Metallicity distributions in the Draco, Ursa Minor and Sculptor dwarf spheroidal galaxies.

摘要

This study used the 3.5m WIYN telescope and its multiobject spectrograph to measure the strengths of the infrared Call triplet in 95, 70, and 34 red giants in the Draco, Ursa Minor, and Sculptor dwarf spheroidal (dSph) galaxies, respectively. The metallicity calibration was derived from observations of red giants in globular clusters of well determined metallicity. The derived abundances from the Call triplet are consistent with the few measurements made by others at high dispersion when allowance is made for the 0.2 dex offset in [Ca/Fe] that the high dispersion studies have detected between the red giants in the dSph galaxies and the globular clusters.; The metallicities from the Call triplet yield mean abundances of ⟨[Fe/H]⟩ = -1.99, -2.03, and -1.86, for Draco, Ursa Minor, and Sculptor, respectively, and wide dispersions; -3.1 ≤ [Fe/H] ≤ -1.1, -3.0 ≤ [Fe/H] ≤ -1.2, and -2.7 ≤ [Fe/H] ≤ -1.0, respectively. Although the abundance ranges are similar, the dSphs exhibit different metallicity distributions. The abundance distribution in Ursa Minor exhibits a narrow central peak. The abundance distribution in Draco may be bimodal, with statistical significance just below 90%. There is evidence for a metallicity gradient in both Draco and Sculptor, with a concentration of more metal-rich stars inside the core radius. This gradient can explain the horizontal branch and red giant branch gradients detected by others. The data in Ursa Minor are consistent with no gradient.; Two variations on the "Simple Model" of chemical evolution have been compared to the metallicity distributions of these galaxies. In one, chemical evolution follows the standard closed-box model until it abruptly ends because the remaining gas is ejected by supernova driven winds. In the second, gas is lost continuously from the galaxy at a rate that is proportional to the star formation rate. Both models adequately fit the abundance distributions of Draco and Sculptor, but the continuous mass loss model fails for Ursa Minor. The sharp distribution in Ursa Minor suggests that star formation was more efficient relative to the others, removing gas more abruptly and preventing a significant metallicity gradient. The differences imply that star formation and chemical enrichment processes proceeded somewhat differently in these dSphs.