SAPPHIRES: A Galaxy Overdensity in the Heart of Cosmic Reionization at z = 8.47
14 affiliations
- 1Chiba University
- 2University of Arizona
- 3Tsinghua University
- 4Center for Astrophysics ∣ Harvard & Smithsonian
- 5Space Telescope Science Institute (STScI)
- 6The Johns Hopkins University
- 7University of Oxford
- 8University of Cambridge
- 9The University of Tokyo
- 10National Astronomical Observatory of Japan
- 11SOKENDAI (The Graduate University for Advanced Studies)
- 12University of Tokyo
- 13University College London
- 14University of Michigan
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.
Related
- First-star imprints in a metal-poor galaxy overdensity near the end of reionization
- Galaxy protoclusters as drivers of cosmic reionization. II. Te-based metallicities of Lyman-α emitters
- RIGEL: Ultra-faint dwarf galaxy diversity shaped by inhomogeneous cosmic reionization
- Can Blue UV-Bright Galaxies Drive Reionization at Cosmic Dawn? An MUV=−19.6 Galaxy with fesc≈0.85 at z=9.244
- Galaxy Protoclusters as Drivers of Cosmic Reionization: II. Te-Based Metallicities of Lyman-α Emitters
BibTeX
@article{fudamoto2026sapphires,
title = {{SAPPHIRES: A Galaxy Overdensity in the Heart of Cosmic Reionization at z = 8.47}},
author = {Yoshinobu Fudamoto and Jakob M. Helton and Xiaojing Lin and Fengwu Sun and Peter Behroozi and Tiger Yu-Yang Hsiao and Eiichi Egami and Andrew J. Bunker and Francesco D’Eugenio and Yuichi Harikane and Masami Ouchi and Yichen Liu and Weizhe Liu and R. Maiolino and Zhiyuan Ji and Xiangyu Jin and Wei Leong Tee and Feige Wang and Christopher N. A. Willmer and Yi Xu and Jinyi Yang and Yongda Zhu},
journal = {The Astrophysical Journal},
year = {2026},
month = sep,
doi = {10.3847/1538-4357/ae743f},
url = {https://doi.org/10.3847/1538-4357/ae743f},
note = {CC BY},
}
RIS
TY - JOUR
TI - SAPPHIRES: A Galaxy Overdensity in the Heart of Cosmic Reionization at z = 8.47
AU - Yoshinobu Fudamoto
AU - Jakob M. Helton
AU - Xiaojing Lin
AU - Fengwu Sun
AU - Peter Behroozi
AU - Tiger Yu-Yang Hsiao
AU - Eiichi Egami
AU - Andrew J. Bunker
AU - Francesco D’Eugenio
AU - Yuichi Harikane
AU - Masami Ouchi
AU - Yichen Liu
AU - Weizhe Liu
AU - R. Maiolino
AU - Zhiyuan Ji
AU - Xiangyu Jin
AU - Wei Leong Tee
AU - Feige Wang
AU - Christopher N. A. Willmer
AU - Yi Xu
AU - Jinyi Yang
AU - Yongda Zhu
PY - 2026
DA - 2026/09/04
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae743f
UR - https://doi.org/10.3847/1538-4357/ae743f
AB - 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.
SN - 0004-637X
SN - 1538-4357
ER -