An emerging baryon cycle in a galaxy 500 million years after the Big Bang

Shengzhe Wang; Xin Wang; Hang Zhou; Zhijie Qu; Zhaozhou Li; Yuxuan Pang; Qianqiao Zhou; Shouyi Wang; Yangyao Chen; Yuguang Chen; Karl Glazebrook; Glenn G. Kacprzak; Nicha Leethochawalit; Houjun Mo; Themiya Nanayakkara; Huiyuan Wang; Weida Hu; Xunda Sun; Chao-Wei Tsai; Hu Zhan

arXiv · 2608.09813

An emerging baryon cycle in a galaxy 500 million years after the Big Bang

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Abstract

The emergence of stellar feedback as a regulator of galaxy growth marks a fundamental transition in cosmic history. At early times, rapid gas accretion and collapse may induce intense star formation before feedback becomes effective, producing feedback-free starbursts. When and how such bursts subsequently develop into self-regulated baryon cycles remain observationally unknown. Here we show that Gz9p3, a merging galaxy at $z=9.311$, is caught in this transition only 500 million years after the Big Bang. Deep JWST spectroscopy reveals a substantial neutral-gas reservoir along its merger-driven tidal structure and a multiphase outflow. Fine-structure absorption provides the first direct measurement of the electron density of the cool outflowing gas at high redshift ($\approx\,17\,{\rm cm^{-3}}$), yielding a mass-loading factor among the highest yet measured for galaxies of comparable stellar mass. The emergence of such efficient feedback after an intense burst is consistent with the delayed onset of feedback expected in feedback-free starburst models. The cool outflowing gas is unlikely to escape the host halo, implying that much of this metal-enriched material may remain available for future recycling through the circumgalactic medium. Gz9p3 therefore provides an early view of a baryon cycle being established through the interplay of merger-driven gas redistribution, bursty star formation and stellar feedback, suggesting that feedback-regulated recycling was already shaping galaxy growth during the epoch of reionization.

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Shengzhe Wang, Xin Wang, Hang Zhou, Zhijie Qu, Zhaozhou Li, Yuxuan Pang, Qianqiao Zhou, Shouyi Wang, Yangyao Chen, Yuguang Chen, Karl Glazebrook, Glenn G. Kacprzak, Nicha Leethochawalit, Houjun Mo, Themiya Nanayakkara, Huiyuan Wang, Weida Hu, Xunda Sun, Chao-Wei Tsai, Hu Zhan. 2026-08-10. An emerging baryon cycle in a galaxy 500 million years after the Big Bang. https://arxiv.org/abs/2608.09813

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