Kavli Affiliate: Anna Frebel
| Summary:
Ultra-faint dwarf galaxies (UFDs, $M_* < 10^5 M_odot$) 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 II K absorption feature, measured from low-resolution ($R sim 1000$) 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 $σ= 0.59 pm 0.06$ dex, with no evidence for distinct subpopulations. 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.
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