The metal abundance range in the Ursa Minor dwarf galaxy

Abstract

A small sample of red giants in the Ursa Minor dwarf spheroidal galaxy has been observed with the multichannel scanner and the digital spectrograph of the Hale 5 m telescope. These observations confirm the large width of the giant branch in the color-magnitude diagram and indicate that it is caused by a range in metal abundance of approx.1 dex (i.e., -2.55< or approx. =(Fe/H)< or approx. =-1.36, according to Cohen's scale for globular clusters). Ursa Minor and the Draco dwarf spheroidal appear to have very similar ranges in metal abundance. These observations and those of the other dwarf spheroidal galaxies are compared with the models of the chemical evolution of elliptical galaxies that include mass loss. The models are consistent with the dispersions in metal abundance that are found in the dwarf spheroidals, but now with the lack of detectable dispersions in globular clusters that are comparable in mass to Ursa Minor and Draco (e.g., M92). Some explanations for this difference between the galaxies and the clusters are discussed.