MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. III. METALLICITY DISTRIBUTIONS OF MILKY WAY DWARF SATELLITE GALAXIES

摘要

We present metallicity distribution functions (MDFs) for the central regions of eight dwarf satellite galaxies ofudthe Milky Way: Fornax, Leo I and II, Sculptor, Sextans, Draco, Canes Venatici I, and Ursa Minor. We use theudpublished catalog of abundance measurements from the previous paper in this series. The measurements are basedudon spectral synthesis of iron absorption lines. For each MDF, we determine maximum likelihood fits for Leaky Box,udPre-Enriched, and Extra Gas (wherein the gas supply available for star formation increases before it decreases to zero) analytic models of chemical evolution. Although the models are too simplistic to describe any MDF in detail,uda Leaky Box starting from zero metallicity gas fits none of the galaxies except Canes Venatici I well. The MDFs ofudsome galaxies, particularly the more luminous ones, strongly prefer the Extra Gas Model to the other models. Only for Canes Venatici I does the Pre-Enriched Model fit significantly better than the Extra Gas Model. The best-fitudeffective yields of the less luminous half of our galaxy sample do not exceed 0.02Z_⊙, indicating that gas outflowudis important in the chemical evolution of the less luminous galaxies. We surmise that the ratio of the importanceudof gas infall to gas outflow increases with galaxy luminosity. Strong correlations of average [Fe/H] and metallicity spread with luminosity support this hypothesis.