The Metallicity Distribution of the Ultra-Faint Dwarf Galaxy Segue 1

Ultra-faint dwarf galaxies (UFDs; stellar masses M * < 10 5 M ⊙ ) offer unique insights into early chemical evolution in low-mass systems. However, interpreting their metallicity distribution functions (MDFs) has been challenging due to limited spectroscopic samples, especially beyond the red giant branch. We present metallicities from the Ca II K absorption feature, measured from low-resolution ( R ∼ 1000 ) Keck/LRIS spectroscopy of 40 stars in the UFD Segue~1 ( M * ≈ 500 M ⊙ ), including both red giant branch and main-sequence turnoff stars, resulting in a metallicity sample more than six times larger than previously published data for Segue~1. The resulting MDF has an average [Fe/H] = − 2.52 ± 0.10 dex and a dispersion of σ = 0.59 ± 0.06 dex, and is best described by a single Gaussian, with no evidence for multimodal substructure. This is consistent with a continuous, short-duration ($$1~Gyr) episode of star formation and chemical enrichment prior to reionization. The nonzero metallicity spread reaffirms its classification as a galaxy. Segue~1 highlights the rich chemical enrichment histories present even in the least massive galaxies, and underscores the importance of deep spectroscopic follow-up to fully characterize these ancient stellar systems.