JWST UNCOVER: Discovery of >9 Galaxy Candidates Behind the Lensing Cluster Abell 2744

The study of galaxy formation in the early universe, particularly during the epoch of reionization, is a crucial area of modern astrophysics. Prior to the James Webb Space Telescope (JWST), observations of galaxies at redshifts greater than 6 were limited, with only a handful known at z > 9. This limitation stems from the near-infrared (NIR) wavelength coverage of the Hubble Space Telescope (HST), which is insufficient to observe the redshifted ultraviolet (UV) light of these distant galaxies. JWST, with its extended NIR coverage and significantly enhanced sensitivity, offers unprecedented capabilities for observing these early galaxies. Several early JWST studies reported the discovery of z > 9 galaxy candidates, pushing the observational limits of galaxy formation. However, these early results, while groundbreaking, also raised questions about the inferred number density and brightness of these high-redshift galaxies, which appeared to exceed theoretical predictions. Some candidates were later confirmed at z ~12-13, while others were identified as low-redshift interlopers. This emphasizes the need for larger, deeper surveys and spectroscopic follow-up observations to confirm these high-redshift identifications and understand the implications for galaxy formation models. The UNCOVER (Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization) Treasury survey, utilizing the gravitational lensing of the Abell 2744 cluster, aims to address these limitations by providing deeper multi-wavelength NIRCam imaging and subsequent NIRSpec spectroscopic follow-up. This paper focuses on the detection and characterization of z > 9 galaxy candidates within the UNCOVER survey, aiming to enhance our understanding of the early universe.

Previous studies using HST and Spitzer had already identified galaxies at redshifts exceeding z~6, but observations beyond z~9 were scarce. Oesch et al. (2018) and Bowler et al. (2020) reported on some early detections of z > 9 galaxies. The advent of JWST observations significantly increased the number of candidates reported in various studies including Atek et al. (2023), Donnan et al. (2023), Harikane et al. (2023), Adams et al. (2023), and Austin et al. (2023). Many of these challenged the distance record set by HST, but spectroscopic confirmation was crucial to rule out low-redshift interlopers. The high number density of luminous galaxies at z>12 was attributed by several studies to a variety of factors, including reduced dust attenuation at higher redshifts, enhanced star formation efficiency, non-standard initial mass functions (IMFs), or even non-ΛCDM cosmologies. However, these interpretations often depend on the accuracy and completeness of the observations and their subsequent modeling. The UNCOVER survey, with its combination of deep imaging and planned spectroscopy, provides an opportunity to address some of the existing uncertainties and provide more robust results.

This study utilizes deep NIRCam and NIRISS imaging data from three JWST observing programs: UNCOVER, GLASS, and a DDT program, along with ancillary HST observations. The total field of view covers ~45 arcmin², reaching a 5σ limiting magnitude of ~29.2 AB. Two photometric catalogs were constructed: a general UNCOVER catalog and a custom catalog optimized for high-redshift galaxy detection. The object detection and photometry employed different aperture sizes and deblending parameters in the two catalogs. The custom catalog is designed to better handle faint, high-redshift sources, minimizing the impact of blending with brighter objects. High-redshift galaxy candidates were selected using a combination of color-color dropout criteria and photometric redshifts. These criteria were designed to minimize contamination from lower-redshift interlopers like cool stars and quiescent galaxies. Specific color-color windows were defined for selecting candidates in the redshift ranges 9 < z < 11 and 11 < z < 15. The color-color criteria were validated by simulating the photometry of various stellar and galactic templates. In parallel to the dropout selection, photometric redshifts were estimated using SED fitting with both BEAGLE and Eazy software, utilizing different galaxy templates and star formation histories. To minimize lensing uncertainties, the initial SED fitting was performed on unlensed fluxes. The gravitational lensing magnification factors were determined using a new UNCOVER cluster mass model based on multiple image systems. A quality assessment was performed on the candidates based on the robustness and consistency of the photometric redshift solutions from the two different codes. Finally, the physical properties of the candidates, including stellar mass, star formation rate, stellar age, and UV continuum slope, were estimated from refined SED fitting with BEAGLE, incorporating Gaussian priors for the redshift and utilizing a more flexible star formation history.

The UNCOVER survey identified a total of 19 high-redshift galaxy candidates: 16 in the redshift range 9 < z < 12 and 3 in the range 12 < z < 13. Seven of these candidates are deemed robust, showing narrow and consistent high-redshift solutions. The lensing amplification for the candidates ranged from μ = 1.2 to 11.5. The lensing-corrected luminosities span a considerable range, and the galaxies show generally young ages (10-100 Myr), low stellar masses (6.8 < log(M*/M⊙) < 9.5), and low star formation rates (SFR = 0.2-7 M⊙ yr⁻¹). Several galaxies at z ~9-10 displayed a clear Balmer break, providing additional constraints on their stellar masses. The UV continuum slopes were measured to be between β = -1.8 and -2.3, typical for early galaxies at z > 9 but not as extreme as the bluest sources recently discovered. The study found evidence for a rapid redshift evolution of the mass-luminosity relation and a redshift evolution of the UV continuum slope for a given intrinsic magnitude. Spectroscopic confirmation was obtained for two candidates at z = 9.76 and z = 9.3, supporting the photometric redshift estimates. The comparison with theoretical predictions from hydrodynamical simulations and semi-analytical models indicates a general agreement in the redshift evolution of the mass-luminosity relation, particularly the steeper mass-luminosity relation at higher redshifts. The observed UV slopes are generally consistent with theoretical models that consider stellar populations with low dust attenuation and metallicities, aligning with expectations for young galaxies at high redshifts.

The findings of this study provide further insight into galaxy formation in the early universe. The detection of numerous z > 9 galaxy candidates, particularly the robustly confirmed sources, strengthens the evidence for a significant population of galaxies at these early epochs. The observed properties of these galaxies, namely their young ages, low stellar masses, and low star formation rates, are consistent with theoretical expectations for galaxies in the early universe. The observed redshift evolution of the mass-luminosity relation and the UV continuum slope supports the theoretical models that predict these trends. The significant number of candidates discovered suggests that the population of galaxies at z > 9 might be more abundant than previously estimated. However, it is important to note that the current sample may not be fully representative of the underlying galaxy population due to the relatively modest magnification factors for most of the candidates. Future deep observations and spectroscopic follow-up are needed to establish a more complete understanding of the galaxy population at these early epochs. These discoveries highlight the power of JWST and the importance of deep surveys combined with spectroscopic confirmation to study galaxy formation during the epoch of reionization.

This paper presents a significant step forward in our understanding of galaxy formation at z > 9, thanks to the JWST UNCOVER survey. The detection of 19 high-redshift galaxy candidates, seven of which are deemed robust, and the confirmation of two via spectroscopy confirms the power of this method. The observed properties are broadly consistent with theoretical expectations. Future work should focus on obtaining deeper JWST observations of a larger area and securing spectroscopic confirmation for all of the candidates, which will help to refine our models and improve understanding of galaxy formation during the early universe.

The primary limitation of this study is the reliance on photometric redshifts. While the use of two independent codes and rigorous selection criteria helps mitigate errors, some uncertainty remains. Furthermore, the sample of detected galaxies may not be completely representative of the overall population of z > 9 galaxies, as the magnification provided by the gravitational lensing is not uniform across the field. The study also relies on theoretical models for interpreting the observed properties, and these models have inherent limitations and uncertainties. Finally, the sample size of spectroscopically confirmed galaxies is limited, hindering a more detailed analysis of individual galactic properties and evolution.