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

Keduse Worku, Tiger Yu-Yang Hsiao, Dan Coe, Abdurro'uf, Tom Resseguier, Rebecca L. Larson, Jacqueline Antwi-Danso, Gabriel Brammer, Vasily Kokorev, Larry D. Bradley, Lukas J. Furtak, Masamune Oguri

First listed 2025-12-12 | Last updated 2025-12-12

Abstract

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.

Short digest

JWST imaging and spectroscopy of the MACS0647 lensing field yield a catalog of 57 z>6 candidates, with 14 spectroscopic confirmations spanning z=6.10–9.25, plus two Little Red Dots and two low‑z interlopers. A tight overdensity at z≈6.1 is confirmed, and G395H resolves two components in one member separated by ~90 km/s, implying Mdyn≈10^8 Msun if bound. The highest‑z system, EBG‑1 at z=9.25, shows a spectral turnover consistent with damped Lyα, while spectroscopic fits indicate low stellar masses (10^8–10^9 Msun) and subsolar metallicities (0.1–0.4 Z⊙). Practically, single‑slitlet NIRSpec nods deliver results comparable to standard 3‑slitlet patterns, enabling denser MSA targeting.

Key figures to inspect

• Figure 1: Map the spatial layout of high‑z candidates and spectroscopic members behind MACS0647 to visualize the z≈6.1 overdensity and lensing geometry.
• Figure 2: Compare PRISM spectra and SED fits for v4 3754 / v7 5191 under single‑slitlet vs 3‑slitlet nods to assess line detections and the equivalence of observing strategies.
• Figure 3: Inspect the PRISM 2D/1D spectra of four z≈6.1 members to see the common emission‑line set (e.g., [O III], Hβ) and the narrow redshift slice defining the overdensity.
• Figure 4: For EBG‑1 (v4 3568 / v7 4922, z=9.25), examine the 2D/1D spectra and SED to locate the damped‑Lyα–like spectral turnover and gauge the modest lensing magnification.
• G395H panels (where shown in the text/figures): Look for the galaxy with two spatially resolved components at z≈6.1 to verify the ~90 km/s offset and infer the ~10^8 Msun dynamical mass from the projected separation.