Abstract
Abstract
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
z
spec
∼ 8.47 via emission-line detections of [O
iii
] 5008 Å (with >5
σ
) and tentatively confirm one additional galaxy (at ∼3
σ
). 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 (
16.5
arcmin
2
) 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.