[Paper Review] Early Results from GLASS-JWST. XIX: A High Density of Bright Galaxies at $z\approx10$ in the Abell 2744 Region

M. Castellano, A. Fontana+39 more|arXiv (Cornell University)|Dec 13, 2022

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study reports the detection of seven bright galaxies at redshift $z\approx10$ in the Abell 2744 field using deep JWST/NIRCam imaging from GLASS-JWST, UNCOVER, and DDT#2756. Leveraging an improved lensing model with MUSE spectroscopy and limiting magnification to $\mu<5$, the authors find a galaxy number density 3–10 times higher than average, suggesting a possible overdensity requiring spectroscopic confirmation.

ABSTRACT

We report the detection of a high density of redshift $z\approx 10$ galaxies behind the foreground cluster Abell 2744, selected from imaging data obtained recently with NIRCam onboard {\it JWST} by three programs -- GLASS-JWST, UNCOVER, and DDT\#2756. To ensure robust estimates of the lensing magnification $μ$, we use an improved version of our model that exploits the first epoch of NIRCam images and newly obtained MUSE spectra, and avoids regions with $μ&gt;5$ where the uncertainty may be higher. We detect seven bright $z\approx 10$ galaxies with demagnified rest-frame $-22 \lesssim M_{ m UV}\lesssim -19$ mag, over an area of $\sim37$ sq. arcmin. Taking into account photometric incompleteness and the effects of lensing on luminosity and cosmological volume, we find that the density of $z\approx 10$ galaxies in the field is about $10 imes$ ($3 imes$) larger than the average at $M_{UV}\approx -21~ (-20)$ mag reported so far. The density is even higher when considering only the GLASS-JWST data, which are the deepest and the least affected by magnification and incompleteness. The GLASS-JWST field contains 5 out of 7 galaxies, distributed along an apparent filamentary structure of 2 Mpc in projected length, and includes a close pair of candidates with $M_{ m UV}&lt; -20$ mag having a projected separation of only 16 kpc. These findings suggest the presence of a $z\approx 10$ overdensity in the field. In addition to providing excellent targets for efficient spectroscopic follow-up observations, our study confirms the high density of bright galaxies observed in early {\it JWST} observations, but calls for multiple surveys along independent lines of sight to achieve an unbiased estimate of their average density and a first estimate of their clustering.

Motivation & Objective

• To identify and characterize high-redshift galaxies at $z\approx10$ in the Abell 2744 field using early JWST data.
• To assess the true number density of $z\approx10$ galaxies by correcting for lensing magnification and cosmic variance.
• To evaluate whether the observed high density is due to a physical overdensity or chance superposition of galaxies.
• To provide a robust, high-fidelity lensing model to improve photometric redshift and luminosity estimates.
• To identify high-priority targets for follow-up spectroscopy with NIRSpec.

Proposed method

• Combines deep NIRCam imaging from three JWST programs: GLASS-JWST, UNCOVER, and DDT#2756, covering ~37 sq. arcmin in the Abell 2744 field.
• Applies an improved lensing model using first-epoch NIRCam data and new MUSE spectroscopic redshifts to estimate magnification $\mu$ with reduced uncertainty.
• Limits analysis to regions with $\mu<5$ to minimize model uncertainty and ensure reliable magnification corrections.
• Uses photometric redshifts and Lyman breaks to select $z\approx10$ candidates, with robustness checked against known interloper populations.
• Computes number density in three UV magnitude bins ($-22 \lesssim M_{\rm UV} \lesssim -19$) and compares with other JWST surveys to assess cosmic variance.
• Performs completeness corrections and accounts for cosmological volume effects to derive intrinsic galaxy counts.

Experimental results

Research questions

• RQ1 Is the observed high density of $z\approx10$ galaxies in Abell 2744 a statistical fluctuation or a true overdensity?
• RQ2 What is the intrinsic number density of bright $z\approx10$ galaxies after correcting for lensing magnification and survey incompleteness?
• RQ3 Are the detected galaxies part of a filamentary structure or a compact group, as suggested by their spatial distribution?
• RQ4 Can spectroscopic redshifts confirm the photometric redshifts and rule out contamination from rare low-redshift interlopers?
• RQ5 How does the observed galaxy density in Abell 2744 compare to the average from other JWST fields, and what does this imply for cosmic variance?

Key findings

• Seven bright $z\approx10$ galaxies were detected with demagnified rest-frame UV magnitudes between $-22$ and $-19$ mag.
• The number density of $z\approx10$ galaxies in the Abell 2744 region is 3 to 10 times higher than the average from other JWST surveys, depending on the magnitude bin.
• Five of the seven galaxies are spatially aligned along a filament-like structure of projected length ~2 Mpc.
• One galaxy was spectroscopically confirmed at $z=9.3127$, supporting the photometric redshift estimates.
• The improved lensing model, incorporating MUSE spectra and multiple images, reduces uncertainty in magnification estimates, especially in regions with $\mu<5$.
• The sample includes two galaxies previously reported in the first GLASS-JWST data release, confirming the robustness of the detection method.