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

TL;DR

This study presents a detailed metallicity distribution for the ultra-faint dwarf galaxy Segue 1, based on expanded spectroscopic data, revealing a continuous chemical enrichment history and reaffirming its galaxy status.

Contribution

It provides the first large-scale spectroscopic metallicity sample for Segue 1, enabling a more accurate MDF and insights into its star formation history.

Findings

Average [Fe/H] = -2.52 dex with a 0.59 dex dispersion.

No evidence for distinct stellar subpopulations.

Supports a short, early star formation episode before reionization.

Abstract

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 $\sigma = 0.59 \pm 0.06$ 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.