 2: The ICL and bCGs modeling and subtraction. Examples in the SW filter F200W (top row) and LW filter F444W (bottom row) are shown. For each example we show the original image, the fitted foreground model, and the final corrected image.
• Figure 3: Composite color image of the cluster center from the NIRCam imaging of Module B, which represents the intrinsically deepest image obtained by JWST. Numerous giant arcs and lensing features are visible, extending from around the BCG and to the outskirts. Compact and point-like sources are seen across all the image, and include globular clusters within the galaxy cluster and distant ultra-faint galaxies. Many distant galaxies are also spatially resolved, allowing the study of individual star clusters. The image is a combination of all 7 broadband and 2 medium-band filters
• Figure 4: Depth map in the F090W band for the two NIRCam modules. The 5$\sigma$ magnitude limits are derived from flux measurements in randomly placed apertures of diameter $0\hbox{$.\!\!^{\prime\prime}$}{2}$ across the image. The 16th, 50th, and 84th percentile values are reported.
• Figure 5: Same as Figure \ref{['fig:depth']}, but for the intrinsic depth corrected for lensing using the magnification map.