High-Redshift Galaxy Candidates at z > 6 as Revealed by JWST Observations of MACS0647

We present a catalog of 57 high-redshift $z>6$ galaxy candidates, including 14 spectroscopic confirmations ($z = 6.10$ – 9.25), 2 Little Red Dots ($z = 4.77$, 5.81), and 2 interlopers ($z = 3.23$, 3.72), based on \JWST\ NIRCam imaging (7 filters), NIRSpec spectroscopy (PRISM and G395H), and archival \HST\ imaging (17 filters) of the strong lensing galaxy cluster MACS0647. Our highest redshift confirmation ($z = 9.25$) is an Extremely Blue Galaxy (presented in~\citealt{Yanagisawa2024}), and here we identify a spectral turnover likely due to damped Lyman-$α$. We identify an overdensity of galaxies with spectroscopic redshifts $z = 6.1$, confirming the $z \sim 6$ overdensity identified in \HST\ images. In one of these galaxies, our high-resolution G395H spectroscopy reveals two spatially resolved components with a velocity difference of $\sim$90 km/s; if these components are gravitationally bound, this would imply a dynamical mass on the order of $\sim 10^8\ M_\odot$ given their projected separation. We present spectral line fluxes, widths, and derived physical properties, including stellar masses ($10^8 - 10^9 \ \mathrm{M}{\odot}$) and metallicities ($10% - 40% \ \mathrm{Z}{\odot}$) for our spectroscopic sample. We note half of our NIRSpec data was obtained with standard 3-slitlet nods and half was obtained with single slitlets yielding similar results, demonstrating the power to observe more sources on a densely packed NIRSpec MSA.

💡 Research Summary

This paper presents a comprehensive study of high-redshift galaxy candidates behind the strong lensing galaxy cluster MACS0647, utilizing the unparalleled capabilities of the James Webb Space Telescope (JWST). By combining JWST NIRCam imaging (7 filters), NIRSpec spectroscopy (both low-resolution PRISM and high-resolution G395H modes), and archival Hubble Space Telescope (HST) imaging (17 filters), the authors have compiled a robust catalog of 57 galaxy candidates at redshifts z > 6.

The candidate selection process was based on photometric redshift fitting using the EAZY code, requiring a signal-to-noise ratio ≥3 in the F356W band and at least two other filters, alongside a best-fit photometric redshift of z ≥ 6. Spectral Energy Distribution (SED) fitting was further refined using BAGPIPES to derive physical properties. Follow-up spectroscopic observations with JWST NIRSpec yielded definitive confirmations for 14 galaxies, with redshifts ranging from z=6.10 to z=9.25. Two lower-redshift “Little Red Dots” (z=4.77, 5.81) and two interlopers (z=3.23, 3.72) were also identified.

A key finding is the spectroscopic confirmation of an overdensity of galaxies at z=6.1, corroborating earlier hints from HST imaging. This suggests that galaxies were already clustering in the late stages of the Epoch of Reionization. Within this overdensity, high-resolution G395H spectroscopy of one galaxy revealed two spatially resolved components with a velocity difference of approximately 90 km/s. If gravitationally bound, this system would have a dynamical mass on the order of 10^8 solar masses, providing a rare glimpse into the internal dynamics or merger state of an early universe galaxy.

The highest-redshift confirmation in the sample is at z=9.25, an “Extremely Blue Galaxy” where the spectrum shows a turnover likely indicative of a damped Lyman-α absorption wing, probing the neutral gas environment at this epoch. For the spectroscopically confirmed sample, the authors present detailed measurements of emission line fluxes and widths, deriving physical properties such as stellar masses (10^8 – 10^9 M⊙) and metallicities (10% – 40% of solar metallicity).

An important technical aspect of this work is the comparative analysis of NIRSpec Multi-Object Spectroscopy (MOS) observing strategies. The data were obtained using both the standard 3-slitlet nod pattern and a more efficient single-slitlet approach. The study demonstrates that both methods yield similar quality spectra for bright targets, validating the single-slitlet method as a powerful technique for observing a larger number of sources on a densely packed Microshutter Assembly (MSA), thereby optimizing future JWST survey efficiency.

In summary, this research leverages gravitational lensing magnification and JWST’s spectroscopic power to confirm a significant population of early galaxies, characterize their basic properties, identify large-scale structure (overdensity), and resolve internal kinematics. It provides a valuable benchmark catalog and methodology for future investigations into the formation and evolution of galaxies during the first billion years of the universe.