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

Ultra-faint dwarf galaxies (UFDs; stellar masses \(M_* < 10^5\,M_\odot\)) 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_IIK absorption feature, measured from low-resolution (\(R\sim1000\)) Keck/LRIS spectroscopy of 40 stars in the UFD Segue~1 (\(M_* \approx 500\,M_\odot\)), 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] \(= -2.52 \pm 0.10\) dex and a dispersion of \(\sigma = 0.59 \pm 0.06\) dex, and is best described by a single Gaussian, with no evidence for multimodal substructure. This is consistent with a continuous, short-duration ($\lesssim$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.