An inefficient dwarf: Chemical abundances and the evolution of the Ursa Minor dwarf spheroidal galaxy

TL;DR

This study analyzes chemical abundances in stars of the Ursa Minor dwarf galaxy to understand its star formation history and chemical evolution, revealing prolonged star formation, low efficiency, and gas loss through winds.

Contribution

It provides new detailed chemical abundance data and introduces tailored galaxy chemical evolution models explaining low-metallicity enrichment and gas loss in Ursa Minor.

Findings

Extended star formation lasted nearly 5 Gyr.

Low star formation efficiency explains chemical signatures.

Gas loss through winds accounts for current gas deficiency.

Abstract

We present detailed chemical element abundance ratios of 17 elements in three metal poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine with extant data from the literature to assess the predictions of a novel suite of galaxy chemical evolution models. The spectroscopic data were obtained with the Keck/HIRES instrument and revealed low metallicities of [Fe/H]=-2.12, -2.13 and -2.67 dex. While the most metal poor star in our sample shows an overabundance of [Mn/Fe] and other Fe-peak elements, our overall findings are in agreement with previous studies of this galaxy: elevated values of the [alpha/Fe] ratios that are similar to, or only slightly lower than, the halo values but with SN Ia enrichment at very low metallicity, as well as an enhancement of the ratio of first to second peak neutron capture elements [Y/Ba] with decreasing metallicity. The chemical evolution models which were tailored to reproduce the metallicity distribution function of the dSph, indicate that UMi had an extended star formation which lasted nearly 5 Gyr with low efficiency and are able to explain the [Y/Ba] enhancement at low metallicity for the first time. In particular, we show that the present day lack of gas is probably due to continuous loss of gas from the system, which we model as winds.