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

Abstract

Ultra-faint dwarf galaxies (UFDs, $M_{\ast }<10^5{M}_{\odot }$) 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 ($M_{\ast }\approx 500M_{\odot }$), 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 $\ddot{I}ƒ=0.59\pm 0.06$ 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.

12 pages, 6 figures. Submitted to The Open Journal of Astrophysics