SAPPHIRES: A Galaxy Over-Density in the Heart of Cosmic Reionization at $z=8.47$

Yoshinobu Fudamoto; Jakob M. Helton; Xiaojing Lin; Fengwu Sun; Peter Behroozi; Tiger Yu-Yang Hsiao; Eiichi Egami; Andrew J. Bunker; 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; Yongda Zhu

arXiv · 2503.15597

SAPPHIRES: A Galaxy Over-Density in the Heart of Cosmic Reionization at $z=8.47$

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Abstract

We report the discovery of a galaxy proto-cluster candidate (dubbed MACS0416-OD-z8p5) at a spectroscopic redshift of $z\sim8.47$, dating back to $\sim550$Myr after the Big Bang. The observations are part of the JWST Cycle-3 treasury program, Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (SAPPHIRES) with NIRCam-grism. Using wide field slitless spectroscopy (WFSS) obtained in the MACS0416 parallel field, we robustly confirm nine galaxies at $z_{\rm spec}\sim8.47$ via emission line detections of [OIII]5008A (with $>5\,\sigma$) and tentatively confirm one additional galaxy (at $\sim3\,\sigma$). This discovery represents the highest-redshift, spectroscopically confirmed galaxy over-density known to date, which is $\sim6$--$8$ times more dense than the average volume density of galaxies at the same redshift. Furthermore, a galaxy hosting a low-mass active galactic nucleus (``Little-Red-Dot'') is found as a member, suggesting an early emergence of active, massive black holes and feedback between these black holes and their surrounding environments. We also discuss the spatial structures connecting the galaxy over-density to nearby massive star-forming galaxies (separated by $\sim 5$pMpc, including MACS0416-Y1 and MACS0416-JD. This finding of a massive dark matter halo hosting a galaxy over-density at $z\sim8.5$ is surprising given that our survey covered only a small, random field ($16.5\,{\rm arcmin^2}$) as part of a pure parallel observation. The comparison with cosmological simulations shows that the likelihood of finding such a large-scale structure is $<5\,\%$ under the current galaxy formation scenario and the observed survey volume. Our results demonstrate the power of WFSS observations to build a complete line-emitter sample and suggest an important role for over-densities in enhancing galaxy formation by funneling large-scale gas supplies into small cosmological volumes.

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Yoshinobu Fudamoto, Jakob M. Helton, Xiaojing Lin, Fengwu Sun, Peter Behroozi, Tiger Yu-Yang Hsiao, Eiichi Egami, Andrew J. Bunker, 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, Yongda Zhu. 2025-03-19. SAPPHIRES: A Galaxy Over-Density in the Heart of Cosmic Reionization at $z=8.47$. https://arxiv.org/abs/2503.15597

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