An Ultra-Faint, Chemically Primitive Galaxy Forming in the Reionization Era

Kimihiko Nakajima; Masami Ouchi; Yuichi Harikane; Eros Vanzella; Yoshiaki Ono; Yuki Isobe; Moka Nishigaki; Takuji Tsujimoto; Fumitaka Nakamura; Yi Xu; Hiroya Umeda; Yechi Zhang

arXiv · 2506.11846

An Ultra-Faint, Chemically Primitive Galaxy Forming in the Reionization Era

Kimihiko Nakajima·Masami Ouchi·Yuichi Harikane·Eros Vanzella·Yoshiaki Ono·Yuki Isobe·Moka Nishigaki·Takuji Tsujimoto·Fumitaka Nakamura·Yi Xu·Hiroya Umeda·Yechi Zhang

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Abstract

The formation of the first stars and galaxies marked the onset of chemical enrichment, yet direct observations of such primordial systems remain elusive. Here we present James Webb Space Telescope spectroscopic observations of LAP1-B, an ultra-faint galaxy at redshift z_{spec}=6.625 +/-0.001, corresponding to a cosmic age of 800 million years after the Big Bang, strongly magnified by gravitational lensing. LAP1-B exhibits a gas-phase oxygen abundance of (4.2 +/- 1.8) x 10^{-3} times the solar value, making it the most chemically primitive star-forming galaxy discovered to date. The galaxy displays an exceptionally hard ionizing radiation field, which is inconsistent with chemically enriched stellar populations or accreting black holes but matches theoretical predictions for an exceptionally metal-deficient stellar population. It also shows an elevated carbon-to-oxygen abundance ratio for its metallicity in the interstellar medium, consistent with nucleosynthetic yields from a stellar population formed in the absence of initial metals. The lack of detectable stellar continuum constrains the stellar mass to <3,300 Msun, while the dynamical mass, derived from emission-line kinematics, exceeds the combined stellar and gas mass and indicates a dominant dark matter halo. Our findings establish LAP1-B as a "fossil in the making", a direct high-redshift progenitor of the ancient ultra-faint dwarf galaxies observed in the local Universe, offering a rare window into the earliest stages of galaxy formation.

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Kimihiko Nakajima, Masami Ouchi, Yuichi Harikane, Eros Vanzella, Yoshiaki Ono, Yuki Isobe, Moka Nishigaki, Takuji Tsujimoto, Fumitaka Nakamura, Yi Xu, Hiroya Umeda, Yechi Zhang. 2025-06-13. An Ultra-Faint, Chemically Primitive Galaxy Forming in the Reionization Era. https://arxiv.org/abs/2506.11846

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