Year: 2026
Type: article
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.
Source: The Astrophysical Journal
Authors: Yoshinobu Fudamoto, Jakob M. Helton, Xiaojing Lin, Fengwu Sun, Peter Behroozi, Tiger Yu-Yang Hsiao, Eiichi Egami, Andrew J. Bunker, Francesco D’Eugenio, Yuichi Harikane, Masami Ouchi, Yichen Liu, Weizhe Liu, Roberto Maiolino, Zhiyuan Ji, Xiangyu Jin, Wei Leong Tee, Feige Wang, Christopher N. A. Willmer, Yi Xu, Jinyi Yang, Yongda Zhu
Institutions: University of Arizona, Chiba University, Tsinghua University, Center for Astrophysics Harvard & Smithsonian, Space Telescope Science Institute, Johns Hopkins University, University of Oxford, University of Cambridge, The University of Tokyo, The Graduate University for Advanced Studies, SOKENDAI, National Astronomical Observatory of Japan, University College London, University of Michigan
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G103270039, G1199145202, G1409812818, G1824424381, G2144673021, G256731684, G3152952368, G3575130984, G4915654406, G5480708011, G5568641063, Comprehensive understanding of the formation history of structures in the Universe, G7935691682, Observational studies of dust-obscured galaxies in the early Universe, G8931419319
OpenAlex ID: W7208793847