SAPPHIRES: A Galaxy Overdensity in the Heart of Cosmic Reionization at z = 8.47

Journal article

We report the discovery of a galaxy protocluster candidate (dubbed MACS0416-OD-z8p5) at a spectroscopic redshift of z ∼ 8.47, dating back to ∼550 Myr after the Big Bang. The observations are part of the JWST Cycle 3 Treasury Program Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (or SAPPHIRES) using NIRCam grism observations. Using wide-field slitless spectroscopy (WFSS) obtained in the MACS0416 parallel field, we robustly confirm nine galaxies at zspec ∼ 8.47 via emission-line detections of [O iii] 5008 Å (with >5σ) and tentatively confirm one additional galaxy (at ∼3σ). This discovery represents the highest-redshift spectroscopically confirmed galaxy overdensity known to date and is ∼6–8 times more dense than the average number density of galaxies at the same redshift. Furthermore, a galaxy hosting a low-mass active galactic nucleus (a “little red dot”) is found as a member, suggesting that black hole growth may already be underway in dense environments at early cosmic times. This finding of a massive dark matter halo hosting a galaxy overdensity at z ∼ 8.5 is surprising given that our survey covered only a small, random field ( 16.5arcmin2 ) as part of a pure-parallel observation. Comparison with cosmological simulations shows that the likelihood of finding such a large-scale structure is <5% under the current galaxy formation scenario and within the observed survey volume. Our results demonstrate the power of WFSS observations for building complete samples of line-emitting galaxies and suggest an important role for overdensities in enhancing galaxy formation by funneling large-scale gas supplies into small cosmological volumes.

Authors: Fudamoto, Y; Helton, JM; Lin, X; Sun, F; Behroozi, P

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 1008
• Issue: 2
• Pages: 217-217
• Publication date: 2026-09-04
• DOI: 10.3847/1538-4357/ae743f
• EISSN: 1538-4357
• ISSN: 0004-637X
• Language: English
• Keywords: Dark matter; Galactic halo; Dark galaxy; Reionization; Redshift; Galaxy formation and evolution; Halo; COSMIC cancer database; Galaxy